Relativistic Speed Travel: What is the Effective Speed?

[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.

 

[Dale]

I think it is called celerity or sometimes proper velocity (which I don't like). https://en.m.wikipedia.org/wiki/Proper_velocity

 

[Smattering]

I think it is called celerity or sometimes proper velocity (which I don't like). https://en.m.wikipedia.org/wiki/Proper_velocity

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?

 

[Dale]

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

 

[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.

 

[Vitro]

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.

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.

 

[Smattering]

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.

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?

 

[PeroK]

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?

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).

 

[Smattering]

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).

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.

 

[PeroK]

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.

And Mars is 10-20 light minutes away!

 

[Smattering]

And Mars is 10-20 light minutes away!

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.

 

[PeroK]

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.

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

 

[m4r35n357]

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

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

Spoiler

The final paragraph is a bit of a downer . . .

 

[Mister T]

So in principle, I could reach travel every finite distance within my lifetime, given that I can achieve the necessary acceleration?

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.