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Comment by avmich | Hacker News Reader

If we're talking of this kind of efforts, why not to discuss this - https://toughsf.blogspot.com/2019/10/the-expanses-epstein-dr... . Seems in the realm of physics, with some more impressing results...

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One would have expected a narrative published in the last decade could have featured a more felicitously named phlebotinum.

Yeah this sounds cool. Essentially you could design a large spacecraft and put a fusion reactor in it (we will have viable, positive yield reactors soonish). It should be possible then to power a fusion drive as well by building a sort of "half-open" fusion reactor chamber into which we dispense some of the plasma without any pressurization. It should essentially cause a massive explosion, that if somehow controlled by magnetic fields, should yield an enormous forward thrust.

Technically, does it even matter how fast we eject? Shouldn't relativity allow us to reach speed of light with any positive thrust velocity? If the speed of the shuttle was of any concern, that should directly invalidate relativity, since passengers would suddenly not perceive any acceleration anymore, even though nothing about the spaceship and its physical reaction has changed.

To my understanding of physics, there is almost no amount of thrust that would actually propel us to the speed of light, in reality. The energy costs alone would simply be too enormous.

"Folding space"/"Warping space", or shrinking the space in front of the craft while expanding the space, would seem to be the only theoretical way to achieve speeds that not only would match light speed but could surpass it and not break the laws of physics as we understand them. The drive Alcubierre proposed in the 80's, I think... maybe the 90's. I'm not sure when. A Spanish mathematician or physicist who was a fan of Star Trek.

EDIT: My understanding of physics is pretty minimal. I'm more than happy for someone to correct me and explain what I got wrong. This type of stuff is fascinating.

Alcubierre is a Mexican physicist, not Spanish.

Whoops, sorry =/ Like I said, my understanding of this stuff is very surface. Thank you.

It's actually that there is no amount of thrust that would propel you to the speed of light - no 'almost' about it.

From the perspective of an outside observer, a constantly-accelerating spaceship will approach the speed of light, but never quite get there. This will happen regardless of the level of thrust.

Alcubierre drives are pretty sweet - the only trick is they seem to require negative mass [0]... at least they've gotten down the requirement from a negative Jupiter mass to a mere -700 kg!

[0] - https://en.wikipedia.org/wiki/Alcubierre_drive#Mass%E2%80%93...

That's cartoon physics.

Even without relativity, the ejection velocity matters. To get the same change in momentum, you need to eject more mass if you have a lower ejection velocity. That means that you have to carry more mass to eject. And that means that, to accelerate that mass, you have to eject more mass. And so on. It just becomes really unworkable really quickly. (This is called the "rocket equation".)

As for the rest of your second paragraph: Look into how relativistic velocity addition works.

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