# Speed of Light

1. Jun 19, 2010

### azice

Lets just pretend we have a spaceship traveling in the vaccum of space with no friction from space dust whatsoever. It accelerates at a meager 10m/s. Eventually in millions of years it will near the speed of light. If nothing can go faster than the speed of light than what prevents said spaceship from passing that barrier when it gets just 10m/s below the 3x10^8 value?

2. Jun 19, 2010

### Gerenuk

To keep up the acceleration you need increasingly higher power. To accelerate to the speed of light you need an infinite amount of energy since being at the speed of light (for an object with mass) corresponds to having infinite energy.

Look at
$$E=\frac{m_0c^2}{\sqrt{1-\frac{v^2}{c^2}}}$$
to understand how the amount of energy necessary scales with the velocity.
http://www.wolframalpha.com/input/?i=plot%281%2Fsqrt%281-x^2%29%2C{x%2C0%2C1}%29
The y axis is the amount of total energy necessary and the x axis is the velocity, where 1 corresponds to the speed of light.

3. Jul 14, 2010

### Grips

If light travels at a certain speed through fiber-optic cable, would the light get to its destination faster if the fiber-optic cable was actually a liquid, and the liquid had a substantial velocity in the same direction as the light?

4. Jul 14, 2010

### FawkesCa

this is one of the toughest concepts to comprehend while starting in physics. the speed of light (C) MUST be a constant and you can see it in this way.

if youre on a spaceship going around at 100,000 km/s (1/3 of C) observers on the ground will see light going 2/3 faster than your ship. but to observers on the ship, light will still be going 300,000 km/s.
not to mention the whole infinite energy thing. took me 3 months to get a handle on this. its just something you have to learn to get past.

5. Jul 14, 2010

### Grips

So, if I have a second spaceship and I speed up to 100,000km and back down to zero, as I observe the light from the other ship, it will always be C? I thought light was constant only in it's local reference frame.

6. Jul 14, 2010

### Niznar

The speed of light is constant in all reference frames, which gives rise to relativity of speed, time and length, among other properties.

No matter how fast you travel, the speed of light remains constant relative to observers in all reference frames.

7. Jul 14, 2010

### DrGreg

Yes. Light through a medium such as fibre-optic travels slower than light in vacuum, so motion of the medium will add to the speed. But the speed will still be slower than the speed of light in vacuum (c).

8. Jul 14, 2010

### Glen Bartusch

OK this is your second 'liquid fiber optic' post in a different thread.
If you want an explanation on the effects of liquids on lightspeeds, read about the Fizeau experiment, where it was proven that light slows down when the light runs thru standing water, and slows down even more if the agua is moving in a direction opposite the light.

If the water (or liquid fiber-optic cable) is moving in a direction the same as light, it would be possible to get that light to equal c, or 300Km/sec, but never more than 300Km/sec.

9. Jul 14, 2010

### Staff: Mentor

Don't hijack another person's thread, it is considered rude.

10. Jul 14, 2010

### Staff: Mentor

This is called the relativistic rocket problem. Here is a good page on it: http://www.phys.ncku.edu.tw/mirrors/physicsfaq/Relativity/SR/rocket.html [Broken]

Essentially it is just due to the geometry of spacetime. No matter how fast you go and how much you accelerate you always wind up with a velocity less than c.

Last edited by a moderator: May 4, 2017