Measuring Speed in Relativity - Einstein's Work Explained

[erik giles]

Good morning.

I've just completed working through Einstein's work on special and general relativity. Do I understand that, in relativity, a moving object cannot measure its own speed without an external reference to compare to? IE, there is no experiment it can conduct to determine its own speed without another object to compare relative speed. Is this true? If so, I'd like to better understand it. What else should I read?

 

[Orodruin]

This is nothing special to SR and GR. It is true also in classical mechanics. There is no such thing as an absolute velocity and all velocities must be given relative to something else.

 

[erik giles]

That's what I thought.

So, in this thought experiment, let's say a man is inside a moving spaceship (or a plan or train or anything else) and cannot look outside. He has no instruments connected to the outside. He therefore has no way to measure his speed, correct?

 

[Orodruin]

Not only can he not measure his velocity. Nobody else can say that he is moving without specifying what he is moving relative to.

He would describe things perfectly well using his rest frame as an inertial frame as long as it is not accelerating.

 

[erik giles]

Right but can he measure his speed in any way?

 

[Nugatory]

He therefore has no way to measure his speed, correct?

Better to say that because all speeds are relative to something else, the "his speed" that you would have him measure is not a meaningful concept so there's nothing to measure. He can measure his speed relative to the interior of his spaceship (zero, unless he's bouncing off the walls), but that's it.

Right but can he measure his speed in any way?

No.

 

[Nugatory]

What else should I read?

You could do much worse than Taylor and Wheeler's "Spacetime Physics".

 

[Ibix]

Speed is only defined as a relationship between two bodies. So nobody can measure anybody's speed ever. They can only measure its speed relative to something.

Typically we use speed relative to the floor in every day life.

 

[erik giles]

Excellent thank you!

Actually I think i have Taylor and Wheeler here somewhere. I have a number of books that I realized I should stop reading until I get through and totally understand SR and GR. I had high school classical physics, and a lot of advanced math in the area of statistics, Operations Research, but after that I am self taught so bear with me. Looking back, I should have at least gotten a minor in physics.

let me clarify one more thing - this question is not one of the 'debated' question of physics? This is a given, in relativity, correct?

 

[Ibix]

Not debated at all. Solid since Newton, basically.

As with all science that's potentially subject to revision in the future, but there aren't even any hints of debate here that I'm aware of.

 

[Vanadium 50]

You're making this too hard. First, don't tackle GR until you have SR down. Second, what is your absolute position? Not position with respect to something else (like the origin of a coordinate system), but your absolute position. If you say "that's silly, there's no such thing", why should velocity, which is distance divided by time, be any different?

 

[Orodruin]

If you say "that's silly, there's no such thing", why should velocity, which is distance divided by time, be any different?

This is a dangerous argument as it risks being extended to ”why should acceleration, which is velocity divided by time, be any different?” But it is different.

 

[erik giles]

No I don't think it's silly at all! You've confirmed what I thought, which is my goal, I only want to be sure I have it right. Thanks!

 

[erik giles]

You're making this too hard. First, don't tackle GR until you have SR down. Second, what is your absolute position? Not position with respect to something else (like the origin of a coordinate system), but your absolute position. If you say "that's silly, there's no such thing", why should velocity, which is distance divided by time, be any different?

Okay, got it. Your question - what is the absolute position - confirms it for me. There is none, therefore such measurement is meaningless in absence of another body, which I excluded at the beginning of the thought experiment.

I am now enlightened on this concept. Thanks and Merry Christmas all!

 

[erik giles]

This is a dangerous argument as it risks being extended to ”why should acceleration, which is velocity divided by time, be any different?” But it is different.

I was thinking the same thing and I am going to tackle it next. I won't come back until I understand it well enough for good questions.

 

[Vanadium 50]

But it is different.

Yes it is, and that difference is important. But let's cross that bridge when we come to it.

 

[Mister T]

What else should I read?

You can easily find on the web Galileo's famous discourse in which he describes being confined in the hold of a ship that moves in a straight line at a steady speed. Newton adopted this idea as his 1st Law, and after thorough testing, most famously in the Michelson-Morley experiment, it has graduated to the Principle of Relativity. As far as we know, Galileo got it right.

 

[m4r35n357]

You can easily find on the web Galileo's famous discourse in which he describes being confined in the hold of a ship that moves in a straight line at a steady speed.

Here's a direct link.