Does Einstein's theory of relativity imply that interstellar space travel is impossible?
The opposite. It makes interstellar travel possible, or at least possible within human lifetimes.
The reason is acceleration. Humans are fairly puny creatures, and we can’t stand much acceleration. Impose much more than
And to travel anywhere significant, we need to accelerate up to your travel speed, and then decelerate again at the other end. If we’re limited to — say —
But relativity helps a lot. As soon as we get anywhere near the speed of light, then the local time on the spaceship dilates, and we can get to places in much less (spaceship) time than it would take in a Newtonian universe. (Or, looking at it from the point of view of someone on the spaceship: they will see the distances contract as they accelerate up to near light-speed — the effect is the same, they will get there quicker.)
Here’s a quick table I knocked together on the assumption that we can’t accelerate any faster than
You can see that to get to destinations much beyond 50 light years away, we are receiving massive advantages from relativity. And beyond 1,000 light years it’s only thanks to relativistic effects that we’re getting there within a human lifetime.
Indeed, if we continue the table, we’ll find that we can get across the entire visible universe (47 billion light-years or so) within a human lifetime (28 years or so) by exploiting relativistic effects.
So, by using relativity, it seems we can get anywhere we like!
Well…. not quite.
First, the effect is only available to the travellers. The earth-times will be much much longer. (Rough rule to obtain the earth-time for a return journey, double the number of light years in the table and add 0.25 to get the time in years). So if they return they will find many thousand years have elapsed on earth: their families will live and die without them. So, even we did send explorers, we on earth would never find out what they had discovered. Though perhaps for some explorers, even this would be a positive: “Take a trip to Betelgeuse! For only an 18 year round-trip, you get an interstellar adventure and a bonus: time-travel to 1300 years in the earth’s future!”
Second, a more immediate and practical problem. The amount of energy it takes to accelerate something up to the relativistic speeds we are using here is — quite literally — astronomical. Taking the journey to the Crab Nebula as an example, we’d need to provide about
That is a lot. But it’s available: the Sun puts out
But I’m just going to airily treat all that as an engineering issue (albeit one far beyond anything we can attack with currently imaginable technology). Assuming we can get our spaceships up to those speeds, we can see how relativity helps interstellar travel. Counter-intuitive, but true.
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