Can black hole entropy be observer-dependent?

[enquirealways]

If our universe has a history of infinity of cause-effects/time, how could it ever traverse that infinity to reach 'now'.

I mean to say, even if our universe started with a big bang, there must have been some cause for it, then a cause for the cause that caused big bang and so on ad infinitum.

How can this infinity be ever traversed to reach the present time?

 

[phinds]

If our universe has a history of infinity of cause-effects/time, how could it ever traverse that infinity to reach 'now'.

Why do you think it does?

I mean to say, even if our universe started with a big bang, there must have been some cause for it, then a cause for the cause that caused big bang and so on ad infinitum.

That becomes "turtles all the way down" and is not science but philosophy

How can this infinity be ever traversed to reach the present time?

Doesn't seem likely does it?

 

[enquirealways]

Why do you think it does

I think it does because there seems no other logical conclusion. In fact this seems inevitable.

 

[micromass]

I think it does because there seems no other logical conclusion. In fact this seems inevitable.

The more you study science, the more you will see that nature has a habit of not following our logic. We always need to adapt our logic and way of thinking in order to understand nature. Many people in the past have had ideas on how our world should behave logically, most of them were wrong.

I am not convinced that everything in our world has a cause. Many occurences in QM seem to happen completely randomly without any cause. Maybe there is an underlying determinism that can give you this cause, or maybe our philosophy that "every effect must have a cause" is just wrong.

 

[enquirealways]

The more you study science, the more you will see that nature has a habit of not following our logic. We always need to adapt our logic and way of thinking in order to understand nature. Many people in the past have had ideas on how our world should behave logically, most of them were wrong.

I am not convinced that everything in our world has a cause. Many occurences in QM seem to happen completely randomly without any cause. Maybe there is an underlying determinism that can give you this cause, or maybe our philosophy that "every effect must have a cause" is just wrong.

Ah! I feel nervous...

Even if the philosophy of 'cause and effect' is wrong ( i can't somehow believe it to be so), time is almost certainly eternal.

If one says time started with big bang, there must have been some 'ttiimmee' before that 'caused' big bang.

Even if big bang was not caused, it must have taken some 'ttiimmee' to come into existence, even if this ttiimmee is infinitely small. It hints time was already there.

So, how we traversed this infinite time to reach the present?

 

[micromass]

time is almost certainly eternal.

Why do you say this? How do you know there was anything before the big bang. Just saying "it is logic" or "it is my intuition" or "I believe it to be so" doesn't cut it. Logic, intuition and beliefs were wrong before.

 

[enquirealways]

Why do you say this? How do you know there was anything before the big bang. Just saying "it is logic" or "it is my intuition" or "I believe it to be so" doesn't cut it. Logic, intuition and beliefs were wrong before.

Let me present the other scenario...

Big bang happened from a primordial ball extremely small in size. Time may have existed, since there must have been some motion in that ball that led to explosion.

if we negate this, big bang then becomes the case of 'something from absolute nothing'. This seems utterly implausible.

The QM thing you talked about has quantum fluctuations to begin with. Particles don't spring out of absolute nothing.

This hints the presence of 'something' before bigbang and this in turn hints at time via the question...How and when this something came about?

 

[micromass]

Let me present the other scenario...

Big bang happened from a primordial ball extremely small in size. Time may have existed, since there must have been some motion in that ball that led to explosion.

if we negate this, big bang then becomes the case of 'something from absolute nothing'. This seems utterly implausible.

You are severely misunderstanding the big bang. The big bang theory never says that something came from nothing. The big bang theory never says that the universe was somehow infinitely dense and then expanded. All it says is that there is a hypothetical point in time where it seems that the universe was infinitely dense. However, the big bang theory only starts working until $10^{-43}$ seconds after the hypothetical point in time. It makes no statements what-so-ever about what happened before $10^{-43}$ seconds after the hypothetical point. Nobody knows. There are a lot of theories, but nobody really knows. So nobody is saying that something came from nothing.

The QM thing you talked about has quantum fluctuations to begin with. Particles don't spring out of absolute nothing.

That was not really what I was referring to.

