24 ms·

The quantum source of space-time

179 points by joshus 11y ago

crontumCombudor 11y ago
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. Big deal. 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?
- aptwebapps 11y ago
  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.
  - crontumCombudor 11y ago
    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.
    - aptwebapps 11y ago
      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.
      - crontumCombudor 11y ago
        You know it's all just an artifact of an expression written on paper, right?
        drdeca 11y ago
        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.
        TeMPOraL 11y ago
        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.
        antimagic 11y ago
        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...
        TeMPOraL 11y ago
- titanomachy 11y ago
  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.
  - crontumCombudor 11y ago
    By observing one entangled particle, you force the other to also take on a definite state. ...on paper.
    - noobermin 11y ago
      In the lab http://arxiv.org/abs/1511.03189 http://arxiv.org/abs/1511.03189
      - reduxotron 11y ago
        We therefore reject the hypothesis that local realism governs our experiment. They reject a hypothesis. Nothing more. They did not prove that they changed an existing state. They merely decided that the state was not present, until measured. They decided a thing was not visible until looked at, because that makes more sense on paper.
        gibybo 11y ago
        Check out https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c It's my favorite video for explaining how we know that the state is not determined until it has been measured. It's not just a decision we thought up on paper. It is real and well proven.
        11y ago
- deleted 11y ago
  [deleted]
- gone35 11y ago
  Read up on it more, if you care: http://physics.stackexchange.com/questions/3158/why-is-quantum-entanglement-considered-to-be-an-active-link-between-particles?rq=1 http://physics.stackexchange.com/questions/3158/why-is-quant... http://physics.stackexchange.com/questions/126622/quantum-entanglement-and-spooky-action-at-a-distance?lq=1 http://physics.stackexchange.com/questions/126622/quantum-en...
  - crontumCombudor 11y ago
    The choice between locality and realism can be easily summed up as thus: 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. But what these two concepts drive at, is whether or not a single meaningful, distinctive, isolated history determined the current state of the universe, and whether or not there is only one deterministic way to arrive at the present moment. 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.
    - cing 11y ago
      crontumCombudor's Dictionary. Science, noun. "A semantic debate."
- cing 11y ago
  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!
- Karellen 11y ago
  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.
- fizx 11y ago
  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.
  - pacala 11y ago
    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?
    - philipov 11y ago
      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.
    - crusso 11y ago
- mathgenius 11y ago
  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.
ajmurmann 11y ago
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?
- abdullahkhalids 11y ago
  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.
- PolLambert 11y ago
  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.
cperciva 11y ago
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?
- zamalek 11y ago
  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.
  - DrScump 11y ago
    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.
- taliesinb 11y ago
  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.
  - tfgg 11y ago
    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.
  - jerf 11y ago
    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.
taliesinb 11y ago
Here's a recent lecture by Erik Verlinde in which he tries to explain both dark energy and dark matter using entropic gravity: https://www.youtube.com/watch?v=PSYXt3Xu3xI 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?
- contravariant 11y ago
  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.
  - semi-extrinsic 11y ago
    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=13256884418314014212 https://scholar.google.no/scholar?cluster=132568844183140142...
tambourine_man 11y ago
What a fascinating article. Thank you.
ars 11y ago
"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.
- cjg 11y ago
  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.
  - mrob 11y ago
    >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.
    - kgwgk 11y ago
      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.
lisper 11y ago
"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!
- zby 11y ago
  'determine' does not mean 'change' - does it?
  - lisper 11y ago
    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 https://www.youtube.com/watch?v=dEaecUuEqfc http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf
    - 11y ago
edem 11y ago
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.
dschiptsov 11y ago
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.
- dools 11y ago
  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.
  - dschiptsov 11y ago
    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?
    - Retra 11y ago
      What would you ask of a proof? If you're reading some text, how do you determine if the text constitutes a proof?
sevenless 11y ago
Does this imply we could possibly manipulate space-time or gravity by creating artificial entanglements (like in a quantum computer)?

