Question about speed of light and information

[Omega0]

I think about the following: For a black hole it seems quite natural to reflect about information loss.
May be we have a firewall etc. We have an event horizont which does not allow any information exchange with
the outer world.

The difference between the universe and the singularity might be clear, the black hole is something local.
The universe is the universe but I think we face a comparable problem.

My question: If we already accept that parts of our universe are out of our scope, what does this means to the model me have about our universe?

Thanks

 

[Dr. Courtney]

Is it valuable to speculate about situations where experiments are not possible and will never be possible?

 

[rootone]

Your questions answers itself.
Our models suggest that there is more to the Universe than we can observe.
It's reasonable to suppose that regions beyond observation are physically similar to the observed universe, but we can't make definite assumptions.
Therefore our models are constrained to being an accurate description of what actually we do in fact observe.
The implication of our models for areas which we can never observe must be considered as speculative, even if the speculation does seem reasonable

 

[Omega0]

Is it valuable to speculate about situations where experiments are not possible and will never be possible?

Physics is only acceptable (for me) if there is a
1,) model which leads to predictable effects
2.) model which decribes nature accurately
The first is the impressing part. The second is the one which I would like to understand in cosmology and I would hope to find the first.

 

[Finny]

If we already accept that parts of our universe are out of our scope, what does this means to the model me have about our universe?

Parts of the universe out of our past light cone, that is, so distant in space or time we can never communicate, means we will never be able to experimentally confirm any theories we may have about such regions.

Quantum gravity, when fully developed, may have more to tell us about information 'loss'. Let's hope so: I saw one estimate [Leonard Susskind, I think] that posited 99% of the information from our universe likely resides there. That was apparently one of his motivations for the great BLACK HOLE WAR, his 'debates' with Stephen Hawking about information loss in such. That is the title of his book, by the way, a great non mathematical discussion for the general public about information loss [or not] in black holes.

 

[rootone]

The relativity theories describe nature very accurately and have made predictions that proved true.
Same can be said for quantum mechanics and it's cousins.
So far though we don't have a testable unified 'theory of everything'.
Work is in progress though, maybe somebody will hit on the right idea eventually, and produce such a theory which is verifiable.

 

[Omega0]

Your questions answers itself.
Therefore our models are constrained to being an accurate description of what actually we do in fact observe.
The implication of our models for areas which we can never observe must be considered as speculative, even if the speculation does seem reasonable

If there wouldn't be dark energy and matter I might agree. I don't think so positive about the model but I have no clue how to solve this issue.

 

[Omega0]

The relativity theories describe nature very accurately and have made predictions that proved true.
Same can be said for quantum mechanics and it's cousins.
So far though we don't have a testable unified 'theory of everything'.
Work is in progress though, maybe somebody will hit on the right idea eventually, and produce such a theory which is verifiable.

I love GRT and I love physics generally. I didn't find a clue to solve the GUT so far, I should take some holidays... the point is that I feel that there is something
going wrong in research but I can't add something constructive so far - so everything is fine.

 

[Chalnoth]

If there wouldn't be dark energy and matter I might agree. I don't think so positive about the model but I have no clue how to solve this issue.

What do dark matter and dark energy have to do with your question?

 

[Omega0]

What do dark matter and dark energy have to do with your question?

Thanks for the remark. I have just in mind "Therefore our models are constrained to being an accurate description of what actually we do in fact observe."
I definitely didn't want to blur the basic question. I think about the dependencies - if there is one. In my eyes we have no accurate description - which begins with measurement and ends with prediction. The problem of missing information is just part of it and my basic remark.

 

[Chalnoth]

Thanks for the remark. I have just in mind "Therefore our models are constrained to being an accurate description of what actually we do in fact observe."
I definitely didn't want to blur the basic question. I think about the dependencies - if there is one. In my eyes we have no accurate description - which begins with measurement and ends with prediction. The problem of missing information is just part of it and my basic remark.

We definitely do have an accurate description. $\Lambda$CDM is a very accurate description of our universe to date. There may be some discrepancies with regard to structure formation, but it successfully describes the relationship with a wide varieties of observation quite well.