 

[Chalnoth]

If our universe has a history of infinity of cause-effects/time, how could it ever traverse that infinity to reach 'now'.

In physics, the idea of cause-effect relationships just doesn't describe reality very well. In fact, it isn't even clear that the traversal of time is anything but an illusion.

For example, in General Relativity, space-time is described as a single entity (called a manifold). This entity doesn't traverse time, and neither does anything within it. It just exists across all time and space. The passage of time arises as an emergent property of certain specific configurations of the manifold. Specifically, if you have a low-entropy state, then you will see events that look like the passage of time in both directions away from that low-entropy state.

 

[Chronos]

To expand on Chalnoth's explanation, a photon has no sense of time. It can traverse any distance you care to imagine in no time whatsoever, with respect to its reference frame.

 

[enquirealways]

You are severely misunderstanding the big bang. The big bang theory never says that something camie from nothing. The big bang theory never says that the universe was somehow infinitely dense and then expanded. All it says is that there is a hypothetical point in time where it seems that the universe was infinitely dense. However, the big bang theory only starts working until $10^{-43}$ seconds after the hypothetical point in time. It makes no statements what-so-ever about what happened before $10^{-43}$ seconds after the hypothetical point. Nobody knows. There are a lot of theories, but nobody really knows. So nobody is saying that something came from nothing.



That was not really what I was referring to.

Thanks for correcting me.

So, we don't know.

But still, there seem to be two options only. Either there was sth before big bang or there was nothing. In both cases we can't avoid infinity.

1. If there was something, how it came about. If we find another something from which the first sth came about, the next question is 'how the second sth came about' and so on...

2. If there was absolutely nothing before bigbang, we can ask, 'what is the mechanism for this.' If we find some mechanism, we can say 'what is the mechanism for the mechanism that brings sth from nothing and so on...

So, how to traverse this infinity?




Moreover, Do you think questions will ever end? Will a state come when there will be nothing else to know? I don't think so.

Isnt this an infinity in itself?

 

[Chalnoth]

It's a bad question. It's not a question that makes sense in terms of physics.

 

[enquirealways]

In physics, the idea of cause-effect relationships just doesn't describe reality very well. In fact, it isn't even clear that the traversal of time is anything but an illusion.

For example, in General Relativity, space-time is described as a single entity (called a manifold). This entity doesn't traverse time, and neither does anything within it. It just exists across all time and space. The passage of time arises as an emergent property of certain specific configurations of the manifold. Specifically, if you have a low-entropy state, then you will see events that look like the passage of time in both directions away from that low-entropy state.

I don't understand this. When i talked about time being an illusion, my thread was closed!

 

[enquirealways]

It's a bad question. It's not a question that makes sense in terms of physics.

Still, it seems a valid question.

 

[Chalnoth]

I don't understand this. When i talked about time being an illusion, my thread was closed!

Whether or not time is an "illusion" is an active debate within physics. But I don't think there's any getting away from the idea that the passage of time is an emergent property.

 

[phinds]

... there seem to be two options only ...

and either one of them as you expressed it is turtles all the way down and we just don't know. At present, this is as I said earlier speculative philosophy, not physics.

 

[enquirealways]

and either one of them as you expressed it is turtles all the way down and we just don't know. At present, this is as I said earlier speculative philosophy, not physics.

Even if we know the answer in future, can we escape this 'turtles all the way down' thing.

Please try to explain.

 

[Athanasius]

Enquire always, here are some thoughts I have had regarding your question. I will describe them in purely naturalistic terms because of their implications for cosmology if true. I think it may be possible to have causality without time. For instance, the flow of sound logic on a printed page is a sort of cause and effect, but all of the thoughts exist all at once on the page. How could time come to exist in a timeless realm of cause and effect? One possibility is the arrow of thermodynamics, as already mentioned. Another is that time may be apparent only to minds riding and observing the advancing edge of causality, like a man on a raft riding the front of a flow of water released from a dam. Whatever time is, both it and causality seem to have a "now," an edge, that flows in one direction. That seems to mean that both are like a finite ray with an advancing edge that we ride, with the potential to continue forever but requiring a beginning. This would mean that neither of them is an infinite continuum. If they were, as you mentioned, we could never have arrived at the present time or state of causality. It would also mean that there must be a beginning to both time and causality. The first cause by definition must be either uncaused or self-causing. If there was a first cause, that is something, not nothing. So you would not get everything from absolutely nothing, but rather from a causative something.

