Perhaps more a physics question, but there is also a philosophical question why physics validate all measurements as the same. Of course one could ask why should one particular measurement be the only 'real' one.

Of course this is all about special relativity. But a bit in line of Ockhams'razor one could also suggest that the measurement with the less variable parameters should be the most right one.

But if one measure the length of something with a ruler just laying it besides it, it should be a more accurate measurement of the length of something than of the measurement of a observer in motion relative to that length. But in physics both measurement are equal in validity according to SR. Only light is absolute.

So the question is, isn't a measurement with less variable parameters not more accurate than one that is depending on the observers'conditions?


You are confusing the empirical act of measuring versus the concept of measurement used in theories such as SR.

In theories such as SR, the idea is that if you take a measurement within any reference frame, it is "valid." It describes the object correctly. What is not included is how difficult this measurement is. When it comes to the world of metrology, where we study how well we can measure things, you are traditionally correct. The best measurements are those where we can lock down every unknown variable possible. However, that is not a fact of spacetime. That's merely an artifact of the hardware we use to measure.

If I may offer an example, consider a case where the stationary observer measuring the length of an object with a ruler is piss drunk. They can barely see straight. Meanwhile, my moving observer* is in a lab with millions of dollars of fancy measurement equipment. I would expect the lab measurement to be more accurate than the drunkard's. That has nothing to do with the extra mathematical details of the Lorentz boost, and everything to do with the fact that the moving lab has better measurement equipment.

* As nir pointed out in the comments, technically an observer in SR is not actually a person, but merely a frame of reference. I use observer here in the colloquial sense because the original question appears to be using "observer" in the colloquial sense as well, and it looks like some of the confusion may stem from the fact that SR doesn't actually need an individual observing.

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  • I upvoted the answer but it currently contains a potential source of confusion that should be fixed. In SR an observer is a frame of reference, not an individual making measurements: "Speaking of an observer in special relativity is not specifically hypothesizing an individual person who is experiencing events" - en.wikipedia.org/wiki/Observer_(special_relativity) – nir May 1 '17 at 8:37
  • @nir Good point. I added an edit accordingly. I think I want to keep the original phrasing, however, because it aligns closer to the way the OP is thinking. – Cort Ammon May 1 '17 at 14:49

I don't think this is about accuracy or some measurement being the right one. The measurement of the length of something with a ruler just laying besides it is called proper length or rest length: https://en.wikipedia.org/wiki/Proper_length

What can you do that an object with 10m rest length flying by your reference frame with terrific speed only measures X where X < 10m? It is just a fact of life.

In your reference frame it only measures X, and at the same time it still has a proper length of 10m. Neither fact is truer or more accurate than the other.

As an illustration consider the Ladder Paradox: https://en.wikipedia.org/wiki/Ladder_paradox

Imagine a 10m long flying ladder, and imagine that due budget constraints engineers are required to build the smallest barn that can momentarily contain the entire ladder. These engineers would be right to build the barn X+ε long rather than a whooping and wasteful 10m.

BTW, Einstein explains special relativity beautifully in Relativity: The Special and General Theory: https://archive.org/details/cu31924011804774

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