It is always worthwhile to consider the question, "Is there an alternative way to describe the same data?" And cosmologists have been asking and trying to answer that question. So far, the alternative models have fallen into two camps:
1. The model fits the data, but introduces even more unknowns, such as a dynamical dark energy or modified gravity.
2. The model doesn't fit the data.

To date, $\Lambda$CDM is the simplest model that fits the observational data. Nobody has yet come up with a simpler explanation. There are a wide variety of speculative explanations that are more complex.

 

[PeterDonis]

If we already accept that parts of our universe are out of our scope, what does this means to the model me have about our universe?

It depends on what you mean by "out of our scope". We can't directly observe what's inside a black hole, but we can get indirect evidence about it. Similarly, we can't directly observe what's beyond our cosmological horizon, but we can get indirect evidence about it.

Also, as Chalnoth has just noted, the simplest model that fits the data is preferred. It is simpler to assume that the laws of physics work the same in regions of the universe we can't directly observe (like inside black holes and beyond our cosmological horizon) than it is to assume that the laws are different in those regions. So in the absence of data which suggests otherwise, we should assume that the same laws of physics we observe to work in regions we can directly see, also work in regions we can't see.

 

[Omega0]

It depends on what you mean by "out of our scope". We can't directly observe what's inside a black hole, but we can get indirect evidence about it.

This is pure speculation. It makes no sense to extrapolate into something you will never enter - and if you are in you'll never get out.
It's like belivieng in a life after death.

Similarly, we can't directly observe what's beyond our cosmological horizon, but we can get indirect evidence about it.

Aha?

So in the absence of data which suggests otherwise, we should assume that the same laws of physics we observe to work in regions we can directly see, also work in regions we can't see.

Which would contradict the discovering of quantum theory, doesn't it?

 

[PeterDonis]

This is pure speculation.

No, it isn't. Indirect evidence is not speculation.

For example, we have indirect evidence that there is a black hole at the center of our galaxy. It's true that we can't directly see inside to verify that it's a black hole; but the indirect evidence is that things fall into a region containing a million or so solar masses of mass (as shown by the orbits of stars around it), but which emits no detectable radiation. When things fall in, they disappear; any radiation they emit quickly gets redshifted to a point where it's undetectable. If the laws of physics work in that region of spacetime the same as they work near us, then the only possible object that could be in that region at the center of our galaxy is a black hole; if it were anything else, what we observe would be different.

Which would contradict the discovering of quantum theory, doesn't it?

Not at all. Quantum theory was not made up in the absence of evidence; it was forced on physicists because they kept getting strong evidence that classical (pre-quantum) physics did not make correct predictions. And that evidence was obtained right here on Earth; nobody had to argue that, while classical physics worked OK here on Earth, it must be breaking down billions of light years away.

 

[Chronos]

Omega0, you are stuck on a failed theory of 'everything'. Observational evidence is the ulitimate trump card in science.

 

[tom aaron]

I'm not as confident as others that we our 'on the right track'. However, it's the only track we have that 'may' be the right track.

Dark matter is an unknown but not really an enigma. It is probably particles that will fit fine within the Standard Model. In contrast, dark energy may throw a wrench into everything we assume about the Universe.

Quantum entanglement is still magic...not really magic but something indicates that there's a whole bunch of 'stuff' we are not aware of and maybe never will. Entanglement is the elephant in the room that zaps my confidence that we know anything at all.

 

[Chronos]

Quantum phenomenon by definition are irrelevant on cosmic scales.

 

[Finny]

My question: If we already accept that parts of our universe are out of our scope, what does this means to the model me have about our universe?

One thing it forces us to recognize is that while we have rather amazing mathematical constructs [models] which we can experimentally verify, sometimes to many decimal places accuracy, the flip side of the coin is we don't know nearly as much as we'd like. Good thing, as PeterDonis points, out we have some indirect evidence for what's going on. Experimentalists turn out to be repeatedly more capable of gathering information and making better observations than initially thought.

Einstein thought black holes merely a mathematical result rather than a physical reality. And consider that we thought we knew about most matter and energy in the universe; Then very recently dark matter and dark energy were 'discovered'. WOW what a surprise. Add that we can only directly observe what is likely an 'insignificant' spec of our universe. Good thing we can see things from the past [light sometimes takes a long time to reach us] or we'd really be in the dark! [pun intended]

 

[tom aaron]

Quantum phenomenon by definition are irrelevant on cosmic scales.