 

[julcab12]

Even if we know the answer in future, can we escape this 'turtles all the way down' thing.

Please try to explain.

... No-maybe, but we sure have a something to make intuitive sense to it. We derived at infinities whether we like it or not. Integral part of math; To string people it is treated as (1/12)-Riemann Zeta function lims→1ζ(s)=−1/12 while to LQC people-'bounce' where* the turtle is somehow slowly turning all the way up at some point flipped back again in an endless cycle (analogy).. Anyway, I couldn't find any mechanism that deals well with divergent functions than bounce.

 

[phinds]

Even if we know the answer in future, can we escape this 'turtles all the way down' thing.

Please try to explain.

I have no idea.

 

[sshai45]

and either one of them as you expressed it is turtles all the way down and we just don't know. At present, this is as I said earlier speculative philosophy, not physics.

So what happens if we discover that "hey, the 'universe' has existed forever!"? Then it seems all the "philosophizing" becomes moot anyway since fact trumps speculation.

Or is someone trying to suggest by this "philosophizing" that it is _impossible_ for the universe to have "existed forever"?

 

[Chronos]

We are fairly confident the universe has a finite age. We are also fairly confident we do not know what preceded the universe.

 

[Drakkith]

To bounce off of Chronos' post, we are confident our models explain the universe back to a point in time about 13.7 billion years ago. What was before this, if anything, is unknown. If anything existed, it was most likely very different from what we have today. Because of this we say the universe, as we know it, has a finite age.

 

[CraigDxHypo]

The "nothing" theory of cosmology.

Let me present the other scenario...
Big bang happened from a primordial ball extremely small in size. Time may have existed, since there must have been some motion in that ball that led to explosion.

By most Big Bang models, the very early (< 10^-43 seconds) universe didn’t have distinct, measurable particles – one could reasonably say the entire universe was a single, indivisible “primordial atom”. In physics, to have motion (which is defined as change in distance), one must have distance (which is defined as the difference in position of two bodies), and to have distance, one must have two bodies, so motion is undefinable in the very early BB universe

if we negate this, big bang then becomes the case of 'something from absolute nothing'. This seems utterly implausible.

Although it can seem implausible to people unfamiliar with scientific literature on the subject, this is just what “nothing cosmogist” like Edward Tryon proposed 40+ years ago. Though the models haven’t attracted much attention – scientists tend to be adverse to simple theories that appear unlikely to be experimentally validatable in the foreseeable future – they’ve not to my knowledge been credibly refuted, either. (This 1994 magazine article has a good non-technical story about philosophical and scientific ideas about the origin of the universe, focused on “nothing theorists” such as Tyron).

The essence of Tyron and other’s proposal – "the universe is simply one of those things that happens from time to time" – is that though the probability of the Big Bang’s “primordial atom” appearing in any a short period of time – say 1 second – is very small, it’s greater than zero. Given sufficient time, the probability of this event approaches one. The “empty” universe preceding the proposed Big Bang large scale vacuum fluctuation event – although no ordinary clock existed in it to measure time – was of sufficiently long duration.

The QM thing you talked about has quantum fluctuations to begin with. Particles don't spring out of absolute nothing.

In quantum physics, particles do spring out of “absolutely nothing”, absolutely nothing being the “classical” vacuum, a volume of space with no matter, light, or other particles in it. In most cases, they “spring out” or existence so quickly after they spring in that they’re not detectable. In some unusual cases, such as with the Hawking radiation that theory states allows small black holes to evaporate rather than growing and consuming stars and planets that have the misfortune to create them, “sprung from vacuum” particles should be detectable.

The ordinary kind of “springing for vacuum” quantum physics predicts happens often is called a “small scale” quantum fluctuation. The extraordinary kind that “nothing cosmologists” propose created the Big Bang is “large scale”.

