Light powered spaceship

**Redbelly98** · Feb12-10, 09:09 PM

Originally Posted by SpectraCat

Isn't there a problem here concerning conservation of momentum? If the wavelength of the photons stays constant, then the momentum stays constant as well, so how can there be any net transfer of momentum?

Since the photon changes direction each time it is reflected, it's momentum changes.
Also, the wavelength is not constant, but gets longer each time it is reflected from the receding spacecraft.

**SpectraCat** · Feb13-10, 09:15 AM

Originally Posted by Redbelly98

Since the photon changes direction each time it is reflected, it's momentum changes.
Also, the wavelength is not constant, but gets longer each time it is reflected from the receding spacecraft.

Ok, I was a bit imprecise ... my contention was that the *magnitude* of the momentum was constant ... clearly the direction changes upon reflection.

I guess I understand what you are saying about the wavelength change too ... you are saying there *is* some momentum/energy transfer to the ship, which causes it to increase its outward velocity a little bit, and so there is a small Doppler (red) shift of the reflected photon, correct? I neglected the Doppler shift in my initial analysis ... so I think I understand how it could work now.

**lightarrow** · Feb13-10, 10:34 AM

Originally Posted by SpectraCat

Ok, I was a bit imprecise ... my contention was that the *magnitude* of the momentum was constant ... clearly the direction changes upon reflection.

I guess I understand what you are saying about the wavelength change too ... you are saying there *is* some momentum/energy transfer to the ship, which causes it to increase its outward velocity a little bit, and so there is a small Doppler (red) shift of the reflected photon, correct? I neglected the Doppler shift in my initial analysis ... so I think I understand how it could work now.

Yes, the electromagnetic radiation exchange momentum with the spaceship. If this one has a speed v and total reflecting back, then the frequency f2 of light (or in general EM radiation) after reflection becoms:

f2 = f1*(1 - v/c)/(1 + v/c)

where f1 is the frequency before reflection.

Note, however, that not only frequency, but intensity too is decreased because of a moving frame of reference (= after reflection), so the previous formula is not enough to to evaluate the energy/momentum change of the radiation. The light intensity changes with a similar formula (don't remember if the same or with a Sqrt).

~~Anti-Meson~~ · Feb14-10, 09:44 AM

Solar sails, as previously mentioned, are already a form of powering a craft by light. It uses radiation pressure to propel itself across space. For more info see:

Bonsor, Kevin. "How Solar Sails Work." 12 October 2000. HowStuffWorks.com. http://science.howstuffworks.com/solar-sail 14 February 2010.

**SpectraCat** · Feb16-10, 08:14 AM

Originally Posted by Anti-Meson

Solar sails, as previously mentioned, are already a form of powering a craft by light. It uses radiation pressure to propel itself across space. For more info see:

Bonsor, Kevin. "How Solar Sails Work." 12 October 2000. HowStuffWorks.com. http://science.howstuffworks.com/solar-sail 14 February 2010.

Thanks for the link. I had thought solar sails were based on an absorptive design .. I didn't realize they could be made using mirrors as well.

However, wouldn't one get more net acceleration if the sail *did* absorb the electrons, since then the net momentum transfer per photon would be much higher? And doesn't that open up other possibilities as well? What if the "sail" were also a photoelectric device? I guess the issue there is that current technology would require a photoelectric sail to be too thick (i.e massive) to be effective, but one could envision that changing in the future.

**ZapperZ** · Feb16-10, 08:23 AM

Originally Posted by SpectraCat

Thanks for the link. I had thought solar sails were based on an absorptive design .. I didn't realize they could be made using mirrors as well.

However, wouldn't one get more net acceleration if the sail *did* absorb the electrons, since then the net momentum transfer per photon would be much higher? And doesn't that open up other possibilities as well? What if the "sail" were also a photoelectric device? I guess the issue there is that current technology would require a photoelectric sail to be too thick (i.e massive) to be effective, but one could envision that changing in the future.

Think of what would happen as this goes on for a while. You have a ship, stuck in the space, isolated from other form of electrical connection. The "sail" will start having charging effects from all that photoelectrons that have left it! It has no "ground" connection to replenish, and as a result, the effective work function will start increasing, making it more difficult to produce photoelectrons as time goes on. And if you want large propulsion, and produce more photoelectrons per second, the charging effect will build up even faster!

There is a reason why this isn't a good idea, and why it isn't done.

Zz.

**Redbelly98** · Feb16-10, 08:36 AM

Originally Posted by SpectraCat

Thanks for the link. I had thought solar sails were based on an absorptive design .. I didn't realize they could be made using mirrors as well.

However, wouldn't one get more net acceleration if the sail *did* absorb the electrons, since then the net momentum transfer per photon would be much higher?

Reflection transfers twice the momentum that absorption does, since the change in momentum of the impinging particle is

p_final-p_initial = (-m*v) - (m*v) = -2mv

whereas for absorption it is

p_final-p_initial = (0) - (m*v) = -mv

A similar argument works for photons, except that the momentum would be ±h/λ, instead of ±m*v

... And doesn't that open up other possibilities as well? What if the "sail" were also a photoelectric device? I guess the issue there is that current technology would require a photoelectric sail to be too thick (i.e massive) to be effective, but one could envision that changing in the future.

The electrical supply would be a separate issue than the propulsion system. I'm not familiar enough with the technology to comment on what the best means of supplying electricity would be.