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## Using Light

I don't know if this is the correct place to put this, but I have sort of a "theory".

Is there a possible way to use light to propel something at speeds of light. I use my 150mW 650nm laser and It'll push a balloon at decent speeds, and when it pops, you can keep the parts of the balloon in the air with the laser.

It goes slow because of the air and forces of gravity...but in space, none of that happens...right? If you could make a big enough laser with enough power, I think in space, you could generate a powerful enough laser and eventually the collimated light would push it along gradually increasing speed until it hit around or even the speed of light. Collimated light in space never diverges as there are no atmospheric elements to "interrupt" the collimation as on Earth.

Sorry, I'm a noob and can't really put things into scientific terms...but I've been thinking about this.

-Ingo

2. Originally Posted by Ingo
Is there a possible way to use light to propel something at speeds of light. I use my 150mW 650nm laser and It'll push a balloon at decent speeds, and when it pops, you can keep the parts of the balloon in the air with the laser.
Let me get the ball rolling for the experts here.
I don't know if there is a way using light, but I suspect in your case, you are heating up the air on one side of the balloon, and using the effects on the atmosphere to move the baloon.

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You can use it to send photons at the speed of light (that's a tautology, though), but nothing with rest mass. Things can recoil from photon absorption/emission, or reflection — photons do possess momentum. That's the basis of laser cooling and a solar sail. But you will never be able to get the target moving at c.

For reflection, you will basically get twice the photon's momentum imparted to the object (2p because it has changed direction). In the object's frame of scenario you present, as the object's speed increases, it will see the light shifted to the red, and longer wavelengths have less momentum. So subsequent photons will impart decreasing amounts of momentum.

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If I use a radiometer, it'll spin like crazy if I put the red on the black

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Most radiometers actually use local heating rather than radiation pressure. If it was radiation pressure, then it will rotate toward the black vanes. if it's heating, then it rotates toward the white. (assuming a source that hits all of the vanes)

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But a self propelled laser? No.
Photons have no mass, so although they leave the 'nozzle' at the ultimate velocity, there can be no Newtonian reaction to propel the rocket.

Light sails, yes, but not to light speed - even a powerful laser will suffer from some spread of the beam, and the light pressure will diminish with distance from the laser source.

John

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Photonic propulsion is an old concept (you could theoretically use even a searchlight or strong lasers pointing backwards):

http://en.wikipedia.org/wiki/Nuclear_photonic_rocket

Here's a guy who thinks he's built one:

http://photonics.com/content/news/20.../22/86585.aspx

http://search.yahoo.com/search?p=pho...p=mss&ei=UTF-8

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I once read a science fiction novel where they sent a craft to Barnard's Star using a powerful, solar-powered laser based on Mercury that propelled the craft by shining on its light sail. It was able to achieve a significant fraction of light speed. The novel was otherwise forgettable, as the characters were 2-dimensional. That's what happens when physicists try to write literature. But it was a "hard-core" SF novel, meaning that the author did the math to make sure it would work. So, yes, lasers can power spacecraft to substantive fractions of light speed.

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Originally Posted by NEOWatcher
Let me get the ball rolling for the experts here.
I don't know if there is a way using light, but I suspect in your case, you are heating up the air on one side of the balloon, and using the effects on the atmosphere to move the baloon.
Bingo, NEOWatcher. Just like the solar-powered black and white dimond-paddled rotating unit.

In space, a much larger power would required to actually ablate the side of a balloon and send it elsewhere.

10. Originally Posted by JohnD

But a self propelled laser? No.
Photons have no mass, so although they leave the 'nozzle' at the ultimate velocity, there can be no Newtonian reaction to propel the rocket.
Actually, photons do have momentum, so they could be used in theory to accelerate a ship without using reaction mass.
From here
http://www.atomki.hu/fizmind/specrel/rocket.html
The most efficient theoretical way to propel the rocket is to use a "photon drive". It would convert mass to photons or other massless particles which shoot out the back. Perhaps this may even be technically feasible if we ever produce an antimatter-driven "graser" (gamma ray laser).

Originally Posted by JohnD
Light sails, yes, but not to light speed - even a powerful laser will suffer from some spread of the beam, and the light pressure will diminish with distance from the laser source.

John
That's right; laser beams do spread, although the spread is less for shorter wavelengths.

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Photon thrust would be Power/c. It doesn't matter what the wavelength is, so there's no need to make a gamma laser just to get to short wavelengths. It's more important to have an efficient lasing medium, for photon thrust, or a high reflectivity, for a solar sail.

12. I don't exactly know why the site I linked to specified gamma ray lasers as a propulsion system; but perhaps I can guess.

