# Q & A: who sees length contraction?

Q:
Is time dilation and length contraction observable?Consider for example a rectangular sheet inside a spacecraft moving at a high velocity.Would an observer inside the spacecraft ever observe the sheet as a square sheet if length is contracted enough along the length?Similarly would a stationary observer in the earth be able to do so?In other words is length contraction observable?Also is time dilation observable?
- Ajmal (age 18)
India
A:

These effects are easily observable but not in the way you are picturing. Here's the key thing to remember: there is no distinction between a "stationary" and a "moving" observer. Any observer can choose to use a coordinate frame in which she is stationary.

So inside that spacecraft, everything looks entirely normal. Squares remain squares, all clocks agree, etc. Someone who chooses to say that the spacecraft is moving will say that all its clocks are slow and that things have contracted along the direction of motion. It's easy to measure those effects.

For example, the lifetime of muons created in the upper atmosphere is long enough for them to reach the surface because, from our point of view, their internal clocks are running slow. From the muons' point of view, they still have the usual short lifetime, but they reach the surface because the atmosphere has contracted into a thin pancake, so the earth doesn't have far to go to reach them.

Who is right? Both are right, because  despite their different coordinate descriptions they get the same results for real physical things, like what things bump into each other.

Mike W.

(published on 11/23/2016)