# B Entropy and the Singularity

1. Jan 12, 2016

### Feeble Wonk

Please have pity for the idiot in the room. I've tried to look into this concept through a few papers I've found on-line, but the mathematics involved is too far over my head.
I'm trying to wrap my head around the general concept of entropy as it applies to a singularity.
In a singularity of infinite density, how does one measure the degree of entropy? It seems to me that, depending on how you look at it, you could argue that the singularity is either at maximum entropy OR zero entropy.
Is this apparent logical ambiguity one of the reasons that physics "breaks down" at the singularity?

2. Jan 12, 2016

### Staff: Mentor

You cannot defined an entropy because physics breaks down. There is simply no way to establish something like phase-spaces with a singularity and current physical models.

3. Jan 12, 2016

### Feeble Wonk

So, the general concept of entropy in a singularity is meaningless... right? That's what I had previously assumed.

4. Jan 12, 2016

### rootone

The mere existence of a singularity in a theory signifies that the model in the circumstances being considered gives rise to a meaningless prediction.
It means 'we don't know what is going on here and the theory doesn't apply', not that a physical object actually can exist which is infinitely dense, or has a negative mass, etc.

5. Oct 27, 2016

### Ltiilika

Well, the 2nd law of thermodynamics states that the intropy of an isolated system increases. Well, if we consider that the whole universe was an isolated singularity before the Big Bang that would mean that in that singularity the entropy of the universe was at its lowest and the entropy has only increased since the Big Bang. That would mean that the entropy of a singularity is low.

6. Oct 27, 2016

### Chronos

7. Oct 27, 2016

### newjerseyrunner

Interesting side note is that string theory independently came up with the same formula for entropy as Bekenstein. (My source was the first in a very specific circumstance but it's since been more generalized.)

8. Oct 27, 2016

### Staff: Mentor

That is not correct. The singularity arises when you take a particular idealized model and extrapolate it backwards in time. But no cosmologist believes that that extrapolation is actually valid back to that point. Some new physics should come into play before then; our best guess at this point is that the new physics is quantum gravity. But in any case, the singularity is not part of our actual physical universe, so trying to reason based on the assumption that it is is meaningless.

9. Nov 6, 2016

### Anthony1948

Is The Big Bang an imagined illusion?
To a simple mind like mine, I cannot not imagine the logic behind the Big Bang. It seems so inconceivable that my brain rejects it over and over again and grasps onto alternative theories... alternative theories, to me, is like being rescued from an unimaginable slow tortuous death.

Can anyone explain the logic and facts behind this incomprehensible theory? For example, How can all the matter in the universe or multiple infinite universes be condensed down to a single point of infinite density?....not very logical.

How can you have a point of infinite density?... Illogical by the very nature of "infinite density"
...and yet respected scientists purport this theory....are they nuts?
All the matter in the universe condensed down to a single tiny ball?...illogical.
I'd say the universe is pretty big with billions of stars in our galaxy and billions of galaxies....How could that possibly happen? Can you explain this? Better yet where is the proof or a facsimile of proof?

As a side note how can you explain the expansion of the universe? everything moving away from each other without a central point to expand from? Then how can they say its expanding? Expanding from what? For example, i you take a star, then each star or galaxy would have be a central point in itself and repel each other? Yes? What do you do? Just have infinite central points? Even the explanations; Ie the raisins in the baking raisin bread explanation, still requires a central point.... its all so confusing. I welcome any feedback.

10. Nov 6, 2016

### Drakkith

Staff Emeritus
That's not what the theory says. The singularity is first and foremost a breakdown of the theory to make useful predictions, much like the graph of 1/x breaks down at x = 0.

Second, as we look backwards into the past, the density of the universe increases everywhere. There simply isn't a center of the universe at all, let alone a single point in space that everything is moving backwards towards.

Because all galaxy clusters/superclusters are moving away from all other galaxy clusters/superclusters and the best way to describe this kind of change is the word "expansion".

Expanding away from all other unbound objects (unbound meaning that gravity isn't strong enough to hold the objects together and overcome the expansion).

Expansion does not occur within galaxies or galaxy clusters. The presence of enough mass or energy sort of "bends space the other way" (the "other way" being gravity) and prevents expansion from occurring.

Indeed. The analogy is of course just an analogy and cannot accurately describe the real situation. Its sole purpose is to help people understand how expansion makes galaxies move relative to one another, not to describe the entire expansion process.

11. Nov 6, 2016

### Anthony1948

12. Nov 6, 2016

### Anthony1948

Thank you for your reply. I appreciate your insight. What a relief. Your answer makes so much sense. Obviously I have many misconceptions.So its back to the books to get a better understanding of inflationary theory and the big bang.

13. Nov 12, 2016