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Using physics to explain time

  1. Mar 14, 2007 #1
    Is it agreed my physicists that time is an emergent property resulting from the 2nd law of thermodynaimcs?

    The universe started from very low entropy and entropy increased from there and will keep on increasing. The result of its increase is heat flow which basically means chagne at all scales in the constitiuents of matter. It is the change in matter flow that has enabled the universe to be what it is and the change will continue as long as entropy is not maximised. This change has produced us in the universe and we observe this change and create an an entity called time to explain this change. As if time allows things to change. But really it is the second law at work and change will stop when entropy is maximum. When the universe achieves this state, time as we know it will also have stopped. In this way it is the second law which creates the effect of the existence of time. Time is an artificial quantity just like the x,y,z spatial coordinates in that it isn't a fundalmental law of physics.

    I have been very unsepcific but is it basically correct?

    If so then the idea of going back in time is nonsense because to do so would mean violating the 2nd law of thermodynamics. This is consistent with SR where in that theory it also argues that travelling back in time is physically impossible. It's good to see two theories giving the same results. However, they both are classical theories applied to macroscopic objects. I am aware some individual atomic entities can travel back in time.

    I have heard that the 2nd law can also explain why a ball falls to the ground when released from a distance from the bottom of the ground. I thought it was because of gravity although time is recquired for gravity to do work on the ball. Is the need for time how the 2nd law comes into it? i.e. the entropic state when the ball was realsed was lower than after it collided with all the air molecules. After the ball collided with the molecules and also the molecules on earth which it would touch when on the ground, entropy has increased which is explained by the 2nd Law. That's why it falls. It seems like gravity isn't needed to explain why a ball falls?! Is gravity an emergent property of the 2nd law? I see a connection in that far away where matter density is low, gravity is low. So the change in entropy after letting go of the ball would be lower than on earth hence it dosen't move as fast which is consistent with the 2nd law.
    Last edited: Mar 14, 2007
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  3. Mar 14, 2007 #2
    I doubt that.

    Entropy increase is often referred to the 'arrow of time', but it's easy to think of a system in which entropy doesn't change- but still has dynamical properties.

    A idealized frictionless bouncing ball seems to be one such system.

    It's true that a real bouncing ball does increase the net entropy of the system, but that can't account for gravitational effects. Why doesn't a balloon filled with helium also drop to the ground?
  4. Mar 14, 2007 #3
    I have said many things in my OP what do you doubt?

    Maybe it's just that by going up, the net total incrase is maximised. Whereas if it went down, entropy wouldn't have increased as much.

    When heavier collide with the ground, there is more heat transferred when a light ballon collides with the ground. So it may be better off entropically to float up the sky and collide with as many molcules of air as possible. There is also the thing with what is going on inside the ballon.
  5. Mar 14, 2007 #4
    Nah- I still don't buy it.

    Release a ball halfway up a wooden room- which has near-identical floor and ceiling.

    If it were only a matter of maximum entropy increase then the ball would hit the ceiling 50% of the time.

    In reality- the ball will always drop to the ground upon release.

    Entropy always increases (globally) whatever you do, but it's not the only principle involved.

    Thermodynamically, the free energy change is minimized at equilibrium- which is internal energy-entropy*temperature (F=U-TS).
  6. Mar 14, 2007 #5
    If you do the experiment near the Earth's surface than it will tend to fall down with a very high chance even if the room has near equal dimensions because the weight of the earth is so much greater so there is much more potential for heat transfer.
  7. Mar 14, 2007 #6
    Well, I don't agree. The heat transfer only depends on the velocity of the ball upon impact and the details of the surfaces. In outer space the entropy would increase whether the ball bounced off the floor or the ceiling.

    You simply can't derive gravity from entropy. Well, if you can then post the details or give a reference.
  8. Mar 15, 2007 #7


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    Well, it is an aspect of "time", probably the one that corresponds to our notion of direction of time. Indeed, we "remember" the past, and that's due to increased "memory" of the past, which can only happen in a situation of increasing entropy.

    Well, "time" enters physics in more than one way. The time you talk about is the time that is a kind of "memory and processing" time. So when the whole of the universe would be in thermodynamical equilibrium, there wouldn't be any processing and remembering anymore. So there couldn't be a kind of Maxwell demon that would still know yesterday from tomorrow.
    That doesn't change that other aspect of time, which is geometrical (in relativity) though.

    :confused: I have never heard that :smile:

    However, remember that if the universe is in a state of thermodynamical equilitbrium, there cannot be a ball that falls to the earth, because there are then still dissipative processes possible, which would increase entropy, which was maximal by assumption.

    But no, gravity doesn't result from the second law.
  9. Mar 15, 2007 #8
    So there are two concepts for time right? What are they and what are their difference?

    What i really understand that time is a measurement.

    If you go by the theory of bing bang, everything started as one black hole. Everything is so densely packed that nothing moves around and so there is no entropy at all. You usually measure time to see how long it takes for an event. Since everything was one, there could only be one movement. But if there is no movement there is no event and no need to measure time. So time starts at the first movement any matter of the big bang or the first appearance of entropy.

    But time, in reality is nothing but an abstract measurement. When formulas produce a shorter time, it means that it will take a shorter time compared with the standard time that is happening at the moment.