Really? You discovered this how?

Not knowing something is far different from 'irrelevent'. The sun has an impact on the orbit of Earth even though we once never knew the relationship...it wasn't 'irrelevant' just because we did not know it.

 

[rootone]

Perhaps 'insignificant' would be better than 'irrelevant'.
A lot of the reasoning underlying QM has to do with probabilities, (and of course measurement).
There is an infinitesimally small probability that all the air molecules in my room will head into one corner, and I could suffocate.
However on macroscopic scales involving billions of molecules that probability is so tiny it's unlikely to occur before the Universe come to an end, (let alone during my life time).
So while it's not strictly impossible, it a very safe bet that it won't happen,
Furthermore there are no examples of such extremely improbable events having ever been observed happening with macroscopic objects.

Where it does become an issue is when considering the very earliest moments of the Universe.
At this stage It is small enough that QM effects might be present, yet simultaneously it is cosmic in scale by definition.
We don't as yet have any theory which can explain what the universe behaves like at this earliest time.
Relativity starts to give nonsense results, and QM doesn't explain much if anything.

 

[Chalnoth]

No, it isn't. Indirect evidence is not speculation.

For example, we have indirect evidence that there is a black hole at the center of our galaxy. It's true that we can't directly see inside to verify that it's a black hole; but the indirect evidence is that things fall into a region containing a million or so solar masses of mass (as shown by the orbits of stars around it), but which emits no detectable radiation. When things fall in, they disappear; any radiation they emit quickly gets redshifted to a point where it's undetectable. If the laws of physics work in that region of spacetime the same as they work near us, then the only possible object that could be in that region at the center of our galaxy is a black hole; if it were anything else, what we observe would be different.

This isn't quite accurate. We have no observations of anything falling into the supermassive black hole at the center of our galaxy. What we do have are the orbits of stars near the black hole, though soon we expect to have the event horizon itself imaged (http://eventhorizontelescope.org/).

 

[Chalnoth]

Relativity starts to give nonsense results, and QM doesn't explain much if anything.

QM explains quite a lot. It's just that we can't yet be sure what the correct theory of QM is in the very early universe. So far, semi-classical models that assume that the quantum behavior has no significant impact on gravity and QM is described by an effective field theory have worked pretty well. This may be incorrect, but it's workable.

If you're stuck on the idea that QM provides predictions of probability, this doesn't distinguish it from classical physics in terms of observation. No experiment has ever had perfect accuracy. QM places some limits on how accurate experiments can possibly be (though few experimental apparatuses push QM limits), but the fact that there is always some level of uncertainty is not specific to QM.

Furthermore, QM's predictions denote a probability distribution, and such probability distributions can be usually observed to very high accuracy through observations of many realizations of the distribution.

 

[tom aaron]

Perhaps 'insignificant' would be better than 'irrelevant'.
A lot of the reasoning underlying QM has to do with probabilities, (and of course measurement).
There is an infinitesimally small probability that all the air molecules in my room will head into one corner, and I could suffocate.
However on macroscopic scales involving billions of molecules that probability is so tiny it's unlikely to occur before the Universe come to an end, (let alone during my life time).
So while it's not strictly impossible, it a very safe bet that it won't happen,
Furthermore there are no examples of such extremely improbable events having ever been observed happening with macroscopic objects.

Where it does become an issue is when considering the very earliest moments of the Universe.
At this stage It is small enough that QM effects might be present, yet simultaneously it is cosmic in scale by definition.
We don't as yet have any theory which can explain what the universe behaves like at this earliest time.
Relativity starts to give nonsense results, and QM doesn't explain much if anything.

Not really

Every experiment shows the validity of QM. It impacts every particle. Something either has an impact or it doesn't. There is no distinction between a big impact or a little impact. That is a human construct. Oceans exist because of the cumulative assembly of individual drops of water. The Universe consists of matter and energy.

 

[PeterDonis]

We have no observations of anything falling into the supermassive black hole at the center of our galaxy.

