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In minutepysics, while explaning length contraction, it is stated that

If something moves while you are measuring its length, it does not represent its length

Question:

Why?

I, somewhat, know that "measuring the length of an object while moving" does not work from this question, but I cannot understand why that is the case; after all, we do a lot of length measurements by lasers, by which we use the time difference between laser beams arriving at different times.

Qmechanic
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Our
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  • I've removed a number of comments that were attempting to answer the question and/or responses to them. Please keep in mind that comments should be used for suggesting improvements and requesting clarification on the question, not for answering. – David Z Apr 22 '20 at 20:25
  • Can you be specific by what you mean by "time difference between laser beams arriving at different times"? Can you sketch it on a spacetime diagram? – robphy Apr 23 '20 at 23:40
  • @robphy Like, you send a laser beam to an object, it reflects from the object and comes backs to you; by this way, you measure the difference between you and the object. If you do this with both ends of the object (provided that the object is oriented properly), you can measure the length of the object – Our Apr 24 '20 at 05:20

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If something moves while you are measuring its length, it does not represent its length.

I think this is not strictly correct. When measuring the lenght of a moving object, the result is the contracted length as a consequence of Relativity. I think it means that if something moves while you are measuring its length, it does not represent its proper length (length at rest). As you may know, the proper length is a relativistic invariant quantity.

falgenint
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