# Entropy and Gravity?

Hi, I am also having problem with the assumption Entropy makes about the movement of molecules in a 3D space. Does it assume that gas molecules could have equal chance of going in any direction? If so then how is it possible outside a free-fall lab as Gravity bias would always make all the molecules point (orient) the same way and give all of them the same final direction and hence their movement could be predicted with 100% accuracy and precision if needed. Einstein predicted the trajectory of photons because of the bias Gravity created for photons so I am sure molecules trajectories could also be predicted and there should be no need to calculate probabilities at all. Entropy states "as we do not know " hence lets "assume equal probabilities".
What if equal probabilities does not exist? That would invalidate Entropy conclusions completely, wouldn't it?
How can 6N have equal probabilities in presence of Gravity?
Why assume equal probabilities to build conclusions upon when there is a universal bias present (Gravity) which could easily explain both the creation of all order we are seeing (e.g. stars, light, planets, moons, galaxies, solar systems, oceans etc) and working of the universe e.g ice floating, hot air rising, oil floating on water, rain, rivers, ships sailing, leaves falling, day and night etc single handedly.
What am I missing?
Does SM really assume absence of bias when building it's conclusions!

Nugatory
Mentor
Hi, I am also having problem with the assumption Entropy makes about the movement of molecules in a 3D space. Does it assume that gas molecules could have equal chance of going in any direction?
No, of course not. If it did, we wouldn't be able to use the theory of gases to explain why the air pressure at sea level is greater than at the top of a 5000 meter tall mountain.
It is true that we don't consider gravitational effects when analyzing, for example, a flask of air one meter on a side. But that's not because we're making the assumption that molecules are equally likely to move in any direction, it is because we we can calculate the magnitude of the bias and see that it's too small to affect the answer in the one meter case but not the 5000 meter case - so we include it in our calculations in one case but not the other. (It would e a good exercise to calculate the pressure difference between sea level and one meter above sea level - that will prove both that we know how to do it when we have to and that there's a good reason why we don't usually worry about it).
What if equal probabilities does not exist? That would invalidate Entropy conclusions completely, wouldn't it?
How can 6N have equal probabilities in presence of Gravity?
Try it..... Find a six-sided die and toss it 120000 times. How much do your results deviate from the results predicted by the assumption that all six outcomes are equally probable?
Why assume equal probabilities to build conclusions upon when there is a universal bias present (Gravity)?
Because we can make that bias arbitrarily small by performing our experiments in free fall or far from any gravitating body - but we require that our laws of physics still work under those conditions.
Does SM really assume absence of bias when building it's conclusions!
To repeat the first sentence of this post..... No, of course not.

hutchphd and Dale
Thanks for explaining it.
So what exactly does "Equal a priori Probabilities" mean in terms of movement of molecules outside a free-falling lab?

Try it..... Find a six-sided die and toss it 120000 times. How much do your results deviate from the results predicted by the assumption that all six outcomes are equally probable?

Here is I look at the die example. Because of Gravity not all 6 sides of the die are equal in size so the die will always try to face the same side up no matter how you throw it. Assuming equal probability would be disastrous. Now let's forget for few seconds the face a die lands on, a die no matter how hard you throw will always have the "same final direction" so you could easily "predict" the trajectory of it.
So what I am saying is that even a die does not move randomly. A die wants to roll downwards in one direction only just like everything else in the universe. All Molecules of a gas (outside a free-falling lab) must be oriented (pointed) the same way and would want to move in the same direction due to Gravity.
Entropy states "As we do not know" hence assume "absence of bias (Equal a priori Probabilities)"
But we can predict with 100% accuracy all molecules final direction as it is the same and there is a bias present!
Also because of this bias (Gravity), time arrow is arranging matter in "an order" where as Entropy states Time arrow moves matter towards more disorder or "randomness"
Take an example of an hourglass, it clearly shows how time is arranging matter in Gravity's "order" according to light/heavy by using a "Bubble sort" method as sand falls down and air rises. All buildings will fall one day because of the bias and order created by Gravity and not because of absence of bias as Entropy claims. Ships sail an ice floats because of this bias too.
I understand how Gravity is not as strong as other variable involved say in a small flask. But It is not the "strength", it is the "orientation" and the "final direction" Gravity imparts on molecules which destroys Equal a priori Probabilities.
Ice will float no matter how small the flask is!
If Gravity is ignored then rivers can't flow and hot air will not rise.
How does Entropy explain rivers?
I appreciate your time. Entropy assumption has been confusing me for a while and it seems like a wrong assumption to make to me as bias of Gravity can explain all order we are seeing (stars, galaxies, planets, moons, solar systems, oceans etc) and it can also explain how the universe runs and time moves too eg rain, ships sailing, clouds, birds flying, apples falling, hot air rising, ice floating.
Peace

And just to clarify again:

What you are saying is that Equal probability (chaos or randomness) does not actually exist in our universe due to bias created by Gravity otherwise universe will not function or would be even created. That is a fact.

Equal probability is only assumed for experiments in free-falling lab or small scale calculations.

Thank you for clarifying it.

But I think many people in the World are confused about what scientific position is on this issue as they believe "equal probability" actually exists and created all order we are seeing. But it is the bias (Gravity) which is creating all orders and making everything work.

Another question though:
If you take Gravity out of the equation then wouldn't time arrow stop? - an hourglass would stop working too in a free-falling lab. So how does Entropy explain arrow of time while it ignores Gravity?
In a free-falling lab - oil and water will not separate and if you stir Oxygen and Helium in an isolated box then they would not layer up either. A sand castle should never fall apart in a free-fall lab as I see it. So if you ignore Gravity as in a free-falling lab time arrow stops because matter stops getting Bubble sorted as everything starts moving in the same direction with same acceleration if not acted upon. But the moment free-fall ends then Helium and Oxygen molecules will go in predictable directions just like sand and air in an hourglass will or bubbles in water will.
In a free fall lab, bubbles would not even bubble to the surface of water as I envision, will they?
And also what about Evolution and Multiverse then? Are they not based on Equal a priori Probabilities ?
How do those theories justify assuming equal probabilities for drawing their conclusions when equal probability does not even exist outside a free-falling lab?
Thanks again for explaining it.

PeterDonis
Mentor
2020 Award
If you take Gravity out of the equation then wouldn't time arrow stop?

Obviously not, since we have plenty of experience now with free-falling conditions in spacecraft and time flows just fine in them.

The fact that some processes don't work the same in the absence of gravity as they do in the presence of gravity has nothing whatever to do with "time's arrow stopping". It's just a simple consequence of, you know, the process requiring gravity to be present to work properly. But there are lots and lots of processes that don't have that requirement.

It seems like you are taking some observations that are specific to particular circumstances and trying to build a theory of the whole universe out of them. That doesn't seem like a promising approach.

Dale
PeterDonis
Mentor
2020 Award
The OP's premises in this thread are based on serious misunderstandings and personal theory, which is off limits for discussion here. Thread closed.