The quantum source of space-time(nature.com)
nature.com
The quantum source of space-time
http://www.nature.com/news/the-quantum-source-of-space-time-1.18797
123 comments
I am a physicist...
The most interesting (to me, at any rate) bit of this article are susskind's observations on computational complexity. As a precocious undergrad I put forward the idea that gravity and time dilation were the same thing, and both stemmed from the universe needing a constant amount of "time" to compute the interrelations between the particles in a volume of space - and if there were more particles more time would be needed as the graph of particle interactions would grow non-linearly - and therefore time "slows down" in a matter (information) dense area of spacetime.
What really got me gunned down was then going on to argue that this suggested a simulated universe, running in a substrate.
Either way, interesting to see someone else who actually has some clout having similar ideas.
The most interesting (to me, at any rate) bit of this article are susskind's observations on computational complexity. As a precocious undergrad I put forward the idea that gravity and time dilation were the same thing, and both stemmed from the universe needing a constant amount of "time" to compute the interrelations between the particles in a volume of space - and if there were more particles more time would be needed as the graph of particle interactions would grow non-linearly - and therefore time "slows down" in a matter (information) dense area of spacetime.
What really got me gunned down was then going on to argue that this suggested a simulated universe, running in a substrate.
Either way, interesting to see someone else who actually has some clout having similar ideas.
Why do you (and others) tend towards arguments suggesting simulations?
To me, the language here is important. Both simulation and computation imply (to me) a tool and a user of the tool. Even a broader interpretation of computation as, umm, the efficient transfer of precise information through spacetime in certain shapes (which is easily providable by our understanding of physics today) requires an observer to extract the computation from the otherwise non-semantic system.
It definitely makes sense to think of the universe as a projection of a higher dimension, or a holograph, or however you want to look at it, but that's a far cry from implying a simulation.
EDIT: hologram -> holograph
To me, the language here is important. Both simulation and computation imply (to me) a tool and a user of the tool. Even a broader interpretation of computation as, umm, the efficient transfer of precise information through spacetime in certain shapes (which is easily providable by our understanding of physics today) requires an observer to extract the computation from the otherwise non-semantic system.
It definitely makes sense to think of the universe as a projection of a higher dimension, or a holograph, or however you want to look at it, but that's a far cry from implying a simulation.
EDIT: hologram -> holograph
A simulation doesn't mean that there is a simulator - just that what we consider real may be a projection of something else. As you say, the holographic universe principle can equally answer this.
That said, the simulation argument strongly pushes in favour of a simulated reality - particularly when you consider that there is no need for it to be real time, or of the entire universe. In short if we ever gain the capability to accurately simulate a small corner of the universe, then it stands to reason that someone further up the hylaean flow (apologies to Stephenson) also has.
Hell, maybe the rotation of distant galaxies is off because they're just sprites ;)
That said, the simulation argument strongly pushes in favour of a simulated reality - particularly when you consider that there is no need for it to be real time, or of the entire universe. In short if we ever gain the capability to accurately simulate a small corner of the universe, then it stands to reason that someone further up the hylaean flow (apologies to Stephenson) also has.
Hell, maybe the rotation of distant galaxies is off because they're just sprites ;)
Hell, maybe the rotation of distant galaxies is off because they're just sprites
So there's probably some other simulation of our universe where they tried to do models, but it was too slow.
If not, then the simulator introduced a pre-mature optimization. The existential ramifications of such a development inefficiency are palpable!
So there's probably some other simulation of our universe where they tried to do models, but it was too slow.
If not, then the simulator introduced a pre-mature optimization. The existential ramifications of such a development inefficiency are palpable!
What do you feel about the mathematical universe hypothesis[1]? If I understand it correctly (and I probably don't), it says that the universe is literally mathematics. Not that it's a manifestation of mathematics or can be modeled by mathematics, but is 100% mathematics.
I think that fits the computation model and doesn't require an observer or operator.
[1]: https://en.wikipedia.org/wiki/Mathematical_universe_hypothes...
I think that fits the computation model and doesn't require an observer or operator.
[1]: https://en.wikipedia.org/wiki/Mathematical_universe_hypothes...
Mathematics is a human language for studying the structure of various experiences. To say the universe "is" mathematics without saying what mathematics is doesn't really say much at all. You might as well say it's magic.
And, saying that it's a "mathematical structure" is just saying that it's a structure that humans can apply mathematics to model. So there's really no mechanism for this to be true, except in the trivial "the universe has structure" sense, which also tells you (basically) nothing, because humans can't even perceive things that don't have structure, nor can we reason about them.
And, saying that it's a "mathematical structure" is just saying that it's a structure that humans can apply mathematics to model. So there's really no mechanism for this to be true, except in the trivial "the universe has structure" sense, which also tells you (basically) nothing, because humans can't even perceive things that don't have structure, nor can we reason about them.
> Mathematics is a human language for studying the structure of various experiences
I think you are getting hung up on the semiotics of mathematics which is indeed a human construct. The idea of "1 + 1 = 2" is true no matter how (or if) you represent the idea.
> saying that it's a "mathematical structure" is just saying that it's a structure that humans can apply mathematics to model
That's not what the theory says (AFAIK). The mathematical universe hypothesis says that there is nothing in the universe, it's all just mathematics.
Edit: Think about it this way: if you could simulate a universe and in that simulated universe you modeled people and all the stuff around them, what would you tell those simulated people with their simulated free well what a table is made out of? It's all mathematics, right?
I think you are getting hung up on the semiotics of mathematics which is indeed a human construct. The idea of "1 + 1 = 2" is true no matter how (or if) you represent the idea.
> saying that it's a "mathematical structure" is just saying that it's a structure that humans can apply mathematics to model
That's not what the theory says (AFAIK). The mathematical universe hypothesis says that there is nothing in the universe, it's all just mathematics.
Edit: Think about it this way: if you could simulate a universe and in that simulated universe you modeled people and all the stuff around them, what would you tell those simulated people with their simulated free well what a table is made out of? It's all mathematics, right?
The math universe hypotheses assumes a very strict Platonist POV, which is not really a justifiable position, in my experience.
>The idea of "1 + 1 = 2" is true no matter how (or if) you represent the idea.
That's really a triviality, though. If you have the idea that 1+1=2, then yes, it's true. Usually people say mathematical theorems are true in "all possible universes", but the way they select which universes are possible is... by applying logic that works in this universe. 1+1=2 is only true because it doesn't contradict our experiences. It's useful. In another universe, it may not be useful, and thus wouldn't be true there.
>if you could simulate a universe and in that simulated universe you modeled people and all the stuff around them, what would you tell those simulated people with their simulated free well what a table is made out of?
I would put words in whatever order was most useful to them to employ for the construction and manipulation of tables. Anything else would be meaningless. You might as well just say "magic" if they can't use it.
>The idea of "1 + 1 = 2" is true no matter how (or if) you represent the idea.
That's really a triviality, though. If you have the idea that 1+1=2, then yes, it's true. Usually people say mathematical theorems are true in "all possible universes", but the way they select which universes are possible is... by applying logic that works in this universe. 1+1=2 is only true because it doesn't contradict our experiences. It's useful. In another universe, it may not be useful, and thus wouldn't be true there.
>if you could simulate a universe and in that simulated universe you modeled people and all the stuff around them, what would you tell those simulated people with their simulated free well what a table is made out of?
I would put words in whatever order was most useful to them to employ for the construction and manipulation of tables. Anything else would be meaningless. You might as well just say "magic" if they can't use it.
> 1+1=2 is only true because it doesn't contradict our experiences.
Not quite. In the mathematical sense, 1+1=2 is true because (a) it is a well-formed sentence in a certain formal system and (b) there exists a proof of it in that formal system. The formal system consists of a grammar for well-formed sentences as well as rules for deriving true sentences.
The key point here is that the definition of "true" is part of the definition of the formal system. Unlike in philosophy, we have a clear and unambiguous definition of what "true" means, and its evaluation does not depend on properties of our physical universe.[0]
So when people say that "mathematical theorems are true in all possible universes", they're well-intentioned but I would argue that they are misleading. The deeper (philosophical) truth is that the (mathematical) truth of mathematical theorems is independent of universes[1].
[0] The fact that our minds are drawn to thinking about the specific type of formal systems that are usually considered to be reasonable foundations for mathematics may well be a consequence of the properties of our physical existence.[2] However, if by some magic the definitions of formal systems studied by alien civilizations in a different physical universe were to be transmitted to us, we would be able to arrive at the same conclusions about those systems as the aliens, and vice versa. There would be no disagreement about what sentences are true in such an alien formal system.
