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Is there an effective life time of the universe where it will reach max entropy?

  1. Jul 7, 2012 #1
    I have been having a discussion with a friend and I think we have reached the limit of our knowledge, so hopefully someone on here can help!

    Basically the question is if a photon is emitted it will travel forever providing it never hits anything. So lets say we are close to the edge of the universe and we are a photon that is emitted, it travels forever and will never return, does this mean there is an effective life span of the universe where it will reach a state of max entropy and lots of energy will be "lost" as photons, kind of like a candle burning and eventually it dies?

    Or does the whole universe constantly expanding (at the speed of light?) come into play and mean this will not happen? Eg the speed of the universe expanding (speed of light) + the speed of the photons leaving the universe (speed of light) = speed of light eg you can't go faster than c (c+c=c), so they effectively never leave, I get the feeling it is more complex than this though.

    Perhaps me and my mates should stick to drinking games instead of theorising about the universe.

    Also I apologise for any gross simplifications or incorrect understandings, please learn me. :)
  2. jcsd
  3. Jul 7, 2012 #2

    Simon Bridge

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    No such thing.
    There is a bunch of confused thinking in your question so bear with me while I tackle it and give some context so I can answer the question I think you are trying to ask about.

    The "edge of the Universe" is usually considered to be the limit that you can see ... no matter where you are in the Universe, this will be the same distance. It is possible for something to be so far away that it is beyond the limit of what we can see but it is not possible to be "near the edge" except under some very unusual understanding of the word "near".

    Distant parts of the Universe are certainly retreating from us, due to Hubble expansion, faster than the speed of light.

    c+c=c makes no sense - surely c+c=2c ... an observer travelling at light-speed sees everything else travelling at light-speed and all distances totally contracted so all places are "here". So for an object to be travelling at c wrt them makes no sense. We can imagine that two bodies pass each other going in opposite directions, each doing some speed v very close to c wrt, say, you watching. In that case you can say that the speed of each wrt the other is still less than c, but it still won't work to say that v+v < c because that is not the correct relationship.

    The max-entropy state of the Universe is sometimes called the "Heat Death" and it is one of the models for the Ultimate fate of Everything.

    In a finite - closed Universe, your photon zipping off on it's own could, in principle, eventually return: the curvature of space-time returns it back to it's origin like the curvature of the Earth does for tourists on round-the-world jaunts. However, the rate of expansion is such that more universe arrives faster than the photon can cross it ... so even without an ultimate heat death, the photon will never make it home the long way round.

    All the cosmologists I've known have been heavy drinkers :)

    Heat Death:

    Shape of the Universe:

    Observable Universe:
  4. Jul 7, 2012 #3
    When I said near the edge of the universe, I meant the matter that is the furthest from the centre. Does that make any more sense?

    Heat death looks very interesting, thank you, appreciate any other comments you have. :)


    This article says that the most likely shape of the universe is the infinite flat model if this is the case then the photos would not indeed return to the origin, they would just continue on forever would they not? So I guess this is essentially heat-death which you described.
    Last edited: Jul 7, 2012
  5. Jul 7, 2012 #4

    Simon Bridge

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    Center of what? Universe has no center.
    Yep - but be careful - that is just a model of the local geometry. You'll see that common pictures of space-time have a bell-shape with space wrapped around in a circle... these are "global" models. It's also not a great article - I just included it to give you an idea of what you are dealing with and you need the others as well.
    Nope. I think you are thinking of all the photons leaving our local space-time and the expansion rate meaning no photons can come in from elsewhere? In the heat death, entropy maxes out. Try working out what happens to the entropy of a system that is slowly emptied of particles.
  6. Jul 7, 2012 #5
    Heat death is not the end... when the point comes that entropy is maximum, there is still random fluctuation. The magnitude of a local fluctuation (decrease in entropy) is not strictly bound... there will be a statistical probability for any arbitrary magnitude of entropy decrease.

