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How far can light travel?

by InfinateLoop
Tags: light, travel
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InfinateLoop
#1
Dec27-07, 04:10 PM
P: 5
I am hoping someone could let me know how far light can travel?
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ok123jump
#2
Dec27-07, 04:15 PM
P: 27
If light (as in photons) were in a vacuum, it would travel forever in a given direction.

In reality however, it is far more probable that light will experience disturbances (absorption, reflection, refraction, etc..) as it moves through space - thus, it's distance of travel is limited to some finite distance. I do not think that there is any good estimation for this realistic finite distance. Clearly, this finite distance depends on the situation of the light being transmitted, direction and regions of space through which it will travel.
InfinateLoop
#3
Dec27-07, 04:23 PM
P: 5
Thanks for the reply.
So do these disturbances destroy the light? I'm a little confused to what actually happens to the energy/photons.

belliott4488
#4
Dec27-07, 05:07 PM
belliott4488's Avatar
P: 666
How far can light travel?

Photons can be "destroyed", although the more usual term is "absorbed". Their energy is then transferred to the material (particles) that absorbed them. They might also be reflected (or re-radiated, if you prefer the quantum-mechanical version), in which case they might have lost some energy to the reflective surface.

Classically, light is simply absorbed, much as a sound wave can be absorbed by a soft material. In Quantum Theory, there are fundamental interactions wherein a photon interacts with a charged particle, e.g. an electron, and the photon is absorbed. Another way this is stated is that "photon number is not conserved", i.e. you can create and destroy photons.

That was somewhat rambling - I hope it helped.
InfinateLoop
#5
Dec27-07, 05:43 PM
P: 5
Yes, very helpful thanks!

So on a quantum level is this a massive collision? A photon is traveling at c and comes to a screeching halt?
ok123jump
#6
Dec27-07, 07:03 PM
P: 27
Quote Quote by InfinateLoop View Post
Yes, very helpful thanks!

So on a quantum level is this a massive collision? A photon is traveling at c and comes to a screeching halt?
[Removed Text] I wasn't correct about the facts.
belliott4488
#7
Dec27-07, 07:05 PM
belliott4488's Avatar
P: 666
I'm not sure what you mean by a "massive collision".

Unfortunately, Quantum Field Theory stops short of describing in detail what happens to the fields that correspond to the particles participating in an interaction like this. One thing that is for sure, however, is that you have to leave behind the classical picture of a particle as a discrete object; the photon and the electron are both represented by quantum fields that carry certain properties. The energy and momentum of the photon are transferred to the electron, and the photon field is no more. I'm speaking very loosely here, and if you want a better picture, I think you'll have to post a question specifically about particle interactions in Quantum Field Theory.

Alternatively, you could just stay in the classical realm, where light is only a wave form of an electromagnetic field (no photons). In this case, the EM field exerts forces on the electrons in the absorbent material, which react by absorbing the energy of the incoming field. Since the electrons have EM fields of their own, you could think of these fields as "swallowing up" the incoming field, although I've never heard a physicist describe it this way!
belliott4488
#8
Dec27-07, 07:15 PM
belliott4488's Avatar
P: 666
Quote Quote by ok123jump View Post
If by "massive" you mean forceful, that is not what the evidence indicates. Conventional Quantum Mechanical Theory says that a photon has no mass. F=m*a implies that there cannot be a force at all - the interaction of an atom with a photon must be an event which is essentially force-less. This if it is true that a photon has no mass, this would allow photons to slip in and out of atoms without affecting the momentum. As far as we can tell experimentally, it appears that this is the case.

Adversarily, I must state that we cannot say for sure. The Quantum Theory of Optical Phenomena by J.C. Slater and David Bohm suggests that a photon has mass and may be treated mathematically similar to an electron. Although I tend to lean in my personal beliefs toward this theory, it is still contentiously debated.