[1] Where I use the word "universe" in the physical sense and not in the sense that is found in set theory and its ilk.
[2] However, I personally don't think so. I believe (though I cannot prove it) that something like the Church-Turing hypothesis also applies to foundational formal systems at least up to basic set theory (possibly minus the axiom of choice). It is conceivable that some alien civilization in a much stranger universe would consider a certain extension of our set theory as the natural choice for mathematical foundations, but they would recognize our set theory as a subset of what they're studying.
Not quite. In the mathematical sense, 1+1=2 is true because (a) it is a well-formed sentence in a certain formal system and (b) there exists a proof of it in that formal system. The formal system consists of a grammar for well-formed sentences as well as rules for deriving true sentences.
The key point here is that the definition of "true" is part of the definition of the formal system. Unlike in philosophy, we have a clear and unambiguous definition of what "true" means, and its evaluation does not depend on properties of our physical universe.[0]
So when people say that "mathematical theorems are true in all possible universes", they're well-intentioned but I would argue that they are misleading. The deeper (philosophical) truth is that the (mathematical) truth of mathematical theorems is independent of universes[1].
[0] The fact that our minds are drawn to thinking about the specific type of formal systems that are usually considered to be reasonable foundations for mathematics may well be a consequence of the properties of our physical existence.[2] However, if by some magic the definitions of formal systems studied by alien civilizations in a different physical universe were to be transmitted to us, we would be able to arrive at the same conclusions about those systems as the aliens, and vice versa. There would be no disagreement about what sentences are true in such an alien formal system.
[1] Where I use the word "universe" in the physical sense and not in the sense that is found in set theory and its ilk.
[2] However, I personally don't think so. I believe (though I cannot prove it) that something like the Church-Turing hypothesis also applies to foundational formal systems at least up to basic set theory (possibly minus the axiom of choice). It is conceivable that some alien civilization in a much stranger universe would consider a certain extension of our set theory as the natural choice for mathematical foundations, but they would recognize our set theory as a subset of what they're studying.
If you received a formal statement from an alien in another universe, it might not even look formal from your perspective. Mathematical theorems are not necessarily independent of universes, because those factors we consider necessary for formalization may not apply in other universes. You don't know if contradictions in this universe must be contradictions elsewhere.
> I would put words in whatever order was most useful to them to employ for the construction and manipulation of tables. Anything else would be meaningless
Is your objection to the mathematical universe mostly that it's not useful to know everything is just mathematics? That our reality is essentially the same as if it were a simulation is just trivia but doesn't help you make a table?
Is your objection to the mathematical universe mostly that it's not useful to know everything is just mathematics? That our reality is essentially the same as if it were a simulation is just trivia but doesn't help you make a table?
Pretty much. You say it's all mathematics, I'll say it's a simulation. Or maybe it's a dream. Or a the manifestation of another being's will. Or something completely incomprehensible.
Why should anyone care that it's mathematics? It's certainly not like any mathematics that we know; we can't do it some other way if we don't understand it. It's too big to fit in a brain, and too complex to simplify accurately. Since we don't understand its axioms, how do we know they form a mathematical structure?
Why should anyone care that it's mathematics? It's certainly not like any mathematics that we know; we can't do it some other way if we don't understand it. It's too big to fit in a brain, and too complex to simplify accurately. Since we don't understand its axioms, how do we know they form a mathematical structure?
> Why should anyone care that it's mathematics?
Well, if the universe is a simulation, then you can start poking around for the limits of the simulation and finding out new things is something that lots of people enjoy.
http://www.technologyreview.com/view/429561/the-measurement-...
Well, if the universe is a simulation, then you can start poking around for the limits of the simulation and finding out new things is something that lots of people enjoy.
http://www.technologyreview.com/view/429561/the-measurement-...
I actually love that theory because it makes me feel warm and fuzzy. However, it is.... Highly hard to defend, to say the least.
This is a fascinating idea. Could you elaborate on how you think the additional computational complexity would give rise to attraction between particles in matter-dense regions?
Spacetime just stretches to reduce the effective density in a flat universe and maintain an effectively constant information density - thus gravity and time dilation.
Isn't that what Wolfram is also getting at with his irreducible complexity concept?
I wonder if the trick about this is not really to find a true explanation but the right perspective and more and more simulation seems to be a very valuable perspective to put on this.
Disclaimer — I am not a physicist so apologies if my question is stupid.
I wonder if the trick about this is not really to find a true explanation but the right perspective and more and more simulation seems to be a very valuable perspective to put on this.
Disclaimer — I am not a physicist so apologies if my question is stupid.
Beautiful stuff, man. One of the more interesting questions I've seen asked in my casual hobby readings lately is: "Is the Universe quantized?" Meaning of course, is there a final limit to how small things get, and, with the passing of time, does it flow freely or pass in very tiny quantized periods?
Tangentially related, this basically makes the argument that some black holes (like low mass X-ray binaries) are actually societies that have managed to control matter near the planck scale, best read as hard sci-fi: http://accelerating.org/articles/Smart-2011-TranscensionHypo...
> is there a final limit to how small things get
AFAIK yes, the limits are known as Planck Constants. It means that we are living in a digital universe.
https://en.wikipedia.org/wiki/Planck_constant
There are also conjectures about a _holographic_ universe.
https://en.wikipedia.org/wiki/Holographic_principle
https://www.youtube.com/results?search_query=holographic+uni...
AFAIK yes, the limits are known as Planck Constants. It means that we are living in a digital universe.
https://en.wikipedia.org/wiki/Planck_constant
There are also conjectures about a _holographic_ universe.
https://en.wikipedia.org/wiki/Holographic_principle
https://www.youtube.com/results?search_query=holographic+uni...
The existence of the Planck length does not imply quantized spacetime, which is still very much an open question in the field of physics. To the best of my knowledge most models still posit a continuous spacetime. cf. https://www.quora.com/Is-time-discrete-or-continuous
I don't see how that would work with length contraction and time dilation. A unit quantum length object in my frame of reference would have different length to one in a frame at motion relative to mine, and time for it would pass at different quantum clock ticks. I don't see how a universe like that could work when it came to interactions between moving particles.
You're assuming that the Lorentz contraction applies unchanged as you move to the Planck length. That's not a given.
[deleted]
Why does it imply a simulation? Perhaps it can be explained by an anthropic argument: Perhaps the kind of mechanics behind spacetime in which time evolution is computed in a stepwise manner depending on density are extraordinarily more likely because they somehow allow for much simpler mechanics (sets of laws with lower entropy are more likely to be the ones we find ourselves in, especially so if we assume that all non-physical sets of laws are realized as well, so that the impact of such a difference in entropy on the probabilties could be vast).
This seems relevant. It's just a ride.
https://www.youtube.com/watch?v=KgzQuE1pR1w
https://www.youtube.com/watch?v=KgzQuE1pR1w
Well, with these kind of articles you can usually safely replace "physicists" with "some physicists".
Anyway, the part that I know, and which is well established, is that there seems to be a correspondence between a d+1 dimensional quantum gravity theories, and d dimensional quantum field theories (e.g the standard model). A while ago this was still purely conjecture, but Maldacena showed that this correspondence was true (under certain conditions) for an AdS space with a conformal field theory on its boundary. As far as I know this is now pretty well established, most of the discussion seems to be on how much it can be generalized.
From my (limited) understanding it seems that (in AdS/CFT) entangled particles tend to correspond to strings traversing from one part of the boundary, through the bulk, to another part of the boundary, the space the string travels through then forms a tube like object. This gives a link between entanglement and geometry, since these tubes exist if and only if the two points on the boundary are connected.
I think it isn't disputed that this happens for AdS/CFT, but whether something similar is true for our universe is debatable. If however it turns out that the holographic principle does hold in general then that would imply that our universe somehow corresponds to a 2+1 dimensional field theory, where entanglement of this field theory is responsible for the fact that our space-time is connected.
Anyway, the part that I know, and which is well established, is that there seems to be a correspondence between a d+1 dimensional quantum gravity theories, and d dimensional quantum field theories (e.g the standard model). A while ago this was still purely conjecture, but Maldacena showed that this correspondence was true (under certain conditions) for an AdS space with a conformal field theory on its boundary. As far as I know this is now pretty well established, most of the discussion seems to be on how much it can be generalized.
