# Length contraction

1. Feb 10, 2016

### kent davidge

(Sorry my bad English). I'd like to know if a body traveling near the speed of light really experiences a length contraction or if it SEEMS to be contracted for an observer in a inertial frame.

2. Feb 10, 2016

### phinds

Time dilation and length contraction are something that is never experienced by an object itself, but only seen by a remote observer.

For example, you, right now as you read this, ARE traveling at near light-speed and are MASSIVELY time dilated and length contracted from the frame of reference in which a particle in the CERN accelerator is at rest. Similarly you are somewhat time dilated and length contracted from the frame of reference in which a really some celestial body is moving, AND you are not at all time dilated or length contracted from the frame of reference in which the chair you are sitting in is at rest. And you are all of these things all at the same time. Do you feel any different?

3. Feb 10, 2016

### kent davidge

Ohh that's crazy... I understood it now. Thank you!

4. Feb 11, 2016

### Orodruin

Staff Emeritus
Well, this was not really the question. The question was if length contraction is something which only appears to happen to an object. While your answer is correct, I think it fails to address this point and may be confusing. The entire point is that in a different inertial system, the object really is shorter. It does not affect the object itself, but it certainly is something physical. Of course, this changes nothing in the object's rest frame and the object will not be any different there. It also is not important that the object is "remote", the only important thing is what frame is used to describe the object, whether the observer is standing right next to the object or is lightyears away.

I think it is also important to stress that what we talk about in SR is not how things seem or appear to an observer. What an observer would actually see is going to be distorted by the finite speed of light and this is a very common misunderstanding.

5. Feb 11, 2016

### phinds

*&%^\$%#, I KEEP using "remote" when it has been pointed out to me over and over that it's a very poor choice of words in this context.

I WAS at least careful to specify in my larger paragraph that everything is relative to reference frames, but your other points are nonetheless good as well. Thanks.

6. Feb 11, 2016

### stevendaryl

Staff Emeritus
Yeah, I think there are maybe three levels of "real" in Special Relativity:
1. Quantities that are the same in all coordinate systems.
2. Coordinate descriptions of those quantities, which vary from coordinate system to coordinate system.
3. What things "look like" to an observer, which is distorted due to finite lightspeed.
To give an example: If someone has a device that periodically flashes a light signal, then:
1. The device has a certain proper frequency, the number of flashes per unit time, as measured in its rest frame: $F_{proper} =$ (whatever).
2. It has a different frequency, due to time dilation, as measured in a frame where the device is moving: $F_{measured} = \sqrt{1-\frac{v^2}{c^2}} F_{proper}$
3. It has yet another frequency, due to Doppler shift, as actually seen by an observer moving relative to the device: $F_{doppler} = \frac{\sqrt{1-\frac{v}{c}}}{\sqrt{1+\frac{v}{c}}} F_{proper}$

7. Feb 11, 2016

### A.T.

To complicate matters, "observer" is often used as a synonym for "coordinate system", while all coordinate systems agree on what things "look like" visually to an actual physical observer.