## Interstellar Travel

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pete
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### Interstellar Travel

I recently bought a telescope and that naturally got me thinking about travelling to the stars. Obviously time-dilation becomes a bit of a problem and it's no point going somewhere if everyone you know is long dead by the time you get back.

So how about you take them with you? And I mean everyone... the whole solar system. What better power source can we build than our sun?

I imagine a series of solar powered electromagnets, in orbit at a safe distance from the sun, that could channel (or focus if you like) charged particles from the sun like a rocket nozzle. Move the entire system nearer to where you'd like to be from the comfort of home. Probes and more sensible transport could be then used when the distances are more manageable.

Obviously there'd be a lot of details to work out such as the effect on our own orbit, but is the concept sound? I mean could it be possible in theory to divert enough energy using magnetism to one side of the sun?

idobox
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### Re: Interstellar Travel

If your satellites deflect solar wind, they will be accelerated (it's called a magnetic sail), but that shouldn't affect the course of the sun.
You need to find a way for the satellite to transfer that force to the sun.

A solution would be to put such a thing in the direction you want to go, deflecting the solar wind 180°, and balancing thrust with gravity. In effect, it would become a sort of gravitational tug, so you need your electromagnet to be tied to the most massive thing you can find.

I checked the wiki for solar wind. The sun looses 4 to 6 billion tons of mass an hour, with speeds from 400 to 700km/s.
If we consider 6 billions tons at 750km/s fired in the same direction in one hour, the ejected mass has a momentum of 4e18 kg.m/s. As a result, the sun is pushed away and gains a deltaV of 2e-12m/s, that's an acceleration of 6e-16 m/s². At this rhythm, it would take 178 million years to reach one light year.

A better alternative would be to cause the Sun to eject more mass, or faster, from one side. Maybe by dropping stuff in it, or by manipulating magnetic fields near the surface.
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pete
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### Re: Interstellar Travel

Thanks. It's easy to lose track of the scale of things if you don't know the math.

PM 2Ring
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### Re: Interstellar Travel

I reckon we'd need to utilise the photons as well as the solar wind. What happens if we put the Sun inside a reflective Dyson sphere that has a hole in it so that all the solar wind and light escapes through the hole? Would that work, or would the sphere just crash into the Sun?

pete
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### Re: Interstellar Travel

Another thought I had was to use the gas giants. Would constantly accelerating them perpendicular to the ecliptic (using the planets as fuel for fusion reactors perhaps) be enough to drag the sun and everything else along with it at a reasonable pace?

By reasonable I mean travel times on the order of thousands rather than millions of years.
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idobox
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### Re: Interstellar Travel

PM 2Ring wrote:I reckon we'd need to utilise the photons as well as the solar wind.

Let's do the math. The total light output of the Sun is about 4e26W, it gives us about 1.3e18N, and an additional push of 6.7e-13 m/s. Which means the solar wind is useless compared to sunlight.
It would still take 5 millions years to reach 1 light year.

pete wrote:Another thought I had was to use the gas giants. Would constantly accelerating them perpendicular to the ecliptic (using the planets as fuel for fusion reactors perhaps) be enough to drag the sun and everything else along with it at a reasonable pace?

The mass of Jupiter is 1.9e27kg, and is 2 times more massive than all other planets combined.
If we assume all this mass is hydrogen, and we fuse it all, we get 1.2e42 J. Fusing the helium into carbon would give an additional 1.11e41J, and the benefit goes down as the mass goes up.
So, 1.3e42J available. If all this energy is somehow transformed in kinetic energy, we can give the Sun a speed of 11423 km/s. Once at this speed, you would travel 1 ly in 262 years.

This looks like the best option. Of course, fusing hydrogen is very hard, and you have to convert that energy into kinetic energy, but it's in the realm of possibility.
Given the scale of the project, it would be better to first get Jupiter much closer to the sun. There is limit on the thrust you can get, because Jupiter is a gas giant, and as such, not really easy to push. Fusion reactors could float on the surface, balancing thrust with buoyancy. I don't know how to compute the kind of stress Jupiter could survive without ripping apart.
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gmalivuk
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### Re: Interstellar Travel

PM 2Ring wrote:I reckon we'd need to utilise the photons as well as the solar wind. What happens if we put the Sun inside a reflective Dyson sphere that has a hole in it so that all the solar wind and light escapes through the hole? Would that work, or would the sphere just crash into the Sun?
The simplest setup would indeed crash into the sun, since there'd be nothing to counter that tendency.

http://en.wikipedia.org/wiki/Stellar_engine has some other suggestions.
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pete
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### Re: Interstellar Travel

idobox wrote:in the realm of possibility

Awesome
Thanks!

idobox
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### Re: Interstellar Travel

I still believe the best option is to affect the activity of the sun asymmetrically. Causing the photosphere to superheat locally, resulting in fusion where it's not supposed to happen, a big explosion/eruption, and net thrust as a result might be more feasible (compared to fusing all the hydrogen in Jupiter).