From my (limited) understanding it seems that (in AdS/CFT) entangled particles tend to correspond to strings traversing from one part of the boundary, through the bulk, to another part of the boundary, the space the string travels through then forms a tube like object. This gives a link between entanglement and geometry, since these tubes exist if and only if the two points on the boundary are connected.
I think it isn't disputed that this happens for AdS/CFT, but whether something similar is true for our universe is debatable. If however it turns out that the holographic principle does hold in general then that would imply that our universe somehow corresponds to a 2+1 dimensional field theory, where entanglement of this field theory is responsible for the fact that our space-time is connected.
I believe Maldacena's 1999 paper [1] proving the AdS/CFT correspondence is the most cited in theoretical physics in the last 50 years. It's an incredibly nice result that is almost impossible to explain to laymen. (You did a pretty good job.)
[1] https://scholar.google.no/scholar?cluster=132568844183140142...
[1] https://scholar.google.no/scholar?cluster=132568844183140142...
Maldacena conjectured a correspondence. It's never been proven.
He managed to prove it quite rigorously for a specific example, which is the paper semi-extrinsic mentioned. He did indeed not prove the correspondence in general, but even this 'limited' example already turned out to be immensely useful.
If any two particles are connected by entanglement, the physicists suggested, then they are effectively joined by a wormhole. And vice versa: the connection that physicists call a wormhole is equivalent to entanglement. They are different ways of describing the same underlying reality.
In other words, there's no such thing as "spooky action at a distance", because with particles joined by a wormhole there is no distance between them?
In other words, there's no such thing as "spooky action at a distance", because with particles joined by a wormhole there is no distance between them?
Information transfer was never a problem with spooky action. But if all entangled particles were linked by some kind of funky wormhole it seems a harder task to explain why FTL information transfer isn't routinely possible.
All of the arguments about why it's not possible remain true, even if connected by some sort of wormhole that means they "really are" right next to each other in some sense. You still have the fundamental problem that you can't select what you collapse, so you still have to send a conventional signal to "decode" the putative instantaneous signal.
Remember the correspondence principle will still be in play; future quantum theories will still have to limit out to what we know today, because the QM of today is arguably the most rigorously tested theory in humanity's history, by number of significant digits. FTL communication still will have all of the problems conventional relativity says it will, for all the same reasons.
Entanglement has suggested for a long time that there may (in clumsy English words) be some sort of "real reality" that isn't necessarily constrained by what we think of as space and time. In fact even relativity looked at in a certain manner has suggested this; you can travel from any point in the universe to any other, barring black holes, along null spacetime intervals. Null spacetime intervals have no distinction between the points in them, because they all come out the same 0 in the metric measurement. I wouldn't expect that any of this new math is going to change anything; it may explain where the constraints of space and time come from, but explaining the constraints doesn't mean that the explanation will come with a way to get around them!
In fact my personal observation is that the fundamental limits of space and time have been getting stronger as we learn more about physics, not weaker; a mathematically rigorous derivation of the fundamental speed-of-light limit from a "more fundamental" level of reality locks the door even tighter, it doesn't open it.
Remember the correspondence principle will still be in play; future quantum theories will still have to limit out to what we know today, because the QM of today is arguably the most rigorously tested theory in humanity's history, by number of significant digits. FTL communication still will have all of the problems conventional relativity says it will, for all the same reasons.
Entanglement has suggested for a long time that there may (in clumsy English words) be some sort of "real reality" that isn't necessarily constrained by what we think of as space and time. In fact even relativity looked at in a certain manner has suggested this; you can travel from any point in the universe to any other, barring black holes, along null spacetime intervals. Null spacetime intervals have no distinction between the points in them, because they all come out the same 0 in the metric measurement. I wouldn't expect that any of this new math is going to change anything; it may explain where the constraints of space and time come from, but explaining the constraints doesn't mean that the explanation will come with a way to get around them!
In fact my personal observation is that the fundamental limits of space and time have been getting stronger as we learn more about physics, not weaker; a mathematically rigorous derivation of the fundamental speed-of-light limit from a "more fundamental" level of reality locks the door even tighter, it doesn't open it.
Indeed. When I saw a novel proposal for embedding QM inside GR using closed time-like curves (rather than the usual vice-versa), the main problem seemed to be that a GR-computer is potentially _much_ more powerful than a QM-computer, so you have to explain where the extra power has gone.
I think many theories start off at "spooky action at a distance." Imagine what Newton thought about how gravity works - two massive objects affecting each other for no apparent reason: spooky action over a distance.
The difference with the Newtonian case is that the effects, although without a visible linkage, were directly related to masses and distance. There was no concept of, say, the lighting of a particular candle in Africa also lighting the one on your desk while the thousands of candles in-between remain unaffected.
Do I understand this right that if the conjecture can be proved correct and the remaining issue about time gets solved we effectively have a grand unified theory?
Probably not. But it will be a big step in the direction.
There are two unifications that need to happen. One is the unification of the strong force with electro-weak force(the unified version of electromagnatic force and weak force). The other is gravity with the unification of the three forces.
As far as I understand, this work is concentrating on the second unification, and not the same. Though it might open the door for the first one too. Someone closer to this area should comment.
There are two unifications that need to happen. One is the unification of the strong force with electro-weak force(the unified version of electromagnatic force and weak force). The other is gravity with the unification of the three forces.
As far as I understand, this work is concentrating on the second unification, and not the same. Though it might open the door for the first one too. Someone closer to this area should comment.
No. Our space-time is definately NOT Anti de Sitter (AdS). AdS/CFT is nothing more than very nice mathematics with applications in different fields of physics. It's not applicable to our universe.
"The theory holds that gravity is geometry: particles are deflected ... not because they feel a force ... but because space and time around the object are curved."
Can anyone explain to me why it can't simply be a force? If you accept that gravity is caused by energy and not mass (which is obviously the case), then I see no reason a photon can not experience a force.
So what is the reason to insist it's geometry?
And related to that, I still have never seen an explanation of how geometry is supposed to cause something to start moving without a force. Explaining how it deflects something, I get. How does it start moving in the first place?
The only explanation I've gotten is that things are always moving - through time, and the geometry just transfers some of that motion into physical motion. But that explanation is very very very lacking since all things do not move through time at the same rate (because of the various types of time dilation that are possible), yet the gravitational force is identical.
Can anyone explain to me why it can't simply be a force? If you accept that gravity is caused by energy and not mass (which is obviously the case), then I see no reason a photon can not experience a force.
So what is the reason to insist it's geometry?
And related to that, I still have never seen an explanation of how geometry is supposed to cause something to start moving without a force. Explaining how it deflects something, I get. How does it start moving in the first place?
The only explanation I've gotten is that things are always moving - through time, and the geometry just transfers some of that motion into physical motion. But that explanation is very very very lacking since all things do not move through time at the same rate (because of the various types of time dilation that are possible), yet the gravitational force is identical.
That explanation that "things are always moving - through time" is exactly correct. You need to think of the full four dimensional spacetime. It is that spacetime which is curved rather than just space.
The crucial concept is Newton's first law (objects continue on their trajectory if a force is not applied). The straight lines in the 4D spacetime (geodesics) - the lines that an object would follow if no force is applied - correspond to the paths that look as if a gravitational force is applied.
The crucial concept is Newton's first law (objects continue on their trajectory if a force is not applied). The straight lines in the 4D spacetime (geodesics) - the lines that an object would follow if no force is applied - correspond to the paths that look as if a gravitational force is applied.
In order to help people think about curvature and gravity, look at the following examples:
- The surface of the earth is a 2 dimensional positive curved space. To see this, draw a triangle with corners on the north pole, on the equator near Somalia and on the equator in Equador. The resulting triangle has a sum of all corners > 180 degrees.
- In a negative curved space, the sum would be less than 180 degrees. In a flat space, it is equal to 180 degrees.
- Another way to see the curvature of the surface of the earth is to observe that it's impossible to draw 2 parallel lines that do not intersect.
- The 2D torus (e.q. the surface of a donut) is flat. Test it with triangles.
- The towers of the Verrazano–Narrows Bridge are wider at their top than at their base. This has nothing to do with the earth have a positive curvature. Test it with a torus.
- 3D space is nearly always flat in the universe, especially at the surface of our planet.
- 4D space-time is not remotely flat. If I throw up a ball, it will come down. This is due the mass of the earth curving its surrounding 4D space-time. The straight line for a ball in the curved space-time looks like the ball changes directions and comes down in our flat 3D space.
