# Question about speed of light and information

1. Jul 6, 2015

### 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

2. Jul 6, 2015

### Dr. Courtney

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

3. Jul 6, 2015

### rootone

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

4. Jul 6, 2015

### Omega0

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.

5. Jul 6, 2015

### Finny

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.

6. Jul 6, 2015

### 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.

7. Jul 6, 2015

### Omega0

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.

8. Jul 6, 2015

### Omega0

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.

9. Jul 6, 2015

### Chalnoth

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

10. Jul 6, 2015

### Omega0

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.

11. Jul 6, 2015

### Chalnoth

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.

12. Jul 6, 2015

### Staff: Mentor

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.

13. Jul 6, 2015

### Omega0

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

14. Jul 6, 2015

### Staff: Mentor

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.

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.

15. Jul 6, 2015

### Chronos

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

16. Jul 7, 2015

### 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.

17. Jul 7, 2015

### Chronos

Quantum phenomenon by definition are irrelevant on cosmic scales.

18. Jul 7, 2015

### Finny

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]

19. Jul 7, 2015

### tom aaron

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.

20. Jul 7, 2015

### 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.

21. Jul 7, 2015

### Chalnoth

This isn't quite accurate. We have no observations of anything falling in to 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/).

22. Jul 7, 2015

### Chalnoth

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.

23. Jul 7, 2015

### tom aaron

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.

24. Jul 7, 2015

### Staff: Mentor

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).

25. Jul 7, 2015

### Omega0

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?