Shooting protons at crazy high speed into the sun should produce heat and muons in the corona. If the protons are fast enough, the muons could reach the chromosphere and cause fusion there.
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ikrase
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### Re: Interstellar Travel

Awwww, come on. Those ideas are TINY.

What you really need is a stellar cluster full of white dwarfs, a black hole to generate a magnetic sail, and triggered supernovas.

That's right: Ludicrously Large MagOrion.
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snowyowl
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### Re: Interstellar Travel

Refuelling would be difficult, and it presumably requires stellar-engineering technology to get a black hole close enough to the white dwarf cluster. But if I ever need to annihilate something by slamming a cluster of neutron stars into it at relativistic speeds, I'll let you know.
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davidstarlingm
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### Re: Interstellar Travel

snowyowl wrote:Refuelling would be difficult, and it presumably requires stellar-engineering technology to get a black hole close enough to the white dwarf cluster. But if I ever need to annihilate something by slamming a cluster of neutron stars into it at relativistic speeds, I'll let you know.

Can an orbiting gravity tractor operate on a black hole? I don't see why it wouldn't.

bouer
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### Re: Interstellar Travel

How about using 2 supermassive black holes, one is active, consuming an accretion disk and ejecting polar jets, the other is held in place over one pole by gravity and radiation pressure, consuming one polar jet completely. Civilization could orbit around the dormant black hole. Magnetic fields could direct interstellar/intergalactic plasma away from the dormant black hole and into the active one. Almost 50% of the mass of the material consumed should be turned into forward motion.

Copper Bezel
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### Re: Interstellar Travel

What keeps the accretion disk on the left and the orbiting solar system on the right bound to the moving black holes? I mean, yes, gravity, duh, but what keeps the objects orbiting stably around a constantly accelerating object? (That is, one that's not, as usual, being accelerated by a gravitational field that affects the orbiting body, too, as in any case of a satellite of an orbiting body.)

And is it right to assume that the jet is going to be continually increasing the mass of the BH on the right as well?
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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bouer
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### Re: Interstellar Travel

I'm pretty sure this would be ridiculously unstable, it might be able to theoretically work, but it could probably never be accomplished in practice.

Tass
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### Re: Interstellar Travel

Copper Bezel wrote:What keeps the accretion disk on the left and the orbiting solar system on the right bound to the moving black holes? I mean, yes, gravity, duh, but what keeps the objects orbiting stably around a constantly accelerating object? (That is, one that's not, as usual, being accelerated by a gravitational field that affects the orbiting body, too, as in any case of a satellite of an orbiting body.)

And is it right to assume that the jet is going to be continually increasing the mass of the BH on the right as well?

The system orbits the front black hole. Since it is near to this one the gravity is strong. The gravity from the other hole and the acceleration of the system are second order effects which will not drastically perturb the orbit. (The orbital plane will just lie slightly behind the center of the front hole.)

Of course it will be terribly unstable. The distance between the two holes will be an unstable equilibrium. That will have to be controlled by active control of the inflow to the acreation disk. Worse will probably be that there is no force maintaining the front hole in the jet. Though maybe the same effect that keeps a ball in a fountain of air or water could come into play. As the hole moves to the side of the jet, parts of the jet will miss the event horizon and swing by on a hyperbolic trajectory, thus it will exert a restorative sideways force on the hole.

Izawwlgood
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### Re: Interstellar Travel

It's always mentioned in sci-fi, the notion of 'tacking against the solar wind'. How do you accomplish this, given that the solar wind is always pushing outward, and you don't have a medium to tack against?
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SlyReaper
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### Re: Interstellar Travel

Izawwlgood wrote:It's always mentioned in sci-fi, the notion of 'tacking against the solar wind'. How do you accomplish this, given that the solar wind is always pushing outward, and you don't have a medium to tack against?

