# Length contraction and direction of tavel

1. Jun 7, 2015

### Amr Elsayed

Hi all, I want to make sure of a particular information. Length is contracted only in direction of motion. If I am on a space craft moving with high speed, I shall see the universe is contracted just in front of me that I am going to, but there would be no contraction If I look to regions that I'm getting further from ." those regions behind me" ??

2. Jun 7, 2015

### Staff: Mentor

There would be contraction behind you also because behind is still parallel to the direction of travel. There would be no contraction to the left or right because those are perpendicular to the direction of travel.

3. Jun 7, 2015

### Amr Elsayed

But if it's the same contraction ahead and behind, how can C stay the same? For instance 2 beams of light are travelling to me from front and behind. Time dilation will not be enough to see C is fixed

4. Jun 7, 2015

### Staff: Mentor

Correct. Time dilation is not enough. You need length contraction and the relativity of simultaneity also. The best way is to use the full Lorentz transform.

5. Jun 7, 2015

### Amr Elsayed

I mean for those 2 beams of light, distances traveled by them are different since I'm moving toward one of them and away from another one. I then should see the one I'm going to is faster. but length contraction will not change it ,because it's same for both distances traveled by beams. If length contracts from front only I shall experience same C from front and behind.
Thanks for response

6. Jun 7, 2015

### phinds

you will see one red shifted and one blue shifted but you will see both moving at c

7. Jun 7, 2015

### Amr Elsayed

Well, I am asking how I will see both travelling at c and length contraction is same for front and behind

8. Jun 7, 2015

### Staff: Mentor

You need the full Lorentz transform. Not just bits and pieces.

Are you familiar with the Lorentz transform?

9. Jun 7, 2015

### Amr Elsayed

Sorry, but Lorentz transformations are time dilation and length contraction, and inertial mass increase is to conserve momentum.

10. Jun 7, 2015

### Mentz114

The speed of light is invariant under Lorentz transformation, which as you say takes into account those things you mention.

(except for 'inertial mass increase')

11. Jun 7, 2015

### Amr Elsayed

Yes, but my question is about how this purpose " invariant C" can be achieved by Lorentz transformations. I get it if length is contracted only from front. But I don't get how it happens if length is contracted from both front and behind.

12. Jun 7, 2015

### Mentz114

If you start with two events $(t_1,x_1)$ and $(t_2,x_x)$ so that $(t_1-t_2)^2-(x_1-x_2)^2=0$, if you transform these events then this relation still holds.

Messing about with 'length contraction' will not lead to better understanding. Use the LT ( as DaleSpam has said).

13. Jun 7, 2015

### Staff: Mentor

No, none of those things are the Lorentz transforms (although they are consequences of the Lorentz transforms). The Lorentz transforms are the more general equations from which length contraction and time dilation (and a whole bunch of other interesting stuff, such as $E=mc^2$) are derived.

To understand this problem properly, imagine two rods joined end to end, each one meter long as measured in a frame in which they are at rest. You are flying past them at great speed. At the exact moment that you are lined up with the junction between the two rods, one them is sticking out behind you and the other is sticking out in front of you.

Where exactly is the front end of the front-pointing rod at that moment? The distance between that point and where you are is of course the length of the front-pointing rod in the frame in which you are at rest, because you're at the other end of that rod at that moment.

Where exactly is the back end of the back-pointing rod at that moment? The distance between that point and where you are is of course the length of the back-pointing rod in the frame in which you are at rest, because you are at the other end of that rod at that moment.

It takes a bit of algebra with the Lorentz transforms, but you'll find that the two lengths are the same and equally contracted from the one-meter length in their rest frame. An easier way of getting this result is simply to work through the derivation of the length contraction formula starting from the Lorentz transforms; you'll see that the derivation works equally well no matter whether some of the object is behind you or in front of you.

14. Jun 7, 2015

### jartsa

In our universe you can adjust whether a light beam hits you earlier or later, simply by adjusting your speed. If you consider that a change of the speed of the beam, then the speed of the beam changes.

Physicists mean something else by speed: How many meters something travels in one second.

15. Jun 7, 2015

### harrylin

See post #4 by Dalespam. It has to do with setting clocks at a distance, as explained in §1 of http://www.fourmilab.ch/etexts/einstein/specrel/www/ as well as the last sentence of §3 . The effects of length contraction and time dilation are small compared to the effect of setting distant clocks differently.