Antimatter reactions are efficient matter-to-energy converters but they give off a lot of gamma rays, presumably in all directions; only if those gamma rays could be directed in one direction (say, in a graser) could they be used as thrust. The neutrinos emitted by antimatter/matter annihilation are even more difficult to convert into directional thrust.

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Originally Posted by eburacum45
I don't exactly know why the site I linked to specified gamma ray lasers as a propulsion system; but perhaps I can guess.

Antimatter reactions are efficient matter-to-energy converters but they give off a lot of gamma rays, presumably in all directions; only if those gamma rays could be directed in one direction (say, in a graser) could they be used as thrust. The neutrinos emitted by antimatter/matter annihilation are even more difficult to convert into directional thrust.
The gammas from annihilation give no net momentum themselves, though, if the parent particles had none.

14. The gammas from annihilation give no net momentum themselves, though, if the parent particles had none.
I don't think that is right; the parent particles are converted into gammas isotropically and if half of the photons can be reflected (no mean feat with a gamma ray) then all the momentum of the light will be concentrated in one direction. It helps if these gammas are also focused, which is also difficult. The ship moves in the other direction, thereby conserving momentum.
The photon drive is highly speculative, but it isn't a reactionless drive.

Here is a weird suggestion;
The Redshift Rocket
http://en.wikipedia.org/wiki/Redshift_Rocket
I really doubt that this one is feasible.

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Originally Posted by eburacum45
I don't think that is right; the parent particles are converted into gammas isotropically and if half of the photons can be reflected (no mean feat with a gamma ray) then all the momentum of the light will be concentrated in one direction. It helps if these gammas are also focused, which is also difficult. The ship moves in the other direction, thereby conserving momentum.
The photon drive is highly speculative, but it isn't a reactionless drive.
It's the reflection that gives you the thrust, though, and you will be limited by the reflection efficiency (which, as you recognize, will be low). The annihilation itself contributes nothing to the thrust.

16. Originally Posted by Warren Platts
So, yes, lasers can power spacecraft to substantive fractions of light speed.
Slowing might be a bit of a problem though.

The Project Rho site states that a photon drive needs 300MW to produce a 1N thrust, which in simple terms is a large power station being able to move a small feather.

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Originally Posted by Sleepy
Slowing might be a bit of a problem though.

The Project Rho site states that a photon drive needs 300MW to produce a 1N thrust, which in simple terms is a large power station being able to move a small feather.
In the novel I was thinking about, they used aerobraking, and it was also definitely a one-way trip that everyone onboard happily volunteered for.

The laser that pushed the craft was powered by a huge solar array based on Mercury. Yes, the amount of juice required was ridiculously large.

18. Originally Posted by Warren Platts
In the novel I was thinking about, they used aerobraking, and it was also definitely a one-way trip that everyone onboard happily volunteered for.
How can you aerobrake from intersteller speeds? A 1,000,000 ton space ship travelling at 0.01c would have to lose 109MT of energy (4.5x1021J) which is only a little less energy than the dinosaur killer.

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Originally Posted by Sleepy
How can you aerobrake from intersteller speeds? A 1,000,000 ton space ship travelling at 0.01c would have to lose 109MT of energy (4.5x1021J) which is only a little less energy than the dinosaur killer.
I don't know. The craft in the story was more like 25-100 tons, though.

1,000,000 tons is way too much. The USS Abraham Lincoln (an aircraft carrier) only displaces about 100,000 tons.

20. You need a ship of a certain size simply to provide a life support system capable of sustaining the crew. Now the faster you travel the less support you need. For a one way trip you also need to provide them with the means to settle on a new world.

Now a 0.01c ship is a generational ship so must be huge cos it would take 1000 years to get to a nearby star [10ly] with a planet So a million tons doesn't seem excessive. Now a 100 ton ship unless you've got the crew in some form of magical stasis is too small to sustain them for more than a few weeks, the space shuttle orbiter masses 68t empty. Even allowing for such a tiny ship the KE is only reduced to 10 GT which is still an immense amount of energy.

Another problem is the enourmous deceleration force required to aerobrake from interstellar speeds. Across a jupiter sized planet from 0.01c you need to slow down at linear rate of 45MN for 10 seconds. So your ship is dumping 1GT of energy per second.

Now if you used a small non g ship then the speed must be much higher, say a max of .25c which would give a journey time to a close star of less than a lifetime, then the KE needing to be lost is 1015M, where M is the ships mass in Kg.

A 100T ship would need to lose 1020J which is back up to a dinosaur killer event and would require even more extreme breaking forces, 700+ times greater than the 0.01c entry

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I agree, it would never work for a manned ship. But what about a small unmanned probe that only weighed 10 to 100 kg? It wouldn't have to worry about having to stop; it would just pick a trajectory that would cause it to loop around the star it was visiting and then head off to the next nearest star. I think something like that might be possible with current technology.

