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A few questions on Thermodynamics

  1. Sep 18, 2010 #1
    This is my first post here and first time visiting here as a non-guest user so this may be in the wrong spot.

    Is the first two laws of Thermodynamics contradicting?

    I will try to explain the question from my rather uneducated viewpoint:

    Energy cannot be created or destroyed, only transferred. This seems to point towards Energy being always in existence. Now according to (my understanding of) the BBT energy came into existence in first moments of the universe, which implies it has an age. Meaning Energy was created and has not been infinitely in existence. How does physics reconcile this, or am I just plain wrong? Also if it doesn't have an age and it's always been in the universe then wouldn't that make it a contradiction to the second law in which it would become infinitely dilute?
  2. jcsd
  3. Sep 19, 2010 #2
    the big bang theory explains that a particle on an infinitely small size........became so dense that it could handle its own mass and so it exploded...this means that energy existed......
  4. Sep 19, 2010 #3
    Okay, so that would mean if all the energy in the universe were to culminate in one area then it would collapse inwardly on itself and then explode creating, essentially, a whole new universe?

    Would that imply that energy always existed, would this not make energy infinitely dilute?

    Or does it simply state that energy existed before the BB, but nothing else before that can even be theorized about?

    This also seems to contradict the notion that the universe is expanding, but that may simply be from my uneducated viewpoint.

    Please excuse my comments if the questions seem idiotic.
  5. Sep 19, 2010 #4
    I think that the physics laws that have been verified "here and now" have no need to be valid "everytime and everywere".
  6. Sep 19, 2010 #5


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    if you add up the total energy of a flat universe, the result is precisely zero. How can this be? When you include the effects of gravity, energy comes in two forms. Mass corresponds to positive energy, but the gravitational attraction between massive objects can correspond to negative energy. If the positive energy and the negative gravitational energy of the universe cancel out, we end up in a flat universe.
  7. Sep 19, 2010 #6
    you are right denis.....it has not been theorized.........and it is specifically not theorized because.what happened before big bang is not our concern.......
  8. Sep 19, 2010 #7
    well its true.but wat happened that even if everything got annihilated.in this universe there was a positive charge which was sustained and hense it continues to exist..........you mey read about Quantum Vaccuum in black holes........you will understand everything....:tongue:
  9. Sep 19, 2010 #8
    From my understanding I was under the impression that the laws of physics were supposed to be universal laws for our universe and that they would remain such even as the Universe tore itself asunder as some theories claim will happen in the distant future?

    Perhaps it's just youthful curiosity, but I think unlocking the mysteries of how our universe came into being, and the process that lead to it, are important enough to be studied and theorized.

    Maybe that's just too far above my current understanding, but what does that this mean for matter? Would matter cancel itself out because of gravity?
  10. Sep 19, 2010 #9
    well then i must tell you........to make a TRue picture of today's universe come into being there were lot of false attempts.....if the force of the explsion was a bit more then...the universe would continue to expand and gravity could not stop it,no creation of matter......if the force was a bit less then the gravity wuld win expansion and the universe would collapse.......it was just too lucky to have that much force and gravity...........
  11. Sep 19, 2010 #10


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    The big bang didn't create any energy, it was already there. We can only theorize and guess as to where all the matter and energy came from. Some believe our universe constantly gets pulled back to a big bang over and over again. Where the matter and energy initially came from is anyones guess. Perhaps it just has always existed.

    Also, what do you mean by asking if the energy would eventually "dilute"?
  12. Sep 19, 2010 #11
    I was referring to Entropy, which would seem to dash the 'always existed' theory, if I understand the second law as well as I think I do.
  13. Sep 20, 2010 #12


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    I guess i can see how it would be confusing. Are you confused because entropy and the 2nd law should mean that matter could never be brought back into a Big Bang because that would violate those laws?
  14. Sep 20, 2010 #13
    Yes, or rather that the next big bang should never be as big as the one preceding it. That would make the 'previous big bang' scenario seem... off in some way. Especially if we are entertaining the notion that it's possible that matter/energy was always in existence then it would mean what we would have now would be infinitely degraded/dilute. This doesn't appear to be the case.

    I hope I am making sense.
  15. Sep 20, 2010 #14


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    Without knowing that much about all this, i can only say that gravity must counteract entropy. While, in general, entropy will tend to increase, the attraction of gravity counteracts it at some point. (Like when/if all matter is pulled back towards a new big bang)

    That is, unless there is some form of energy that gravity has no effect on. Take all this with a grain of salt, as i am definitely not an expert on this.
  16. Sep 20, 2010 #15
    I wouldn't imagine there was because gravity influences matter, all forms of energy are essentially also forms of matter and vice versa, so if Gravity would have no effect on something then I would think it would have to be something that's not matter/energy. That said, I know a lot less than you do being the one asking the questions so I'm not really qualified to state pretty much anything as fact.
  17. Sep 20, 2010 #16
    Sure, it is true that physical laws are supposedly universal. However it's hard to define universal. What physicists usually talk about is called the symmetry of physical law.

    If I drop a ball in China and a ball in the US then we would expect the same result. This is translational symmetry. If we do the same experiment a week apart, we would expect the same result, etc..

    There are many symmetries in physics and they have interesting consequences. In using quantum mechanics physicists were able to discover that symmetries correspond to conservation laws.

    Translational symmetry -> Linear momentum conservation
    Temporal symmetry -> Energy conservation

    Just to name two. However at the same time it is very interesting to consider time as symmetrical forwards/backwards. A basic understanding of thermodynamics will tell you that while time is symmetrical it is biased if we take into account entropy. It's rather easy to see this if you consider a ball laying on the ground.

    If you didn't have the concept of entropy you might reasonably suggest that the ball might absorb heat energy and change it into kinetic energy and then the ball might start bouncing up and down. It seems quite absurd however there is nothing that would suggest it wouldn't happen if you don't consider entropy. If this scenario could happen then we reasonably could say that time is not biased.

    Back to the question you posed, it's not really possible to answer your question. Physicists don't really know why before the big bang the entropy was really low.

    The reason it's hard to figure out why entropy was supposedly real low is that physical laws are essentially approximations and we don't have an approximation that works well for the conditions of the big bang. Physics is based in empirical roots, so our "laws" are just theories that hold to a great approximation. Newton's laws work for speed much slower than the speed of light and objects in size of the order of meters.

    If we have a single atom Newton's laws wont hold, we need quantum mechanics.

    If a ball is thrown to almost the speed of light Newton's laws wont hold, we need relativity.

    There are tons of scenario's in which certain theories don't hold and we need to resort to other theories. A problem with the big bang is that we don't really know what happens when you mix extremely large masses on extremely small scales, which would presumably be the conditions of the big bang.

    My point is that while it is true the physical laws exhibit symmetries it's hard to say why the big bang would have a low entropy because we don't have a good understanding of the big bang. It's not to say we never will, but at the present time that's just the way it is.

    I hope that in reading all of this a part of your question was cleared up. It would be impossible to completely explain it to you, but the whole concept of the big bang really is fascinating. Just from this post you can see that what you've asked is a thought that people are currently researching.
    Last edited: Sep 20, 2010
  18. Sep 20, 2010 #17


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    Thanks feldoh!
  19. Sep 21, 2010 #18
    Thank you, I think you may have just added a new layer to my understanding of the universe.
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