Yes, you're right. I believe we do have observations of matter falling into suspected black holes of stellar mass (for example, binaries where one of the two objects is massive but dark).

 

[Omega0]

Yes, you're right. I believe we do have observations of matter falling into suspected black holes of stellar mass (for example, binaries where one of the two objects is massive but dark).

Just a remark: It is not that I don't believe in the existence of black holes. Perhaps this wasn't clear from my statement. I just wanted to say that I find it sort of astonishing to speculate what happens if you could be beyond the event horizon. The loss of information seems to be critical. At least it is very controversial. See Joseph Polchinskis firewall.
I wanted to point out that in my eyes one should think about what this means to the universe. I would say the issue is comparable although the scale is not easy comparable. Why the following argument should be wrong: Information loss is not allowed, so there is no part of the universe expanding relative to us with v > c?

 

[PeterDonis]

I just wanted to say that I find it sort of astonishing to speculate what happens if you could be beyond the event horizon.

Why? All we are doing is applying the same laws of physics that work everywhere we have already tested them. It would be more astonishing to maintain that the laws of physics somehow change at the horizon.

The loss of information seems to be critical.

First, the information loss problem is only a problem if you accept the GR prediction about event horizons in the first place. If you don't believe it's justified to apply the standard laws of GR to an event horizon, then there is no information loss problem to begin with. The problem arises from the fact that what GR says about horizons and what happens inside them (that a singularity forms and anything that hits the singularity is destroyed, including quantum information) appears to conflict with what QM says about information being preserved by unitarity. In order to have that apparent conflict, you need to first apply what GR says in the standard way.

Also, the schemes that have been proposed to try and avoid this issue all involve some sort of "new physics" at the horizon; but that means the word "speculation" should be applied to these schemes, not to the standard GR understanding of the horizon, which is, as I've already noted, simply apply the same laws of GR that work everywhere else. We have experimental evidence that those laws work; we have no experimental evidence at all about "firewalls" or any other speculative proposal about things that could happen at the horizon to "fix" the information loss problem, if it is indeed a problem.

Why the following argument should be wrong: Information loss is not allowed, so there is no part of the universe expanding relative to us with v > c?

Because there is no conflict between the FRW model of an expanding universe (including there being a cosmological horizon) and quantum unitarity. The quantum information in the FRW model never gets destroyed. The fact that it can't be communicated back out through the cosmological horizon doesn't pose a problem, because all that means is that you and anything behind your cosmological horizon must be spacelike separated. Quantum theory has no problem dealing with spacelike separated events, as long as unitarity is preserved.

The problem with a black hole is that, as I said above, quantum information being destroyed in the singularity violates unitarity. In other words, the problem is really the singularity, not the event horizon. It is in principle possible that a model could be devised that removed the singularity and preserved unitarity, but still included an event horizon (in fact, some proposed "baby universe" models, where what would have been the singularity instead spawns a new universe, pretty much do that). Such a model would not be subject to the information loss problem.

 

[Chalnoth]

Just a remark: It is not that I don't believe in the existence of black holes. Perhaps this wasn't clear from my statement. I just wanted to say that I find it sort of astonishing to speculate what happens if you could be beyond the event horizon. The loss of information seems to be critical. At least it is very controversial. See Joseph Polchinskis firewall.

Many physicists find it interesting to determine what various theories predict about the interior of black holes. It's true that we probably can't ever experimentally confirm those predictions, but when experiments are impossible it is still possible to use consistency as a check.

For example, from examining what General Relativity says about the interior of a black hole, we can be certain that General Relativity is incorrect. Exactly where it breaks down is uncertain. If there exists a theory which describes the interior of a black hole and has no singularities or other inconsistencies, then that theory has a chance of being correct. Then, if we're lucky, that theory will have observational consequences outside of a black hole that we can test.

It would be foolish to conclude that any theory is correct merely because it provides a consistent description of the interior of a black hole, but it turns out that it is surprisingly difficult to produce such a consistent theory, which makes it a useful exercise in eliminating possible theories of physics. More than that, a lot of theorists just find this kind of question interesting, even as they realize that it may be unsolvable.