Well, you can angle the sail to change the direction the solar particles are deflected, and thereby change your direction of thrust. While the radial component of thrust will always be outward from the sun, you can reduce the radius of your orbit by angling the transverse component in a retrograde direction, thereby getting closer to the sun. Or "sailing against the solar wind", as it were. Technically, it's not tacking, but people like the idea of spacecraft being analogous to sea vessels, so they use similar terminology.

What would Baron Harkonnen do?

Izawwlgood
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### Re: Interstellar Travel

Ohhhh, I get it. Thanks.
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idobox
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### Re: Interstellar Travel

Also, a well designed length could be attracted.
The rays arriving to the lens are not perfectly parallel, and by making them parallel, and perpendicular to the lens itself, you can get a tiny pull. Optical cages work on that principle, but use highly focused light to increase the effect.
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ikrase
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### Re: Interstellar Travel

Izawwlgood wrote:It's always mentioned in sci-fi, the notion of 'tacking against the solar wind'. How do you accomplish this, given that the solar wind is always pushing outward, and you don't have a medium to tack against?

I think that's mainly an interplanetary travel thing. A magnetic solar wind sail can be directed anywhere in the solar system (albeit very slowly) just by turning it on and off.
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Copper Bezel
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### Re: Interstellar Travel

...

Isn't a solar sail "mainly an interplanetary travel thing" already? Because if you want to talk about "very slowly"....
So much depends upon a red wheel barrow (>= XXII) but it is not going to be installed.

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SlyReaper
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### Re: Interstellar Travel

Depends on the sail. If it's made of some hypothetical type of unobtanium, is hundreds of thousands of miles across, is one atom thick, and has 100% reflectivity, then you could get some fairly impressive thrust out of that.

What would Baron Harkonnen do?

ikrase
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### Re: Interstellar Travel

*MAGNETIC* solar sail. Not reflective. Very very large superconducting open-frame coil. Creates drag w/ solar wind, or can be used as an 'airbreak' for interstellar ships.
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zenten
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### Re: Interstellar Travel

Could we hook up a bunch of these small black holes to the sun somehow to push the whole solarsystem? http://arxiv.org/pdf/0908.1803.pdf

bouer
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### Re: Interstellar Travel

Thanks for the link zenten, that was an interesting read. What I got from this is that SBHs are a very efficient and fast method to convert mass to energy, I don't see any reason one couldn't put a few million of the drives they propose onto the sun. Something to note, however; the paper suggests that the SMB and the payload be roughly the same weight, this is clearly not feasible for moving the sun, so the sun will take much longer to accelerate.

davidstarlingm
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### Re: Interstellar Travel

zenten wrote:Could we hook up a bunch of these small black holes to the sun somehow to push the whole solarsystem? http://arxiv.org/pdf/0908.1803.pdf

The trouble I had was how they propose to stabilize the small black holes. It's not like you can simply "pause" Hawking radiation until you want to "go". Or maybe I just didn't read far enough.

Sizik
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### Re: Interstellar Travel

Reflect it back in, perhaps?
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davidstarlingm
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### Re: Interstellar Travel

Sizik wrote:Reflect it back in, perhaps?

I shudder to think of the outrageous properties of unobtanium capable of perfectly reflecting all the Hawking radiation from a small black hole.

We're dealing with pressure and energy and radiation many orders of magnitude greater than a supernova, no?

Speaking of which, what's the feasibility of building an Orion Drive, but using Type-Ia supernovae as the explosive? If we're moving stars and whatnot around, it shouldn't be that tricky to carry along a sufficient number of binary white dwarves that can be slammed together to trigger the blasts.

gmalivuk
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### Re: Interstellar Travel

davidstarlingm wrote:
Sizik wrote:Reflect it back in, perhaps?

I shudder to think of the outrageous properties of unobtanium capable of perfectly reflecting all the Hawking radiation from a small black hole.

We're dealing with pressure and energy and radiation many orders of magnitude greater than a supernova, no?
Not even close, actually.

It would be difficult to reflect energy back, though, that's true. A big enough black hole could more easily be kept stable by shooting atoms into it from time to time.
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davidstarlingm
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### Re: Interstellar Travel

gmalivuk wrote:
davidstarlingm wrote:We're dealing with pressure and energy and radiation many orders of magnitude greater than a supernova, no?
Not even close, actually.

It would be difficult to reflect energy back, though, that's true. A big enough black hole could more easily be kept stable by shooting atoms into it from time to time.

