# Speed of light

1. Jul 26, 2010

### Brett13

I just want to make sure I understand this clearly. Light travels at 186 000 miles/second right? So if something was 10 light years away would that mean that it would take 10 years going at the speed of light to get there? Wouldnt it be possible to make a probe go even half that speed? I dont know how fast voyager is going but would it be possible we we just strap on a plasma rocket ( a really big one) and let it build up speed. Is that a dumb question?

2. Jul 26, 2010

### magellanic3

no it is not a dumb question.
no i don't think it is possible.
u said 'a really big one' however if the mass increase then the acceleration will ultimately decrease.F=m*a.

3. Jul 26, 2010

### Chronos

See relativity 101.

4. Jul 26, 2010

### fatra2

Hi there,

Voyage 1 is now travelling at a tad more than 10 miles/second. It is still very very far from anywhere close half the speed of light.

Of course many people in this forum will tell you about relativity, which they are right. But way before any major relativistic effect kicks in on the mass of the space shuttle, other problems have to be solve to go faster.

One of the major problem (from my point of view) is that outer space is empty. The only way to go faster is to push the rocket on something. But in outer space there is nothing. Therefore, it becomes very hard to get a good grip, to add some extra speed.

Cheers

5. Jul 26, 2010

### Chronos

A rocket pushes against its own ejecta, so being in a vacuum is an advantage [no resistance]. Still, it would require an impractical amount of fuel to accelerate a craft to anywhere near light speed. An ion rocket could achieve a respectable fraction of light speed, but, accelerates very slowly.

6. Jul 27, 2010

### fatra2

Hi there,

No resistance both ways. No resistance on the rocket, but the ejecta is pushed with no resistance neither. On top of it, the ejecta, being mostly in the gas state, dissipates very quickly in outer space. Therefore, like you said, an incredible amount of fuel would be needed to accelerate to anything close to the speed of light (0.001% of c).

Cheers

7. Jul 27, 2010

### Staff: Mentor

It seems like you're intending to imply that a rocket is less powerful in space than in the atmosphere. That isn't true. The lack of backpressure in space means rockets are more efficient in space.

Last edited: Aug 1, 2010
8. Jul 27, 2010

### Brett13

What exactly is an ion rocket? Is it like one of those photon sail type deals?

9. Jul 28, 2010

### Staff: Mentor

No, an ion rocket is simply a particle accelerator: it uses strong electromagnets to shoot charged particles out the back of the rocket at very high speed.

10. Aug 1, 2010

### JED777

You folks should really review your physics basics before replying to a person about rocket propulsion. “For every action there is an equal and opposite reaction of the exact same force." The surface of the Earth has nothing to do with lifting a rocket. It is the result of a force blasting out that pushes the rocket in an opposite direction. This works flawlessly in outer space because there is no gravity to hold the ship down. A push as small as a fart will change your course accordingly. What should be considered is the mass of the space craft and the amount of force required to accelerate it. I remember reading that if an acceleration of 1g is maintained for one Earth year, the vehicle will approach the speed of light in that time. The amount of thrust required to produce 1g depends on the mass of the ship. Remember, this is a constant acceleration
(as in x miles per hour, per hour) This would enable people in the space craft to stand planted to the floor where the 1g is being applied and feel as if there were gravity. The problem is that we do not have the technology to carry a fuel light enough that is capable of producing this continuous thrust. The mass of the fuel defeats the power ratio. In the future when we harness nuclear fusion, it is reasoned it will be possible. This is the power of the stars. Finally, the more mass the harder it is to approach the speed of light. I hope this answers your question. There is lots of math on this you can look up on-line if you are keen on it.
Best wishes,
JED

11. Aug 1, 2010

### Staff: Mentor

Welcome to PF, Jed.

You are right that if the force is the same the acceleration will be greater far away from the earth because of the lack of gravity.

But that's not fara's misunderstanding. Fara's misunderstanding is believing that the same engine will produce more thrust in the amosphere than in space, when the reality is the opposite.

12. Aug 1, 2010

### JED777

Yes Russ, Fatra2 misunderstands this basic rule of physics. It is a common misunderstanding if you have not studied elementary physics in HS. I hope we answered the original question for the thread.

Good to be onboard here at the forum.

JED
.

13. Aug 2, 2010

### fatra2

Hi there,

I can only agree with your comment. But tell me again on what does your small fart pushes against??? If there is nothing in the empty space to push against, where will you accelerate to??? And then, if you let a big one rip, but there is still nothing to push against, will you go any faster??? Remember that we are talking about EMPTY space, in the meaning that there is nothing out there.

Cheers

14. Aug 2, 2010

### Dmitry67

fatra2, any rocket sacrifices part of its own mass to accelerate: it ejects fuel at high speed (becoming lighter) and the rest of the mass accelerates.

The problem for the relativistic rockets is the fact that space is not empty - one sand grain will destroy the ship.

15. Aug 2, 2010

### fatra2

Hi there,

There is no misunderstanding from my point. All I was saying is that in the atmosphere there is something to push against, while in the empty space, well there is nothing. And it is kinda hard to accelerate without any kind of propulsion.

Cheers

16. Aug 2, 2010

### Chronos

A rocket does not 'push' against anything other than its own mass. Think 'every action has an equal and opposite reaction'. Rocket exhaust throws out mass at high velocity and the rocket reacts by moving in the opposite direction.

17. Aug 2, 2010

### fatra2

Hi there,

Thank you for reminding me. But it comes right back to my first comment. The amount of mass "thrown" out of the rocket implies that to reach a great speed, you need an outstanding amount of mass. Therefore, the speed limit reach by a rocket is very much set on the amount of fuel (mass) that it can cary, rather than relativistic effects that would make the ship's mass increase.

Cheers