# Something you want to

1. Nov 7, 2008

### hakovala

I have a little problem.. Just a little mind problem, that has bothered me for a while..

Lets say there is three points(lets say objects or something) in space. They are all in line relative to each other. Two of them (other than, the middle one) are moving at the light speed relative to each other. (this is how i have understand the whole thing, two points can travel at the speed of light relative to eatch other)
So..
If one of those two points send signal to the other with the highest speed known, light speed. Would that information reach the other object, even if it's moving at the speed of light, away from the receiving object, relative to each other.
With common knowledge it is sure that if two objects/information/(anything) move relative to each other with the same speed, then the object/information/(abything) that was sended (the signal that was send by the first object) is moving with the same speed, so the signal(sende information) won't never reach the receiver point, because it is traveling the space with the same velocity.
Or is it..??
Lets say that those two points( from the line that is made from three points (two of those points that are the most left and the most right points.) !! And dont even try to argeu with me to be too simple. I just want to make my self clear. ) are traveling with speed of light to each other.. so this is my question..:
The third point.. when does the signal pass it.. never..??
The third point in the middle of those two points are moving less than the speed of light relative to to each of those other two points.. are they..??

If left most point sends a signal to the right most point.. Then the signal would reach the middle point and because the middle point isn't moving away from the right most point at the speed of light, then the the signal would reach the right point..

I'm just asking these questions cause i'm interested in physics.. (LOL) I just want the answer for my questions.. Plah..

P.S. If i suck in my enlish, then tell me so.. i don't mind.. lol.. HEHEH

2. Nov 7, 2008

### hakovala

I have wondered this for a while.. this is one of those problems that I want some kind of answer..

Last edited: Nov 7, 2008
3. Nov 8, 2008

### HallsofIvy

No, if two object are moving at light speed in the same direction, no signal from the second can reach the other. It is outside the "space-like" cone of the other. A signal from the second object would reach the middle one at the same time the object itself did.

4. Nov 8, 2008

### yuiop

Hi Hakovala,

I think I see what you are getting at and it is an interesting question.

Point A is stationary.
Point B is moving at 0.8c relative to A.
Point C is moving at the speed of light relative to A.

A light signal sent from A catches up with B but never catches up with C.

That seems a little odd because if B sends a signal to C it appears the signal from B should catch up with C because on the face of it C is only going 0.2c faster than B, right? Well in fact it is not right. C is going at the speed of light relative to A and is also going at the speed of light relative to B. That is one the odd properties of the speed of light in relativity and can be understood when you understand relativistic velocity addition (or subtraction). See http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html

If you draw the spacetime diagram you will see that signals sent from A or B are parallel to the wordline of C and that makes it clear they never catch up with C.

5. Nov 8, 2008

### Fredrik

Staff Emeritus
I assume that in the first sentence the objects are actually both moving away from the middle object at light speed, and in the last sentence they're moving towards the middle object at light speed.

6. Nov 8, 2008

### HallsofIvy

Only light can move at the speed of light relative to anything.

7. Nov 8, 2008

### MeJennifer

That is not correct, for instance gluons also travel at the speed of light.

In general all elementary particles that are massless travel at the speed of light.

8. Nov 8, 2008

### Jonathan Scott

I think the more useful illustrative case is when two objects are moving in opposite directions at very slightly less than the speed of light. In that case, a signal emitted by one CAN catch up with the other one eventually, even though their rate of separation in the observer's frame can be nearly 2c. The signal travels at c in the observer's frame regardless of the speed of the emitting object.

9. Nov 8, 2008

### HallsofIvy

Yes, as soon as I wrote that I realized I should have said "massless objects".

10. Nov 8, 2008

### MeJennifer

True.

The only exception is when two objects are separated by a certain minimum distance and they continuously accelerate in opposite directions. In such situations it can be the case that a light signal emitted from one object will never reach the other object.

11. Nov 8, 2008

### HallsofIvy

No. At the instant the light signal is emitted from object A the two objects are are a fixed distance apart. Also from that instant, the light is moving with speed c relative to object B. The light signal must cross that distance and the distance B will have moved. Since B is moving with speed less than c, that will happen in finite time.

12. Nov 8, 2008

### MeJennifer

You might want to study the characteristics of a Rindler horizon.

13. Nov 8, 2008

### Saw

Could you briefly explain what a Rindler horizon? Does it produce the effect that one observer does not measure c as the speed of light?

14. Nov 9, 2008

### Fredrik

Staff Emeritus
The brief explanation is that if your world line is one of the hyperbolas in the first picture in the Wikipedia article, then you won't be able to receive any light signals that were sent at t=0 from locations with x<0 or x=0. The reason for that should be obvious. (What does the world line of a ray of light look like in a spacetime diagram?).

A measurement of the speed of light always yields the result c if it's done right.