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Understanding how energy disappears

  1. Jun 23, 2008 #1
    I've been reading about the possible ends to the universe and I came across the "big freeze".


    In this scenario, all the energy in the Universe goes out and the Universe is cold and desolate with no stars. But the law of conservation of energy states that energy cannot be destroyed. Where did all this energy go then?

    It's like when I punch something imaginary in mid air. What happens to the energy momentum in my fist after I am done punching? Does it vibrate the air molecules around me on such a small level into infinity?

    I see energy disappear around me all the time, such as when a light bulb runs out. What happens to the photons that the light bulb was giving off the entire time before it burnt out?

    Any help would be appreciated. Thanks!

    How can electrons go through houses and what not. I was wondering about radio waves and how they're just electrons moving out of the broadcasting location. Do these electrons also continue forever into space? And how do they have enough energy to go on forever?? More importantly, can these radio wave electrons get bonded to something in some sort of electric-chemical reaction?
  2. jcsd
  3. Jun 23, 2008 #2


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    Radio waves are not electrons. They are electromagnetic waves, just like visible light, only with longer wavelengths and can be "diffracted" rather easily through openings like doors and windows.

    Next time, you should not ask two very different questions in a single post/thread. It makes it confusing when people start responding to it.

  4. Jun 23, 2008 #3


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    You are right - the energy of the universe has to be constant. In a "Big Freeze" scenario, as it says in the Wik link, the low temperature is due to the size of the univers: as the universe expands, the energy spreads out into more and more space, eventually becoming so diluted that the energy density is very low.

    This, btw, is the reason that the cosmological micrwoave background radiation is so low of an energy (2.4 K iirc) despite being an 'image' of moments after the big bang.
    Most forms of energy are eventually dissipated as heat. A light bulb emits light, which is absorbed by your walls and floor and heats them.

    When you punch the air, the energy is absorbed by your shoulder and muscles. That's why it hurts!
  5. Jun 23, 2008 #4
    The energy does not vanish. It is just spread out more or less uniformly throughout an ever-growing universe. At the end of this scenario the whole universe will have the same temperature (slightly above 0 Kelvin), almost all matter will have collapsed to black holes which are so far apart (and still drifting away) from each other that there is almost no gravitational force left between them. The energy has just developed to a state where it cannot be transferred anymore because it is in thermal equilibrium.
    But this scenario can only take place if the total mass in the universe is less than some critical value. If there is enough mass the universe will collapse due to its gravitation.

    Yes. The kinetic energy of your fist will transform mostly into molecular chaotic movement = heat energy. Everything eventually ends up as heat
    The photons are absorbed somewhere and transform their energy to heat eventually.

    Radio waves are electromagnetic waves - just the same as light. Electromagnetic waves do not consist of electrons but of photons.
  6. Nov 19, 2008 #5
    It is not generally known the fact that, even under the theory of BigBang, energy is not constant, is not conserved at large scale.
    Only at local scale the conservation of energy is stated.
    quoting from here:
    "Is energy conserved in an expanding universe?
    This is a tricky question, depending on what you mean by "energy." Usually we ascribe energy to the different components of the universe (radiation, matter, dark energy), not including gravity itself. In that case the total energy, given by adding up the energy density in each component, is certainly not conserved. The most dramatic example occurs with dark energy -- the energy density (energy per unit volume) remains approximately constant, while the volume increases as the universe expands, so the total energy increases. But even ordinary radiation exhibits similar behavior; the number of photons remains constant, while each individual photon loses energy as it redshifts, so the total energy in radiation decreases. (A decrease in energy is just as much a violation of energy conservation as an increase would be.) In a sense, the energy in "stuff" is being transferred to the energy of the gravitational field, as manifested in the expansion of the universe. But there is no exact definition of "the energy of the gravitational field," so this explanation is imperfect. Nevertheless, although energy is not really conserved in an expanding universe, there is a very strict rule that is obeyed by the total energy, which reduces to perfect conservation when the expansion rate goes to zero; the expansion changes the rules, but that doesn't mean that anything goes.
  7. Nov 19, 2008 #6
    The original question of this post is about Big Freeze supported on 2nd law of thermodinamics.
    But have a look to the universe and what do we see ?
    If 2nd law holds than Life itself is not possible.
    The universe is going on the path of constructing more and more complex structures. Starting on CMB quasi-homogeneus state the universe is now a much more non-homogeneus site.
    So the 2nd law of Thermodynamics is not holding.
    Universe decided to evolve the opposite way.
  8. Nov 19, 2008 #7
    You're thinking of entropy in the wrong way. Entropy is not a system tending to disorder - disorder is merely a metaphor for entropy. A better way of thinking of it is as a degradation of energy. Amounts of energy are becoming unusable due to irreversible processes.

    As an aside - the entropy decrease due to life on Earth is much lower than the increase caused by general processes. There's an quantative article on it somewhere. . . .

    Found the abstract: http://scitation.aip.org/getabs/ser...00076000011001031000001&idtype=cvips&gifs=yes
  9. Nov 19, 2008 #8
    Quoting from from the abstract (I dont have access to the paper without paying) :
    "Quantitative estimates of the entropy involved in biological evolution demonstrate that there is no conflict between evolution and the second law of thermodynamics."

    I dont know what is the contents of that paper, but I'm able to say:

    The living organisms use/consume free energy to construct complex structures.
    They can hold only in a free and rich energy environment, not a closed system.

    As you say,it is true "Amounts of energy are becoming unusable due to irreversible processes"
    it holds clearly that the universe is agreggating matter/energy to provide such rich energy environments.
    Smart Universe.
  10. Nov 22, 2008 #9

    If my understanding is correct, which it may not be, wouldn't super-massive black holes be attracted to one another? So although a thermal equilibrium may be reached, the gravitational force of the black wholes would draw them all together? Leading to another possible big bang?

    I mean, space, in essence, it a very complicated homogenous mixture, so although heat may be thoroughly dispersed, the density is only an average, not constant?

    I'm sorry if my post was confusing :)

  11. Nov 22, 2008 #10
    The black holes would be attracted to each other. But because the distance between them may be so great the force between them will be less than the dark energy expansion "forces" - just the same way that galaxies behave just now.

    The density is average but the fate of the univers depends on this average density. If it is greater than the so called critical density then there will be enough force to pull the universe in together leading to a "Big crunch". If it is less tha critical density then what I first described will occur. And if it equals the density then there will be a form of equilibrium.
  12. Nov 23, 2008 #11
    Ah, thank you. That cleared things up a bit.

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