# Spaceships and light speed

1. Apr 28, 2004

### mee

Why does the time on the planet left not appear to slow from the spaceship as it is accelerating at great speed away from the spaceship, relative to the spaceships perception. I mean, could traveling at close to light speed also be viewed as slowing down the spaceship more and more so that the rest of the universe, traveling at close to light speed, rapidly moves in relation to the now practically "still" spaceship.

2. Apr 28, 2004

### LURCH

As I understand it, it does. Special Relativity states that each observer (the one on the ship and the one on the planet) will see the other slowing down.

3. Apr 28, 2004

### mee

So how does the twin paradox hold up if both slow down relative to the other? -Feeling ignorant.

4. Apr 29, 2004

### Hyperreality

One important property most people miss while discussing the twins paradox is that Special Relativity only applies for inertial reference frames, in other words it does not deal with change of reference frames or non inertial reference frames.

So, suppose observer A stays on earth, and observer B aboard the spaceship travelling close to the speed of light to a distant star and back. The fact is, there is no contradiction.

While it is true that both are slowing down relative to each other, it is important to realise, it is impossible for observer A to tell whether he/she is moving since he/she is always travelling at a constant speed while observer B can tell whether he/she is moving due to acceleration, in fact observer B undergoes a transition of inertial reference frames by acceleration. So it is possible to distinguish the type of reference frames observer A and B are in and hence apply special relativity to the appropriate reference frame (in this case it only applies to observer A). Therefore there would be an apparent age difference between observer A and B.

Last edited: Apr 29, 2004
5. Apr 29, 2004

### mee

?

What if the spaceship is not seen as accelerating but as slowing down more and more until it reaches a certain speed and stops, the rest of the universe now seen as moving at a great speed in relation. Are you saying that if the whole universe were moving at close to the speed of light in a particular direction that its inertial reference frame would be to make it as if it were still as it is not accelerating?

6. Apr 29, 2004

### LURCH

Yes, the universe would continue as though it were not moving. And in reality, it isn't. When you say,
, You are giving an accurate description of what's really happeneing. Any change in volocity is an acceleration. To say that the universe is holding still while the ship accelerates is no different than saying that the ship decelerates while the universe continues on at a constant volocity.

7. May 1, 2004

### mee

What if the universe is seen as accelerating and the ship is seen as standing still? Is that possible?

8. May 1, 2004

### Antonio Lao

All things in the universe, including the vacuum, possess the dual inseparable properties of permeability, $\mu$ and permittivity, $\epsilon$. The speed of light c is determined by these physical variables.

$$c = \frac{1}{\sqrt{\mu \epsilon}}$$

The reason that the speed of light is a constant in vacuum is because the permeability and permittivity of vacuum are constants.

The rocket is made of matter. So it has its own unique values of permeability and permittivity. Their values also determined the speed attainable by the rocket. When these values are the same as that of the vacuum then and only then can the rocket attain light speed of 186,000 mi/s.