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The quantum source of space-time

nature.com

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Re: The quantum source of space-time

#21

Post by taliesinb »

cperciva wrote: post #15

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 dista…

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.

Re: The quantum source of space-time

#22

Post by cing »

crontumCombudor wrote: post #16
gone35 wrote: post #10

Earlier quoted context omitted.

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...

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…

crontumCombudor's Dictionary. Science, noun. "A semantic debate."

Re: The quantum source of space-time

#23

Post by gibybo »

reduxotron wrote: post #19
noobermin wrote: post #14

Earlier quoted context omitted.

In the lab http://arxiv.org/abs/1511.03189

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 .

Check out 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.

Re: The quantum source of space-time

#24

Post by DrScump »

zamalek wrote: post #20
cperciva wrote: post #15

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 dista…

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.

Re: The quantum source of space-time

#26

Post by Karellen »

crontumCombudor wrote: post #2

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. Y…

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.

Re: The quantum source of space-time

#27

Post by ars »

"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.

Re: The quantum source of space-time

#28

Post by cjg »

ars wrote: post #27

"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…

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.

Re: The quantum source of space-time

#29

Post by lisper »

"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!

Re: The quantum source of space-time

#30

Post by contravariant »

taliesinb wrote: post #17

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 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…

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.

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