# How far can a photon travel?

1. Mar 23, 2004

### jimma

We know that photons can travel tremendous distances in a vacuum, but exactly how far can they travel in our atmosphere?

Thank you for any assistance.

Jimma

2. Mar 23, 2004

### Ed Quanta

From the viewpoint of a photon, distance has no meaning because of special relativity. The closer you travel towards the speed of light, the more length contracts in the dimension you are travelling in, and the more time slows down.

From our view point, photons travel 3 * 10^8 m/s in a vacuum. So the distance they travel is equal to whatever time interval you are observing multiplied by this speed. Note that using bose einstein condensates, photons have been slowed down to speeds approaching zero. But I assume you are talking about photons in a vacuum.

3. Mar 23, 2004

### jimma

The case is concerned with atmospheric attenuation. If, say, a 150W xenon arc light source were to emit photons in a direction of a detector (intensified CCD camera or photomultiplier) how far away would the detector have to be before it registered zero sugnal, i.e. when the photons no longer reached it! Would they reach 26KM? This is of course assuming that the light was pulled around the curvature of the earth due to gravity.

Thank you for any assistance

Jimma

4. Mar 23, 2004

### LURCH

I am only a layman and not particularly knowledgeable about optics, but I am fairly certain this question involves too many variables to be answered. I can say with certainty that earth's gravity is nowhere near powerful enough to curve light to such a degree that will conform to the curvature of the planets surface. But, perhaps the light source and detector could be on very tall towers?

At any rate, the word "atmosphere" covers a very broad range of gaseous conditions. Also, the sensitivity of the detection device would be a major determining factor (I am not familiar with the equipment you name in your last Post). In but I do know that any detector will register a reading of "0" when the light source becomes in detectable from background radiation. Light from an arc light, just like light from a bulb or from a star, will be visible and far greater distances in the dark then in the daytime.

5. Mar 23, 2004

### Integral

Staff Emeritus
It is not a matter of the photons "not reaching" the detector.

At a large distance you can consider your lamp a point source, this means that the same number of photons pass through the surface of any sphere which is centered on the source. That means that the number of photons passing through any given portion of the sphere drops off as 1/r2. When the number of photons passing through the area captured by your detector drops below the sensitivity of the detector you will no longer be able to "see" the source. This is when your signal is lost in the noise.

6. Mar 23, 2004

### Staff: Mentor

As anywhere, they travel freely at C until they hit something.