Ah, I was thinking in terms of REALLY small black holes....like, skyscraper-mass black holes.

gmalivuk
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### Re: Interstellar Travel

Do you think a supernova releases less energy than a skyscraper's mass * c^2?
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davidstarlingm
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### Re: Interstellar Travel

gmalivuk wrote:Do you think a supernova releases less energy than a skyscraper's mass * c^2?

Hah! No, certainly not. But the intensity -- power per unit area -- is, I'm guessing, a lot higher.

Intensity is typically Watts/m2, so let's see about an order-of-magnitude estimate.

Supernova: 1044 Joules over 15-20 days with an initial surface area of 7e14 m2....1.1e23 Watts per square meter.

Skyscraper black hole (using the Empire State Building's 340,000 tonnes and the power equations for Hawking radiation): 3e15 Watts with an initial surface area of 3.2e-36 m2....9.375e50 Watts per square meter.

Yeah, that's a lot higher.

gmalivuk
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### Re: Interstellar Travel

Sure, but that's only relevant if we're trying to reflect things back right at the radiating surface, which would indeed be a stupid way to go about it, but I don't think was what anyone was suggesting. If you put the reflective shell a mere 1m away from the black hole, you'd have 10^36 times as much surface area, bringing the total intensity down to a mere billionth or so of what you get even all the way out at the surface of a star going supernova.

According to the paper linked earlier, the total energy released in the last second before a black hole completely evaporates, the most energetic second of its lifetime, is about 1/230 the amount of energy released by the Sun every second.

Sure, that's a lot of energy if you're trying to contain it all within a normal-sized starship, but if instead you jettison "used" black hole drives when you get to your destination, just do so on the opposite side of the star from where you'll be setting up shop and you'll barely even notice when it evaporates.
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davidstarlingm
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### Re: Interstellar Travel

gmalivuk wrote:Sure, but that's only relevant if we're trying to reflect things back right at the radiating surface, which would indeed be a stupid way to go about it, but I don't think was what anyone was suggesting. If you put the reflective shell a mere 1m away from the black hole, you'd have 10^36 times as much surface area, bringing the total intensity down to a mere billionth or so of what you get even all the way out at the surface of a star going supernova.

According to the paper linked earlier, the total energy released in the last second before a black hole completely evaporates, the most energetic second of its lifetime, is about 1/230 the amount of energy released by the Sun every second.

Sure, that's a lot of energy if you're trying to contain it all within a normal-sized starship, but if instead you jettison "used" black hole drives when you get to your destination, just do so on the opposite side of the star from where you'll be setting up shop and you'll barely even notice when it evaporates.

Good point.

Alternately, you can save up the "used" black holes with only a few seconds of life left, and use them for either a hyper-Orion drive or as very very unpleasant tactical nukes.

zenten
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### Re: Interstellar Travel

davidstarlingm wrote:
zenten wrote:Could we hook up a bunch of these small black holes to the sun somehow to push the whole solarsystem? http://arxiv.org/pdf/0908.1803.pdf

The trouble I had was how they propose to stabilize the small black holes. It's not like you can simply "pause" Hawking radiation until you want to "go". Or maybe I just didn't read far enough.

You don't. What you do is keep on pumping matter into the black hole to keep it at a constant mass. Although the authors of the paper say that they're not sure that's possible or not, because the density needed is still an open question that would take quantum gravity to solve.

gmalivuk
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### Re: Interstellar Travel

A point that's come up a few times before on the forum when this is discussed, is that the pressure from the super intense radiation you get from small black holes would likely overpower the force of gravity trying to pull any potential "feed" matter close to the event horizon.
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bouer
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### Re: Interstellar Travel

That's why lasers are usually suggested. There is no known force that limits how close photons can get to each other, as opposed to matter.

gmalivuk
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### Re: Interstellar Travel

Yeah, that would work for maintaining a large black hole close enough to a star to have energy to waste like that. I don't see how it would be useful between stars, though, if you wanted to keep the thing stable. If you already have a way to generate that much laser power, why not just use that to run the ship?
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davidstarlingm
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### Re: Interstellar Travel

gmalivuk wrote:Yeah, that would work for maintaining a large black hole close enough to a star to have energy to waste like that. I don't see how it would be useful between stars, though, if you wanted to keep the thing stable. If you already have a way to generate that much laser power, why not just use that to run the ship?

If you have access to that much laser power, is there anything more efficient (or higher-thrust, or whatever) than just pointing the laser away from where you want to go?