 

[marcus]

I wonder if Enquire is still around, or checking this thread. I thought the questions were good and somebody should steer him towards the real research going on about what happened at the start of expansion. There is normal science being done.

I think anybody interested in that would do well to watch the 28 minute video of the talk Paul Steinhardt gave on 23 June. https://www.physicsforums.com/showthread.php?t=763608

...
I mean to say, even if our universe started with a big bang, there must have been some cause for it,...

Sure. A lot of excellent researchers feel like that---that the start of expansion (quite possibly a bounce from contracting phase) should be explained. Testable models should be proposed. It's being studied. Plausible ideas are out there some of which can be FALSIFIED by further observation (which is a sign that it's normal science.)
Abhay Ashtekar and Ivan Agullo are two active researchers in this area, a PF member filmed interviews with them and posted this:

Someone else got his permission to repost, so there are two URLs that have it:

...If one says time started with big bang, there must have been some 'ttiimmee' before that 'caused' big bang.

Even if big bang was not caused, it must have taken some 'ttiimmee' to come into existence, even if this ttiimmee is infinitely small. It hints time was already there...

Sure. That's an intelligent intuition. A lot of good cosmologists don't accept the idea that "time began" at the start of expansion. If you are interested, you should get to know some of the leading ones by watching their talks or interviews.

...
if we negate this, big bang then becomes the case of 'something from absolute nothing'. This seems utterly implausible.

The QM thing you talked about has quantum fluctuations to begin with. Particles don't spring out of absolute nothing.

This hints the presence of 'something' before bigbang and this in turn hints at time ...

"Something from nothing" appeals to some researchers, and not to others. As does the idea of a "quantum fluctuation" in something quite unlike our universe. But those people are not the only ones now working on the problem of understanding the early universe, what happened at the start of expansion.
And why wouldn't there have been time before the start of expansion? It hasn't been shown that there was NOT. That is just something they say in popular media--it gets public's attention--a "beginning of time!" is a catchy idea.

If you watch Steinhardt's 28 minute video you will see some critical analysis of some of the other views. The first 15 minutes or so is critique.
http://physics.princeton.edu/strings2014/videos/talk1h.mp4
The one problem is maybe a lot of people are trying to watch that talk and overload makes the streaming a bit balky. When I watch it sometime I have to click on pause and go do something else while the buffer memory fills up,and then restart.

 

[Chronos]

I still feel like pushing 'time' back before the BB is little more than kicking the 'can' down the road. Its main appeal is to those unwilling to consider the universe may have had an acausal origin.

 

[marcus]

I still feel like pushing 'time' back before the BB is little more than kicking the 'can' down the road. Its main appeal is to those ...

I can't look into other people's minds to see what appeals to them about trying to understand the start of expansion in terms of physical process.

But it is normal science to try to understand what is near to us, and from which we have a lot of observable/measurable evidence.

I am happy to see progress in understanding just this one (hypothetical) big bounce. It does not bother me that this seems to kick the existential can back in time. Let there always be big questions for the philosophically inclined---but let's see some testable models explaining THIS expansion that we see going on around us and have an abundance of signals from, to analyze.

I'm glad to see models being developed to describe the beginning of THIS expansion, and to see people thinking about ways to test and falsify them. It is a natural thing to be curious about, and it is normal science (as Steinhardt points out.)

 

[Chronos]

No disagreement intended. I like the idea of time and space as emergent properties of the gravitational field, and the gravitational field as an emergent property of the big bang. From that perspective, the question of time before the big bang is moot. I admit the case for this viewpoint is not beyond a reasonable doubt. Cyclical models make me cringe because they seem to force entropy back to zero, which I find objectionable on thermodynamic grounds.

 

[skydivephil]

Mdoern bouncing comsolgies are not necessarily cylic models. They basically change the Friedman equation at veyr high curvature regimes only . We know that the unvierse was in such a regime 13.8 bio years ago and so there may have been a bounce then. If the universe exapnds forever ,as is the case if dark energy is a comsoloigcal constant and not something dynamic like quintessence, then the universe in these models will not recollapse and they will not be cyclic.

