# Can a human travel to distant stars? And other questions.

## Main Question or Discussion Point

1. Can a human travel to distant stars? If the faster you go the slower the time passes for you, then you could technically travel to stars several thousands of years distant from you. It could take thousands of years for someone on Earth, but just a few days for you.

2. Why can't we travel faster? Why are human rockets so slow in relation to light? What stop us from accelerating it more and more?

3. Is it, or will it be praticable to travel to the future using that method - like, travelling at near speed of light around earth until certain age?

Related Special and General Relativity News on Phys.org
phinds
Gold Member
2019 Award
1. Can a human travel to distant stars? If the faster you go the slower the time passes for you, then you could technically travel to stars several thousands of years distant from you. It could take thousands of years for someone on Earth, but just a few days for you.
This has been beaten to death. It is theoretically possible but there are no practical drives to make it happen. Do a forum search or an internet search if you'd like to find a few thousand pages of discussion.

2. Why can't we travel faster? Why are human rockets so slow in relation to light? What stop us from accelerating it more and more?
Pretty much same answer as above but I'll add that it's basically a matter of fuel/payload ratio plus if you go too fast, hitting a spec of dust will wipe you out.

3. Is it, or will it be praticable to travel to the future using that method - like, travelling at near speed of light around earth until certain age?
"Near the speed of light" is just nonsense as far as any practical human technology is concerned now and it's not likely to change much in the future.

If it is a fuel/payload ratio problem what would be the expected speed limit of a human if we manage to use antimatter as fuel? Are there calcs on that or is it just nonsense?

Another limiting factor is acceleration. Our bodies can handle more than 1g temporarily but I do not think we can handle it for an extended time.

Astronuc
Staff Emeritus
If it is a fuel/payload ratio problem what would be the expected speed limit of a human if we manage to use antimatter as fuel? Are there calcs on that or is it just nonsense?
We only produce picogram quantities of anti-matter - at most - and usually less. There is not a lot of energy in that. Anti-matter would be combined with an equal amount of matter for anihilation, and that energy is not entirely charged particles which is necessary for magnetic confinement. Concepts would have the anihilation energy deposited in a propellant, usually hydrogen. Ultimately though, rockets have to carry a mass of propellant which must be accelerated, and to go long distances would require a lot of mass the consequently energy.

To minimize propellant mass, concepts using magnetic sails have been been proposed, but the thrust is so low due to low particle density of solar wind that it would take a long time to get any appreciable acceleration. Approaching another star means traveling against its stellar wind.

There is no practical interstellar travel that would get anyone to a nearby star in a lifetime, or several lifetimes for that matter.

phyzguy
Given the energy limitations, I think that the only practical scenario for traveling to the stars is slow, self-sustaining multi-generational ships that travel to the nearby stars, followed by setting up a civilization at the new star systems, then repeating the process. It might be possible a few hundred years from now to launch a ship that could travel to Alpha Centauri in ~100 years. For example, imagine a ship the size of an aircraft carrier, carrying ~1000 people. It could mass ~10^8 kg, and given a large solar-powered laser based here in the solar system, it could travel to the nearby stars in a time on the order of 100 years. While this is a far-fetched scheme, it is within the realm of the possible. Schemes of accelerating to near the speed of light and relying on relativistic time dilation to allow you to travel to distant stars in a single lifetime just look energetically out of the question.

Dale
Mentor
If it is a fuel/payload ratio problem what would be the expected speed limit of a human if we manage to use antimatter as fuel? Are there calcs on that or is it just nonsense?
I really like this suggestion about using the Hawking radiation from small black holes to power the space craft. It has the advantage that it is, in principle, much easier to convert matter entirely to energy by dropping it into a black hole than by anhillating it with antimatter.

http://arxiv.org/abs/0908.1803v1

Note, this is not peer-reviewed, so take it with a grain of salt.

Very impressive ideas. I like the idea with using photons to create a 'black hole'. Reminds me of that older idea of using atom bombs as the 'propellant' for a spaceship, maybe you could create 'unstable', extremely small black holes in that manner too?

DaveC426913
Gold Member
Another limiting factor is acceleration. Our bodies can handle more than 1g temporarily but I do not think we can handle it for an extended time.
Accelerating at a constant 1g would be sufficient. At 1g, a spaceship could get up to relativistic velocities in a matter of weeks.

D H
Staff Emeritus
http://arxiv.org/abs/0908.1803v1

Note, this is not peer-reviewed, so take it with a grain of salt.
Apparently, even the authors take it with a grain of salt. One sentence from the middle of page 10 summarizes my thoughts:
It is not clear if this is feasible.​

Dale
Mentor
I agree. But I find the idea uniquely thought provoking and appealing.

ghwellsjr
Gold Member
I didn't read the entire paper but I'm just wondering how you contain a black hole and how you get it to move where you want it to go without falling into it. I know it's very small but it's also very massive.