- If you try to find the triangle of a sphere with the biggest sum of corners, you'll discover that the outside and inside of a triangle are interchangeable. We've entered the field of topology now and this has nothing to do with its curvature.
- The surface of the earth is a 2 dimensional positive curved space. To see this, draw a triangle with corners on the north pole, on the equator near Somalia and on the equator in Equador. The resulting triangle has a sum of all corners > 180 degrees.
- In a negative curved space, the sum would be less than 180 degrees. In a flat space, it is equal to 180 degrees.
- Another way to see the curvature of the surface of the earth is to observe that it's impossible to draw 2 parallel lines that do not intersect.
- The 2D torus (e.q. the surface of a donut) is flat. Test it with triangles.
- The towers of the Verrazano–Narrows Bridge are wider at their top than at their base. This has nothing to do with the earth have a positive curvature. Test it with a torus.
- 3D space is nearly always flat in the universe, especially at the surface of our planet.
- 4D space-time is not remotely flat. If I throw up a ball, it will come down. This is due the mass of the earth curving its surrounding 4D space-time. The straight line for a ball in the curved space-time looks like the ball changes directions and comes down in our flat 3D space.
- If you try to find the triangle of a sphere with the biggest sum of corners, you'll discover that the outside and inside of a triangle are interchangeable. We've entered the field of topology now and this has nothing to do with its curvature.
>It is that spacetime which is curved rather than just space.
This is the crucial point that many discussions overlook. The concept of "curvature of space" is obvious nonsense because curvature is measured with respect to space. For space itself to curve implies the existence of meta-space (and as many nested metaspaces as you like). But "spacetime" does not work like "space", and there actually is no metaspace. Talking about "space" curving or expanding is unnecessarily confusing.
This is the crucial point that many discussions overlook. The concept of "curvature of space" is obvious nonsense because curvature is measured with respect to space. For space itself to curve implies the existence of meta-space (and as many nested metaspaces as you like). But "spacetime" does not work like "space", and there actually is no metaspace. Talking about "space" curving or expanding is unnecessarily confusing.
>> It is that spacetime which is curved rather than just space.
> This is the crucial point that many discussions overlook
I agree completely with this (and have said as much in the past --https://www.reddit.com/r/cosmology/comments/2zy19s/bowling_b...) but
> The concept of "curvature of space" is obvious nonsense
This is emphatically not correct. Space can be curved without being embedded in a higher "meta-space". You can have a 2D surface that is curved as if it were the surface of a sphere without it actually being on an actual 3D sphere -- it just has to "connect up" the right way and have "parallel" lines bend towards each other and so on. Ditto in more dimensions.
> This is the crucial point that many discussions overlook
I agree completely with this (and have said as much in the past --https://www.reddit.com/r/cosmology/comments/2zy19s/bowling_b...) but
> The concept of "curvature of space" is obvious nonsense
This is emphatically not correct. Space can be curved without being embedded in a higher "meta-space". You can have a 2D surface that is curved as if it were the surface of a sphere without it actually being on an actual 3D sphere -- it just has to "connect up" the right way and have "parallel" lines bend towards each other and so on. Ditto in more dimensions.
The concept of "curvature of space" is obvious nonsense because curvature is measured with respect to space.
This is not really true. If you take a flat two dimensional space, a sheet of paper, this space has no intrinsic curvature. You surely now imagine this sheet of paper laying flat on a table in front of you embedded in our usual three dimensional space. Now pick it up and role it to form a cylinder. This gives extrinsic curvature to the space, curvature in the space the sheet is embedded in. But the sheet has still no intrinsic curvature, if you were a two dimensional creature living on the sheet you can not tell whether it is laying flat on the table or is rolled up to a cylinder, at least ignoring the fact that the cylinder connects two opposite edges of the space.
The surface of the earth on the other hand has intrinsic curvature, angles of triangles don't add up to 180 degrees for example. And this intrinsic curvature is a feature of the space not of the embedding into a higher dimensional space. It is not easy to visualize if possible at all, but spaces can have intrinsic curvature independent or even without an embedding, our three dimensional space can be curved without being embedded in a higher dimensional space and the same of course holds for space-time.
This is not really true. If you take a flat two dimensional space, a sheet of paper, this space has no intrinsic curvature. You surely now imagine this sheet of paper laying flat on a table in front of you embedded in our usual three dimensional space. Now pick it up and role it to form a cylinder. This gives extrinsic curvature to the space, curvature in the space the sheet is embedded in. But the sheet has still no intrinsic curvature, if you were a two dimensional creature living on the sheet you can not tell whether it is laying flat on the table or is rolled up to a cylinder, at least ignoring the fact that the cylinder connects two opposite edges of the space.
The surface of the earth on the other hand has intrinsic curvature, angles of triangles don't add up to 180 degrees for example. And this intrinsic curvature is a feature of the space not of the embedding into a higher dimensional space. It is not easy to visualize if possible at all, but spaces can have intrinsic curvature independent or even without an embedding, our three dimensional space can be curved without being embedded in a higher dimensional space and the same of course holds for space-time.
"Extrinsic curvature" is normally called just "curvature". "Intrinsic curvature" isn't normally called anything because it has minimal relevance to everyday life. If you talk about "curvature of space" the natural assumption is that you mean extrinsic curvature.
But in general relativity extrinsic curvature is the irrelevant thing, it's all about intrinsic curvature. We don't think of space-time being embedded in a higher dimensional space but we still talk about curvature of space-time, its intrinsic curvature.
These demonstrations with balls rolling on a stretchable rubber sheet to visualize gravity are really misleading in this regard because they use a good deal of extrinsic curvature to make things work but in reality mass doesn't deform space-time into a fifth dimension or at least it doesn't necessarily do so.
These demonstrations with balls rolling on a stretchable rubber sheet to visualize gravity are really misleading in this regard because they use a good deal of extrinsic curvature to make things work but in reality mass doesn't deform space-time into a fifth dimension or at least it doesn't necessarily do so.
Curvature is not "measured with respect to [an embedding] space". If you have a triangle and the sum of the angles is not 180 degrees that's a non-Euclidian (curved) space.
But that's not how the word "curved" is used in everyday life. Call it non-Euclidian if it's non-Euclidian. Redefining common words only leads to confusion.
Participating in a discussion about an article titled "the quantum source of space-time" to say that basic concepts are nonsense because some words do not have the same meaning as in everyday life also leads to confusion.
Take a 50-kg object (110 pounds) and carry it in your arms for an hour on a flat road. You will have done no work against gravity. You may complain that this is not how the word "work" is used in everyday life - and that you will have done a lot of "work" against gravity (preventing the object from falling to the ground). However, if you wish to discuss Physics and Mathematics, using terms that have precise definitions in those fields, you cannot object on the basis that these words do not have the same meaning as non-Physicist and non-Mathematicians ascribe to them.
Why stop there? Field, black hole, wormhole, string, force, spin, colour, charm, strangeness. All of these terms and more are used by physicists and have very different meanings to those used in "everyday life".
Good questions.
It could be a force - if someone finds a theory that describes all the observational data. They haven't.
The reason to insist geometry is that general relativity manages to predict extremely accurately the result of all gravitational experiments/observations as of 2015. In science, prediction comes before explanation, meaning if you have to accept whatever explanation the best predictive theory offers and not vice versa.
Well, its has been explained - you just haven't gone through the right books. There are two answers, which I will simplify. One, the Big Bang threw everything all over the place and now masses are moving so as to reach the state of minimum energy. Two, there are 3 other forces in the universe which can cause masses to move. For example you can use electromagnetic forces during the combustion of rocket fuel to send a satellite in space. The satellite then moves in orbit because of the geometry of spacetime. Same when stars explode - this involves all three forces.
It could be a force - if someone finds a theory that describes all the observational data. They haven't.
The reason to insist geometry is that general relativity manages to predict extremely accurately the result of all gravitational experiments/observations as of 2015. In science, prediction comes before explanation, meaning if you have to accept whatever explanation the best predictive theory offers and not vice versa.
Well, its has been explained - you just haven't gone through the right books. There are two answers, which I will simplify. One, the Big Bang threw everything all over the place and now masses are moving so as to reach the state of minimum energy. Two, there are 3 other forces in the universe which can cause masses to move. For example you can use electromagnetic forces during the combustion of rocket fuel to send a satellite in space. The satellite then moves in orbit because of the geometry of spacetime. Same when stars explode - this involves all three forces.
" In science, prediction comes before explanation"
I'm going to be a little picky about your choice of language here.
