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The quantum source of space-time

nature.com

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Re: The quantum source of space-time

#81

Post by Confusion »

lisper wrote: post #52
Confusion wrote: post #39

Earlier quoted context omitted.

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…

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?

Re: The quantum source of space-time

#83

Post by madaxe_again »

duaneb wrote: post #74
madaxe_again wrote: post #46

Earlier quoted context omitted.

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 m…

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 extrac…

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 ;)

Re: The quantum source of space-time

#84

Post by pacala »

fizx wrote: post #42
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 yo…

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?

Re: The quantum source of space-time

#85

Post by jerf »

taliesinb wrote: post #21
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.

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.

Re: The quantum source of space-time

#86

Post by dave_sullivan »

hellofunk wrote: post #57
madaxe_again wrote: post #46

Earlier quoted context omitted.

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 m…

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...

Re: The quantum source of space-time

#87

Post by [deleted] »

hellofunk wrote: post #57
madaxe_again wrote: post #46

Earlier quoted context omitted.

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 m…

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?

[deleted]

Re: The quantum source of space-time

#88

Post by taliesinb »

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…

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.

Re: The quantum source of space-time

#89

Post by jessriedel »

semi-extrinsic wrote: post #37
contravariant wrote: post #30

Earlier quoted context omitted.

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…

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...

Maldacena conjectured a correspondence. It's never been proven.

Re: The quantum source of space-time

#90

Post by flatline »

progman wrote: post #68
hellofunk wrote: post #57

Earlier quoted context omitted.

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?

> 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...

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
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