Personally, the following exercise was most useful for me. Imagine a system that according to you is moving (such as a train, with a guy who is sitting in the train and who assumes that the train is "in rest". Now work out how that guy will set clocks in the train "on time" with the help of light or radio signals. You will find that he will set clocks in front of the train, according to your reckoning, behind on clocks that are in the rear.
As a result of his synchronization he will next "measure" what he first made, which is that the speed of light wrt to the train in forward direction is the same as in backward direction. According to you that is not true, and it's due to his wrong settings of clocks. But he will say the same about you.

Note that this clock setting is not automatic. When the train stops, the clocks will be found to be wrong (or it will be found that according to the clocks, the speed of light is not c in both directions). In that sense it is not so that "c stays the same"

16. Jun 7, 2015

### Amr Elsayed

I now know why length is contracted from both front and back. But the problem is how I would measure same speed of light.
What I know is because C must be same, there is time dilation and length contraction. I don't get how for distant moving clocks with same velocity there is a change in speed of time flow but what I got from you Harald is he will set those clocks to run for different speeds to measure same C according to them.

But let me explain what exactly I mean. suppose there is a space craft moving at 0.9C and at an exact moment it is far 3*10^8 meters from a space station. craft is moving away from station .At that moment the station lights a beam into the space craft. The distance between the craft and the station measured by the moving craft is 1307669683 using length contraction. and since C must be 3*10^8, then time for light beam to reach the craft must be 0.4358898944 second from craft's perspective. This means It should take 1 second from perspective of station according to time dilation, but in deed because the craft is moving away by 0.9C it should take 10 seconds for light beam to reach craft from station's perspective not 1 second. I feel confused about it

17. Jun 7, 2015

### Staff: Mentor

You are still confused because you still trying to use time dilation and length contraction.

You must stop using time dilation and length contraction. You must learn what the Lorentz transformations are - they are not time dilation and length contraction. After you have learned the Lorentz transformations, you can use them to understand this problem.

18. Jun 7, 2015

### jartsa

Take two clocks, one meter stick, and two assistants. Set the clocks to the same time. Order the assistants to pick a clock and to go to the ends of the meter stick. Tell them to move slowly to avoid disturbing the clocks by time dilation. Tell the assistants to stop their clock when they see a light pass by. To calculate the speed of the light that passed, divide the distance of the clocks by the time difference of the clocks.

A moving person will disagree with you about these three things:
1: the length of the meter stick
2: how fast the clocks tick
3: whether the assistants disturbed the clocks by time dilation while moving the clocks

19. Jun 7, 2015

### Staff: Mentor

The formulas for time dilation and length contraction are simplified special cases of the Lorentz transform. They only apply in special circumstances.

The time dilation formula applies for a clock that is at rest in one frame and moving in the other. Light is not at rest in any frame, so the time dilation formula does not apply.

The length contraction formula applies for two points at rest wit respect to each other which are at rest in one frame and moving in another. Again, this does not apply for light.

The formula you need to use is the Lorentz transform. The simplified formulas just don't apply as-is. With the Lorentz transform we simply start with:
$$x=ct$$ then we substitute in the Lorentz transform to obtain $$\frac{t'v+x'}{\sqrt{1-v^2/c^2}}=\frac{c^2 t'+v x'}{c\sqrt{1-v^2/c^2}}$$ which simplifies to $$x'=ct'$$

Last edited: Jun 7, 2015
20. Jun 8, 2015

### harrylin

Sorry if it was not clear (did you read the clock synchronization procedure?), but those clocks run at the same speed. What you were supposed to get from Einstein is that you set the times of the clocks such that it looks as if the speed of light with respect to the train is the same in both directions.

Did you try to do what I suggested you to do? In order to really understand it, such things should not be said but done, as an exercise! Once more: please sketch for yourself the observer in the moving train with a clock in every wagon, and determine for yourself what happens when he uses Einstein's clock synchronization procedure. You should find that according to you, he sets the clocks at wrong times.

PS this is how it should look like. You draw two lines, one for the train and one for the train station.

Along the lines you indicate a few clocks on the platform, and also directly next to them also in the train (for example one on each end, and one in the middle):

C'1-------------------C'2-----------------C'3_train

C1-------------------C2-----------------C3_station

Below them you put clock readings. For example your three clocks (on the platform in the station) indicate 0, 0 and 0 seconds. Assuming that C'2 also reads 0 at that instant, what will the other clocks in the train read? They will not read 0!

Last edited: Jun 8, 2015