It seems to me that, at least that your are predicting randomly, you can't possible make predictions with a good model, that is, a good explanation.
I'm going to be a little picky about your choice of language here.
It seems to me that, at least that your are predicting randomly, you can't possible make predictions with a good model, that is, a good explanation.
Without leaving the armchair there actually is a simple fact that gravity-as-geometry explains more naturally than gravity-as-force, and that is the equivalence of inertial and gravitational mass.
After reading this article I somehow feel a little correlation between this and the short story "That alien message" on LessWrong which suggested that we are living in a simulated universe.
"Entanglement lets the measurement of one particle instantaneously determine the state of a partner particle, no matter how far away it may be"
No. No! No!!! This meme has really got to die. A measurement of one entangled particle does NOTHING to the other particle. Its state is exactly the same as it was before. In fact, the whole concept of "before" and "after" a remote measurement doesn't even make sense because it depends on your frame of reference!
No. No! No!!! This meme has really got to die. A measurement of one entangled particle does NOTHING to the other particle. Its state is exactly the same as it was before. In fact, the whole concept of "before" and "after" a remote measurement doesn't even make sense because it depends on your frame of reference!
You are asserting something as true that is actually under dispute. Whether the state of the particle was already determined is the million dollar question. Importantly, under non-local theories, it doesn't have to be. Superpositions can be real and not just mathematical entities.
It's only disputed by people who haven't actually looked at the math.
And I didn't say the state of the particle was already determined. It wasn't. Yes, of course superpositions are real. Yes, of course the Bell inequalities are violated. Yes, this eliminates all local hidden variables theories. Yes, it seems like this necessarily leads to the conclusion that there is spooky action at a distance. But that's wrong. To see why, read the paper or watch the video.
And I didn't say the state of the particle was already determined. It wasn't. Yes, of course superpositions are real. Yes, of course the Bell inequalities are violated. Yes, this eliminates all local hidden variables theories. Yes, it seems like this necessarily leads to the conclusion that there is spooky action at a distance. But that's wrong. To see why, read the paper or watch the video.
I think this is merely a terminological difference.
What happens is:
- before observing one particle, observations on the other particle are described and predicted by an entangled, superposition, state.
- after observing one particle, observations on the other particle are described and predicted by a non-entangled, non-superposition state. The possible outcomes of the observations on the other particle, both predicted and measured, are different after the observation on the first particle.
Of course the observation causes a change in the state of both particles simultaneously. And I understand that strictly speaking, in the physical sense of the word, there is no 'action'. But does it really matter if people say they are 'acting on' the remote particle by observing the local particle, as long as they mean the exact same thing?
What happens is:
- before observing one particle, observations on the other particle are described and predicted by an entangled, superposition, state.
- after observing one particle, observations on the other particle are described and predicted by a non-entangled, non-superposition state. The possible outcomes of the observations on the other particle, both predicted and measured, are different after the observation on the first particle.
Of course the observation causes a change in the state of both particles simultaneously. And I understand that strictly speaking, in the physical sense of the word, there is no 'action'. But does it really matter if people say they are 'acting on' the remote particle by observing the local particle, as long as they mean the exact same thing?
In some sense, yes, this is just quibbling over terminology (as all pedagogical issues ultimately boil down to terminology). But the terminological quibble is very subtle and profound.
> after observing one particle, observations on the other particle are described and predicted by a non-entangled, non-superposition state.
This is the misleading part. It is not true in an absolute sense. It is only true for the observer that makes the "first" measurement (with "first" in scare quote because relativity). It is not true for the observer of the "other" particle. There is no measurement that can be made on the "other" particle that will tell you if the first particle has been measured.
> The possible outcomes of the observations on the other particle, both predicted and measured, are different after the observation on the first particle.
This too is misleading because it assumes that "possibility" is a universal property and it isn't. The "possible" outcomes for the "other" particle change for the observer of the "first" particle, but not for the observer of the "other" particle. And even this is not quite right because it depends on whether the observations are time-like or space-like separated, and whether or not there is a classical communications channel open between the two observers. It gets complicated. Read the paper.
> after observing one particle, observations on the other particle are described and predicted by a non-entangled, non-superposition state.
This is the misleading part. It is not true in an absolute sense. It is only true for the observer that makes the "first" measurement (with "first" in scare quote because relativity). It is not true for the observer of the "other" particle. There is no measurement that can be made on the "other" particle that will tell you if the first particle has been measured.
> The possible outcomes of the observations on the other particle, both predicted and measured, are different after the observation on the first particle.
This too is misleading because it assumes that "possibility" is a universal property and it isn't. The "possible" outcomes for the "other" particle change for the observer of the "first" particle, but not for the observer of the "other" particle. And even this is not quite right because it depends on whether the observations are time-like or space-like separated, and whether or not there is a classical communications channel open between the two observers. It gets complicated. Read the paper.
You do realize this is an open question right?
No jokeprod paper or video is going to put it to rest. You sound like a religious nutcase.
No jokeprod paper or video is going to put it to rest. You sound like a religious nutcase.
I realize that a lot of people think it's an open question. These people are wrong.
And the difference between me and a religious nutcase is that I can back up my position with math.
And the difference between me and a religious nutcase is that I can back up my position with math.
Important to remember, especially if you're practicing physics or engineering in any form: models are not the real world. You're (kind of) right about the (typical interpretation of) the models we use for entanglement, but open questions in this field include things like whether a wavefunction is "real" or just a mathematical tool that seems to get the right results at the length/energy scales we look at.
tl;dr: Just because you have an equation doesn't mean that equation corresponds to anything real. This is the same mistake people make all the time with statistics -- the math is easy, finding the right math to use can be very hard.
tl;dr: Just because you have an equation doesn't mean that equation corresponds to anything real. This is the same mistake people make all the time with statistics -- the math is easy, finding the right math to use can be very hard.
No, these are not open questions. QIT answers all of them. It describes exactly how classical reality emerges from the quantum wave function, and hence settles the question of whether or not it is "real". The answer is: the question of whether or not the wave function is "real" is based on the false a assumption that "real" is a binary predicate. It isn't. Whatever the mathematics of the wave function describes does indeed exist, but it exists in a separate ontological category from classical reality.
See blog.rongarret.info/2015/02/31-flavors-of-ontology.html for more details.
See blog.rongarret.info/2015/02/31-flavors-of-ontology.html for more details.
You sound a lot like Sean Carrol in this video:
https://www.youtube.com/watch?v=GdqC2bVLesQ
Being so sure about your position when it isn't proven is a pretty bad way to do science. You're essentially implying that 3/4 people on that stage are wrong or idiots. I don't buy it.
https://www.youtube.com/watch?v=GdqC2bVLesQ
Being so sure about your position when it isn't proven is a pretty bad way to do science. You're essentially implying that 3/4 people on that stage are wrong or idiots. I don't buy it.
I'm on the road with extremely limited internet connectivity so I can't watch a video at the moment. But may I make a suggestion? Why don't you read my paper or watch my video before you decide that I'm wrong. In fifteen years, no one who has actually read it has taken issue with it. (And, BTW, the only reason it isn't a published peer-reviewed publication is that when I submitted it, it was rejected on the grounds that it wasn't anything new. Which is true. Which is why I stopped trying to publish it.)
Never said you're wrong. You may be right, I don't know. I just know that I don't believe you when you say extremely intelligent experts in the field are all wrong, except for one camp (they can even be a majority). I just have an epistemological problem with your views.
Again, hard to say without seeing the video, but my guess is that they're not wrong, they're just engaged in (IMHO) bad pedagogy.
'determine' does not mean 'change' - does it?
The issue is not so much with the word "determine" as it is with the word "instantaneously". You could say "Entanglement lets the measurement of one particle instantaneously frob the state of a partner particle" without knowing or specifying what the word "frob" means and you'd have the same problem: you're claiming that measuring one particle does something to the other particle, and does it instantaneously and, moreover, that this is mysterious because the two particles are far apart (hence the slogan "spooky action at a distance"). All this is wrong. There is no "spooky action at a distance" because there is no action. Whether that action is "determining" or "frobbing" the state of the other particle is irrelevant.
The correct story is that measurement and entanglement are the same physical phenomenon. The creation of an EPR pair is the first step in any measurement process. The correlations in EPR measurements derive from exactly the same physical process as the correlations in "ordinary" measurements (I put "ordinary" in scare quotes because, as I said, even "ordinary" measurements start with the creation of an EPR pair). When you "measure" the two halves of an EPR pair what you are really doing is performing two measurements on whatever system produced the EPR pair to begin with. When you look at it that way it is not at all surprising that the measurements should be correlated.
