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Interstellar space propulsion...if a spaceship were to get beyond our Sun's gravitational pull and since there is no atmosphere/wind/friction in space...does that mean, if an engine was constantly thrusting, that the spaceship would continue to accelerate to the speed of light, as long as the engine was propulsing forward? In other words, does speed constantly increase in a vacuum? or are the two ideas (Space/Vacuum) separate ideas?

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migrated from scifi.stackexchange.com May 3 '11 at 22:51

This question came from our site for science fiction and fantasy enthusiasts.

Most people say that achieving/surpassing the speed of light is not possible (due to energy constraints). but to quote Star Trek, "Something is only impossible until it is not." That said, I have no real answer for your question but it seems like it would be better suited if it were migrated to physics.SE like ghoppe said. –  Xantec May 3 '11 at 21:46
This is purely a science question, not tied to any particular science-fiction work, therefore it's off-topic on Science Fiction & Fantasy Stack Exchange. ObSF: *Tau Zero by Poul Anderson, which features a spaceship that accelerates on and on, and I think gets the physics right on this particular issue. –  Gilles May 3 '11 at 22:51
The wording of this question is slightly imprecise. For instance, the expression constantly thrusting just implies a positive rocket engine thrust, that may vary in time, not necessarily a constant thrust. Similarly speed constantly increase (in 1D) means positive acceleration, not necessarily a constant acceleration. Also the question can be interpreted as just asking about approaching the speed of light, not actually achieving light speed. –  Qmechanic May 4 '11 at 16:11

3 Answers 3

The answer is no, according to Einstein's Theory of Relativity.

As an object approaches the speed of light, more and more energy is needed to accelerate it further. To reach the speed of light an infinite amount of energy would be required.

Also, there is wind/friction in space. There is no absolute vacuum, there is an interstellar medium. Indeed, relativistic rockets would have to account for the medium, and perhaps even use it as a source of fuel, see: Bussard Ramjet.

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That would be great except Einstein's theory will be proven to be inaccurate or incomplete, if not wholly wrong, within 50 years. In science, no is just another word for haven't figured it out yet. –  BBlake May 3 '11 at 21:53
@BBlake, if it can't be proved wrong today, how can you put a time-frame on when it will happen? –  Sam May 3 '11 at 22:03
Because I'm an optimist! –  BBlake May 3 '11 at 22:04
@ghoppe, And at relativistic speeds, tiny particles become like atomic bombs. –  Sam May 3 '11 at 22:08
@BBlake Your optimism is admirable, but the chances of the Theory of Relativity being proven "wrong" within 50 years are about equal to the chances of the Theory of Evolution being proven "wrong." It's been quite rigorously tested. The ability to travel faster than light opens quite a few paradoxes that are rather difficult to resolve. –  ghoppe May 3 '11 at 22:32

You asked three questions...

1.) ... does that mean, if an engine was constantly thrusting, that the spaceship would continue to accelerate to the speed of light, as long as the engine was propulsing forward?

No. For reasons ghoppe gave.

2.) ... In other words, does speed constantly increase in a vacuum?

Yes, speed would increase continuously, for the example you gave, asymtotically approaching (but never reaching) the speed of light.

3.) ... or are the two ideas (Space/Vacuum) seperate ideas?

No. A vacuum is an excellent and practical description of what most space is.

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I would disagree slightly with your third point. Space is mostly a vacuum, but there is enough matter that if you were going, say, 90%+ the speed of light, it would start to make a difference. –  PearsonArtPhoto May 4 '11 at 14:57

Assuming that our perceptions of Relativity hold up in interstellar space...

The ship would continue to accelerate as long as fuel held out.

Speed would continue its asymptotic clime towards C...

until drag was sufficient to end that climb. Drag would also increase due to interstellar medium collisions as speed increased. But note also: a sufficiently high thrust will not be exceeded by drag.

Note that as speed increases, and drag increases, so does radiation. The interstellar medium results in Alpha & Beta radiation increases, and high energy photon releases from same producing X-ray and even gamma radiation.

Note that several science fiction works have relativity have a cutoff point. In a few more, Relativity is ignored entirely; the most egregious offender is E.E. "Doc" Smith's Lensmen series. John Ringo's Looking Glass series has heliopause boundary changes to the nature of physics, including relativity

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You seem to contradict yourself; you say that drag will end that climb, yet you say that high thrust will beat the drag. The true issue here are the relativistic kinematics at hand. Velocities don't add like we expect them to at relativistic speeds. –  Justin L. May 6 '11 at 8:09
Drag is limited due to the asymptotic approach; the drag is the mass of particles hit, which is directly proportional to speed. If thrust is low (like ion drives), it will limit speed to .99C or so; if thrust is high, the balance point would be superluminal, and thus not actually stop the speed climb towards C. –  aramis May 6 '11 at 15:13

protected by Qmechanic Dec 25 '12 at 16:13

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