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The quantum source of space-time

nature.com

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Re: The quantum source of space-time

#41

Post by kgwgk »

lisper wrote: post #36
zby wrote: post #34

Earlier quoted context omitted.

'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, mor…

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?

Re: The quantum source of space-time

#42

Post by fizx »

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…

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.

Re: The quantum source of space-time

#43

Post by richmarr »

zby wrote: post #34
lisper wrote: post #29

"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 i…

'determine' does not mean 'change' - does it?

> '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.

Re: The quantum source of space-time

#44

Post by danbruc »

mrob wrote: post #33
cjg wrote: post #28

Earlier quoted context omitted.

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 app…

>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. Talk…

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.

Re: The quantum source of space-time

#45

Post by repsilat »

mrob wrote: post #33
cjg wrote: post #28

Earlier quoted context omitted.

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 app…

>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. Talk…

>> 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.

Re: The quantum source of space-time

#46

Post by madaxe_again »

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…

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.

Re: The quantum source of space-time

#47

Post by lisper »

semi-extrinsic wrote: post #40
lisper wrote: post #36

Earlier quoted context omitted.

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, mor…

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.)

Re: The quantum source of space-time

#48

Post by lisper »

kgwgk wrote: post #41
lisper wrote: post #36

Earlier quoted context omitted.

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, mor…

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?

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.

Re: The quantum source of space-time

#49

Post by mrob »

danbruc wrote: post #44
mrob wrote: post #33

Earlier quoted context omitted.

>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. Talk…

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" 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.

Re: The quantum source of space-time

#50

Post by mathgenius »

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…

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.
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