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

[Foliage]

I have been having a discussion with a friend and I think we have reached the limit of our knowledge, so hopefully someone on OCAU 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.

[Danske]

Interesting question, I can't pretend to know the answer but from my limited knowledge, I don't think a photon can simply lose it's energy from travelling through a void of space. Why? I believe it might have something to do with not having a relative time reference, we perceive everything relative to our frame of reference, photons might like have it's own time frame .

There's light from very early stages of the universe, most stars at this point (or standard candle) are red shifted which means they are on the red band of the light spectrum, but I dont think that even indicates the photon is losing energy, it just means it's on the other end of the spectrum due to distance. Hmmm...

We need a science version of Zoogwah And hopefully, he'd look like this:


Wish I could understand: http://en.wikipedia.org/wiki/Photon
This might be useful, I havent read it yet: http://profmattstrassler.com/article...les-decay-why/
This is a great read by New Scientist (damn it's just a preview, I got the physical edition here somewhere): http://www.newscientist.com/special/light

[Foliage]

Photons don't lose energy over time, red shift is just their speed relative to us.

So photons definitely travel forever, the question is just whether they "escape" the universe or not.

[GTR27]

Remembering that space, although a vacuum and technically nothing, actually holds (on average of course) around 1,000,000 hydrogen atoms per sq meter - so the photon's chance of running into something is actually quite good - which means the initial assumption of running into nothing is a very unlikely scenario.

But seeing how we don't know how big the universe is (only where we can see to) or what shape it is - its quite hard to define the problem.

[Foliage]

Originally Posted by GTR27

Remembering that space, although a vacuum and technically nothing, actually holds (on average of course) around 1,000,000 hydrogen atoms per sq meter - so the photon's chance of running into something is actually quite good

That is a very interesting fact, regardless there must be an atom that is the further from the centre of the universe, eg there is nothing beyond that atom, so if it releases a photon that photon will be lost.

It might be very low chance, but all you need is 0.000000000001% of photons to be lost into the abyss to put a time limit on the universe, unless of course something to do with the ever expanding nature of the universe stops it.

Though even if photons aren't lost if the universe is simply ever expanding and nothing else changes, it will reach a state of maximum entropy where the universe is effectively dead, at least I think it will.

Bit depressing!

[Foliage]

http://www.physicsforums.com/showthread.php?p=3986483

Got a very interesting answer over here:

Originally Posted by Simon Bridge

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.
http://en.wikipedia.org/wiki/Heat_death_of_the_universe

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:
http://en.wikipedia.org/wiki/Heat_death_of_the_universe

Shape of the Universe:
http://en.wikipedia.org/wiki/Edge_of_the_Universe

Observable Universe:
http://en.wikipedia.org/wiki/Observable_universe

So looks like what I was trying to describe is "heat-death" of the universe.

[Smegger]

If we really simplify and release your imaginary photon, in an arbitrary direction we will call 'thataway'. And the universe is actually a torus - highly probable - then the photon should come back, eventually, from 'overthisaway' or the opposite direction.

Or if the universe is an undefined, blobby kind of a shape - improbable - then it should keep cruising. Well, unless the universe does end up collapsing back on itself, in which case it will come back at some point and you'll get another chance to release it. After the next big bang.

Or if the universe keeps expanding, is not a torus and dies slowly and pitifully from heat death, separation and outright boredom. It should keep cruising.

Mind you all this is moot, as you well know. The further it goes the greater the chances of it hitting something. Or many somethings.

[Foliage]

Yeah definitely depends on the shape of hte universe

http://en.wikipedia.org/wiki/Shape_of_the_Universe

The Wilkinson Microwave Anisotropy Probe (WMAP) has confirmed that the observable universe is flat with only a 0.5% margin of error.[1] Within the Friedmann–Lemaître–Robertson–Walker (FLRW) model, the presently most popular shape of the Universe found to fit observational data according to cosmologists is the infinite flat model,[2] while other FLRW models that fit the data include the Poincaré dodecahedral space[3][4] and the Picard horn.[5]

so there are a few highly probable cases.

[Foliage]

Ok the last post in that physics forums thread has answered my misconceptions about the universe.

The part I was going wrong with is the big bang was space itself expanding, not al matter being condensed in an infinite vacuum and expanding within the vacuum. Hence no matter the shape of the universe, there will be matter everywhere, it won't be confined to one spot on the universe.

[cbwolf]

Originally Posted by Smegger

And the universe is actually a torus - highly probable - then the photon should come back, eventually, from 'overthisaway' or the opposite direction.

Actually the currently generally accepted theory is that the universe is flat and infinite - which specifically rules out a torus.

Originally Posted by Smegger

Or if the universe keeps expanding, is not a torus and dies slowly and pitifully from heat death, separation and outright boredom. It should keep cruising.

This is again, currently the generally accepted theory.

To the OP:

As the universe is most likely infinite and expanding - if you released a photon and it happened not to hit anything, it would keep moving forever in the expanding universe.

[cbwolf]

Originally Posted by Foliage

Though even if photons aren't lost if the universe is simply ever expanding and nothing else changes, it will reach a state of maximum entropy where the universe is effectively dead, at least I think it will.

Bit depressing!

While it's a bit depressing it's also kind of interesting.

If the universe is indeed expanding and infinite with everything moving away from each other (keep in mind that this refers more so to whole galaxies - the stars within galaxies won't move away from each other), then we will eventually get to a point where the only stars you'll be able to see from Earth will be those in our galaxy - leaving the astronomers of that time to believe that the universe is made up of our galaxy surrounded by nothing.

As Lawrence Krauss put it - we live in a very special time, because it's the onlt time that we can know, that we live in a very special time.

[FerrisXB9R]

Another thing to consider in the "max entropy" equation is proton decay, Foliage.

http://en.wikipedia.org/wiki/Proton_decay

[Luke212]

fbro you might like this audio book
http://en.wikipedia.org/wiki/Parallel_Worlds_%28book%29

[irR4tiOn4L]

Don't see why you're focussing on photons leaving. The real trouble is entropy itself only rising and an expanding universe meaning less dense concentrations of matter to begin with.

Put together, assuming the Universe does not collapse back in on itself and keeps exanding, the universe will trend toward a point of maximum entropy and slow death.

The way I think about this is like an animal in a winter blizzard. While a significant amount of heat is concentrated in bodies such as that of the animal, life and the dynamic processes we associate with a 'living' universe can continue. But entropy means such heat will tend to spread evenly throughout the environment, rather than remaining concentrated, and as heat is lost from the body and spread across the winter wilderness, it slowly reaches a state of equilibrium from which it can no longer deviate. And so the animal freezes and life dies as the animating heat leaves its body into an unrecoverable form spread more evenly throughout the environment. Life and dynamism die and the result is everlasting winter.

[Foliage]

All very interesting replies guys, I remember asking qquestions like this when a kid but never knowing where to look or who ask