How light travels such long distances?

[Mohammad Hadi]

light can travel light-years of distances.. its a EM wave which needs no medium to travel, but how can it travel for such long distances. what keeps it moving... does the oscillating magnetic fields and electric fields itself a manifestation of some force??

 

[adjacent]

Its a EM wave which needs no medium to travel

Space is a vacuum. Light can travel through it with very little energy losses(Or no energy loss). If you keep a wall in between the source and your eye, what will happen?

 

[Mohammad Hadi]

keeping a wall means to block the path of light.. i.e the photons from the source aren't reaching my eyes.. so i won't be able to see it... Can we say that in Vacuum, light once emitted from the source will travel forever to the infinite distances..? that's what which makes us see the distant galaxies which are thousands of light years away from us.

 

[phinds]

keeping a wall means to block the path of light.. i.e the photons from the source aren't reaching my eyes.. so i won't be able to see it... Can we say that in Vacuum, light once emitted from the source will travel forever to the infinite distances..? that's what which makes us see the distant galaxies which are thousands of light years away from us.

Yes, as long as it doesn't run into anything.

 

[vociferous]

light can travel light-years of distances.. its a EM wave which needs no medium to travel, but how can it travel for such long distances. what keeps it moving... does the oscillating magnetic fields and electric fields itself a manifestation of some force??

We launched Voyager in the 1970's, and it is likely it will travel for at least tens of millions of years. If we had given it more velocity, it could actually leave the galaxy and eventually wind up somewhere billions of light years away.

Newton's first law of dynamics is essentially, once you give an object a certain momentum, that momentum does not change until it encounters a force that changes it.

Photons are governed by quantum mechanics, but quantum mechanics has an analog to all of Newton's laws of motion.

 

[bahamagreen]

You have never seen anything with the original photons that actually came from what you saw.

Mean free path is the average distance until absorption. Photons have a MFP of only a couple of millimeters between absorptions and re-emissions in air near the Earth's surface. Much longer in space... but not infinite.

The photon that enters your eye is a fresh new one emitted from about 2mm in front of your pupil, but the MFP is much shorter in the aqueous humour, then even shorter through the vitreous humour getting to the retina.

 

[CKH]

So what is the mean free path of a photon in the intergalactic medium, assuming such a notion makes sense? Is it so long that most photons from galaxies billions of light years away never interact?

Light is supposed to be slowed by the absorption and reemission in a medium due to a delay between absorption and reemission (at least in an atomic medium). In a gas, is this interaction assumed to be a lossless process in which no net momentum is transferred?

It's interesting because absorption and reemission must be perfectly symmetric to avoid any change in momentum.

 

[vociferous]

So what is the mean free path of a photon in the intergalactic medium, assuming such a notion makes sense? Is it so long that most photons from galaxies billions of light years away never interact?

Light is supposed to be slowed by the absorption and reemission in a medium due to a delay between absorption and reemission (at least in an atomic medium). In a gas, is this interaction assumed to be a lossless process in which no net momentum is transferred?

It's interesting because absorption and reemission must be perfectly symmetric to avoid any change in momentum.

I believe it is on the order of the radius of the visible universe in the visible range in the extragalactic ISM. It is low enough that extragalactic astronomers do not have to worry about the ISM between the Milky Way and the target galaxy (to the best of my knowledge).

 

[phinds]

I believe it is on the order of the radius of the visible universe in the visible range in the extragalactic ISM. It is low enough that extragalactic astronomers do not have to worry about the ISM between the Milky Way and the target galaxy (to the best of my knowledge).

I think that has to be about right as a minimum (although I suppose it COULD be less on average), otherwise the Hubble Extreme Deep Field would never have been able to collect enough photons to take those amazing pictures of a part of the universe so far away.

 

[CKH]

Light does interact with a transparent medium, but, for example, you can still see clearly through a pane of glass. Light even travels through kilometers of fibre-optic glass.

I believe that along lines of sight to most of the distant quasars we detect absorption by intervening H clouds at different redshifts. Presumably if some of the light is absorbed then some may well interact without change as it does in a dense transparent medium like glass.

I don't know how the magnitude of the absorption lines compare with that of the transmitted light. If we know the optical properties of diffuse H gas, then we can probably predict how many perfect transmissions of photons through H atoms are occurring from the magnitude of the absorption lines.

That we can see distant galaxies does not necessarily imply that there no medium along the path.

 

[bahamagreen]

Entry "Outer Space", section 3 "Environment", first paragraph, Wikipedia sites Davies, P. C. W. (1977), The physics of time asymmetry, University of California Press, ISBN 0-520-03247-0

"...the mean free path of a photon in intergalactic space is about 10^23 km, or 10 billion light years."

 

[phinds]

That we can see distant galaxies does not necessarily imply that there no medium along the path.

I agree. Even the Hubble Extreme Deep Field photon collection could have collected only a modest percentage of the photons that would have reached it had there not been intervening dust/gas/whatever.

 

[phinds]

Entry "Outer Space", section 3 "Environment", first paragraph, Wikipedia sites Davies, P. C. W. (1977), The physics of time asymmetry, University of California Press, ISBN 0-520-03247-0

"...the mean free path of a photon in intergalactic space is about 10^23 km, or 10 billion light years."

Very cool. Thanks for digging that up so we're not all just shooting in the dark here.

So that means the Hubble Extreme Deep Field collected way less than all of the photons that would have arrived if nothing had been in the way. AND it was do over a collect time of 50 days to get enough light to make a meaningful image. Impressive achievement.