Does light last forever?

shinbits · Nov 25, 2010

I sometimes read about a star or supernova or galaxy that is discovered because the light from it just recently hit earth. But then I read that the light may be several million or several hundred million years old.

Why doesn't the light decay? Is it because there's no air in space? Comets decay and give off light as a result, giving them that "shooting star" effect. Is light even subject to decay?

Cabal · Nov 25, 2010

shinbits said:
I sometimes read about a star or supernova or galaxy that is discovered because the light from it just recently hit earth. But then I read that the light may be several million or several hundred million years old.

Why doesn't the light decay? Is it because there's no air in space? Comets decay and give off light as a result, giving them that "shooting star" effect. Is light even subject to decay?

Comets have a tail as they're actually streaming reflective particles behind them. They're solid reflective objects like the moon, not luminscent themselves.

As far as I know, to all intents and purposes light doesn't attenuate in the vacuum. And as you correctly pointed out, space is essentially empty, so it's very likely that light will pass through large swathes of it unimpeded.

Light itself doesn't "decay", but it can be absorbed by other materials.

Lillen · Nov 25, 2010

I am not a physicist but my notion is that light do decay or absorb. Even in space. Remeber it has no matter to interact with! or do it?

shinbits · Nov 25, 2010

Cabal said:
Light itself doesn't "decay", but it can be absorbed by other materials.

So would it be correct that light can indeed last forever?

Cabal · Nov 25, 2010

shinbits said:
So would it be correct that light can indeed last forever?

Yup. It's considered a "stable" particle, so it won't decay into anything else.

Chesterton · Nov 25, 2010

Light disperses, and there's no way to keep it from dispersing. If it weren't for dispersion, you could flick a flashlight off and on, and the light from it would travel away across the universe forever (until it hit a gas cloud or body). I think all light actually does travel virtually forever, but it becomes so dispersed that it becomes negligible in terms of human measurement. Only the individual photons could be detected.

Lillen · Nov 25, 2010

I raised this question to wiccan child in his thread...

But how can light be visible in space... it has nothing to interact with? no mediums to pass through?

shinbits · Nov 25, 2010

Cabal said:
Yup. It's considered a "stable" particle, so it won't decay into anything else.

Thank you for your input, I really appreciate it.

I have one last question.

Since light doesn't decay shouldn't the universe be bombarded by light with few dark places, since there are trillions of galaxies with trillions of stars, giving off light for billions of years? and that's in addition to supernovas and everything else. Or is it because, as you said, that light gets absorbed?

Cabal · Nov 25, 2010

Chesterton said:
Light disperses, and there's no way to keep it from dispersing. If it weren't for dispersion, you could flick a flashlight off and on, and the light from it would travel away across the universe forever (until it hit a gas cloud or body). I think all light actually does travel virtually forever, but it becomes so dispersed that it becomes negligible in terms of human measurement. Only the individual photons could be detected.

I'm assuming when you mean dispersed, you mean spread out.

It's just that "dispersion" does have quite a specific meaning in optics, and it doesn't refer to the spread of light.

shinbits · Nov 25, 2010

Chesterton said:
Light disperses, and there's no way to keep it from dispersing. If it weren't for dispersion, you could flick a flashlight off and on, and the light from it would travel away across the universe forever (until it hit a gas cloud or body). I think all light actually does travel virtually forever, but it becomes so dispersed that it becomes negligible in terms of human measurement. Only the individual photons could be detected.

whoops, I didn't see this before my last post. thanx for your input.

EDIT: Seems llike Cabal bought up an interesting point in response to this.

Chesterton · Nov 25, 2010

Cabal said:
I'm assuming when you mean dispersed, you mean spread out.

It's just that "dispersion" does have quite a specific meaning in optics, and it doesn't refer to the spread of light.

Well I know there is an exact mathematical formula for the "spreading". Maybe I used the wrong word. Is it "diffraction" or what?

Cabal · Nov 25, 2010

shinbits said:
Thank you for your input, I really appreciate it.

I have one last question.

