Science Questions

Can laser beams propel vessels in space?

Tue, 18th Mar 2014

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siyabulela asked:

Can laser beams propel vessels in space?


Tamela - There is a pressure from light, isnít there?Argon-ion and He-Ne laser beams

Richard - Yeah, there is pressure from light. So yes, in theory, it should be able to. So, weíve got the development of solar sails for instance.

Tamela - Exactly. That's what I was thinking of as well.

Richard - Yeah. You can certainly use the propulsion from the stream of charged particles that are coming from the sun to move along. So, if you imagine the sun, itís belting out these particles all the time. So, if you can use those, you can actually use that to push you along.

Tamela - But itís radiation as well. Itís actually photons providing the pressure. When you look at pictures of stellar nurseries, we have lots of really bright young stars. You see these clouds that have been hollowed out because of that radiation pressure just pushing the matter away. So, definitely, photons do provide a pressure. in terms of lasers, that's a different..

Chris -   Can I show off?

Richard - You know do you?

Chris - I know something that enables me to show off very slightly, which is there's this lovely thing called the YORP effect which is the YarkovskyĖO'KeefeĖRadzievskiiĖPaddack effect, which is probably why itís called YORP. This actually is exactly the phenomenon you're describing. We know for instance where the impactor that wiped other dinosaurs came from. It came from the asteroid belt. It was dislodged in the vicinity of Mars probably about 70 million years ago. The thing that probably did the dislodging was light, because these asteroids, if you've got light impacting on one side of them, the photons of light are going to impart momentum to them and give them a nudge. This can cause the things to move a little bit. This can make them then bash into other things and that's exactly what happened. We think it nudged the asteroid a little bit and caused it to break up and ultimately impact on the Earth. But this effect is very real and with lots of light falling on a big surface for a long time, it does impart a nudge. That nudge can ultimately move very big things along a course.

Richard: - I can answer a different question on lasers in space. It doesnít address the propulsion system. They are using lasers in space right now as a new satellite, alpha sat, which was launched around about this time last year. That uses lasers for communication because you can do then speed of light communication. Itís in geostationary orbit so itís way very high above the Earth, sort of sitting there above the Earth. It can communicate. Itís using laser to communicate with satellites which were in low earth orbit, which were only a few hundred kilometres above the Earth.

Chris: - Probably, one of the most famous laser beams is the one which is bouncing off of a mirror on the surface of the moon because lots of people say, ďHow do we know how far it is to the moon? How do we know that people went to the moon?Ē Well, they put a mirror there and there's a laser beam being bounced every day between the Earth and the moon, and that's how we know that the moon is getting about 2 cm further from the Earth every year. Why is it doing that? Because, as it goes around the Earth, itís attracting water on the Earthís surface towards it in a tidal bulge. But, because the Earth is turning, that bulge is slightly ahead of the moon. So, itís exerting a pull on the moon and this means the moon rather like a slight shot, is speeding up a bit. The Earth is losing a bit of energy. The moon is gaining a bit of energy. And if itís going faster, and the gravity between the two isnít changing, so the moon therefore, is going to move away very slightly. So, year on year, itís slowly escaping. So in fact, tides are getting slightly smaller on Earth. So, if we were to wait a billion years or so, we might have very, very small tides which would probably come as a relief to people who live along the east coast the next time there's a storm surge.

Richard: - And isnít it amazing that there's an Apollo era, more than 40-year-old, experiment still working. Weíre still using Apollo science on the moon.


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Yes. This is because there is stress in the EM field. Another way to look at it is that them field can carry momentum. In fact photons have momentum. If they didn't then there'd be no Compton Scattering where a photon scatters of an electron. Pmb, Thu, 20th Mar 2014

One day, lasers may be propelling craft into space.

The momentum of photons is too low to lift a sizeable object against Earth's gravity, but the photon has far more energy than is absorbed by a solar sail. If I understand it correctly:

Modern Physicists work with momentum:

A photon striking a black solar sail is absorbed, imparting its momentum to the sail.

A photon striking a reflective solar sail is reflected, changing the sign of the momentum; imparting twice the photon's momentum to the sail (momentum is a vector, which is conserved in different frames of reference).

And so it is better to make a solar sail from a thin reflective film than an absorbent surface

Old time school physics looked at Energy (which differs depending on your frame of reference)

the kinetic energy transferred to the spacecraft is contributed by the slight redshift of photons reflected from the solar sail.
You can also decelerate an approaching solar sail, which will slightly blueshift the reflected photons.

The idea is to build a space elevator, but you don't want the elevator to be carrying its own fuel, so you could use ground-based lasers to power the elevator.

(I would expect that you could also power it by electricity from the ground - only a conductive space elevator is likely to be destroyed by lightning...) evan_au, Thu, 20th Mar 2014

My understanding of the laser powered elevator, was that the cab has a photovoltaic bottom, which absorbs the photons, and converts the energy to electrical power which can be used for locomotion (Basically a directed, wireless energy transmission). The available energy from the red shift would not be nearly enough to lift anything of the surface of the Earth. chiralSPO, Thu, 20th Mar 2014

Sorry if this is a gormless question but if a photon loses energy by pushing something - its wavelength increases and its frequency falls - what happens when it does this several times and the frequency falls virtually to nothing and wavelength approaches infinity? Is it absorbed long before then?

CD13, Thu, 20th Mar 2014

In theory, a solar sail in space could reach a significant fraction of the speed of light - if you have a really big sail, and enough really big lasers. A solar sail traveling at (say) 1% of the speed of light would cause the reflected photons to be red-shifted by 1%, representing a 1% reduction in photon energy.

It would take many such reflections to reduce the photon energy to such an extent that it provided no more thrust than the cosmic microwave background - or the light from nearby stars.

Until we can build some really big lasers in space (and resist turning them on ourselves), solar sails will be restricted to reflecting sunlight, and so they are really only useful for travel within the solar system, at interplanetary speeds (say 30km/s or so). This is a tiny fraction of the speed of light (around 0.01%), so each reflection will absorb a tiny fraction of the photon energy.

If you have ever stood between two parallel mirrors, you can see multiple reflections of yourself - but it fades out after 10 or so reflections. This is because no mirror is perfectly reflective (especially if mass is important, such as on a spacecraft).

The biggest solar sail humans have deployed into Earth orbit to date was about 20 meters across. So it's highly unlikely that a photon reflected from any solar sail has ever impacted a second solar sail. I doubt that this will become a problem any time soon. evan_au, Fri, 21st Mar 2014

I suppose it depends on what it's pushing. Suppose the photon keeps scattering off electrons by Compton Scattering. In that case the frequency decreases every time the photon scatters but it will never equal zero. This is different than with matter made of atoms since it that case atoms can absorb the photons  and in some cases not readmit them. Not true with a photon scattering off an electron. Pmb, Fri, 21st Mar 2014

The effect is pretty small if I recall correctly 300 MW of radiation at visible light frequency produces a thrust of one newton so you need a high powered laser with a narrow beam and a large sail to catch it all. syhprum, Fri, 21st Mar 2014

So the faster a solar sail is moving avay from a light source, the more efficiently it is going to accelerate from the photons energy? (Keeping in mind that you need more energy to gain a value of speed the faster you are moving)

If we would just absorbe the photon, instead of reflecting, then the energy would heat up the sail and not push it as much. McKay, Mon, 31st Mar 2014

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