# Terminology question w.r.t. relativistic speed travel

1. Nov 18, 2015

### Smattering

Dear all,

Let's assume I was planning an interstellar journey with relativistic speed. Being at rest on earth, the distance to the destination is $d$. Let's further denote the proper time I will need to reach the destination as $\tau$.

Is there an official term for the effective speed $\frac{d}{\tau}$?

I understand that this mixes up measures from two different frames of reference, but still the traveller who is planning his trip can reach the destination with distance $d$ within proper time $\tau$, right?

Edit: Fixed a typo.

Last edited: Nov 18, 2015
2. Nov 18, 2015

### Staff: Mentor

3. Nov 18, 2015

### Smattering

Thanks a lot. That seems to be what I was looking for.

Just to be sure: This celerity is not bounded, is it? So in principle, I could reach travel every finite distance within my lifetime, given that I can achieve the necessary acceleration?

4. Nov 18, 2015

### Staff: Mentor

Correct. It is not bounded. As the velocity approaches c relative to some frame the celerity approaches infinity relative to that same frame.

5. Nov 18, 2015

### Smattering

Hm ... then advanced aliens from a distant star system would actually not need any kind of warp drive, multi generation spaceships or hypersleep chambers to visit us. The only thing they needed would be some kind of space drive that allows them to achieve the necessary acceleration.

6. Nov 18, 2015

### Vitro

Not acceleration but speed. We've already achieved plenty more than enough acceleration but cannot sustain it long enough to reach sufficient speed. It all comes down to how much thrust (magnitude and duration) you can get per unit mass of fuel, and of course the mass of the spaceship.

7. Nov 18, 2015

### Smattering

You are right. What I meant is "the necessary acceleration for the required amount of time".

But to come back to the previous point: Relativity is actually not preventing superadvanced aliens from interstellar journeys within their lifespan, is it?

8. Nov 18, 2015

### PeroK

No. But it stops them getting back home. In principle, you could travel as far as the technology would allow in your lifetime, but if you travel 100 light years, then at least 100 years must have passed on your home planet (and 200 years by the time you get home).

9. Nov 18, 2015

### Smattering

That's clear. But look how many volunteers applied for the Mars One project. And that seems not only to be a one way trip but also a suicide mission.

10. Nov 18, 2015

### PeroK

And Mars is 10-20 light minutes away!

11. Nov 18, 2015

### Smattering

If I had to choose between a one way trip that leads me to a camp of tents on Mars and a one way trip that leads me to an extrasolar planet with intelligent life, I would probably not choose Mars.

12. Nov 18, 2015

### PeroK

I doubt you'll ever be faced with that choice.

13. Nov 18, 2015

### m4r35n357

Well there are some fairly major technical "issues" that need solving, but it's a fascinating idea. Have you read about the Relativistic Rocket?
The final paragraph is a bit of a downer . . .

14. Nov 18, 2015

### Mister T

But you don't need to look at that ratio to understand that. All you need to understand is length contraction. As your speed approaches c, the distance to your destination approaches zero.