# Do the distance between two objects contract in length contraction

1. Jan 27, 2014

### Trojan666ru

Imagine two asteroids which are separed by 1Ly distance. They are in uniform velocity (.9c) with respect to an observe in space. So from the observers point of view (rest frame), does the distance between the asteroids (ie. 1Ly) appear contracted?

2. Jan 27, 2014

### dEdt

If the distance between two asteroids is measured to be $d_0$ in the asteroids' rest frame, then in a reference frame where the asteroids are moving at $0.9c$ the distance will be about $0.44d_0$.

3. Jan 27, 2014

### Trojan666ru

So i can see the asteroid just behind the first one within a less amount of time after seeing the first asteroid. But what if there was no asteroid in front? Will that take more time to see the last asteroid?

4. Jan 27, 2014

### Staff: Mentor

The presence or absence of one object does not alter the coordinates of another object unless they are exerting forces on each other or something.

5. Jan 27, 2014

### ghwellsjr

You've left a few unspecified parameters here to give a definitive answer. When you say they are separated by 1Ly distance, we assume you mean in their mutual rest frame. When you say they are in uniform velocity, we assume you mean in a direction in line with the two asteroids. And when you ask about what the observer observes, we assume you mean what is defined according to his rest frame and not according to what he actually sees. Then dEdt's answer is correct. Notice that he also made two of those assumptions:

This is really a simple question that is best answered by applying the Lorentz Transform at 0.9c to the mutual rest frame of the two asteroids. You don't need any observer or any implication of his "point of view".

Here's the rest frame for the two asteroids where they are separated by 1Ly along the x-axis:

And here's a frame moving at 0.9c along the x-axis with respect to the first one:

Can you see that at the time of zero years, the red asteroid is displaced by 0.44Ly with respect to the blue asteroid?

Do you understand that nothing has changed in space simply by applying a different set of coordinates to the scenario?

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6. Jan 27, 2014

### ghwellsjr

If you insist on sticking an observer in the scenario, you have to say where he is with respect to the asteroids.

7. Jan 27, 2014

### Trojan666ru

I need the equations to calculate length contraction time dilation and equation for two objects closing together and receding

8. Jan 27, 2014

[STRIKE]The observed separations in position will change for different observers related by Lorentz boosts, but not by the usual length contraction relation (the simple gamma factor). The easiest way to see this is by brute force: write down expressions for x(t) = ... in some frame for each of the two objects, define x1(0) - x2(0) (the position separation) there, and then apply a Lorentz boost to both equations and subtract them to get the position separation in the new frame (for the coordinate observer).[/STRIKE]

Last edited: Jan 27, 2014
9. Jan 27, 2014

### Staff: Mentor

10. Jan 27, 2014

### Mentz114

I tried this ( back of envelope) and got γd0. Is this correct ?

11. Jan 27, 2014

### ghwellsjr

This isn't the correct way to determine "separations in position" which is why you get a different answer than the "usual length contraction". The bottom two dots in both of my diagrams above follow your scheme and end up with a separation of 2.3Ly
(the reciprocal of the correct answer, 0.44Ly) because the new time coordinates are different. Separations, distances, and lengths between two events have to be with the same time coordinate (even though the Proper Times may be different).

12. Jan 27, 2014

### ghwellsjr

It's the correct answer for his incorrect scheme and gives the reciprocal of the correct answer. This is one reason why drawing diagrams is so useful.

Last edited: Jan 27, 2014
13. Jan 27, 2014

### Mentz114

Thanks ! I was interested because that is also the result of a certain experiment. Not on topic to talk about here.

14. Jan 27, 2014