# B Speed of light or speed of causality?

1. May 28, 2017

### alghe

[Moderator's note: this was originally the second part of a post in another thread; it has been moved to this separate thread.]

PLEASE BE ADVISED: I'm a complete ignorant with regards to the details of what I'm about to say and I've never looked at the mathematics of the subject, let alone understand it. So most of what I am about to say comes from pop science. Excuse me in advance if I sound stupid or rhetorical.

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"Nothing can travel faster than the speed of light" - we hear this again and again. I've heard it and it stuck in my head and it became a law.

Then you gradually get to QM and then you find out, actually, it's the speed of causality that matters while "c" is just a convenient constant (AFAICT). So, entanglement breaks this law at the cost of no (valuable) information being transmitted[1].

Anyways, that's fine, entanglement. And then you learn about dark energy and the rate at which spacetime expands in an accelerated way, which seems to not care about "c", however, this is affecting information. Let's say we've a blackhole, near it we've virtual particles ala Hawkin's radiation. "Normally" if this blackhole would be lucky, it would eat something.

Let's imagine that this blackhole is a sad blackhole and instead of getting lucky in devouring some cluster it instead speeds away from all the rest eventually evaporating.

I want to know: around that blackhole, how is space behaving when it's "expanding" (what the fudge does this even mean? how is this happening and what are the underlying mechanism for driving this? post-BB of course).

Moreover, in the vecinity of that blackhole and within the blackhole, the singularity or the firewall of the blackhole or whatever you want to call it distorts spacetime in quite a dramatic way, what would the interaction be with the expansion rate/dark energy?

DISCLAIMER: I know all of this is still yet to be discovered/known and most of what I asked could be answered with "we don't know yet" -- however, my reason for posting is not to get his answer but to at least get some references from you with papers of what we know and what are the current hypothesis/theories in more detail (as in, with some maths in it).

I'm sick of watching popular discussions simply saying "we don't know yet", "we need a version of quantum gravity", "general relativity just doesn't want to play well with QM", "we're quite stuck since discovery of Higgs as we were hoping for SUSY to be discovered and right now looks like the Standard Model just explains about everything we know but we have these glitches in our model with regards to the graviton" etc. etc. etc.

Any pointers to some papers published in trying to reconcile these in the recent times (i.e. this year) would be awesome for me to understand where we are at the moment.

Thanks also for your time and patience here!

[1] (this is for me still a bit confusing since you can still do a basic computation of 0 and 1 where 0 means no signal transmitted yet 1 means transmitted depending on observing or not the entangled particle at either of the side -- correct me if I'm wrong: this is implied if the angle of the prepared particle doesn't give 50% chance and the spin is correct -- I don't remember which one was exactly but probably not 1/2)

Last edited by a moderator: May 28, 2017
2. May 28, 2017

### Staff: Mentor

As noted in the other thread that originally spawned this post, this is a red flag that you should be looking for sources that are not pop science. You can't learn actual science from pop science sources.

At this level of approximation, yes. But there's a more fundamental law that is somewhat different. See below.

Usually the "convenient constant" is just $1$, since in "natural" units the units of time and the units of space are the same. The number "c" is best thought of as a conversion factor that comes from us historically not recognizing that we should be using the same units for both time and space.

No, it doesn't, because the more fundamental law does not assign a "speed" to causality. The more fundamental law is just that measurements that are made at spacelike separated events must commute, i.e., that the results of the measurements cannot depend on the order in which the measurements are made. Spacelike separated measurements made on entangled particles satisfy this law.

"Space" is not a physical thing, it's an artifact of coordinates. (More details on that in the original thread where I responded to the first part of your post, on dark energy.)

A black hole is no different from any other object as far as how it moves in the universe as a whole.

These are not the same thing. The singularity is the point (more precisely, the spacelike surface) inside the hole at which the standard classical GR model of a black hole as a vacuum spacetime breaks down; but in that standard classical GR model, as just noted, the spacetime is vacuum all the way down to the singularity. There are no "firewalls".

The "firewall" is a hypothesized "surface" just at the hole's horizon that (hypothetically) destroys any object falling in and transfers its quantum state to outgoing radiation--in this model, the spacetime is not vacuum at the horizon (and in fact there is no true event horizon at all). So at the classical level it's a different model.

None of the stuff happening inside the hole, or at the "firewall" if they exist (see above) changes the fact that, as far as the rest of the universe is concerned, the black hole is just like any other object, as above. Black holes don't require any special treatment when you are talking about the expansion of the universe as a whole; they just contribute to the overall mass density.

3. May 30, 2017

### Khashishi

It means that if you try to establish two objects at different points in space to be at rest to each other, you will run into some problems. Let's say you use some instantaneous calibration to set the instantaneous relative velocity to zero at time t. Then at time t+1, you will find that they have some relative velocity, and they are moving apart. Also, the distance between them is greater than it was before.

Near a black hole, dark energy is negligible compared to the mass energy of the black hole, so it can be ignored. Dark energy is only dominant when averaging over large scales, since on average space is nearly empty of matter.

4. Jun 4, 2017

### alghe

OK, thanks all for your replies. By pop science I meant the kind of documentaries on Nova or whatever, these American types that really dumb it down. I've watched a lot of them as a kid and up until now (so roughly, 15 years) but at this point I kind of got sick and am trying to push my fear of math away.