179 points by joshus 11y ago

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. Big deal. 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?

aptwebapps 11y ago
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.
- crontumCombudor 11y ago
  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.
  - aptwebapps 11y ago
    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.
    - crontumCombudor 11y ago
      You know it's all just an artifact of an expression written on paper, right?
      - drdeca 11y ago
        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.
        TeMPOraL 11y ago
        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.
        antimagic 11y ago
        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...
        TeMPOraL 11y ago
titanomachy 11y ago
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.
- crontumCombudor 11y ago
  By observing one entangled particle, you force the other to also take on a definite state. ...on paper.
  - noobermin 11y ago
    In the lab http://arxiv.org/abs/1511.03189 http://arxiv.org/abs/1511.03189
    - reduxotron 11y ago
      We therefore reject the hypothesis that local realism governs our experiment. They reject a hypothesis. Nothing more. They did not prove that they changed an existing state. They merely decided that the state was not present, until measured. They decided a thing was not visible until looked at, because that makes more sense on paper.
      - gibybo 11y ago
        Check out https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c It's my favorite video for explaining how we know that the state is not determined until it has been measured. It's not just a decision we thought up on paper. It is real and well proven.
        11y ago
deleted 11y ago
[deleted]
gone35 11y ago
Read up on it more, if you care: http://physics.stackexchange.com/questions/3158/why-is-quantum-entanglement-considered-to-be-an-active-link-between-particles?rq=1 http://physics.stackexchange.com/questions/3158/why-is-quant... http://physics.stackexchange.com/questions/126622/quantum-entanglement-and-spooky-action-at-a-distance?lq=1 http://physics.stackexchange.com/questions/126622/quantum-en...
- crontumCombudor 11y ago
  The choice between locality and realism can be easily summed up as thus: 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. But what these two concepts drive at, is whether or not a single meaningful, distinctive, isolated history determined the current state of the universe, and whether or not there is only one deterministic way to arrive at the present moment. 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.
  - cing 11y ago
    crontumCombudor's Dictionary. Science, noun. "A semantic debate."
cing 11y ago
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!
Karellen 11y ago
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.
fizx 11y ago
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.
- pacala 11y ago
  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?
  - philipov 11y ago
    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.
  - crusso 11y ago
mathgenius 11y ago
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.

ajmurmann 11y ago

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?

abdullahkhalids 11y ago
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.
PolLambert 11y ago
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.

cperciva 11y ago

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?

zamalek 11y ago
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.
- DrScump 11y ago
  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.
taliesinb 11y ago
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.
- tfgg 11y ago
  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.
- jerf 11y ago
  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.

taliesinb 11y ago

Here's a recent lecture by Erik Verlinde in which he tries to explain both dark energy and dark matter using entropic gravity: https://www.youtube.com/watch?v=PSYXt3Xu3xI 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?

contravariant 11y ago
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.
- semi-extrinsic 11y ago
  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=13256884418314014212 https://scholar.google.no/scholar?cluster=132568844183140142...

tambourine_man 11y ago

What a fascinating article. Thank you.

ars 11y ago

"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.

cjg 11y ago
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.
- mrob 11y ago
  >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.
  - kgwgk 11y ago
    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.

lisper 11y ago

"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!

zby 11y ago
'determine' does not mean 'change' - does it?
- lisper 11y ago
  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 https://www.youtube.com/watch?v=dEaecUuEqfc http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf
  - 11y ago

edem 11y ago

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.

dschiptsov 11y ago

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.

dools 11y ago
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.
- dschiptsov 11y ago
  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?
  - Retra 11y ago
    What would you ask of a proof? If you're reading some text, how do you determine if the text constitutes a proof?

sevenless 11y ago

Does this imply we could possibly manipulate space-time or gravity by creating artificial entanglements (like in a quantum computer)?