    That probability may be small, but once there is heat death there is lots of time (infinite?) for these fluctuations to develop. So eventually a big enough local fluctuation occurs such that the resulting slide back down might look exactly like what we observe as our universe... that is to say that our current universe may be a local fluctuation of a previous heat death...
  7. Jul 8, 2012 #6
    Let me give an example, the universe is an infinitely large 3d area, all the matter is spread out in this 3d area. If you were to take the two particles that were the largest distance apart and draw a line between them, the midpoint would be the "centre" for my discussion.

    So all the matter in the universe is contained between these two points as they are the two farthest away particles, if these particles were to emit a photon outwards, eg away from each other, the photon would be lost into the abyss would it not? This is assuming the universe contains a finite amount of matter/energy.

    Yep so for my discussion we are assuming the model that is correct/true is the infinitely flat model, if it was the others then it would indeed be possible for them to return.

    When I say leaving the universe, I mean they are leaving the place where the majority of matter is contained, they aren't actually leaving our local space-time. So they system isn't emptied of particles, the photons just eventually spread infinitely far away from the bulk of the matter.

    A good example of what I am trying to describe is a candle floating in space magically burning, the bulk of this "universes" matter is within the candle itself, there is no more matter apart from the candle, this space/universe is infinitely large and the photons being emitted from the flame are sent out into the abyss to never return, eg energy is slowly lost from the candle in the form of photons into empty space.

    Does that make any more sense what I am trying to describe? Thanks for taking the time to respond.

    That is very interesting example, there could indeed be far more matter out side of our limits of the observable universe, and this could be the case.
  8. Jul 8, 2012 #7
    Assuming that the photon will still move as long as there is time, your question can be translated as, "Will time of our universe end?"

    There is nothing about an "edge" that would confer the magical property of being able to end the universe. Just how many photons do think would have to hit the edge in order to reach maximal entropy? What is on the other side of the edge? It's like what's on the other side of the lake, the other side of the border, or the other side galaxy. If I move everything that exists in my house outside of my house, including the things that make it up, do you think the time of my house will end? What in the world does THAT have to do with physics?

    There is no maximal entropy definable for an undefined mammoth expanse called our universe. People can claim its finite and that it thereby could have a finite limit to the entropy, assuming that there is a temperature that is above absolute zero which also serves as a minimum temperature, but claim of a finite universe is riddled with assumptions.

    Entropy can double, and double, and double, and double, and still never reach the maximum, if there is none. A maximal entropy would take, well, a maximal amount of time to happen in that case - or should I say, it wouldn't even happen!

    Of course, some people could theorize that the universe has a maximum finite entropy, but really, what sense does THAT make, and how could any one of us possibly know the answer to something so ridiculous as a "maximum entropy of the universe"?

    Well it is.

    Honestly, if you want a real answer to this question, you going to have meet some advanced aliens. Everyone that will talk to you here will not have the answer.

    Enjoy your life.
    Last edited: Jul 8, 2012
  9. Jul 8, 2012 #8
    I did not mean to imply there would be an end, just a point where the energy is so sparse it is as if the universe has died, you could define some artificial limit where it is classed as dead if you like.

    Edge was the wrong word, I don't mean an actual edge, I mean the matter that is furthest from all the other matter, eg on one side there is all the other matter, on the otherside there is vast emptyness.

    I definitely did not mean to imply time would end, just that there would be a point where via emission a large amount of photons would be shot out into the vast emptyness of space, beyond the furthest matter so they are effectively lost.

    Like how a candle loses energy to its environment via photon emission, think of all the matter in this universe as the candle, and the photons as the energy that is lost from the candle. Eg the "Edge" of all the matter would be the edge of the candle, it is matter that is furthest from my self defined "Centre of the universe" eg the centre of the candle.

    Does that make more sense?
  10. Jul 8, 2012 #9
    Yes it does. You mean a point of no return. Well, in that case, that doesn't mean that entropy reaches a maximum. Instead, the entropy just keeps on increasing.

    If the light were to not reach a point of no return, then it has a chance of returning, in which case either something must be holding it back, and/or something unknown could reflect, refract, or otherwise disturb the light towards aggregating it back so compactly that it could lead to the rebirth of the universe. In such a case, there would have to be something else other than photons in the mix, which would make the situation rather different.
  11. Jul 8, 2012 #10

    Simon Bridge

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    How would you determine a "largest" separation of two points in an infinite[/i volume? Surely the largest separation is infinity?

