The life of 'Light'

[ballaratdragons (Ken)]

I have, for many years, pondered the issue of 'when does light run out'?

Light itself must be an amazing thing as it appears to never diminish . . . or does it?

We look up at the Heavens with our unaided eye and see up to billions of years into the past, and even further with our Telescopes!! Our cameras catch light even further back in time and distance!!!!!!

OK, here's the thing: Light left a star billions of years ago, travelled for those billions of years, over multiple-trillions of kilometres, and is still shining bright!

When does 'light' actually wear out?
It MUST eventually.
If so, what else is out there that we are not quite seeing that has faded due to time and distance.

[[1ponders] (Paul)]

As I understand it Ken (and I may have misremembered it as Uni Physics was about 30 odd years ago), light does "wear out" but doesn't disappear. From what I understand, as light propagates through space (ignoring the reddening effect of dust and gas) it uses up energy. This lost energy reduces the frequency of the light. So if the light at a galaxy on one side of the universe started out as blue, at some point it will become red and then invisible to us as infared and then as radio waves. I don't know the distances and time frames involved but they are considerable.

This is different from the doppler shift of light.

[ballaratdragons (Ken)]

Thanks Paul,

but you would think at some stage it all wears out. It may take a trillion trillion years, but eventually

[[1ponders] (Paul)]

I guess the closest analogy I can give is the cosmic background radiation that started with the big bang. Its taken something like 15 billion years but the initial radiation is still there but only warm (3 degree Kelvin or somet such) enough to produce the faintest of microwave radiation.

[wasyoungonce (Brendan)]

now now boys in what medium does light propagate in space??

is it a wave or a particle

cannot have it both ways

[Kal (Andrew)]

Originally Posted by [1ponders]

As I understand it Ken (and I may have misremembered it as Uni Physics was about 30 odd years ago), light does "wear out" but doesn't disappear. From what I understand, as light propagates through space (ignoring the reddening effect of dust and gas) it uses up energy. This lost energy reduces the frequency of the light. So if the light at a galaxy on one side of the universe started out as blue, at some point it will become red and then invisible to us as infared and then as radio waves. I don't know the distances and time frames involved but they are considerable.

This is different from the doppler shift of light.

This change of wavelength is due to the expansion of the universe, or something else? Reading up on CMB, for example, I come across this: " As the universe expands, the CMB photons are redshifted, making the radiation's temperature inversely proportional to the Universe's scale length."

I would imagine all light to be redshifted by the act of the universe expanding, which could be construed as photons losing energy as they travel through space.

[ballaratdragons (Ken)]

Originally Posted by Kal

I would imagine all light to be redshifted by the act of the universe expanding, which could be construed as photons losing energy as they travel through space.

Eventually getting to Zero energy?

[Kal (Andrew)]

Originally Posted by ballaratdragons

Eventually getting to Zero energy?

That depends on if the universe continually expands (open universe) or not. There is still (and I don't forsee an answer to this question any time soon!) alot of debate over whether we live in an open or closed universe.

[cahullian]

I don't think light runs out , I think it gets absorbed. If you are in a room with no windows or doors the light inside the room won't be seen from the outside as the walls absorb said light.
Just my layman term for something I have no understanding of.

Gazz

[robagar]

I guess for redshifted photons to have zero energy the wavelength would have to be infinite. That's never going to happen no matter how much spacetime is stretched by the cosmic expansion. Unless the expansion is infinite, in the "big rip" of some cosmological models perhaps?

[Paddy (Patrick)]

Originally Posted by cahullian

I don't think light runs out , I think it gets absorbed. If you are in a room with no windows or doors the light inside the room won't be seen from the outside as the walls absorb said light.
Just my layman term for something I have no understanding of.

