« Why Should I Defend My Faith? | Main | Links Mentioned on the 3/11/14 Show »

March 11, 2014

Comments

What cosmology tells us. The universe is expanding. We can wind the clock back so that the universe looks smaller and smaller as we look back in time. However, that only works back to the Planck Time. At that point general relativity breaks down and the universe is so small quantum effects take over. Unless I have missed something really rather radical in physics, theories of quantum gravity and the effects on spacetime have not progressed to allow us to make ANY conclusions about whether the universe had a beginning or not.

So the standard model works POST Planck Time, not before

"From this description you can see how dangerously close this cause sounds to a theistic description of God. Perhaps this is why many researchers and cosmologists seek to find a cosmological model avoiding a cosmological singularity (a model denying the beginning of space, time and matter"

Its a little rude to impute an agenda to deny God. Rather it's because our current physics doesnt work so they are trying to find ways to understand what's going on. The fact that this direction doesn't fit your particular brand of religion is neither here nor there and I doubt that factor ever enters their conciousness.

So P2 of the Kalam "The universe began to exist" is not supported currently by science. The best you can say if you support Vilenkins work (and I quote from his paper):

"Did the universe have a beginning? At this point, it seems that the answer to this question is probably yes" http://arxiv.org/pdf/1204.4658v1.pdf p5

You may also like to consider Turok and Steinhardts Ekpyrotic model for completeness, which is empirically testable and an interesting alternative to answering the question.

Without going into the other problems with the cyclical universe, we can at least scratch the Second Law of Thermodynamics of the list.

The first thing to remember about that law is that it is a statistical law. It is immensely probable that entropy will increase, not certain.

The reason for this bias for increased in interactions is not that somehow that is how it must be. The bias exists because the universe is expanding. Things tend to spread out (i.e. entropy increases) because there is more space to spread out into.

In a contracting universe, this is not the case. In a contracting universe, the Second Law of Thermodynamics is false. As such, the Second law has no overarching application on the system of the cyclical universe itself.

So, a world that cycles between big bang, universe, big crunch and back to big bang seems to be possible. That does not, of course, make it true

"The reason for this bias for increased in interactions"

Rule #13 of good writing:

Make sure you haven't any words out.

The phrase above s/be:

"The reason for this bias for increased entropy in interactions"

The Great Suprendo: I have read some of Turok and Steinhardt's work on the Ekpyrotic Model, but in fairness to this topic, it still offers no explanation for how the infinite, empty universes (or branes) necessary to allow the Ekpyrotic Model to work came into existence. Their model starts with the assumption of at least two infinite, empty universes and proceeds from that point to attempt to provide an explanation for what we observe in our universe. Their model does not answer the question of how our universe (empty or otherwise) came to exist. We are still left with the question of "what caused our universe to begin to exist?"

Wisdomlover,

You said, "In a contracting universe, the Second Law of Thermodynamics is false." This is news to me. As long as the universe remains a closed system, I don't see why this would be true. Why do you think it's true?

Entropy is a measure of how much space things have to move around in (not, as popularly characterized, of disorder).

In an expanding universe, there is more and more space for the same things to move around in.

In a contracting universe, there is less and less space for the same things to move around in.

Because of this, entropy increases in an expanding universe, but decreases in a contracting universe.

Since the Second Law of Thermodynamics is that entropy always increases it is, therefore, false in a contracting universe.

WL, where are you getting this? I went through the Naval nuclear power program, took all kinds of physics and thermodynamics, then went to college and took more physics, chemistry, and thermodynamics, and I've never heard this.

My physics coursework was many years ago. But I seem to recall getting it either in the classical thermodynamics courses or the follow-on statistical mechanics courses that I took.

To be sure, any such discussion in those classes was informal in nature. You can do thermodynamics and statistical mechanics perfectly well without assuming that the second law doesn't apply in a contracting universe. This is because, of course, we live in an expanding universe.

But it would be quite surprising to me if the second law were still true in a contracting universe.

It's tempting to think of entropy as 'stuff' in the way that energy is. So that the second law would seem to be saying that you just get more and more of it as time goes by.

That strikes me as misguided.

Entropy is a measurement of how much space things have to move around in.

That's all.

The second law is simply an expression of how grossly improbable it is that things will spontaneously move into a smaller spaces. They will tend instead to expand out into a larger spaces.

But, obviously, if there's less and less space for things to move around in, they just can't move into a larger space and must move into a smaller space. So that measurement that we call entropy, goes down.

How else could it be?

Entropy is a measurement of how much space things have to move around in.
Maybe you mean phase space?

Ron-

I wasn't going for technical perfection. I was trying to move past the first order misconception of entropy as mere disorder.

If you have a sample of helium in a 10 liter container at 0 Celsius and you slowly let it expand into a 20 liter container keeping the temperature at 0 Celsius, you increase the entropy in the sample...even though both the initial and final state have about the same level of disorder. The helium atoms just have more space to move around in.

Look here, for example. It says, "[Bolzmann] interpreted rho as a density in phase space".

What is 'phase space'?

A point, P, in space is identified by its 3 spatial coordinates (x, y, z).

A 'point' X in phase space is identified by those same coordinates (x, y, z) PLUS 3 momentum components, call them (p, q, r).

So you can say that 'phase space' consists of every possible combination of position and momentum. That is, phase space is every possible (x, y, z, p, q, r).

Knowing a particle's 'location' in phase space means knowing its position AND its momentum.