    What abyss? Do you imagine that there is a part of space that has all the galaxies etc in it that is expanding into a great emptyness? That's not how it works.

    That is meaningless - there is no volume of the universe which can be said to contain the majority of matter
    In which case, your picture of the Universe is badly in error. The Universe is not like a candle in a larger darkness. A better (but not by much) picture would be an infinite room with candles evenly, but not regularly, distributed through it. There is no center and no finite volume containing the majority of matter.
    Since the observable Universe can only be a small fraction of the whole Universe this must be the case.

    I suspect the difficulty you are having with these concepts is tied in with your ideas about what we mean by the big bang and the Hubble expansion. These are not expansions of matter into space, they are expansions of space dragging the matter along for the ride. The luminous part of the universe is not a finite volume in a greater void - but spread (on the large scale) evenly throughout the entire volume of the Universe. There is no center, there are no edges. Grasp this and you'll have a better time.
  12. Jul 8, 2012 #11
    This is pretty much what I was imagining.

    Thanks for that, explains why my question made no sense to anyone :p

    So space is expanding, not that the matter in it is accelerating away on its own accord, the space it is within is expanding. That makes more sense to me.

    So does this mean that if space is infinite, that there is infinite matter spread in this universe? and that if space is finite eg a toroid or one of the various other shapes that the amount of matter would also be finite? if it is the former how do we know that the amount of matter is indeed infinite and not finite like I described?

    Once I get my head around this I can probably answer my own question, thanks again for bearing with me.

    edit: If the big bang contained all matter originally and it was actually space expanding (what you said), not matter expanding in an infinite vacuum (my incorrect understanding) then I can see that whatever the length of this universe, whether it be finite, infinite, a weird shape etc there will be mass spread evenly so there will be no "furthest away particle" to emit a photon into an abyss.

    I think that answers my question. Thanks.
    Last edited: Jul 8, 2012
  13. Jul 8, 2012 #12
    You are talking about three different universes here.

    "The Universe"
    "the observable Universe"
    "the whole universe"

    It is not even clear if the "observable Universe" is centered on Earth.

    In fact, if it is asymmetric, it can be many millions of light years away.

    Even the definition of "observable" can change.

    What if we zoomed so deeply some day that we could see a normal spiral galaxy like ours, with just as many stars, having one-billionth the angular area in the sky than even the small, blue, hot galaxies (of the Hubble Deep Field) that dominated "the early universe" billions of light years away and billions of years ago? Such would be clear evidence of asymmetry.

    Of course, we haven't ruled that out yet either.

    Keep in mind that our Milky Way IS a like a candle in a larger darkness. In fact, it applies to objects on many scales. At least 20 orders of magnitude!
    When Andromeda and other galaxies were first pictured, they looked like nebulae. In fact, the Milk Way used to be called an island universe due to the lack of information about what these "nebulae" really were!
    See how flexible that word "universe" really is!

    What would be VERY strange is if there weren't a larger object somewhere out than we know about already there that acts just like such a candle. More likely, there are some rather large "candles" out there whose signature exists at wavelengths too long for us to pick up using our present day instruments, or perhaps their angular size in the sky is so mammoth, and their mass is so large, that they distort the very image which makes them up as the light escapes them, causing it to fill the whole sky with an additional cosmic background that's rather homogenous.

    And of course, there wouldn't be a single solitary "center" either. Just regions of higher density dotted across the vastness of space.
    Last edited: Jul 8, 2012
  14. Jul 8, 2012 #13
    Yep the milky way is like my description, however I was talking about the entire universe.

    Perhaps my question would be better phrased "Is there an effective life to our galaxy" because our galaxy is indeed like a candle in space where the emissions will leave and never come back, however all the other universes are also doing the same thing and their light is almost certainly offsetting any lost photons.

    Would you say this is correct, the loss of energy from our galaxy via photons would be offset by incoming photons from other galaxys? If not then I guess our galaxy would indeed eventually "die" due to loss of energy from photon emission, this is of course assuming that no galaxys, stars etc collide with our galaxy.

