IIRC, the limit with humans on-board is like .3c due to the red/blue shifting of the background radiation of the universe. At about .3c, it turns into microwaves/radar/x-rays and such, becoming quite bad for humans. There’s so much background radiation, there may be no way to adequately shield from it and/or travel may be limited only in the direction of less background radiation.

Maybe someone has a link to more information, that’s just what I remember from years ago.

Right, also the hull must resist the impact against hidrogen atoms at near the speed of light, which would apply tremendous forces. Dust will be like exploding nukes.

That's a good point. I have no idea actually where the practical limits would be, especially considering options that arise if the ship has access to a lot of electrical power to generate a protective magnetic field. But even at 0.3c, the trip to Proxima could still be worthwhile for an unmodified human.

You still have the deceleration issue.

You need to use the same amount of energy and time to decelerate, as you did to accelerate.

So you'd only be able to sustain constant acceleration for half the distance. Then deceleration for the second half of the journey.

That would make the travel time much, much longer than your calculations, since you'd be traveling at relatively slow speed for both the first & last 20-25% of the distance.

Here’s a link to a Gizmodo article talking about the study… unfortunately I can’t get the original study to load: https://gizmodo.com/this-is-what-it-would-really-look-like-t...

> If we ever figure out light-weight fusion drives, we could build light hugger space ships that accelerate to a decent fraction of the speed of light. At 1g acceleration it takes about one year to get to 0.9c, which would make the Proxima system accessible with a travel time of about 6 years. Or 8 years at 0.5g. Or 9 years if we 'only' get to 0.5c.

No, we couldn't. The energy density of fusion isn't anywhere near close enough to make a lighthugger, you'd need insane mass ratios[1] and ramjets are probably unfeasible (as in, even-if-your-a-Kardeshev-II-civilization-unfeasible[2]).

[1]: http://www.projectrho.com/public_html/rocket/slowerlight3.ph...

[2]: https://www.sciencedirect.com/science/article/pii/S009457652...

Another atomic rockets enjoyer. I've killed so much time on that site. I don't agree with every one of Winchell's conclusions. But they are small things. And I don't know of a more comprehensive and grounded take on future spaceflight technologies.

I was saddened to hear about Winchell's cancer diagnosis last year[1]. I've wasted a ton of time there too, it's an oddity compared to the rest of the modern internet. I know very few other useful sites like it, and I love it all the more for its strange aesthetic.

I don't agree with everything there either, but usually it's only very minor gripes. In this case though, it's hard to argue with the reality of the rocket equation.

[1]: https://twitter.com/nyrath/status/1436010987086286853?s=20

Oh man, I hadn't heard about that. Best of luck to him but he's right prostate cancer is a pretty bad one.

What are the other few sites you do know of?

And you are absolutely right about the rocket equation. You can't really get around it.

I'm sure there's more but the first few that come to mind are Greg Egan's homepage[1], Xenology[2], and on a completely different topic "Gramadach na Gaeilge"[3] (Irish Grammar).

[1]: https://gregegan.net

[2]: https://xenology.info

[3]: http://nualeargais.ie/gnag/gram.htm

It's too late to edit, but the link to xenology should be http and not https:

[2]: http://xenology.info

This is exciting stuff indeed. Imagine a year-4,000 spaceship reaching Centauri before another spaceship dispatched 1,000 years before it?

For the cynical or pessimists: I'm not optimistic about humanity reaching year-4,000, I'm just possibilistic about it (as in the philosophical meaning [1]).

[1] https://en.wikipedia.org/wiki/Actualism#:~:text=possibilism,...

I can't imagine humanity not making it to 4000. There are very few true existential threats.

This is literally the plot of a very old computer game Alien Legacy. [0]

[0]: https://en.m.wikipedia.org/wiki/Alien_Legacy

This is more or less the plot of a story by Brazilian science fiction writer Roberto Luiz Calife.

And the story "Far Centaurus" by A.E. Van Vogt.

Will have to look that one up.

Thank you for your optimism, now let me balance that out a little.

Propulsion maybe the easier part of engineering. Afaik we are nowhere near to life support and environment control that could be considered stable for a time period of maybe 10 years or more.

The biggest biological advantage, adaptation is meaningless for large organism on such a timescale and maybe dangerous on a microbial level.

You are correct that we are nowhere near that in life support technology, however, there don't seem to be an fundamental hurdles there. If we end up opting for a biological life support system, adaptation would not even enter the equation - it'd be a matter of genetic engineering. An obvious danger would always be onboard ecosystem collapse, of course.

Whenever I read discussions about space travel and trying to approach the speed of light for human travel I have the feeling there's a lot of tunnel vision (excuse the pun). It always seems to assume travelling along a linear path through space.

In my mind, using the admittedly poor analogy of space-time as a membrane that is distorted by gravity, my thinking is we'll find some way to effectively bring two points in space-time adjacent to each other and 'step' between them. Wormholes, Warp, or whatever you want to call it.

Think of a flexible membrane 10 meters long and we are going to travel from one end to the other. We can either travel in a linear fashion over its surface for 10 meters or we can wrap the membrane so both ends are next to each other and travel 0.01m across the gap between them.

Yes, I know there are all sorts of obstacles and arguments against this simplistic visualisation but this is how I've always imagined the physics of any science fiction faster-than-light travel - worm-holes, warp drives, etc.

If you want an actual, real, possible-right-now analogy, think of our current situation where the fastest way to travel from London, UK to Sydney, Australia (1/2 way around the world) is through the atmosphere in an aeroplane travelling ~17000 kilometers and taking at best 19.5 hours.

SpaceX (Musk) Virgin Galactic (Branson), other companies [0], and space agencies [1] have talked about, and are actively working toward, stepping outside the atmosphere into space to reduce the time element to around an hour.

So just like with supposedly faster-than-light travel, by 'stepping outside' the linear/conventional thinking/physics we reduce the time of travel element dramatically. The start and end points don't move but the distance travelled by the transport vehicle itself changes in order to enable the reduction in time.

[0] https://www.telegraph.co.uk/travel/news/new-flights-space-lo...

[1] https://edition.cnn.com/travel/article/hypersonic-flight-air...

I've never bought it. It will still involve being able to transfer information FTL, and then you're on to the Grandfather Paradox.

On top of it, let's handwave all of this away, poof. Any aliens anywhere in the Universe would also be using it once their technology got to that level. Just from the lack of visitors, we have three possibilities that are immediately evident:

1) Humans are among the first intelligent species to get to this technology in the entire Universe.

2) Other species have it, but all of them, every last far-flung one of them, even the renegade civilizations or the mad scientists at the individual level, abide by not using it around humans. Despite all of their numerous reasoning capacities and utterly inhuman motivations, every last sophont with access to the tech has agreed upon this, and has for thousands of years.

3) That tech can't exist.

The first and second are unlikely, statistically.

Bits of dust or a pebble hitting ship at 0.5c would make for a bad day.

Is the 6 years measured in the time-frame of a passenger on the spacecraft, or as observed from Earth?

At 0.9c the relativistic time dilation is such that 2.3 days on Earth pass for every day shipboard time, meaning the crew would experience less than 4 years of travel in total: https://www.fourmilab.ch/cship/timedial.html

Since we aren't too fast yet, it would basically be the same.