Speed, mass increase & outside observers.

[Endervhar]

Imagine you are in your spacecraft , and within your observable universe there is no other body. As far as you are aware, you are stationary. If you shine a light inside your craft you will measure its speed as 300,000kps, as you also will if you shine a light outside the craft. Nothing gives you any information about your possible state of motion, so you are quite justified in thinking that you are at rest. Now, if you accelerate your craft so that your instruments tell you that you are traveling at approaching the speed of light, what happens to the mass of your craft? You will be aware of no change, if you were, you would know that you were in motion. If there were an outside observer, she would measure your craft’s mass as approaching infinity as you neared light speed, but there is no external observer, and in your frame of reference nothing has changed. As there is no-one in whose frame of reference your craft’s mass has increased, can it, in fact, be said to have increased?

 

[HallsofIvy]

Imagine you are in your spacecraft , and within your observable universe there is no other body. As far as you are aware, you are stationary. If you shine a light inside your craft you will measure its speed as 300,000kps, as you also will if you shine a light outside the craft. Nothing gives you any information about your possible state of motion, so you are quite justified in thinking that you are at rest. Now, if you accelerate your craft so that your instruments tell you that you are traveling at approaching the speed of light, what happens to the mass of your craft?

Your speed relative to what? Assuming that your craft has external sensors so that for its "observable universe" there are other bodies then it can give your speed relative to one of those bodies. But, of course, it would really tell you that the other bodies are going faster relative to you. From the point of view of the craft, you are still stationary.

You will be aware of no change, if you were, you would know that you were in motion. If there were an outside observer, she would measure your craft’s mass as approaching infinity as you neared light speed, but there is no external observer, and in your frame of reference nothing has changed. As there is no-one in whose frame of reference your craft’s mass has increased, can it, in fact, be said to have increased?

There is absolutely no reason to say it has increased or, in fact, that your speed has changed. Why is that a problem?

 

[Endervhar]

Even with no other observable body, would you not be aware that you were in motion while you were accelerating?