For more details see:
https://www.youtube.com/watch?v=dEaecUuEqfc
http://www.flownet.com/ron/QM.pdf
The correct story is that measurement and entanglement are the same physical phenomenon. The creation of an EPR pair is the first step in any measurement process. The correlations in EPR measurements derive from exactly the same physical process as the correlations in "ordinary" measurements (I put "ordinary" in scare quotes because, as I said, even "ordinary" measurements start with the creation of an EPR pair). When you "measure" the two halves of an EPR pair what you are really doing is performing two measurements on whatever system produced the EPR pair to begin with. When you look at it that way it is not at all surprising that the measurements should be correlated.
For more details see:
https://www.youtube.com/watch?v=dEaecUuEqfc
http://www.flownet.com/ron/QM.pdf
No one denies that when a measurement is performed, the measuring system and the measured system become entangled (and the aggregate system containing both continues evolving unitarily).
> When you "measure" the two halves of an EPR pair what you are really doing is performing two measurements on whatever system produced the EPR pair to begin with.
Here's the problem with that. Consider the measurements of a pair of space-like separated entangled photons. A choice can be made about the details of these measurements by the experimenters at the last second possible. For example, the rotation angle of a polarizer can be randomly chosen the instant before a photon strikes it, yet correlations with this choice show up in the other measurement that was taken far away. (Note that these correlations still don't allow you to send information faster than light.) But the "which angle" information was never contained in whatever system produced the two photons.
> When you "measure" the two halves of an EPR pair what you are really doing is performing two measurements on whatever system produced the EPR pair to begin with.
Here's the problem with that. Consider the measurements of a pair of space-like separated entangled photons. A choice can be made about the details of these measurements by the experimenters at the last second possible. For example, the rotation angle of a polarizer can be randomly chosen the instant before a photon strikes it, yet correlations with this choice show up in the other measurement that was taken far away. (Note that these correlations still don't allow you to send information faster than light.) But the "which angle" information was never contained in whatever system produced the two photons.
>No one denies that when a measurement is performed, the measuring system and the measured system become entangled (and the aggregate system containing both continues evolving unitarily).
It's not true that no one denies this. Adherents of the Copenhagen interpretation deny it. They claim that measurement involves a non-unitary phenomenon called "wave function collapse."
> I can't quite tell exactly what you all are arguing about.
Exactly this. You may not realize it, but not everyone understands that measurement and entanglement are intimately related (in fact, the exact same physical phenomenon). In fact, some people vehemently deny it. There are even some card-carrying physicists who vehemently deny it.
> entanglement is not a phenomenon that can be explained classically.
That is certainly true (though I've met people who deny this as well).
It's not true that no one denies this. Adherents of the Copenhagen interpretation deny it. They claim that measurement involves a non-unitary phenomenon called "wave function collapse."
> I can't quite tell exactly what you all are arguing about.
Exactly this. You may not realize it, but not everyone understands that measurement and entanglement are intimately related (in fact, the exact same physical phenomenon). In fact, some people vehemently deny it. There are even some card-carrying physicists who vehemently deny it.
> entanglement is not a phenomenon that can be explained classically.
That is certainly true (though I've met people who deny this as well).
My apologies, I was editing my post while you were replying to it.
> the aggregate system containing both continues evolving unitarily.
The Copenhagen interpretation is that the measured subsystem collapses in a non-unitary way. That doesn't imply the overall system did. I have no opinion* on whether a measured subsystem collapses unitarily or not — the answer to that question does not currently seem to be experimentally testable. But it is testable that the overall system continues evolving unitarily during a subsystem collapse (well, at least up until the overall system is measured), and this has been tested and verified many times.
(* Ok fine, I do have an opinion. I don't think unitarity breaks — ever. I think it appears to break through the process of decoherence. But I'm certainly not going to claim that as fact unless experiment can prove it somehow.)
> the aggregate system containing both continues evolving unitarily.
The Copenhagen interpretation is that the measured subsystem collapses in a non-unitary way. That doesn't imply the overall system did. I have no opinion* on whether a measured subsystem collapses unitarily or not — the answer to that question does not currently seem to be experimentally testable. But it is testable that the overall system continues evolving unitarily during a subsystem collapse (well, at least up until the overall system is measured), and this has been tested and verified many times.
(* Ok fine, I do have an opinion. I don't think unitarity breaks — ever. I think it appears to break through the process of decoherence. But I'm certainly not going to claim that as fact unless experiment can prove it somehow.)
Sorrt, but you're flat-out wrong, and the sources you link aren't credible. (That's not even a journal paper.) I suggest you read Scott Aaronson's explanation of why you're wrong (he understands this better than me): http://www.scottaaronson.com/blog/?cat=33
Here's the credible citation:
http://arxiv.org/abs/quant-ph/9605002
(It's ironic, BTW, that you should complain that my citation is "not even a journal paper" when yours isn't either.)
And BTW2, Aaronson's blog post doesn't contradict my position. The Bell inequality violations are indeed real. They just aren't caused by "spooky action at a distance." They are caused by the classical correlations that arise when you trace over the quantum wave function of the universe in order to isolate a subsystem.
And BTW3, I actually did submit that paper to Physics Today back in the day. It was rejected, not because it was wrong, but because it wasn't anything new. (It's amazing how the Physics world bifurcates into two camps: those who think that the connection between entanglement and measurement is common knowledge, and those who adamantly deny that it is true.)
http://arxiv.org/abs/quant-ph/9605002
(It's ironic, BTW, that you should complain that my citation is "not even a journal paper" when yours isn't either.)
And BTW2, Aaronson's blog post doesn't contradict my position. The Bell inequality violations are indeed real. They just aren't caused by "spooky action at a distance." They are caused by the classical correlations that arise when you trace over the quantum wave function of the universe in order to isolate a subsystem.
And BTW3, I actually did submit that paper to Physics Today back in the day. It was rejected, not because it was wrong, but because it wasn't anything new. (It's amazing how the Physics world bifurcates into two camps: those who think that the connection between entanglement and measurement is common knowledge, and those who adamantly deny that it is true.)
>It's amazing how the Physics world bifurcates into two camps: those who think that the connection between entanglement and measurement is common knowledge, and those who adamantly deny that it is true.
The fact that there are two legitimate camps means that no one knows for sure what is happening. And you are obviously in one of those camps. So to insist the world is a particular way is to deny how much is still unknown and acknowledged by others in the physics community.
The fact that there are two legitimate camps means that no one knows for sure what is happening. And you are obviously in one of those camps. So to insist the world is a particular way is to deny how much is still unknown and acknowledged by others in the physics community.
The difference is that one camp has the math solidly on its side, and the other camp has to invoke the extra-physical hypothesis that measurement is somehow different (non-unitary, non-reversible) from everything else.
(Not constructive, but OK: Obviously a blog is not a journal paper, but it doesn't try to look like one either. It does have references to the journal papers it discusses though. As for the Cerf and Adami paper you linked above: from my searching it appears this is not published anywhere, it's just a 15-year-old preprint on the arXiv.)
So, to be clear: you are advocating some interpretation of quantum mechanics (for instance, but not limited to, the many worlds interpretation) where the measurement process is really measuring the system that produced the entangled pair?
I believe this interpretation fails to explain experiments where people have entangled particles with timelike separation [1], and then have shown that measurement of the second particle (which has never co-existed with the first particle in any reference frame) collapses the wavefunction of the first particle.
I'm not saying that the Copenhagen interpretation is the absolute truth, in particular many-worlds and superdeterminism are valid alternative interpretations, but I think the one you're advocating doesn't work.
[1] http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.110... (non-paywall: http://arxiv.org/abs/1209.4191 )
So, to be clear: you are advocating some interpretation of quantum mechanics (for instance, but not limited to, the many worlds interpretation) where the measurement process is really measuring the system that produced the entangled pair?
I believe this interpretation fails to explain experiments where people have entangled particles with timelike separation [1], and then have shown that measurement of the second particle (which has never co-existed with the first particle in any reference frame) collapses the wavefunction of the first particle.
I'm not saying that the Copenhagen interpretation is the absolute truth, in particular many-worlds and superdeterminism are valid alternative interpretations, but I think the one you're advocating doesn't work.