 

[Omega0]

Why? All we are doing is applying the same laws of physics that work everywhere we have already tested them. It would be more astonishing to maintain that the laws of physics somehow change at the horizon.

Also, the schemes that have been proposed to try and avoid this issue all involve some sort of "new physics" at the horizon; but that means the word "speculation" should be applied to these schemes, not to the standard GR understanding of the horizon,

I agree that this is speculative. I just wanted to say if this a problem then we have the problem twice where you counter:

The quantum information in the FRW model never gets destroyed. The fact that it can't be communicated back out through the cosmological horizon doesn't pose a problem, because all that means is that you and anything behind your cosmological horizon must be spacelike separated. Quantum theory has no problem dealing with spacelike separated events, as long as unitarity is preserved.

Okay, so: How can you preserve quantum entanglement for a cosmic scale? Just a theoretical point. If the universe is pumped up by "dark energy", wouldn't this mean that in the very end quantum entanglement ends? For sure, several years after me pushing up the daisies.

 

[PeterDonis]

Okay, so: How can you preserve quantum entanglement for a cosmic scale?

The same way quantum entanglement is preserved between any set of spacelike separated events.

If the universe is pumped up by "dark energy", wouldn't this mean that in the very end quantum entanglement ends?

No. Why would it?

 

[Chalnoth]

I agree that this is speculative. I just wanted to say if this a problem then we have the problem twice where you counter:

Okay, so: How can you preserve quantum entanglement for a cosmic scale? Just a theoretical point. If the universe is pumped up by "dark energy", wouldn't this mean that in the very end quantum entanglement ends? For sure, several years after me pushing up the daisies.

Quantum entanglement is ended by decoherence. Anything that interacts with photons will decohere pretty quickly due to the amount of light that is moving through our universe. If the particle in question doesn't interact with photons, it may take quite a bit longer to decohere, possibly allowing for large-scale entanglement. But it's going to be really difficult to ever observe that entanglement precisely because those particles don't interact much with photons.

 

[Omega0]

For example, from examining what General Relativity says about the interior of a black hole, we can be certain that General Relativity is incorrect. Exactly where it breaks down is uncertain. If there exists a theory which describes the interior of a black hole and has no singularities or other inconsistencies, then that theory has a chance of being correct. Then, if we're lucky, that theory will have observational consequences outside of a black hole that we can test.
...
More than that, a lot of theorists just find this kind of question interesting, even as they realize that it may be unsolvable.

I completely agree. For me it has the bad taste of ruling out nature. The constant hope to circumvent the speed of light. As mentioned before, I think this has to do with the problem of the end of our existence. Call it resignation, I call it realism - which helped physics a lot.

 

[Omega0]

Quantum entanglement is ended by decoherence. ...
But it's going to be really difficult to ever observe that entanglement precisely because those particles don't interact much with photons.

I agree, that's why I wrote, from a very therotical point of view. I am thinking about "the end" of our universe. What will happen if space expands more and more?
What if the decoherence will be seldom? If there is no photon doing this?

 

[Chalnoth]

I agree, that's why I wrote, from a very therotical point of view. I am thinking about "the end" of our universe. What will happen if space expands more and more?

The universe will become empty.

What if the decoherence will be seldom? If there is no photon doing this?

If there are no particles around, then there's nothing to set up any sort of entanglement. But as long as there are particles that interact with photons, there will also be photons.

 

[Chalnoth]

I completely agree. For me it has the bad taste of ruling out nature. The constant hope to circumvent the speed of light. As mentioned before, I think this has to do with the problem of the end of our existence. Call it resignation, I call it realism - which helped physics a lot.

I don't understand what you're trying to say. Determining what various models say about the interior of a black hole doesn't violate the speed of light. It's just a thought experiment. Different people find different things interesting, and that is ultimately very important for the progress of science. The fact of the matter is that we just can't know what avenues of investigation are going to be fruitless before pursuing them, so it's important for people to pursue a diversity of directions for future research.

 

[Omega0]

The same way quantum entanglement is preserved between any set of spacelike separated events.
No. Why would it?

How does for example the Dirac propagator look like if you have 2 particles where one has v > c? Wouldn't you say it makes no sense to couple two particles if they have no exchange of information?