A 'volume' of phase space is a set of possible combinations of location and momentum.

(The momentum of a particle is its mass times its velocity.)

It sounds like you may have heard about Bolzmann's formula in your course, intuitively carried the ideas of expansion and contraction over into your thinking about phase space, and then drawn your conclusions about the second law.

You can't do that.

No Ron. I'm not confusing phase space and regular space. I just chose not to over-complicate the discussion.

You're, more or less, right about what phase space is. There is one dimension of phase space for each degree of freedom in a system.

In a classical gas sample, for example, that works out to six times the number of particles. Each position can vary along the x, y and z axes and each momentum can vary along the same axes.

Now think about the phase space of the simple example of the Helium sample I just described.

Since the temperature is held constant the range of possible momenta of each Helium atom remains unchanged. But since the volume of the gas increased, the range of possible positions increased. As such the volume in phase space increased. Entropy goes up. Not because of disorder, but simply because particles have more space to move around in. (And, of course, the opposite is the case if the volume is halved instead of doubled...entropy goes down in that case). It turns out not to be a linear increase (or decrease) in entropy. The entropy doesn't double when we double the volume, nor does it halve when we halve the volume. The relation is logarithmic, but that's neither here nor there.

Now think about the 'big crunch'. All the particles in the universe are coalescing into a smaller and smaller volume forced there by the ever increasing mass at the center of that volume. So the average range of their positions on each axis is decreasing. What is more, as they are constrained into that smaller and smaller space, their average velocity begins to approach zero because more and more of them are being crunched down into a single point where all motion becomes impossible. So the average range of their momenta is also decreasing. Since both the position and momentum ranges are decreasing, the volume of the system in phase space is decreasing. As such the total entropy of that system is going down.

(That last bit is still highly informal, obviously. There are more degrees of freedom in the real world than just x-y-z position and momentum.)

Certainly, an isothermal expansion will result in a change in in entropy. It will be: delta S = Q/T. Q is the heat the gas absorbs and T is your constant temp. But a gas need not stay at constant temperature!

Entropy is a measurement of how much space things have to move around in.
An expansion (or compression) with no change in entropy is an isentropic process. Such a process is thermodynamically possible making it clear that your definition is wrong. Entropy is not defined by or tied to 'how much space things have to move around in'.

"What cosmology tells us. The universe is expanding. We can wind the clock back so that the universe looks smaller and smaller as we look back in time. However, that only works back to the Planck Time."

Does that not simply fuzz out the start, rather than eliminating it?

IF there was nothing 1 sec prior to getting back that far, you know there was a beginning some time during that one second.

IF there was nothing 1 sec prior to getting back that far
Why would you think there was nothing?

Isentropic processes are reversible processes. They can only occur when a system remains in equilibrium throughout the process. This is, in practice, impossible.

But just to be clear, my claim is not that every decrease in entropy involves a reduction in volume. Nor that every reduction in volume results in a reduction in entropy.

The point is that the entropy of the entire universe during the time leading up to a big crunch is going down.

But let's suppose that I'm wrong. This number called entropy just keeps going up and up and up.

And then...what?

Is there a suggestion that if the universe cycles back to a big crunch that there will be some sort of problem?

What exactly would that be? All the energy in the universe would coalesce back into the crunch. And it's not like the energy in the crunch will be differentiated into some light, some heat, some mechanical energy. If anything, it's probably closer to think that it will just be mass. If there's a big bounce from there back into a big bang, would some of the ergs from the new big bang be 'second-class' ergs? Would there be more of these 'second-class' ergs in the new bang than the prior bang?

"But our entropy is higher than before" you cry.

Who cares? We won't run out of numbers to express this terrible entropy inflation.

They can only occur when a system remains in equilibrium throughout the process.
There are no isothermal processes either. Same reason.
But let's suppose that I'm wrong.
OK

"There are no isothermal processes either. Same reason."

Isothermal processes can, but need not be, reversible. Isentropic processes must be reversible. Reversibility can't exist in the real world. Isothermality can.

But again, neither here nor there, since my general point is not that entropy depends simply on volume. The entropy of a sample of water obviously goes down when it freezes...even though the volume increases. My point was that the events leading up to a big crunch involve the total decrease of entropy of the universe and that is linked to the contracting volume of the universe. (Just as having all the gas in a chamber spontaneously move to the center is a decrease in entropy, having all the matter in the universe spontaneously move to the center is a decrease in entropy.)

As for the "OK", cute.

You think you won a point there?

I'm not sure whether to laugh or cry when Christian apologists make a mess of cosmology to try to prove Christianity.

brgulker-

How about starting with identifying the mess you perceive being made?

some things humans cannot understand because they are over our heads. maybe rotating around the earth's center and revolving around the sun prevent us from thinking straight.

“Perhaps the best argument in favor of the thesis that the Big Bang supports theism,” the astrophysicist Christopher Isham has observed, “is the obvious unease with which it is greeted by some atheist physicists. At times this has led to scientific ideas, such as continuous creation or an oscillating universe, being advanced with a tenacity which so exceeds their intrinsic worth that one can only suspect the operation of psychological forces lying very much deeper than the usual academic desire of a theorist to support his or her theory.”

Berlinski, David (2009-08-26). The Devil's Delusion: Atheism and its Scientific Pretensions (Kindle Locations 988-991). Basic Books. Kindle Edition.

The comments to this entry are closed.