# Light Drive.

Discussion in 'Physics & Math' started by Jolonar, Jul 5, 2004.

1. ### JolonarBeing of intellect.Registered Senior Member

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I am curious about a drive that uses light as its means of propultion.

Is it possible to create? Or is there some law of physics that prevents it from being achieved?

I have just finished reading through my physics books and have been thinking about this.

My knowledge of physics is limited, but still expanding. I wish to know more about a light drive. About the possibilities of getting one to work.

It seems simple enough, but then the calculations are harder to get through.

I may be well out of my league, but I thought that it would be worth asking.

Regards,
J.

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3. ### CrispGone 4everRegistered Senior Member

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Yes, this is possible, because of conservation of momentum; every time you emit a photon to the back of your spacecraft, you eject p = E/c of momentum, where E = h&nu; is the energy of the photon (&nu; is the frequency of the light and h is Planck's constant). Because of conservation of momentum, the spacecraft gains an equal amount of momentum in the opposite direction. The problem here is that this is a miniscule amount for each photon; you need to have a very, very, very strong light source in order to accelerate any realisticly spacecraft mass.

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5. ### HallsofIvyRegistered Senior Member

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Using a "light sail" is one way of using photons as propulsion. Unfortunately there appears to be no way to "tack" into the solar wind.

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7. ### CrispGone 4everRegistered Senior Member

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The solar wind consists mainly of ejected charged matter (protons); spacecraft using solar wind use the conventional sailing principle of being pushed by a medium colliding with the sail.

8. ### JolonarBeing of intellect.Registered Senior Member

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Hiya Crisp,

My brother has also done physics, when I asked him about this problem, he said that it was impossible to do, but I told him it wan't. I'm glad that I can now show him that my thories were right.

Although, I think that my calculations may need a bit of work.

I have created a design that uses a concave mirror and light absorbant material. The light absorbant material should absorb that momentum from the the reflected light and create a forward force. The mirror will create a "reverse thrust" but the projected light will create an equal forward thrust.

I had been thinking about a laser light, that had plenty of charge. However, the beam may not have enough density, so I have tried to find another source of light that has a much higher density. Once I find out the density that the required light would need to be for the photon count, it should be poxsible.

I like the light sail, but it is too hard to use. Solar wind is one thing that I had forgotten to take into account. that may cause a shift in the way the device needs to be shaped.
Thanks!

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How big would the sail need to be to collect enough matter to create a forward accelleratino great enough to be noticed quickly?

I gather it depends on the density of the particle stream, but how can the matter be caught in such a way that it is not reflected or absorbed, but allowed to pass through to allow no distortion in the quantum flux of the energy pattern? *(Does this question make sense?)*

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Thanks very much.

Regards,
J.

9. ### NasorValued Senior Member

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If you do the calculations I think you'll find that it's impossible to create any appreciable thrust with anything like our current technology. The amount of light that would have to be used to propel any substantially sized object is almost inconceivable.

10. ### JolonarBeing of intellect.Registered Senior Member

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Hello Nasor,

You are correct, there is no appreciable thrust with primitive technology like what we have access to. It is not an inconcieveable amount. It's the density of the particles that matters the most. A spectrum that is not visible

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to the naked eye has different densities.

If one has a very high density and is accessable by current technology, then it is possible.

What size of an object would you be talking about? A "Galaxy sized Star Ship" from Star Trek or a Shuttle that we send up into space? Maybe the size of a house .?.

I am not talking about using it within the atmosphere, only in a void. Otherwise it would not work the correct way, and as you say, would produce a miniscule amout of Newtons to any object.

Regards,
J.

11. ### hyperdogRegistered Senior Member

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Keep in mind that any design in which the ship absorbs its own emitted light will result in a net momentum change of zero.

12. ### NasorValued Senior Member

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It's the energy of the photons that matter. A single x-ray photon would have far more 'pushing power' than several infra-red photons, for example. Light doesn't really have a density, since it has no mass.
Well, let's put it this way; if you continuously outputted light at a rate of a megawatt (which you would need a healthy-sized nuclear reactor to power - like the big ones that power cities, not the small ones that power navy ships and space stations) it would take 5 hours to accelerate an object massing only 1 kg to 1 meter/second.

13. ### JolonarBeing of intellect.Registered Senior Member

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Hello Hyper,

Not nessicarily. It all depends on when it is absorbed, and how.

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Regards,
J.

Hiya Nasor,

Ahh, good point. Light does have a mass. But it is extremely minute. If it didn't have a mass, then it couldn't move anything or have any force acting upon it, and thus, a black hole would not affect it. Thus, light does have mass.