Since light doesn't decay shouldn't the universe be bombarded by light with few dark places, since there are trillions of galaxies with trillions of stars, giving off light for billions of years? and that's in addition to supernovas and everything else. Or is it because, as you said, that light gets absorbed?

That's a fairly old argument against the universe being infinite and thus containing infinite matter, that the sky would be then infinitely bright. I don't see why it couldn't be reworked for a finite universe with a particular level of matter - but there are probably other factors to think about it.

The following seems to suggest that expansion of the universe is the reason why we don't see a bright sky in a finite universe.

Olbers' paradox - Wikipedia, the free encyclopedia

Cabal · Nov 25, 2010

Chesterton said:
Well I know there is an exact mathematical formula for the "spreading". Maybe I used the wrong word. Is it "diffraction" or what?

Divergence, if you're just talking about the physical spread of a light source.
Diffraction is a specific type of spreading when light passes through a gap comparable to its wavelength.

Regarding your flashlight example, you're right in that the light spreads out, but then flashlights aren't exactly a wonderful example of straight light. A searchlight is a better example, as the mirror and the bulb are set up so that the light all reflects out parallel to the same axis, so that would probably go for longer without diverging so much (although not on the scale of the size of the universe).

A laser would be even better again, and is used in some astronomical measurements like earth-moon distance. Again though, I don't think it's possible to have a perfectly collimated laser beam. You'd be limited by the precision of the construction of your optics, and I suspect we're not skilled enough to get an entire beam of photons across the universe (yet, maybe...!).

Cain Spencer · Nov 25, 2010

No, it can't since it is energy that travels in waves.

Cabal · Nov 25, 2010

Cain Spencer said:
No, it can't since it is energy that travels in waves.

If that wave interacts with nothing, then it will remain at that energy.

shinbits · Nov 25, 2010

Cabal said:
That's a fairly old argument against the universe being infinite and thus containing infinite matter, that the sky would be then infinitely bright. I don't see why it couldn't be reworked for a finite universe with a particular level of matter - but there are probably other factors to think about it.

The following seems to suggest that expansion of the universe is the reason why we don't see a bright sky in a finite universe.

Olbers' paradox - Wikipedia, the free encyclopedia

wow. this was a great read. thank you.

Cain Spencer · Nov 25, 2010

Cabal said:
If that wave interacts with nothing, then it will remain at that energy.

"If" and even so it does very slowly loose inner parts eventually collapsing its structure, as a figure of speech.

Cabal · Nov 25, 2010

Cain Spencer said:
"If" and even so it does very slowly loose inner parts eventually collapsing its structure, as a figure of speech.

But the only reason that would happen is if it interacts with something, which is quite unlikely in space; or if, as Chesterton pointed out, the actual beam is divergent, in which case it's still arguable that the individual photons carry on their individual trajectories forever. Apart from that there is no natural decay of the energy of a light wave in a vacuum that you describe.

Cain Spencer · Nov 25, 2010

Cabal said:
But the only reason that would happen is if it interacts with something, which is quite unlikely in space; or if, as Chesterton pointed out, the actual beam is divergent, in which case it's still arguable that the individual photons carry on their individual trajectories forever. Apart from that there is no natural decay of the energy of a light wave in a vacuum that you describe.

But if light gets weaker from traveling distances then how exactly can it last forever? By saying forever you would have to assume that it will eventually collide with something, but that is beside the point.

Cabal · Nov 25, 2010

Cain Spencer said:
But if light gets weaker from traveling distances then how exactly can it last forever?

But that's the point, the vacuum DOESN'T weaken the intensity of a beam of light. If nothing interacts with a photon, then it will never decay.

By saying forever you would have to assume that it will eventually collide with something, but that is beside the point.

No, it's the opposite - it lasts forever provided it doesn't interact with something. And I agree, it's quite possible that it will (although I've no idea what supposed to happen to a photon at the edge of the universe or whether that counts as interaction), though not before travelling a staggeringly vast distance in some cases, which is why we are able to observe light from older stars with little to no loss of intensity. But photons, in and of themselves, are classified as stable, which means they don't decay into anything. In that sense, they are indefinitely existent.

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Does light last forever?

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Whats So Funny bout Peace Love and Understanding

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