[1] http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.110... (non-paywall: http://arxiv.org/abs/1209.4191 )
> I believe this interpretation fails to explain experiments where people have entangled particles with timelike separation [1]
I don't have time to read that paper right now, but it sounds like simple entanglement transfer, not unlike what is done for "quantum teleportation." In any case, the math behind QIT is simply the math of QM, so anything that QM can explain, QIT can explain.
[UPDATE:] There was a published version of the C&A paper but I can't look it up right now (I'm on the road with very limited internet connectivity). But it turns out that the C&A position is essentially the same as decoherence/many-worlds. There is essentially no dispute over the facts any more. The problem is with the rhetoric in which those facts are wrapped. Even "many-worlds" is highly misleading.
I don't have time to read that paper right now, but it sounds like simple entanglement transfer, not unlike what is done for "quantum teleportation." In any case, the math behind QIT is simply the math of QM, so anything that QM can explain, QIT can explain.
[UPDATE:] There was a published version of the C&A paper but I can't look it up right now (I'm on the road with very limited internet connectivity). But it turns out that the C&A position is essentially the same as decoherence/many-worlds. There is essentially no dispute over the facts any more. The problem is with the rhetoric in which those facts are wrapped. Even "many-worlds" is highly misleading.
I'm not sure what you mean by "There is essentially no dispute over the facts any more." As far as I understand, no-one has been able (yet) to make a prediction based on the many-worlds interpretation that could (even in theory, with infinite resources and time) be tested experimentally. Same goes for the Copenhagen interpretation. Thus neither of these are proper scientific theories (in the sense of Popper), and so we call them "interpretations". In light of this I don't understand how you can claim it is settled in favor of one side or the other?
Read my paper. It describes exactly such an experiment.
This message clarifies what you mean. But you said before "A measurement of one entangled particle does NOTHING to the other particle. Its state is exactly the same as it was before.".
By measuring (one of the particles in) the system you are collapsing its wave function. Doesn't that mean the state of the system (both particles) is affected?
By measuring (one of the particles in) the system you are collapsing its wave function. Doesn't that mean the state of the system (both particles) is affected?
No. Collapse doesn't actually happen. Collapse is an approximation to the truth. It's a very (very!) good approximation for systems with a large number of mutually entangled degrees of freedom, but it is an approximation nonetheless. See the links I pointed to earlier to understand why.
Doesn't "the truth" involve any change in state at all? I'll look at your answer to quantum mysteries when I have time. But I suspect your original comment could have been phrased "the standard interpretation of quantum mechanics has to die"...
My complaint is more about rhetoric and pedagogy than it is about facts. There is essentially no serious dispute about the facts. But yes, it is true that the Copenhagen interpretation (by which I mean the idea that measurement involves some mysterious non-unitary process called "collapse") is untenable except as an approximation. There is no serious dispute over this.
> 'determine' does not mean 'change' - does it?
Yeah, it does, amongst other things. Unfortunately in that context two of its meanings (to discover & to decide) could have been appicable.
Yeah, it does, amongst other things. Unfortunately in that context two of its meanings (to discover & to decide) could have been appicable.
Does this imply we could possibly manipulate space-time or gravity by creating artificial entanglements (like in a quantum computer)?
What a fascinating article. Thank you.
Entanglement lets the measurement of one particle
instantaneously determine the state of a partner
particle, no matter how far away it may be — even
on the other side of the Milky Way.
If I have a bag of coins, and I take out all of the copper coins, it's not very mysterious that the bag now contains only silver coins, no matter whether I hide it under my bed, or in the freezer, or hang it out the window.
Yes, all the copper coins are now absent from the bag, forcing it into an all silver state, and I can know this, without even looking at the bag. I take the bag, and I drive it across town. I hide it under a rock. I go home. I think about the bag under the rock. I can instantaneously know that the bag contains only silver coins, even though it's across town, hidden under a rock. Instantaneous. I determine the state of the bag. With my mind. Just by thinking. Even across town. So amazing.
Why do journalists and scientists so deeply covet the seeming appearance of the arcane?
Say we're measuring spins of entangled particles with random entangled spins.
So the real problem isn't that "when one is up, then the other will magically be up too." That could be accomplished with local hidden variables (e.g. shared seeds on a PRNG, or your examples).
The real problem is that when you measure A in the "up" direction, and then B in the "10 degrees east of up" direction, then B seems to know that you measured A in the "up" direction.
That is to say: B's probability distribution as a function of the direction its being measured is correlated to the direction that A is measured. There's no way to construct an "A-independent" probability distribution of B's results for arbitrary directions. The probabilities won't sum to 1 and still match experimental results.
It's unfortunate that "A up" therefore "B up" is a degenerate case of this reality where classicality actually works, because it leads to confusion.
(Also the reason you can't use this magic to communicate FTL is that you can only ask one yes/no question of each particle, and because B's probability distribution is distorted in a symmetric way based upon A's measurement, you're still going to get a 50/50 response for yes/no questions asked of random entangled particles)
Feel free to comment, as I paste this on every misunderstanding of Bell's Theorem here, and I edit to make more clear each time.
So the real problem isn't that "when one is up, then the other will magically be up too." That could be accomplished with local hidden variables (e.g. shared seeds on a PRNG, or your examples).
The real problem is that when you measure A in the "up" direction, and then B in the "10 degrees east of up" direction, then B seems to know that you measured A in the "up" direction.
That is to say: B's probability distribution as a function of the direction its being measured is correlated to the direction that A is measured. There's no way to construct an "A-independent" probability distribution of B's results for arbitrary directions. The probabilities won't sum to 1 and still match experimental results.
It's unfortunate that "A up" therefore "B up" is a degenerate case of this reality where classicality actually works, because it leads to confusion.
(Also the reason you can't use this magic to communicate FTL is that you can only ask one yes/no question of each particle, and because B's probability distribution is distorted in a symmetric way based upon A's measurement, you're still going to get a 50/50 response for yes/no questions asked of random entangled particles)
Feel free to comment, as I paste this on every misunderstanding of Bell's Theorem here, and I edit to make more clear each time.
What if I have a million A/B pairs, measured in parallel. Each bit is 100 A measurements, 1 for up and 0 for down. Why is this not FTL communication of 10k bits?
Because you can't use it to pass information to someone listening to only a single end of the pair. You have to measure both particles to detect the correlation, or else all you're getting is random noise.
Imagine I'm flashing a light at you on and off, randomly. Some of the flashes aren't random and contain a message. To read the message, or even detect its existence, you have to know which flashes were random, and this is what observing both particles in an entangled pair allows us to do. There is no signal with only a single particle.
Imagine I'm flashing a light at you on and off, randomly. Some of the flashes aren't random and contain a message. To read the message, or even detect its existence, you have to know which flashes were random, and this is what observing both particles in an entangled pair allows us to do. There is no signal with only a single particle.
It's FTL communication of 10k random bits, whatever that gets you besides a great symmetric encryption key to share between you and the other end.
I also went through a phase where quantum entanglement seemed mundane and easily explained. However, I realized that this was merely because I was taking the analogies too literally.
The key difference between a bag of coins and a microscopic quantum system is that the contents of the bag before being observed is unknown, whereas the quantum state is actually indeterminate. By observing one entangled particle, you force the other to also take on a definite state. Read up on Bell's inequality for the experimental justification of this idea.
The key difference between a bag of coins and a microscopic quantum system is that the contents of the bag before being observed is unknown, whereas the quantum state is actually indeterminate. By observing one entangled particle, you force the other to also take on a definite state. Read up on Bell's inequality for the experimental justification of this idea.
By observing one entangled particle, you force the
other to also take on a definite state.
You didn't even give a decent troll argument against entanglement. What you should've said is that: "I have two bags of coins, one full of silver and one full of copper. I have no idea which is which. I put one bag across town, hidden under a rock. I open up the other bag at home, it's all copper. Thus I know the other bag is all silver. So amazing." In your scenario, you made an observation of what was inside the bags, which is not allowed. It's still a troll argument though, because entanglement is arcane as heck!
To stretch your analogy to breaking point, the explanation that makes the most (least?) sense to me that I've read is thus: First, imagine all the coins have no mass. And a mischevious quantum cat either puts all the copper and sliver coins in the same bag leaving the other empty, or puts all the copper coins in one bag and the silver in the other.
Not knowing which placement has been done, you put one bag in your freezer, and drive across town with the other bag.
Then you ask yourself the question, or perform an experiment to tell you, "Which bag did the cat put all the coins in?", at which point you open the bag and find it full of coins or empty, determining the state of the bag you have, and the state of the bag in your freezer at home.