 

[marcus]

Modern bouncing cosmologies are not necessarily cylic models. They basically change the Friedman equation at very high curvature regimes only . We know that the unvierse was in such a regime 13.8 bio years ago and so there may have been a bounce then. If the universe exapnds forever ,as is the case if dark energy is a cosmological constant and not something dynamic like quintessence, then the universe in these models will not recollapse and they will not be cyclic.

True enough! There's also an entropy "mindset" issue. If you look at how entropy is defined you see it is observer-dependent because it depends on the observer's coarse-graining---the macrovariable versus microvariable distinction. Entropy is the logarithm of the number of microstates (based on degrees of freedom irrelevant to the observer) comprising one grand macrostate (based on d.o.f that he actually interacts with and which affect him).

Any observer has a coarse-graining map corresponding to the lumping together of microstates into macrostates (consolidating all those which don't make any difference to the observer). Entropy measures the "size" in the particular macrostate we're in. The amount of information in it, that we ignore.

There's a group of people who think of entropy as absolute, who don't think that when you talk about it you have to specify a coarse-graining map. It is difficult for them to accept bounce cosmology because it looks to them as if "the entropy" (an absolute quantity) was reset to zero at the bounce. And there are other people who don't have that problem.

If you think of entropy as defined for a particular coarse-graining, then you don't encounter that mental obstacle. There is a pre-bounce guy and according to his coarsegraining the entropy increases astronomically as you go into the bounce, and it never thereafter declines! Because everything post-bounce is irrelevant to him, like it was inside the horizon of a black hole, the whole universe.
The post-bounce guy has a DIFFERENT coarsegraining and he sees the entropy initially low, everything about the bounce matters to him, is of vital importance, affects him thru variables he interacts with. Then as the U expands and diversifies regions of phase space become indifferent and irrelevant to him and entropy (for the post-bounce guy) increases.

The second law holds for any particular guy's entropy---defined based on his coarse-graining of the world.

This has been pointed out by various people. I think probably it would have come up in your Abhay&Ivan interview documentary video. As I recall Thanu Padmanabhan stated it clearly. Entropy is observer-dependent, or words to that effect. I've lost track of all the people who have made that point. Recently it came up here:
http://arxiv.org/abs/1407.3384
Why do we remember the past and not the future? The 'time oriented coarse graining' hypothesis
Carlo Rovelli
(Submitted on 12 Jul 2014)
Phenomenological arrows of time can be traced to a past low-entropy state. Does this imply the universe was in an improbable state in the past? I suggest a different possibility: past low-entropy depends on the coarse-graining implicit in our definition of entropy. This, in turn depends on our physical coupling to the rest of the world. …

Some more reading, if curious:
http://arxiv.org/abs/gr-qc/9901033
http://arxiv.org/abs/hep-th/0310022
http://arxiv.org/abs/hep-th/0410168

 

[marcus]

In previous post I mentioned some articles about entropy in general relativity and for possible entertainment or simply to give a bit of the flavor will quote a passage from Don Marolf's 2004 paper

==quote http://arxiv.org/abs/hep-th/0410168 from conclusions==
the realization that observers remaining outside a black hole associate a different (and, at least in interesting cases, smaller) flux of entropy across the horizon with a given physical process than do observers who themselves cross the horizon during the process. In particular, this second mechanism was explored using both analytic and numerical techniques in a simple toy model. We note that similar effects have been reported³⁵ for calculations involving quantum teleportation experiments in non-inertial frames. Our observations are also in accord with general remarks^36,37 that, in analogy with energy, entropy should be a subtle concept in General Relativity.
We have concentrated here on this new observer-dependence in the concept of entropy. It is tempting to speculate that this observation will have further interesting implications for the thermodynamics of black holes. For example, the point here that the two classes of observers assign different values to the entropy flux across the horizon seems to be in tune with the point of view (see, e.g., Refs. 38,39,40,41,42) that the Bekenstein-Hawking entropy of a black hole does not count the number of black hole microstates, but rather refers to some property of these states relative to observers who…

==endquote==