    Correct me if i am wrong.
    Last edited: Mar 15, 2007
  10. Mar 15, 2007 #9


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    There's a great little book on all these issues, by Zeh:


    It used to be freely accessible but apparently now you can only get some parts.
  11. Mar 15, 2007 #10

    In here they explain thigns in general principles

    A key highlighted passage in blue on that site is "Our psychological sense of time is based on the second law. It summarizes what we have seen, what we have experienced, what we think will happen." So as you say the 2nd law covers one aspect of time but not the other one to do with the time in spacetime?

    They don't use the word entropy but describes the 2nd law in the most basic terms which is interesting. Although also correct?
    "The big deal [presumably the 2nd law] is that all types of energy spread out like the energy in that hot pan does (unless somehow they're hindered from doing so) They don't tend to stay concentrated in a small space; they flow toward becoming dispersed if they can -- like electricity in a battery or a power line or lightning, wind from a high pressure weather system or air compressed in a tire, all heated objects, loud sounds, water or boulders that are high up on a mountain, your car's kinetic energy when you take your foot off the gas. All these different kinds of energy spread out if there's a way they can do so."

    "The second law of thermodynamics summarizes that totally different events involving all kinds of energy have a common cause. A blowout in a tire or a car battery shorting out or slowly running down -- what could seem to be more unlike than those! Yet the reason for their occurring is the same, the tendency for concentrated energy not to stay localized, to disperse if it has a chance and isn't hindered somehow."

    As for the falling ball, they explain it here "Sinking ships are like rocks rolling down a mountain -- as they sink, their potential energy due to being high above sea-bottom is diffused, spread out to the water that they push aside (or, in the case of mountain rocks, diffused as they roll down to the valley and hit other rocks, give those a bit of kinetic energy, and warm them slightly by friction.)"

    If the universe was in thermodynamical equilirburim than as you said, there wouldn't be any ball to fall. If in that situation of equilibrium than also there wouldn't be any gravity either because the mass density will be equally spread out and no net gravititational forces on any matter. In that way the two theories are consistent. Gravity may not result from the second law but the site seems to suggest that the second law could explain gravitational energy changes.
    Last edited: Mar 15, 2007
  12. Mar 15, 2007 #11
    OK- but your quote doesn't imply that the second law leads to gravity.

    Again- the system tends towards an equilibrium in which the free energy, F=U-TS is minimized, where S is the entropy, S=S(U), U is the internal energy (kinetic+potential) and T is the temperature.

    The entropy is always increased, but that doesn't tell you what U is doing.
  13. Mar 15, 2007 #12


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    Yes, that's correct, these are the dissipative phenomena I talked about. However, this is something different than saying that the second law causes gravity or something like that. Rather, there *is* (independently) an interaction called gravity (let's keep the Newtonian viewpoint here), which is a conservative interaction and hence has potential energy. There are also other interactions such as electromagnetic etc...
    Now, a certain macro configuration (rock on top of the mountain) is less probable than other configurations (rock down the mountain, and particles vibrating some more in neighbouring rocks). So if a certain process, *that is allowed by the interactions of gravity, electromagnetism ....* makes us go from the first state to the second state, it will be much less probable to go from the second state to the first (which will also be allowed). THIS is the content of the second law. But the second law "needed" the interactions in the first place for there to be a possibility for the rock to fall and heat its neighbours. It wasn't the origin of it. You cannot derive the laws of gravity or of electromagnetism from the second law.
  14. Mar 15, 2007 #13
    The second law seems to be stated in a very general way which could be why the site explained that it was "The biggest, most powerful, most general idea in all of science". So general that it may be more like a mathematical axiom than a physical one in that it is true in any universe. i.e The most probable state will be the state in which the system will most likely to be in.
  15. Mar 15, 2007 #14
    That's a tautology.
  16. Mar 16, 2007 #15
    Exactly that is why the 2nd law exists any universe!?. I pointed that to our lecture and he agreed that the second law is taughtological. Although to quote our more exactly His notes stated that an alternative version of the 2nd law is that "Any large system in equilibirum will be found in the macrostate with the greatest entropy (aside from fluctuations too small to be measured)."

    Greatest entropy <=> the most probable state

    You could argue that a system by definition will be in the state it is most probable to be in if given the chance. The equilibrium situation shows that it has had the chance.

    You could also say that the 2nd law dictates that the system will tend to change to states with a higher probability of existing or higher entropy hence entropy tends to increase for macrosopic systems. So the 2nd law sounds very taughtological to me.
    Last edited: Mar 16, 2007
  17. Mar 16, 2007 #16
    Just as a side note, if we view time solely from an entropic point of view which is adequate since it is the entropy side which affects our psychological notion of time than today is always better than tomorror. And yesterday is better than today because yesterday will not come again but tomorrow will always lay awaiting. However, I am aware that the universe is a big place and we are locally in a part where things are not as stringent because of the sun which could temporarily create distortions to the 2nd law and that is why tomorrow is not always worse than today. In fact the western civilisation is much better off than 100 years ago due to different ways of harnessing energy and hence fool the 2nd law temporily for our benefit. Despite this, we haven't worked out a way to fool aging so in some ways older people will always want to be younger because becoming older is always guranteeded by the 2nd law.
    Last edited: Mar 16, 2007
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