No problem, huh?
Except, if you'd asked a different question, or performed a different experiment, to tell you "Which bag contains all the copper coins, and which contains the silver coins?" - you would have still got a valid response, and found the bag either full of copper or silver coins, again determining the state of the other bag at home.
That's the nature of entanglement. Without knowing the state, the nature of the question you ask of one particle will determine what sort of answer you get, in a way that instantaneously affects the bag at home too. i.e. someone opening your freezer just after you ask your question will see the other bag in a consistent state with the bag you have.
That's why it's so gorram weird.
Not knowing which placement has been done, you put one bag in your freezer, and drive across town with the other bag.
Then you ask yourself the question, or perform an experiment to tell you, "Which bag did the cat put all the coins in?", at which point you open the bag and find it full of coins or empty, determining the state of the bag you have, and the state of the bag in your freezer at home.
No problem, huh?
Except, if you'd asked a different question, or performed a different experiment, to tell you "Which bag contains all the copper coins, and which contains the silver coins?" - you would have still got a valid response, and found the bag either full of copper or silver coins, again determining the state of the other bag at home.
That's the nature of entanglement. Without knowing the state, the nature of the question you ask of one particle will determine what sort of answer you get, in a way that instantaneously affects the bag at home too. i.e. someone opening your freezer just after you ask your question will see the other bag in a consistent state with the bag you have.
That's why it's so gorram weird.
Read up on it more, if you care:
http://physics.stackexchange.com/questions/3158/why-is-quant...
http://physics.stackexchange.com/questions/126622/quantum-en...
http://physics.stackexchange.com/questions/3158/why-is-quant...
http://physics.stackexchange.com/questions/126622/quantum-en...
The choice between locality and realism can be easily summed up as thus:
If you're satisfied that their might be multiple past states which could all produce the same result, determining the current state, then you can safely ignore these semantic debates, about whether there is a concrete-but-unknown value, or whether the present value remains uninitialized and undefined.
Either you believe the state exists but is unknown, and
cannot be known until measured,
or, you believe that you know the state does not exist,
and materializes upon measurement.
If you're satisfied that their might be multiple past states which could all produce the same result, determining the current state, then you can safely ignore these semantic debates, about whether there is a concrete-but-unknown value, or whether the present value remains uninitialized and undefined.
crontumCombudor's Dictionary. Science, noun. "A semantic debate."
If you could put the bag into the all-silver state after leaving it across town by taking all the silver coins out of another bag, that would be a big deal.
So magnetize a sample of iron as a permanent magnet, cut the sample in half, send the second half to the moon, and re-polarize first half here on earth, and show me that it's lunar partner has spontaneously re-magnetized itself in agreement with its earthbound phantom amputated Siamese twin. And then back again, ad infinitum.
Is this all a round about way of saying you don't believe in quantum entanglement? Because that would be simpler and I wouldn't bother arguing.
You know it's all just an artifact of an expression written on paper, right?
What do you mean by that?
(Obviously you don't mean necessarily literally on paper)
Do you claim that the predictions that are made about what is observed are false, or that the interpretation is wrong, or?
Surely you don't mean that by interpreting it in a particular way, it causes different things to happen, so I can't think of anything other than those two.
(Obviously you don't mean necessarily literally on paper)
Do you claim that the predictions that are made about what is observed are false, or that the interpretation is wrong, or?
Surely you don't mean that by interpreting it in a particular way, it causes different things to happen, so I can't think of anything other than those two.
I think he means that the "entanglement" as understood by general public, i.e. as "I do something to particle A, and suddenly particle B changes" is an invalid way of reading the math; you didn't do anything to B, you merely figured out, by interacting with A, in which world you're in.
You see, this is the thing that kills me when thinking about the quantum world. The Many Worlds interpretation is so clean and obvious, except for one thing - it requires creating a new universe for each quantum entanglement event. Gah!
Of course that does make me think about forking processes and copy-on-write, and the idea that our reality is just a simulation in a computer and that maybe creating new universes is not as expensive as we might at first think...
Of course that does make me think about forking processes and copy-on-write, and the idea that our reality is just a simulation in a computer and that maybe creating new universes is not as expensive as we might at first think...
I'm starting to think about this along the "zero-worlds interpretation" - i.e. there is only one world, but it's not based on discrete particles with states, but on probability distributions. We perceive the world as if it was already mostly simple, but it's because we too are entangled with stuff. There are no many worlds of particles, because particles are not the building blocks of a world. If you look at it from the hypothetical computer simulating us, it's kind of like storing the set of all natural numbers - you can store slices of it in lists (and then quantum entanglement under MWI works by figuring out in which list you are) vs. storing a generator function.
I admit I'm still in the process of learning and figuring this stuff out.
I admit I'm still in the process of learning and figuring this stuff out.
[deleted]
I believe this is the Bertlmann's socks argument. If you care to dig into this a bit deeper you might just have a braingasm.
What is the proof of existence of 'universal time' as physical phenomena instead of being a 'derived property' of some or other physical process?
Rotation of Earth around Sun is just a process, there is no time in it apart from the mind of an external observer. Similarly with any so called "atomic clocks" or any other physical process, including so called expansion of the Universe.
Time, of course, could be derived as a concept from an observable change (a process) by mind, but this does not imply its physical existence, like it is with what we call physical forces.
Space is more subtle, but it is also require an observer and at least two particles related via this or that set of forces.
Two random Photons share nothing, and without an observer there is neither space nor time among them.
Mathematics does not imply existence of described abstraction.
Rotation of Earth around Sun is just a process, there is no time in it apart from the mind of an external observer. Similarly with any so called "atomic clocks" or any other physical process, including so called expansion of the Universe.
Time, of course, could be derived as a concept from an observable change (a process) by mind, but this does not imply its physical existence, like it is with what we call physical forces.
Space is more subtle, but it is also require an observer and at least two particles related via this or that set of forces.
Two random Photons share nothing, and without an observer there is neither space nor time among them.
Mathematics does not imply existence of described abstraction.
Mathematics doesn't imply anything. You can make a model using mathematics and then use it to make predictions, then test those predictions experimentally. By doing this a lot you can discover things and give them names like time, and proper time, and space and whatnot.
Is this a proof?
Western philosophy, so far, succeed in separating these concepts as being a priory which is perfectly reasonable, considering that any sensory input which is subsequently used to train and condition our mind is coming serialized by out sense organ, so, for a mind, which is a result of conditioning by the senses, the notion of succession is a priory. Conditioning by shared physical environment gives us cycles, so the notion of a cycle is also a priory. Succession days into nights in the environment and ageing of other people's bodies gives us notion of a continuous change. But change is not time.
Eastern philosophy (and modern cognitive neuroscience) would suggest that this a priory is related to our minds, not to photons or forces. So?
Western philosophy, so far, succeed in separating these concepts as being a priory which is perfectly reasonable, considering that any sensory input which is subsequently used to train and condition our mind is coming serialized by out sense organ, so, for a mind, which is a result of conditioning by the senses, the notion of succession is a priory. Conditioning by shared physical environment gives us cycles, so the notion of a cycle is also a priory. Succession days into nights in the environment and ageing of other people's bodies gives us notion of a continuous change. But change is not time.
Eastern philosophy (and modern cognitive neuroscience) would suggest that this a priory is related to our minds, not to photons or forces. So?
Right so there's a thing we call time which is to do with how we experience the world around us. There is also a mathematical object of the same name which is part of a model which provides accurate predictions about how things behave. The more successful predictions a model has, the less likely it is that the model is fundamentally inaccurate. But although our perception of time is undoubtedly related to the thing represented by the mathematical object we also call time, they are not the same thing. One is a mathematical object, the other is a subjective experience.
What would you ask of a proof? If you're reading some text, how do you determine if the text constitutes a proof?
To "see" Universe as it is we have to remove an observer - a mind which creates appearances and conditions itself with them.
It is a hack, so to speak.)
It is a hack, so to speak.)
https://www.youtube.com/watch?v=PSYXt3Xu3xI
I'm not a physicist, but he seems to be invoking similar ideas to Raamsdonk: that spacetime and gravity are emergent properties that boil down to entanglement somehow. But the loop quantum gravity people have been saying something like that for ages. And Stephen Wolfram and Konrad Zuse etc. And we have people like Lubos Motl on the other hand to ridicule all of the above.
Anyone able to give a big picture overview of what's been happening the last few years in this somewhat rarefied world? Who are we, the intelligent lay public, to trust?