If Slater and Bohm are correct, then a photon does have mass - if so, the absorption or emission of a photon would result in a slight change in momentum and mass. However, we have not been able to experimentally verify this suggestion.

In summary, it appears that because the photon is massless, the velocity does not truly matter because the momentum of the atom in question is not changed by the absorption/emission of a photon.
Sorry, I don't think that's right. EM fields most definitely carry momentum and can transfer it to massive particles. Invoking F=ma isn't quite appropriate since it describes the acceleration of a massive particle, which, as you've said, the photon is not. Better is to use F=dp/dt, i.e. a force produces a proportional change in momentum (which reduces to ma for a particle with constant non-zero mass).

In any case, the momentum transfer from light (i.e. EM waves) to massive objects is well-known, and is responsible for the solar radiation pressure that is exerted on satellites in orbit, or which would drive the solar sails that have been suggested as a form of propulsion in space.
thomas pesek
#9
May22-10, 03:45 PM
P: 1
OK so light will stop if some force interacts with it.

but what about light that misses an event, and keeps traveling in the vacuum of space ?

was just wondering, in SETI, instead of searching for sound, wouldn't it be wiser to search for light ? wouldn't that travel farther and faster ?

instead of looking out wards, why don't we look in wards ?

I mean, why not send out messages ourselves, piggy backed on lazer beams ?
if found, maybe they'll reply ?

maybe green men are out there and are just waiting for us to call them first ?
dgtech
#10
May22-10, 03:52 PM
P: 70
Quote Quote by ok123jump View Post
If light (as in photons) were in a vacuum, it would travel forever in a given direction.

In reality however, it is far more probable that light will experience disturbances (absorption, reflection, refraction, etc..) as it moves through space - thus, it's distance of travel is limited to some finite distance. I do not think that there is any good estimation for this realistic finite distance. Clearly, this finite distance depends on the situation of the light being transmitted, direction and regions of space through which it will travel.

So then it is possible for the observable universe to be limited by distance rather than time?
Kynnath
#11
May22-10, 03:57 PM
P: 17
Quote Quote by dgtech View Post
So then it is possible for the observable universe to be limited by distance rather than time?
It's limited by both, in a sense. Light travels at a fixed speed. So you can describe the observable universe in terms of the distance the light we can see travelled, or the time it took it to travel that distance. The result is the same either way.
dgtech
#12
May22-10, 04:01 PM
P: 70
But how do we make sure? The only thing we know for sure is light gets absorbed and decays, everything else is just unproven hypothesis.
Kynnath
#13
May22-10, 04:07 PM
P: 17
We don't need to be sure, and proving is not possible. So long as the model we have is internally consistent and agrees with observations, it's good enough.

I'm don't know myself, but astronomers have figured out ways to measure the distance of the farthest emissions we can see (observable universe is, after all, how much of the universe we can see). Then again, for astronomers, a couple orders of magnitude of error is 'precise'. But seeing as I don't want to do the math myself, I'm happy to take them at their word.
dgtech
#14
May22-10, 04:10 PM
P: 70
Yes, I know that, but those ways of measuring are also based on plenty of assumptions, plus there was so much controversial evidence discovered, which was hurried to be "reevaluated" and reinterpreted in a more convenient form. IMO there is a strong element of "believing" and wishful thinking in that area :)
pallidin
#15
May22-10, 05:15 PM
P: 2,292
Does a photon, given enough space and time(and totally unimpeded), eventually "flat line"?
dgtech
#16
May22-10, 05:19 PM
P: 70
It's not a practical question, perfect vacuum likely does not exist

However, even in perfect vacuum gravity will affect photons - a very weak effect but present, at least in the current model
pallidin
#17
May22-10, 05:28 PM
P: 2,292
OK, unimpeded with respect to physical blocking atoms/objects.
What happens to that photon over VERY EXTENDED time? Decrease in wavelength?
dgtech
#18
May22-10, 05:32 PM
P: 70
you mean redshift?


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