Gazz

I'm with Gazz, also as a layman. If photons travel at the speed of light, doesn't that mean that they must have zero mass? And if so, doesn't that mean that they can travel without losing energy unless they are absorbed by something that they encounter on the way? And here I may really reveal my astronomical ignorance - don't even massive objects like planets continue through space without losing momentum unless they encounter another object? If all these are true, light would not run out unless it is caught out. I think. Grateful for education from the more knowledgeable

Patrick

[robagar]

You certainly can't accelerate a non-zero mass to the speed of light, so yes I *think* it means you can say that anything going at light speed must be massless.

Another way of looking at it is this: if a photon can disappear without hitting anything else, then it must have a finite lifetime. But the time dilation effect at light speed is infinite, so we could never see it happen. Therefore photons must live forever

[Paddy (Patrick)]

Thanks Rob, that second last sentence in particular will give me something to contemplate!

[robagar]

Relativity's good like that

[janoskiss (Steve H)]

Light does not "run out" or change its characteristics as it travels through space. However space is not empty so it will eventually interact with something and lose some or all of its energy in the process.

For example, each photon that makes up the light you see through your telescope has traveled millions of years to be finally extinguished as it hits the back of your eye, where its energy is released to induce a conformational change in one of the many millions of rhodopsin molecules in the retina, which in turn causes transfer of charge - a single proton - that sends and electrical signal up the optic nerve to the visual cortex which triggers a whole series of neural impulses in the brain to manifest to you as the conscious awareness of seeing starlight.

Here is a trippy thing: in the photons frame of reference there is no passage of time because light is travelling at the speed of light where time slows to a halt. As far as the photon is concerned its creation in the nuclear fusion inside the star, its long journey to you, and its annihilation in your retina all happened in the same instance. Its only in your frame of reference that many millions of years had passed.

[Paddy (Patrick)]

Originally Posted by janoskiss

Here is a trippy thing: in the photons frame of reference there is no passage of time because light is travelling at the speed of light where time slows to a halt. As far as the photon is concerned its creation in the nuclear fusion inside the star, its long journey to you, and its annihilation in your retina all happened in the same instance. Its only in your frame of reference that many millions of years had passed.

It is indeed an extraordinary thing! I have just come in from some stargazing under a beautiful clear dark sky. As I was observing NGC 1097 I reflected on your comment that the photons that have traveled 40 million years to enter my eye and become part of my experience would themselves have done the trip in one instant. Quite a koan really.

[ballaratdragons (Ken)]

Hmmm, thanks for the explanation Steve, and believe it or not it all made sense to me.

Pity we can't turn a spaceship into photons and send it off at 'light' speed. If light photons travel at light speed so would whatever was transformed into photons. The trick would be converting it back into a spaceship.

Bit Star Trekkish, but they refer to a different form of 'Beaming up'.

[Paddy (Patrick)]

Originally Posted by janoskiss

Here is a trippy thing: in the photons frame of reference there is no passage of time because light is travelling at the speed of light where time slows to a halt. As far as the photon is concerned its creation in the nuclear fusion inside the star, its long journey to you, and its annihilation in your retina all happened in the same instance. Its only in your frame of reference that many millions of years had passed.

If, from the point of view of the photon, no time has passed between its emission at light speed and its absorption, does this mean that, again from its frame of reference, it never existed? Even though it would have existed from the perspective of the observer.

[Kal (Andrew)]

Originally Posted by Paddy

If, from the point of view of the photon, no time has passed between its emission at light speed and its absorption, does this mean that, again from its frame of reference, it never existed? Even though it would have existed from the perspective of the observer.

Light is really just energy travelling through space in discrete amounts. We see the results of this energy, but we don't see it by itself. In fact, we can't even say where a photon is with certainty, we can only state a probability of where it will be.

Consider throwing a rock. The energy was transferred from your body to the rock, but does the energy really exist from it's frame of reference? Light is similar to this, except instead of a rock it is travelling in what we call a photon, and instead of going faster if I throw it with more energy, it will travel at the same speed but it will have a different wavelength.

[robagar]

Not sure if the photon's frame of reference is valid in any case - after all, the basis of Special Relativity is the idea that light goes at c in *all* inertial frames.