Anyway, congratulations again, Sean. Coming across well on national television and usefully promoting some subtle science is well more than most of us could probably do.

But I did pick it up, and I’m so glad it’s been explained!

3) It’s not completely obvious to me that structure formation is not reversible, at least in the case that no black holes are formed. If we were to take a Universe similar to the present-day and flip the sign of the expansion (“Don’t make me turn this cosmos around!”), then the photon gas would increase in temperature and eventually drive all particles back to being relativistic and hence unbound, ie galaxies and stars would all boil away. Once we’re past the hadron/QGP transition on he way back up, nothing is gravitationally bound after that, correct? And so we get back to essentially the same macro state we started with, ie a hot relativistic gas filling the Universe; to first order this looks like a reversal to me.

Thanks for the clarifications,

Paul

Thanks very much for the quick reply. However, there’s still something I’m missing here following your remark on structure formation through gravitational collapse:

1) In my question at #52 I specified zero chemical potentials for heavy (ie non-massless) particles, so there would be no net density/number of non-relativistic particles. If this were the case, then would structures still form? Could one really have a galaxy that’s half matter and half antimatter? I suppose one could postulate an arbitrarily low annihilation cross section — could that describe dark matter? — but that seems like a funny condition for an increase in entropy to hinge on.

2) Even if there is a small net density for some heavy, stable species, such as baryons in our own Universe, there is some finite early period when the temperature is high enough that all those heavy particles are relativistic and so will not collapse gravitationally. That probably describes our own Universe, for example, from the end of the inflation up through the QGP/hadron transition — which is quite a long time in a logarithmic sense. So, was there any gravitational structure formation in our Universe before the QGP/hadron transition? If not, then how was entropy increasing during that period?

Meanwhile I’ll try to save lunch money for the book…

I can get the Colbert Report from Munich, Germany:
http://www.colbertnation.com/full-episodes/wed-march-10-2010-sean-carroll

Let me know if this works.

Does anyone know of an alternate location or working proxy workaround?

I watched it 4 times over the course of a few days now. I feel that it’s hard to beat your answers given the circumstances. They were absolutely great. The only thing I would say that affects a television audience was your non-verbal expressions in the beginning. When Colbert says “hit me hit me” you sort of get taken aback, which doesn’t look comfortable and it made me feel uncomfortable too. This might be a strong statement, but you may have gotten into taking his idiot character on too serious a level.

One of the commenters got it right when they said that Colbert usually outwits his guests, but here he was sitting in wonder at your explanations (especially the cake question). You made him so engaged in your response that he forgot to be a comedian host, and I think that “takes the cake.”

Also, kudos for the very witty response at the end where you mention that Colbert coming before Stewart may happen more often in the multiverses.

Overall, a very good interview! I hope they invite you back/you go on more shows/some producer tries to get you to do a regular TV show. I think you would make an extremely successful TV science popularizer personality.

Interesting but (resorting to tautology), existence is existence; “The universe just is” (Hawking). We share the characteristics of the universe which, past, presently and will define our existence…we just are.

The 4D projection of eternal existence is obvious…ourselves. You were yourself as a boy and you are yourself now…as you will also be yourself when you are older; but you have changed, and will change in the future.

It is human to fear the unknown, but fearing the unknown is like fearing tomorrow…fearing growing older and the process of change which actually defines our lives and consciousness.

I don’t believe that the fact we forget our previous existences and therefore cannot compare them with the one we are now experiencing has foundational importance. Cosmologically past “existence” is part of the record…personal fossils. If we had the technology we could “dig for them”, in fact by studying and observing carefully we can get clues as to what the cosmological past might have been like.

We remember a part of the story of how we got to this place. We cannot remember the future, though that fact in no way discredits our knowledge that the future exists…it is out there. Historical records help extend our knowledge of the past but as these records become more ancient, they become less accurate.

The cosmological models which work are deterministic and dual.

When we observe the universe- and ourselves- we view eternity. What we see- including ourselves- has taken an eternity to develop into what we see today…Sean really got us thinking in 5 minutes! The book is just great…I’m about 3/4 finished.

Best Wishes…

Overall I thought it was a great success! You almost certainly pulled in some people at the margin, from all walks of life, who might now be persuaded that deep questions about time are not just philosophical and undecidable, but can be investigated and discussed in a concrete and accessible way. The Colbert slot was a great opportunity, and you took excellent advantage of it.

Now that you have more than a minute to think about it, I’d like to get your answer to a more exact form of SC’s question about the early Universe.

Imagine an early thermal Universe, pretty similar to how we imagine our own, evolving over some stretch of time with the following specifications: (i) the cosmological constant is exactly zero, (ii) there are no primordial black holes, and none are created, and (iii) the chemical potentials for all massive particle species are vanishing, ie no net number of heavy particles. This seems like a perfectly reasonable situation, at least over a finite interval. The question is, during such a finite interval while this Universe is evolving, is its entropy increasing? It’s hard to see how, since the expansion of a well-thermalized universe should basically be reversible; ie start from a given macro state but switch the sign of the expansion and the contracting universe will go through pretty much the same set of macro states in reverse order. On the other hand, if entropy (suitably defined) is not increasing during an early thermalized phase then it’s hard to justify the statement that initial entropy is low. So how should one think about this?

(I apologize if the answer is spelled out better in the new book, as I can’t afford to buy one and so haven’t read it yet — are you giving away courtesy copies to interested physicists?)

Best regards,

Paul