22. Originally Posted by Sleepy
How can you aerobrake from intersteller speeds? A 1,000,000 ton space ship travelling at 0.01c would have to lose 109MT of energy (4.5x1021J) which is only a little less energy than the dinosaur killer.
In the book they split the sail into two sections, then used the more massive sail to focus the light backwards to decelerate the other one still connected to the spacecraft. The big sail accelerated out of the system while the spacecraft slowed to a more reasonable speed.

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eburacum,
I quote, "photons or other massless particles".

As you well know, mor eand more power is required to accelerate an object with mass as it approaches the speed of light. The mas of the object incrases, with a limit that is infinite. If photons had mass, then each would have an infinite mass.

On SF; "The Mote in God's eye", Larry Niven, supposes a light sail, launched and accelerated by laser. Rather good characterisation, IMHO, especially the Moties.

John

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Originally Posted by loglo
In the book they split the sail into two sections, then used the more massive sail to focus the light backwards to decelerate the other one still connected to the spacecraft. The big sail accelerated out of the system while the spacecraft slowed to a more reasonable speed.
What book is that? I can't for the life of me remember the title or author of the Barnard's Star book I was remembering--I tried finding it on Amazon but couldn't.

I may have to try the Mote in God's Eye, John. I need to escape from this planet for a while.

Cheers,
Warren

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My friend has a very tiny Pulsed YAG laser that gives off around 4MW of light every pulse.

I can push a balloon with 150mW of 650nm collimated light, it certainly doesn't take 400MW to push a feather along

Lower wavelengths have more of a "burning power", I forgot why, but they do.

Anyways, I'm not as scientifically gifted as this thread wants me to be, so I'll probably just watch :surprised

26. I can push a balloon with 150mW of 650nm collimated light, it certainly doesn't take 400MW to push a feather along
As stated before, that is not the radiation pressure, but instead a localized heating effect. 400MW for 1N is about right for actual radiation pressure. This would mean that the largest laser currently in existence (that I can find), a 120TW pulsed laser, would produce roughly 90,000 lbf for the very tiny instant that it operated. So, it is possible. Admittedly, the entire US power usage only averages about 3TW, so this is a ridiculously huge quantity of energy. The specific impulse is incredible though - using a simple nuclear reactor to power the cycle, you could easily achieve at least 25,000 seconds, more with some extra processing and care. So, if you could boost the components all into orbit, and didn't mind an acceleration time on the order of decades, it would be quite an efficient method of getting a long range ship up to speed.

27. Originally Posted by JohnD
eburacum,
I quote, "photons or other massless particles".
I am still not entirely sure that I am with you. Photons do not have mass, but they do have have momentum and could in theory be used to propel a spacecraft using an on-board emitter (they do need to be reflected, as Swansont points out); but the amount of thrust produced would be minute.
On SF; "The Mote in God's eye", Larry Niven, supposes a light sail, launched and accelerated by laser. Rather good characterisation, IMHO, especially the Moties.

John
One of my favourite books.

But any light-sail or photon drive ship would be limited by the tiny momentum of light. A possible alternative would be a particle beam, streams of tiny particles with mass which could transfer much more momentum than a light beam to any craft.

28. Originally Posted by Warren Platts
What book is that? I can't for the life of me remember the title or author of the Barnard's Star book I was remembering--I tried finding it on Amazon but couldn't.

I may have to try the Mote in God's Eye, John. I need to escape from this planet for a while.

Cheers,
Warren
It was Robert Forward's Rocheworld.

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Originally Posted by swansont
It's the reflection that gives you the thrust, though, and you will be limited by the reflection efficiency (which, as you recognize, will be low). The annihilation itself contributes nothing to the thrust.
Let's assume there's a way to use the laser energy to power a powerful ion rocket. Now a much larger percentage of the laser energy is being used for propulsion.

Using the momentum of light is horrendously wasteful.

For example, it takes a 3,000 W light to barely move a tiny piece of extremely thin foil hung by a long, nearly weightless thread in a very tall vaccuum tube.

Pour that much energy into a linear accelerator and you can fire off pea-sized chunks at very high velocities many times a second.

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Originally Posted by mugaliens
Let's assume there's a way to use the laser energy to power a powerful ion rocket. Now a much larger percentage of the laser energy is being used for propulsion.

Using the momentum of light is horrendously wasteful.

For example, it takes a 3,000 W light to barely move a tiny piece of extremely thin foil hung by a long, nearly weightless thread in a very tall vaccuum tube.

Pour that much energy into a linear accelerator and you can fire off pea-sized chunks at very high velocities many times a second.

But eventually you will run out of the pea-sized chunks, and you have to haul all of them into space in the first place.

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