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Tangled Up in Spacetime

scientificamerican.com

75 pointstaralx26 comments

Scientific American has turned into such a rag... their recent bit about "entangled black holes" was especially cringy.

It's Michio Kaku level of hand wavery and lack of rigor.

Meh... I thought the article did a decent job given the readership. The goal of putting quantum mechanics on an information-theoretic foundation is indeed a hot research topic lately.

"In the AdS/CFT correspondence Maldacena showed that one can completely describe a black hole purely by describing what happens on its surface. In other words, the physics of the inside—the 3-D “bulk”—corresponds perfectly to the physics of the outside—the 2-D “boundary.”"

Very interesting!

I thought quantized spacetime had already been disproven below a testable range? I.e. Any theorized particles wouldn't be seen by particle accelerators smaller than say a solar system or drastically larger.

I believe there's a few other models of LQG and others which can still be tested. But I'm skeptical we'll see any positive results. The only reason I hope I'm wrong is that if spacetime can't be quantized then we're stuck with singularities which aren't much fun all things considered.

I seem to remember some discussion about this with GEO600 results a couple years back. Their detector was seeing a bunch of noise and the conversation included asking if we had begun to see granularity of spacetime at their resolution.

Yeah I thought so too. AFAICT the standard model and general relativity are incompatible with any spacetime quantization larger than the Planck scale and that's very far outside of our testable range.

The end of the article states that everything it discussed prior is only part of a "toy". It is known that the toy being studied does not fully correspond to our universe.

Think of coming up with one of these theories like writing a complicated computer program. Sometimes the whole task is too big to even think about, it you sort of understand one part of it, so you work on getting that part sorted out as best you can, and hope you can somehow figure out the rest of the program later, and it'll be useful.

Sort of depends what the theory predicts, as to whether that's relevant.

I always thought of Einstein's theory of General Relativity as a model. For me it doesn't explain the execution.

This to me makes more sense that at a quantum level there are discreet particles that are being acted on according to relativity. I think of it more like a CPU where this is a clock input.

I don't know if this discredits General Relativity in any way. From what I understand, it underlies General Relativity by defining the nature of space and time.

I didn't suggest this discredits general relativity.

Quantum mechanics is pretty field oriented (doubly so for QFT, which is what you're dealing with at that scale) so I don't understand why it would be any less of a "model" than general relativity.

Space and time are very macro concepts according to general relativity. It doesn't actually explain how time elapses differently on one atom vs. another atom only that it does.

I am traveling close to the speed to light how is my watching running slower? How do the atoms in my watch know to run slower?

I am not suggesting that current quantum mechanics is more than a model, but I think we are along the right path to figure out the answer to my above question.

The ideas here lean heavily on entanglement, which is very macro (not to mention unexplained) itself. So I don't see a path forward as far as figuring out how pieces of the system "know" to do what they do via the it from qubit program.

"quantum entanglement of whatever the underlying ‘atoms’ of spacetime are"

If we use this idea there are some space-time atoms, to explain entanglement. That same concept could explain space-time itself better.

It is similar to how Einstein used anomalies in the application of Newtonian physics to light to postulate a more accurate model. Explaining entanglement might be the gateway to explaining space-time more accurately.

That's not what they're doing at all though... they're looking to use entanglement to explain space time, not use space time to explain entanglement.

>I am traveling close to the speed to light how is my watching running slower? How do the atoms in my watch know to run slower?

I think in some sense this is the wrong question to ask. Your watch isn't running slower. The point is that if you're moving, you must be moving relative to something else. Their watch will appear slower to you, and your watch will appear slower to them. The counterintuitive part is that you're both correct.

The reason all this comes about is that both observers measure the speed of light as traveling at the same speed, but they can't agree on the path that the light has taken.

Yes, I should have been clearer. Running slower compared to a stationary observer.

Your last point doesn't explain why or how time dilation occurs, just that we know that the speed of light is a constant to all reference frames.

There is a famous experiment with three synchronized atomic clocks. Two were flown in opposite directions around the globe the other stationary. The eastward flown clock lost 59ns and the westward flown clock gained 273ns, both relative to the stationary clock. The reason is that the clock flown eastward is travelling faster since it is in the direction of the earth's rotation. Measurements are consistent with the theory of relativity.

https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...

> Your last point doesn't explain why or how time dilation occurs, just that we know that the speed of light is a constant to all reference frames.

But that's what the light clock experiment [1] is all about. The two observers see the light moving at the same speed, but one observer thinks the light takes a longer path than the other observer does. If both observations are equally valid, the inevitable conclusion is time dilation.

[1] http://www.emc2-explained.info/The-Light-Clock/#.WBxO8vnLdPA

Isn't part of it the spacetime geometry, specifically your path through it?

It makes absolute sense for my gas tank to be lower if I take a long roundabout path from A to B while you take a short one, even though our cars have no idea about what the other one is doing.

Yes, general relativity is all about the "metric tensor" of space, which is just a fancy way of saying it's about assigning distances to paths. For paths you can actually physically take (i.e. movement slower than the speed of light relative to local space) this ends up corresponding to the time experienced along that path.

I'm not sure that the idea here is that spacetime is made up of particles, but that it consists of nothing more than (mathematical) relationships between fundamental bits of information. In a sense, the only things that actually exist are the physical things that populate spacetime. Spacetime itself is effectively an illusion that can only be properly described as a set of relationships between the actual real stuff.

I think the article is suggesting the opposite to what you are saying.

"quantum entanglement of whatever the underlying ‘atoms’ of spacetime are"

You are arguing Einstein's version of the universe. The disproval of his EPR paper [1] led to the discovery of entanglement.

[1] https://en.wikipedia.org/wiki/EPR_paradox

I don't think I am. Entanglement and nonlocality are the best illustrations of the concept. Rather than spacetime being like a fabric, or an actual thing, it is defined only by relationships. This allows particles to be entangled without being right next to each other. In some sense, that entanglement makes those particles right next to each other.

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