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Ltryptophan's avatar

What effect does only distance travelled have on light?

Asked by Ltryptophan (9105 points ) December 12th, 2010

Not the effect that light occurs from bumping into anything along the way. I mean if light is emitted in a vaccuum does distance travelled affect light?

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14 Answers

koanhead's avatar

Light, like any other form of electromagnetic radiation, decreases in intensity the further it travels from the point of emission. The mathematical relation that describes this is an inverse square- that is to say the intensity of the radiation at distance x is proportional to the reciprocal of the square of x.
This describes aggregate behavior. An individual photon or a collimated beam (like a laser) does not decay over distance according to the inverse-square relation, but some other.

RealEyesRealizeRealLies's avatar

Theoretically, vacuum or not, light continues to travel forever. Our ability to detect it is limited to the degree our instrumentation allows.

Odysseus's avatar

@RealEyesRealizeRealLies , Why do we need improved instrumentation depending on the light sources distance?
What is changing over that distance, is the light scattering or becoming weaker ? can this mean that it looses energy over distance?
Hmmm, time for me to brush up on my physics.

RealEyesRealizeRealLies's avatar

As I understand it, the photon has no counter particle, thus it does not lose energy. It can be converted to another form of energy depending upon what it collides with. But if it does not encounter an object, the photon should continue onward.

ETpro's avatar

@koanhead True for black body radiation. Collimated light from a laser, or collimated radiation from a Gamma Ray Burst would not decrease intensity over distance except for the influence of dust particles or blocking agents in the medium it is passing thorough.

Odysseus's avatar

@RealEyesRealizeRealLies , so scientifically speaking ‘it hits space shit’ ?

koanhead's avatar

@ETpro I did mention collimation as an exception. However, even though it does not follow the inverse-square relation, in the real world collimation is never perfect so the beam will diverge and be diluted over distance- it will just take a much greater distance.

@RealEyesRealizeRealLies we can’t see light from beyond a certain distance regardless of instrumentation, since once the intensity of the radiation decays to a value equal to the cosmic background radiation, we can’t detect it.

flutherother's avatar

Because we are in an expanding universe light weakens with distance travelled. It loses energy and becomes redshifted

kess's avatar

We have the appearance of space because of Light.
So we seen that light and space is one and the same.

Different degrees of Distance appears when there is a comvination of Light and darkness.

For where light is not there is no space at all
for it is infinitely small, but when darkness is not Space is infinitely Large.

LostInParadise's avatar

As I understand it, from a non-quantum viewpoint, as the light travels, it disperses over an increasingly wider area, and so its intensity decreases proportionately. If you imagine the light as a point source, at any point of time the light will be spread out over a spherical surface whose area is proportional to the square of the radius, so the intensity of the light will diminsh as the square of the distance traveled.

The_Idler's avatar

Okay imagine a photon is travelling over a very large distance in space, and happens to not hit anything, like all the light from the stars and galaxies we can see from Earth.

now this light exists as a wave between two points, like this
A/’’\../’’\..B

The wave is not hit or modified by any matter or forces on the way that would affect its energy/wavelength/“colour”. other photons from the same source may be interfered with, decreasing the intensity of that source, when viewed from Earth, but we’re talking about the photon that “makes it through”

Now here’s the funny bit. Space itself is expanding. So, over large distances (&times) the length of the wave itself becomes stretched (along with the space it is occupying along the way) so the wave exists between the same two points, but the distance between them has increased. Now we cant just get more waves of light from nowhere (to fill the gap), so the original number of waves is stretched out, like this
A/’’’’\..../’’’’\....B

So, due to the expansion of space, light that travels very long distances is appreciably modified, its wavelength becoming longer. This is called “redshift” because longer wavelengths are towards the red end of the spectrum. So a galaxy which is getting further away from you (all of them, due the expansion of space) looks a bit more red than it really is.

Now if you think about it, speed of light being constant, a longer wavelength means fewer waves hitting the target per unit time (lower frequency). Considering the fact that the waves are bearing the energy, it is clear that redshifted light will be of lower energy than in its original state.

This is a principle known as “Doppler Shift” and is exactly the same as that, which explains the modification of tones of sound, as objects move towards or away from the observer.

e.g. The formula 1 car sounds more high pitched, as it approaches, because the sound waves are squashed into higher frequencies, and lower as it departs, because the sound waves are stretched out into lower frequencies.

So, although the actual effect is due to increase in distances, i.e. the expansion of space, it is impossible to travel in space, without being subject to it, so I’d say this is an effect of “distance travelled”

See Hubble Constant and @flutherother‘s redshift link for more detail.

Rarebear's avatar

To be clear, the photons may be redshifted and be of lower energy because of the lower frequency, but they still travel at the same speed.

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