# Can objects catch up with their own photons?

1. Apr 9, 2006

### eweast

Can objects "catch up" with their own photons?

I have a strange question concerning whether objects can "catch up" with their own photon omissions. By this I don't mean actually catch up to them, just get closer.

I'll first use an example using sound to illustrate what I'm asking. Consider two objects, one moving relatively slow and the other moving 90% the speed of sound. All other things be equal, one would expect to hear the slower object at a greater distance from the observer.

What about the same scenario with light? The faster object moving 90% the speed of light and being watched for optically. Would the slower object be detected at a greater range? Or would the faster object be seen at the exact same range as the slower, only more blue looking?

I'm not a physicist, but I just know since light is involved the obvious answer isn't.

-Thanks!

Last edited: Apr 9, 2006
2. Apr 9, 2006

### mathman

The main determinating factor is where the two objects were at the time they starting emitting light - their motion will effect color, but not when they are first seen.

3. Apr 9, 2006

### eweast

Thanks for the reply!

Now I'm going to store that factoid away for future use and try and not think about the significance of it.

Thanks again!

4. Apr 9, 2006

### JesseM

If two objects are moving inertially (not accelerating), then if both objects were pursuing a light beam, and relative to a third inertial observer one was moving at 90% the speed of light while the other was moving at only 20%, then the following would be true:

1. In the third observer's inertial reference frame, the distance between the faster object and the light beam increasing more slowly than the distance between the slower object and the light beam. For example, according to the third observer's clocks and rulers, in one year the distance between the light beam and the faster object increased by only 0.1 light-years, while the distance between the light beam and the slower object increased by 0.8 light-years.

2. In the inertial reference frames of each of the two objects, the distance between themselves and the light beam would be increasing at exactly the speed of light. So according to the faster observer's clocks and rulers, in 1 year the distance between the light beam and itself increased by 1 light-year, and the slower observer would observe the same thing using his own clocks and rulers.

Last edited: Apr 9, 2006
5. Apr 9, 2006

### selfAdjoint

Staff Emeritus
If the light is slowed in a medium, the object can overtake its own radiation, this causes the optical analog of a sonic boom, called cerenkov radiation. Look it up.

6. Apr 9, 2006

### pervect

Staff Emeritus
It would be nice if you would think about the significance of mathman's reply and attempt to clarify your original question a bit, rather than tossing his reply in some mental filing system and turning off your brain.

As nearly as I can figure, you are possibly assuming that the fast object is emitting the "same amount" of sound as the slow object, and that at some particular distance d which is the same for both the fast object and the slow object, that the sound will become inaudible.

Unfortunately, this is a rather questionable assumption. And it's not particluarly all that clear if that actually was your assumption, though I can't think of any other assumption, offhand, that would allow you to conclude that you'd hear the slow object first.

If we try and translate the question into terms of light, we run into similar issues.

7. Apr 11, 2006

### eweast

Well, I lied about not thinking about it and did so for the better part of an hour while away from a computer after I got some sleep.

I realized that I had misinterpret mathman's reply. After some thinking I came up with the exact same conclusion as JesseM did.

As for pervect, I did phrase my question completely wrong. I was very tired and thoughts weren't coming out like I wanted them to. With the sound example, I was trying to state that the faster object would make it closer, than could the slower, to the observer before the observer could hear it. I was wondering if the same was true for light.

I figured it is true for light, just as with sound. The oddity being that the moving objects measured the light beam at the same speed as the stationary observer. That being explained by the moving objects' slower clocks, or so I would assume.

selfAdjoint, thanks for pointing out Cerenkov Radiation. I did look it up. I was surprised to learn that the electromagnetic wave of an electron was carried by photons. I knew photons were electromagnetic waves, but I thought that the electron had a field of its own independent of photons.

Last edited: Apr 11, 2006
8. Apr 11, 2006

### pervect

Staff Emeritus
Thinking is good! :-)

It is possible to pin the problem down without making extra assumptions about when a signal can be detected.

The threshold of detection of a signal depends on the signal's intensity, and the details of the receiver, making this approach rather difficult. Both the sound wave and the light wave will be distorted by the motion via mechanisms such as the doppler shift, which will influence the detectability of signals from the source.

One can avoid the need for considering these side-issues by considering a slightly different problem.

Imagine having a slow moving object, a fast moving object, and a stationary object all at the same (almost the same) position at one point in time.

i.e.

A.......................D
B
C

A is stationary, B is moving towards D slowly, and C is moving towards D rapidly.

As the diagram shows, think of A,B, and C as being at essentially the same position (they are actually slightly apart in the vertical direction).

Then we can say that light signals emitted from A,B, and C all reach D at the same time irrespective of their state of motion.

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