So I've found recently a few discussions which kind of go in the direction I was aiming at with these two posts:
1) World Science Festival, a section about entanglement and the holographic principle:
2) Susskind's talk regarding entanglement, information and blackholes:

For some reason I'm still not satisfied by these two remarks so far as to how exactly spacetime interacts with quantum mechanics. In any case, I'll work some more on it and probably come back when I've something to say instead of blabbering about and wasting your time.

At first, I thought: if we accept the Big Bang theory, then at the very beginning (at least), we would have every particle entangled with each other. That should at least include the elementary particles:

5. Jun 4, 2017

### Staff: Mentor

Note that this is only true for accelerating expansion, i.e., in the presence of dark energy (and whether the effect is observable depends on how far apart the two objects are at time t). In an expanding universe with no dark energy, the objects will be falling towards each other at time t+1.

6. Jun 4, 2017

### Staff: Mentor

Since we don't understand at this point how spacetime interacts with quantum mechanics--we don't have a good theory of quantum gravity--any remarks anyone makes are likely to seem unsatisfactory. All we have are various speculations and no way to really test them.

7. Jun 4, 2017

### DennisN

It's already addressed by PeterDonis above, but I'd like to add that this is one of the biggest open questions of physics.
There are different schools of thought, and two big ones are/have been String Theory/Theories and Loop Quantum Gravity (see e.g. in the subforum Beyond the Standard Model). There are tons of papers on these. EDIT: But I noticed this thread was marked with a "B" so I should add that what I mentioned is likely not easy to wrap your head around if you are a beginner .

Last edited: Jun 4, 2017
8. Jun 5, 2017

### alghe

I like this approach from Van Raamsdonk: https://arxiv.org/abs/1005.3035 and have just watched today a talk by him about it:

Do I get the message that string theory is pretty much dead? In the sense that it offered many important tools, maybe, as Sean Carroll highlights in his talks, but the bridge we're getting at is this quantum entanglement giving rise to spacetime? Of course, much still yet to discuss on higher scales where we have decoherence etc. But I wonder what PF community thinks about this approach and the future possible work on it?

I surely find it exciting, more than quantum loop gravity for example as it goes much to fundamental taste without any exotic crazy stuff needed (even though this proposal seems quite crazy in it's own right, at least to a wanna-be physicist like me [not even an amateur, let alone someone to understand the complexity of all of this]). What do you guys think?

9. Jun 5, 2017

### DennisN

I can't answer any of those questions, I know too little about these things. But I will watch the clips you have posted, since I am interested in it.

10. Jun 5, 2017

### alghe

OK, I think I finally found what I was looking for and what I was intuitively aiming for (unknowingly):

I think this is a huge hint towards quantization of space (at least) from my PoV. I wonder how Ziggs and Higgs play their roles into this effect. A few other questions to that video:
1) What happens when two blackholes merge. LS said that they will still be entangled, but his example was with multiple blackholes and describing them as being separate and still entangled, that is, 1:1 entanglement; what I'm asking however is: what happens when two blackholes merge and we detect their gravitational waves? What goes on to the entanglement itself? Obviously, the system will still be entangled, but now let's assume, we had two blackholes created from particle pairs (let's say, top quarks after smashing protons) and we create two black holes. They will be entangled, now what happens when one of these merge with another blackhole that's wasn't related to either of those? So A and B start entangled, B interacts later on as time evolves (Δt etc. etc.) with a C. B and C get entangled, any entanglement between A and B is now lost. A is now "unentangled" with B.

2) We assume a physical system is time reversible, that is, arrows go both forward in time and backwards in time. Accepting this axiom, let's rewind to the BB. Let's assume we're at the era where we have elementary particles. I wonder here (speaking strictly of spin): all of the particles are entangled (seeing the BB at that moment as a single system as if it would be a blackhole)? Or they interact with one another and thus create, destroy and thus recreate the entanglement all the time, and thus the space?

3) There's a paradox here for me with BB that I can see. We've concluded that as entropy grows and the Universe as a closed system grows and gets more chaotic, it cooled down and thus allowed for atoms to happen. But why didn't enough protons create blackholes as they interacted? OK, it was too hot for that to happen, but then, this means that there's a crucial moment, time Ω when the system evolves to atoms etc. and parts of it are entangled (it remains entangled with the latest particles or w/e that could interact and get entangled and then when it cooled down enough, these "flew off" in different directions). Now, observing one of a pair would affect the other one and thus now we're entangled with that pair. So, assuming dual slit experiment, when I measure a particle, my detector gets entangled with that particle and I see the wave-function as if it would collapse, if I don't measure the particle, I know everything there's about to know about the system (entangled) but nothing about the details of the system (i.e. spin up or spin down, I just know that there's the superposition as we understand from uncertainty principle very well). Would this mean, we're affecting the state of the system (Universe) as it evolved from the crucial time Ω when atoms could be formed and thus us? Or we can only affect particles which already interacted and thus created the solar system etc. etc. and thus we only affect the local part of our space? If that's the case, this would be consistent still with what LS is saying in the talk linked above: let's say one of my particle pair gets into the blackhole nearby Earth, then I can find out what I want by measuring my other particle: wouldn't this destroy any previous entanglement of that blackhole and thus collapse the wormhole?

Most of the above is silly and please apologize if so, I am still unsatisfied with this.

By the way, as I was alluding to the firewall, it seems it was also pointed out in the presentation by LS in the linked video above.