A simple power convertion could do that for you, so a small nuclear reactor could do the trick. With one unit, yes, but I would be using about 20 units to accelerate a ship. It would weigh about 2 tonnes, It should only take a few centuries to move it.

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My calculations appear to need a little bit of work, so I shall do that and return with my findings in a day or so.

Thanks Nasor!

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Regards,
J.

14. ### Boris2Valued Senior Member

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jolonar, light has no mass. it has momentum.

15. ### NasorValued Senior Member

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Nope, light has no mass - only energy and momentum. It isn't necessary for something to have mass to be affected by gravity.
I don't understand what you mean by this.

16. ### hyperdogRegistered Senior Member

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A ship that absorbs its own emitted light is a closed system. If it starts with zero momentum, it will always have zero momentum.

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How about what a good fusion reactor can create in the way of a few hundred gigawatts of beamed energy, somewhat of photon dump-trucks loaded with as much mass as possible. In which case we may need to know about the following;

What's the realistic Terminal Velocity of Space Travel?

I'm speaking of actual physical stuff, such as for a craft that's displacing 1000 m3, whereas such, how fast is fast enough?

Obviously at 3e8 m/s * 1e3 = 3e11 m3/s being displaced, and obviously that's either expecting way too much, or of simply requiring way more energy than any 1000 m3 craft could possibly muster.

It takes energy as to displace your way through that many m3/s, through a substance that's perhaps containing as many as 1e6 atoms/m3.

So, of what's possible without our having to utilize anything in the twilight zone?

Can the likes of a sufficient laser/plasma cannon give us 50% Light Speed?

If so, then what's the amount of energy/m3 as for driving and sustaining 50% LS?

Since many of you keep insisting that you know all there is to know about such things, perhaps you'd care as to share in the notions of our capability of transporting ourselves through said expanse of space, or otherwise as to utilizing that of a laser beam of creating those spinning atomic Oort zones reaching all the way out to a given point of nullification, such as the 0.7 light year distant one that's situated between us and Sirius, and from that established photon conduit or waveguide we might then utilize Dr. Wang's 310X packets as to communicate with the likes of whomever is still associated with Sirius.

Or isn't interstellar communications permitted by the space-time-continuum of this forum?

18. ### Brandon9000Registered Senior Member

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Actually, using a light beam for rocket propulsion would be very efficient, since specific impulse increases as the reaction "mass" is more and more finely divided. In fact, I think that photonic propulsion is probably the only means of rocket propulsion which would enable one to reach even the nearest stars in a reasonable amount of time.

One technical challenge would be that the light beam would have to be extraordinarily intense, and any equipment through which it passed on its way out of the ship would be vaporized instantly if it were not almost perfectly reflective.

19. ### swansontRegistered Senior Member

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Naval vessels have reactors that are many tens of megawatts (thermal). Land-based plants are several hundred MWth, up to a thousand MWth or so.

20. ### NasorValued Senior Member

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A couple of point here.

The terminal velocity of your ship will depend mainly on its cross-section (the area of the ship that faces the direction of motion). The volume doesn't really matter. Imagine the difference in wind resistance between a frisbee that's traveling through the air normally with its edge facing the direction of motion vs. a frisbee that's traveling through the air with its wide, flat surface facing along the direction of motion – both frisbee disks have the same volume, but one will have far more drag.

The density of interstellar space is only about 1 proton/cubic meter. Obviously that would change once you got in closer to a solar system, but for the vast majority of an interstellar ship's journey 'space drag' wouldn't be a problem.

Assuming your 1000 m^3 craft was a cube that was 10 meters on each side, then traveling at the speed of light through interstellar space it would only have to displace 100 m^2 * 3*10^8 m/s = 3*10^10 cubic meters of space/second. At a density of 1 proton/ m^3, that would mean the ship would displace 3*10^10 m^3 / 6.022*10^23 g/m^3, which would only be about 5 * 10^-11 kg/second.

Even if you wanted your ship to actually collect all that mass (rather than simply knocking it out of the way) it would only require about 2.2 megawatts of power, which would presumably be easy for such an advanced ship.

So space drage really shouldn't be a problem.

Last edited: Jul 8, 2004
21. ### NasorValued Senior Member

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Thanks for the correction, swansont.

22. ### JolonarBeing of intellect.Registered Senior Member

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Light does have mass. I can assure you of that. Although, noone has been able to register the mass because it is so minute....

Last edited: Jul 8, 2004
23. ### JolonarBeing of intellect.Registered Senior Member

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The mass of light is too small to be registered on any current earthly device.
I mean, "power convertion" Get the powersource, and increase the voltage to a couple of thousand megawatts.