# A question about the thrust of light

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1. Aug 8, 2014

### pberrett

Hi everyone

I am a layman at physics am would be very grateful if someone could assist me with a question about propulsion.

The question arises from an article I recently saw discussing NASA's testing of a Cannae Drive

http://www.techtimes.com/articles/1...ight-make-deep-space-exploration-possible.htm

I am also aware of solar sail use as a method of propulsion

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

I can see how a solar sail would work. The momentum of the photons hitting the sail imparts some of their kinetic energy into the sail and thus pushes the spacecraft.

If this is so, couldn't one simply drive a spacecraft by attaching a large focussed light/torch to the rear of the spacecraft? In other words the light (generated from an onboard energy source) shines photons in one direction or to put it another way, provides momentum to photons going in one direction causing the spacecraft to go in the other direction. If so, would this constitute a propellantless drive for a spacecraft? I say propellantless because light is simply a form of electromagnetic radiation at a higher frequency.

I could be wrong about this so lets consider the alternative. If shining a light in a given direction doesn't impart thrust to the attached spacecraft in the opposite direction, then couldn't I simply put a solar sail on the front of my spacecraft and shine a light on the back of it to pull the sail forward?

I think the former is more likely correct but I am interested to hear why light could not be used to propel spacecraft in this way.

cheers Peter

Last edited: Aug 8, 2014
2. Aug 8, 2014

### Matterwave

You certainly can use light as a propulsion mechanism. But the thrust imparted is very low. Light has momentum p=E/c which gives me F=P/c where P is the power of the lightbulb (divide both sides of the equation by time). and so if I shined a 100 watt light bulb in one direction, then I am only generating 100W/c~3*10^-7 Newtons of force. If I upped it to a 1 megawatt lightbulb, I'd still get only .003 Newton's of force on my craft, upping it to a 1 GIGAWATT lightbulb gives me a measly 3 Newton's of force... so you can see...this may not be the best way to do it because I have to find the power to power all those lightbulbs!

3. Aug 8, 2014

### Staff: Mentor

Yes, certainly.

4. Aug 8, 2014

### pberrett

Thanks for that.

May I pose a followup question then?

To have momentum the photons must have mass, albeit very small.

I know that ion drives get drive power by accellerating ions to very high speed.

Lets say our spacecraft had long tube with a very bright light at one end and shining down the tube. Would it be possible to accellerate the photons travelling down the tube beyond the speed of light using electromagnetism? As photons could not travel faster than light speed it would be like pushing against a brick wall. Thus would the spacecraft be propelled forward?

Would this work?

5. Aug 8, 2014

### Staff: Mentor

Photons have no mass. The equation relating mass energy and momentum is
m^2 c^2 = E^2/c^2 - p^2

They are uncharged, so they cannot be accelerated using EM.

6. Aug 8, 2014

### pberrett

I just found this interesting video which touches on all of the above but approaches it from a mathematical point of view.

Interestingly it concludes that momentum = planks constant divided by wavelength.

Does this mean that, if you instead of transmitting the light using a torch as detailed above (thus giving a very small force in the opposite direction) , you instead transmit a very very low wavelength radio signal eg 1 hertz, you will generate more momentum in the opposite direction ie thrust of your spacecraft?

Last edited by a moderator: Sep 25, 2014
7. Aug 8, 2014

### Staff: Mentor

Hertz is the unit of frequency, not wavelength. Low frequency means long wavelength.

But this just gives the energy per photon. If you have a radio transmitter and a torch which each produce one watt of power (one joule of energy per second), and manage to aim all the energy output from both of them backwards (which is harder to do with a radio antenna than a torch), they should produce the same thrust. The torch produces fewer photons, each with higher energy; the radio antenna produces more photons, each with lower energy.

8. Aug 9, 2014

### pberrett

Lol. I,m an amateur radio operator so I should know that at least. I got mixed up and thought that the equation was talking about frequency , not wavelength.

Cheers Peter

9. Aug 9, 2014

### mishima

Photons are bosons. One interesting property of bosons is that there can be 2 or even more in the same location in space (with the same quantum state). So I think the "coagulation" of photons you are hinting at there would not happen.

Further, im no aeronautical engineer, but a normal rocket does not lift off the ground because its exhaust pushes on the ground. It is the combustion reaction inside that pushes a rocket up. So even if photons were somehow slamming into each other some distance from your craft I don't think the craft would care.

Photons can be accelerated in the presence of large gravitational fields like stars and galaxies, but it is a change in direction not speed (as far as I know).

Cool thoughts though.