Can light travel faster than itself?

[InfinateLoop]

I am hoping someone could let me know how far light can travel?

 

[ok123jump]

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]

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]

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]

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]

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]

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]

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]

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 laser beams ?
if found, maybe they'll reply ?

maybe green men are out there and are just waiting for us to call them first ?

 

[dgtech]

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]

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]

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]

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]

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]

Does a photon, given enough space and time(and totally unimpeded), eventually "flat line"?

 

[dgtech]

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]

OK, unimpeded with respect to physical blocking atoms/objects.
What happens to that photon over VERY EXTENDED time? Decrease in wavelength?

 

[dgtech]

you mean redshift?

 

[pallidin]

you mean redshift?

No. Redshift requires that the emmiter and observer be separate and moving away from each other.
I just want to know what happens to a photon if it goes on, and on, and on.
Does it change?

 

[dgtech]

if there is nothing to take away its energy potential theoretically it shouldn't decay
or at least I think so

I don't really have an idea what the photon actually is made of, maybe it can decay if there are some internal dynamics in it, that interact with each other and displace some form of energy

 

[Kynnath]

Photons don't decay, far as I know and was able to search.

Relativity kind of requires that they don't, though not explicitly. There's some experimental evidence to suggest, in any case, that if they do their half-life is well over the age of the universe.

From relativity, for them to decay they'd have to experience time. But photons can't experience time in that sense as they are moving at the speed of light, and time-dilation would mean that they only ever experience an instant.

From astronomy, if photons decayed then there should be a decrease in intensity the further away objects are. Astronomers would have noticed by now if that were the case, seeing as they use the luminosity of stars to determine aspects of them such as mass, which they then double check by seeing that other stars near them are affected by the correct amount of gravity.

If they were getting it wrong, then it would imply that the farther away we go in every direction, stars are increasingly brighter, and it is only the decay of photons making them seem as bright as closer star, or that stars that seem faint and far away are actually a lot closer, which would mean they are miscalculating their mass. Either situation would have been caught by now.

Thus, photons do not decay. Not in 14 billion years, at least.

 

[diazona]

A photon is a fundamental particle... well, I guess "particle" doesn't really do it justice. It's a quantum of the electromagnetic field. So the question is, if you shoot out an electromagnetic wave in some direction, and if there were literally nothing else for it to interact with, and if there weren't any cosmological or gravitational redshift to worry about, would it keep going forever, unchanged? As far as I know, yes it would.

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 ?

Who gave you the idea SETI was looking for sound? That would be silly, since sound waves can't travel through space. SETI does look for light signals, mostly radio waves.

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

From an alien planet's perspective, even a close one, Earth is nearly indistinguishable from the Sun. So any laser we build would have to outshine the sun, or we'd have to count on the aliens having much better telescopes than we do.

This is actually sort of possible with radio waves, which the Sun doesn't emit very much of, and people are trying. But currently there's not much hope that any aliens are close enough to receive the signals.

 

[pallidin]

So, a photon of a specific wavelength does not change at all over a large expanse and time of uncluttered space?

 

[Kynnath]

There is no 'photon of a specific wavelength'. The wavelength of a photon depends on the reference frame from where you're observing it. But yes, it won't change until it interacts with something.

 

[minomax]

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

I was taught that gravity distorts the space that the photon is traveling through and this is why at the total eclipse the light from the stars directly behind the sun "bends" and those stars are visible. They told me that a photon is massless and gravity does not effect the photon, just the space it travels in. I've never really understood how gravity slows 'time' and distorts 'space' but apparently this is what black holes do for a living.

Does gravity (ie.) black hole capture the photon or the space it is traveling through as it passes the event horizon?

It seems that the more I learn the, the less I know.

 

[TwistedMister]

It's not a practical question, perfect vacuum likely does not exist

I ran across this quote while researching..

some believe our universe is some 28 billion light years across depending on where you believe the milky way is located within our universe...

considering space is infinite, if one were to travel 14 billion light years beyond the edge of our universe then looked back, our universe would appear as a tiny star amidst a gigantic mass of darkness, at that point in space where one is looking back at our universe 14 billion light years away, would that place be near a perfect vacuum?

 

[pallidin]

...considering space is infinite,...

Remember that cosmology is not completely understood, especially with respect to the concept of "infinite space"

 

[epenguin]

You can see right now some that have traveled for nearly 14 billion years. They have been traveling since about 380,000 years after the Big Bang, when the universe became transparent due to charged plasma condensing into atoms.

If you turn on your TV on a channel where there are no transmissions about 1 in 100 of the speckles of the snow you see are (I have read) photons that have been traveling since that time - or rather photons traveling since that time ended their uneventful lives hitting your antenna and triggering current that ended up as a bunch of electrons hitting your screen. I don't know of a simple way to know which flashes are those ones though. However when some of them hit the appropriate instruments they tell us the story of their birthplace. Anyway if we don't collect this now, they are not about to stop arriving - tomorrow by my calculations will arrive some which are now 25,000,000,000 Km away.

 

[epenguin]

OK, unimpeded with respect to physical blocking atoms/objects.
What happens to that photon over VERY EXTENDED time? Decrease in wavelength?

The wavelength increases due to the expansion of the Universe they are in. The original ones are now bunched around 6cm - radio wavelengths. If you had seen them when they started you would have seen red - redder and cooler than our sun which after all is not transparent inside.

 

[jonpaulv]

But this would make time irrelevent. If you think about light years in distance, then that would mean I'm able to calculate 30000000000000 light years. And it only takes you a day to calculate the distance of a light source to you, yet that light source took that long to get to you. So how can we measure light in distance if traveling at that distance is observable in an instant. And if something does abosord the light in space then that proves the existence beyond the life span of light. Meaning if light stops because it is effected means that which effects it is there and more.