# I Standing completly still

1. Sep 3, 2016

### Ronnu

What if we would be able to speed up and object to the the speed of light. Then at the speed of light that object could be moving only in one direction because otherwise it would need to move faster than the speed of light. Now say we would de-accelerate that object but so that the de-acceleration would only be in the opposite direction of the movement of the object and would cease when the objects velocity reached zero. Also let's assume that there are no other forces (gravity etc.) acting upon the object. Wouldn't that body now be standing completly still in the universe?

2. Sep 3, 2016

### Ibix

You can't. Anything based on the assumption that you can is therefore nonsense, I'm afraid.

3. Sep 3, 2016

### Staff: Mentor

Why bother speeding it up and slowing it down? It is already standing completely still in its rest frame.

4. Sep 3, 2016

### Ronnu

Ignoring that fact, if you prefer

5. Sep 3, 2016

### Ibix

You can't ignore a fundamental law of physics, not and get any sense out of it. It's like asking what would maths be like if 1+1 were not equal to 2. You can invent any answer you like. It won't have any relation to the real world, however.

6. Sep 3, 2016

### Ibix

A bit more detail. The fact that a massive object cannot be accelerated to the speed of light is a direct consequence of one of the two postulates of relativity, the invariance of the speed of light. If you want to ignore that fact you are throwing out relativity theory, then asking what it would predict about a universe where it doesn't apply. The question is self-contradictory, so there isn't a coherent way to answer it.

The only way to make the question coherent is to make up a complete physics for a universe where relativity does not apply. But that's an imaginary universe, so there are absolutely no restrictions on what those rules are and the results are not related to the real world.

7. Sep 3, 2016

### bahamagreen

Instead of trying to speed up an object to light speed, just use some mirrors and clocks to measure the light speed itself and adjust your inertial reference frame so that you always measure the light speed to be c (which you will find surprisingly easy to do)... in fact you should measure light radiating out in orthogonal directions to be sure you don't overlook any displacements with a lateral component with respect to the original light's beam direction (getting all the other light direction measures to be c will also be surprisingly easy), so now you are standing completely at rest with respect to light beams moving at c away from you in various directions.

Is it a coincidence that this was so surprisingly easy that you needed no velocity adjustments at all to get the light beams all moving away at c?