D H
Staff Emeritus
I didn't read the entire paper but I'm just wondering how you contain a black hole and how you get it to move where you want it to go without falling into it. I know it's very small but it's also very massive.
Oh ye of little imagination, the answer is right there in the paper!
It is not hard to see how we might satisfy requirement 1. We simply position the SBH at the focus of a parabolic reflector attached to the body of the ship.​
In other words, ignore all scientific and engineering issues, wave your hands, and say presto!

ghwellsjr
Gold Member
Oh ye of little imagination, the answer is right there in the paper!
It is not hard to see how we might satisfy requirement 1. We simply position the SBH at the focus of a parabolic reflector attached to the body of the ship.​
In other words, ignore all scientific and engineering issues, wave your hands, and say presto!
I did read that but since they had previously bad-mouthed antiparticle solutions and from what they said right after your quote, I thought they weren't serious:

It is not hard to see how we might satisfy requirement 1. We simply position
the SBH at the focus of a parabolic reflector attached to the body of the ship.
Since the SBH will radiate gamma rays and a mix of particles and antiparticles,
this is not simple. The proposal has been made in the context of antimatter
rockets, to make a gamma ray reflector out of an electron gas [11].
It is not clear if this is feasible (e.g., [2]).​

In any case, I still don't see how a black hole at the focus of a parabolic reflector works. Is this actually part of the propulsion system where the BH is giving off a lot of energy and the parabolic reflector just makes sure that it all goes behind you?

I'm also not clear on whether the BH is actually generating energy or is simply a place to store it. It must be the latter.

D H
Staff Emeritus
I certainly wasn't serious in bad-mouthing you. They did not address the containment issue. They did not address a lot of issues. They instead hand-waved solutions into existence, hand-waved problems away. This isn't a physics paper. It's a lousy science fiction story.

In any case, I still don't see how a black hole at the focus of a parabolic reflector works. Is this actually part of the propulsion system where the BH is giving off a lot of energy and the parabolic reflector just makes sure that it all goes behind you?
That seems to be the case. You just hover the reflector over the black hole in the direction you want to go to. The big problem is towing the black hole. The only way I can see to do that is to use gravity. To accelerate a 10^9 kg black hole with 0.01 g, you'd have to hover another mass of 10^9 kg less than a meter away. (and you can't use another black hole)

The black hole gives off 3.56 * 10^14 W, so it would have to withstand 2.8 * 10^13 W/m^2, more than 10^10 times the intensity of sunlight here on earth.

How to feed anything into a 10^-18 m radius black hole, that gives of 3.56 * 10^14 W of energy is left as an exercise for the reader.

http://xaonon.dyndns.org/hawking/[/URL] [Broken]

Last edited by a moderator:
But it is such a sweet idea :) And a black hole is no different from any other mass, outside the event horizon, as I understands it? The radiation is assumed to 'propagate' outside the Event horizon isn't it? But there is also the idea of a black hole being 'colder' than the space surrounding it though, stopping that (Hawking) evaporation as I understands it?

But it's still one of the best and coolest ideas I've seen so far. It would sit perfectly in a hardcore SF, I only wish it was me writing it :)
==

Assume that we can create them using lasers. Then assume that you are moving, relative some frame, then let it radiate (and ignore it being 'colder' for this.) Then vaporise it and recreate it, at very short time frames. You will need to get the 'energy' to do so from somewhere though. So we will have to use the mysterious 'zero point energy' :) Heh.

Last edited:
DaveC426913
Gold Member
This thread has stopped being about relativistic travel and is now about sci-fi propulsion. Requesting it be moved to a more appropriate forum.

ghwellsjr
Gold Member
This thread has stopped being about relativistic travel and is now about sci-fi propulsion. Requesting it be moved to a more appropriate forum.
What's the difference?

phinds
Gold Member
2019 Award
But there is also the idea of a black hole being 'colder' than the space surrounding it though, stopping that (Hawking) evaporation as I understands it?
This has been discussed here a couple of times. The existance of Hawking Radiation does NOT depend on the temperature of the space surrounding it being any particular value, high or low.

D H
Staff Emeritus
Assume that we can create them using lasers.
We can't.

This proposal requires that all of the energy required to create the black hole be focused on one tiny, tiny, tiny spot in one tiny, tiny, tiny interval of time. The lasers need to be focusable to the sub-attometer level, the pulses need to be smaller than an attometer long, and all the energy needed to create the black hole needs to be packed into those very small pulses. None of these is even close to achievable. Not now, maybe not ever.

The assumptions go on and on and on. "Cool" is not the right word to use to describe this concept. "Half-baked" is a much better term. That's the problem with anything that could send humans to distant stars. The underlying ideas are half-baked, at best.

One way to think of it: Humanity has a grand total of four spacecraft that are well beyond Pluto's orbit. These spacecraft are tiny, tiny, tiny compared to something needed to carry humans. They have been traveling for 35 to 40 years, and not one has yet crossed the threshold that marks the outer reaches of the solar system.

Dale
Mentor
I think that it is both "cool" and "half-baked". It is primarily interesting by comparison with other proposals which are quarter-baked or less IMO. It is definitely only science fiction, but it is my favorite science fiction in the genre.

DaveC426913
Gold Member
What's the difference?
One is science, the other is engineering.

We can talk about the science. We know how fast, how far and how much energy it would take. We don't need to "suppose" anything.

But any ideas about executing it are currently just fantasy.

Which is fine. But it doesn't belong in the Relativity forum.

ghwellsjr