# I Measuring speed question

1. Dec 23, 2017

### 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?

2. Dec 23, 2017

### Orodruin

Staff Emeritus
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.

3. Dec 23, 2017

### 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?

4. Dec 23, 2017

### Orodruin

Staff Emeritus
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.

5. Dec 23, 2017

### erik giles

Right but can he measure his speed in any way?

6. Dec 23, 2017

### Staff: Mentor

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.
No.

Last edited: Dec 23, 2017
7. Dec 23, 2017

### Staff: Mentor

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

8. Dec 23, 2017

### 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.

9. Dec 23, 2017

### 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?

10. Dec 23, 2017

### 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.

11. Dec 23, 2017

Staff Emeritus
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?

12. Dec 23, 2017

### Orodruin

Staff Emeritus
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.

13. Dec 23, 2017

### 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!

14. Dec 23, 2017

### erik giles

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!

15. Dec 23, 2017

### erik giles

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.

16. Dec 23, 2017

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

17. Dec 23, 2017

### Mister T

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.

18. Dec 23, 2017