    Thoughts on my rephrased question?

    edit: I guess I am essentially proposing how long until all the suns in our galaxy die, of course ignoring the massive black hole in the centre.

    It is all certainly very interesting that is for sure!
  15. Jul 8, 2012 #14
    The answer to your question is, "No."
  16. Jul 8, 2012 #15
    It would take over a trillion years for the smallest of stars to die off. However, in that period of time, one can only imagine the encounters such a star could have which could add gas to it over time, causing it to live a much shorter life.

    In this area of the universe, matter is being converted to electromagnetic radiation faster than the other way around, and light has no effective means to "precipitate" back on the galaxies here. Therefore, in this case, I would not say it is correct to say that the photons from other galaxies are making up for it. However, that doesn't mean there isn't some place whether things are all different and weird where the light could precipitate into the black hole of a galaxy, forming matter again - who knows - not anyone here.
  17. Jul 8, 2012 #16
    Cool, well this has been one big learning experience for me. Thanks guys.
  18. Jul 8, 2012 #17

    Simon Bridge

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    Yeah well, it's tricky to communicate these ideas succinctly when there is such a big misunderstanding to begin with :) I am, of course, not being hugely precise.
    I'm trying to use the language already introduced by the OP ... once the basic understanding has been established, the language can be shifted to be more precise. The above sounds more like the frustrated rant from the Parrot Sketch :)

    Erm ... you mean the "center of the observable universe" can be many millions of light-years away. Hmmm... that would be the case if it takes light a different time to travel in different directions. Which is to say - the Universe may not be isotropic.

    I usually find it helps to start with the simpler models and build up but each to his own.
    Fortunately we seem to have helped OP. So... no worries aye?
  19. Jul 8, 2012 #18
    I think there are lots of negative connotations with "big misunderstanding to begin with". I personally have studied engineering but never as anything to do with the universe, spacetime or anything above first year physics so I probably have a better grasp than a lot of the population but it was obviously still greatly lacking compared to the actual reality.

    I am here to learn as are many people, perhaps a little but more positive reinforcement might be good for peoples motivation to keep learning, when it is worded that way it might turn some people off.

    Just a thought, regardless I appreciate your help. :)
  20. Jul 8, 2012 #19
    Lot of incorrect postulations here. The universe is actually expanding FASTER than light but due to relativity (and this addresses the c+c = 2c) it doesn't matter what speed a photon is emitted at or what speed you are going at when you observe a photon (unlike conventional mechanics) the speed of a photon is always c. The reason behind this is irrelevant to the question but if you're interested do a search on Special Relativity.

    As others have said the universe has no edge, you're thinking of it as a 3D object with boundaries and this is not the case for space, in fact many theorists hold that if you set off in one direction (assuming space isn't expanding) and went in that direction long enough you'd end up back where you started. So photons 'leaving' the universe is impossible.

    However the universe does have a maximum life time, because as the expansion of space continues to accelerate at the moment the space between galaxies is widening, but soon the space between stars IN galaxies will widen rapidly, then the space between nucleons in stars before matter is ripped apart by space's expansion (Big Rip Theory)
    Another theory - Heat Death, speculates that as entropy increases with time the universe will continue and as stars eventually die and we leave the 'Stelliferous age' around 1014 years after the beginning of the universe then first 'degenerate' forms of matter will populate the universe - brown dwarfs that never became stars, white dwarfs from partial collapse of matter, neutron stars from further collapse and black holes from total collapse. Then at about 1040 only black holes will remain as an significant form of matter but even these 'evaporate' away through emission of radiation and in about 10100 after even the supermassive black holes have evaporated the universe will enter a Dark era populated by nothing but lonely photons traveling the vast cosmic distances.

    There still however cannot be such a thing as maximum entropy because as the second law of thermodynamics states than in any closed system (e.g. the universe) - entropy always increases with time, and that means as long as time continues to proceed in a forwards direction entropy always increases. EDIT: Unless the Universe reaches thermal equilibrium and with all the photons equally spaced that may happen, I'm not sure
  21. Jul 8, 2012 #20
    Thanks but read up, I am now learned.
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