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In a nutshell, does Newton's "action = -reaction" law apply to massless particles? If a spaceship directs a condensed light beam on its own heat-resistant photon sail, what would happen?
Can a space vessel generate its own photon wind?
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One only need to determine the momentum (p = E/c = hν/c) from the source and on the sail, and consider the change in momentum of the system. Is there a net momentum or force on the spacecraft in the desired direction of travel?
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Is the sail reflective? And what is the desired direction of travel?EnumaElish said:
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jbriggs444 said:
Let's say the sail is 100% reflective and the direction is "any which way."
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What do you think? And why?EnumaElish said:
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EnumaElish said:
I really do not understand this. Is this what you are describing? The spaceship shoots a beam of light (yellow arrow) onto a sail (curved line in front of the ship) that is attached to the ship itself?
If it is, do you not see why this doesn't work?
Zz.
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ZapperZ said:
It does work. It just works no better than shining a flashlight out the back.
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Vanadium 50 said:
But this one is different than what you described. This is the same as trying to lift yourself by pulling up on the platform that you're standing on.
Zz.
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ZapperZ said:
If it is, do you not see why this doesn't work?
Of course it works, but is in reality (non-ideal reflectivity) less efficient that just shining the light back.
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I don't see it.
Make everything horizontal to make it simple. Shine the flashlight back, and you gain momentum E/c. Shine it forward, and you get a recoil of -E/c, and then when it strikes the mirror and reverses direction, a recoil of 2E/c. Add them up and you get E/c, The mirror doesn't help (it can only hurt), but essentially this is a complicated way to shine a flashlight out the back.
Make everything horizontal to make it simple. Shine the flashlight back, and you gain momentum E/c. Shine it forward, and you get a recoil of -E/c, and then when it strikes the mirror and reverses direction, a recoil of 2E/c. Add them up and you get E/c, The mirror doesn't help (it can only hurt), but essentially this is a complicated way to shine a flashlight out the back.
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I should have put the sail and the light going backwards, but the concept is still the same. You get recoil when the light leaves the source, but then you get the opposite impulse when it bounces off the sail. If you do this colinearly, and assume ideal condition, they all cancel out.
BTW, we still don't know yet if this is what the OP is thinking of! We might already be debating something irrelevant here.
Zz.
BTW, we still don't know yet if this is what the OP is thinking of! We might already be debating something irrelevant here.
Zz.
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No, they don't cancel.ZapperZ said:
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A.T. said:
Sorry, you are correct since I stated the light bounce. What I said is true if it is absorbed. I think I understand what Vanadium is saying now.
Zz.
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The arrangement as depicted is a flashlight. A parabolic reflector is how you arrange for the beam to be emitted in a particular direction in the first place.Vanadium 50 said:
Edit: Or, at least it was, back in the days when flashlights used incandescent filaments.
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Right, but with reflection as your picture shows you get net thrust. It works with air too, if you "divert" some of the air backwards:ZapperZ said:
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But then, as Vanadium stated, why not just simply shoot it out the back and get the same effect?
Zz.
Zz.
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ZapperZ said:
For the record: yes, this is as accurate a representation, as any, of what I meant.
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ZapperZ said:
Okay thanks. Back to the sketchbook [emoji13]
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I guess this does not imply Newton's 3rd law does not apply to photons? The space traveler could eject any object to propel the ship in the opposite direction. And the same applies to light.
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jbriggs444 said:
Some modern flashlight with LED sources also use parabolic reflectors.
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EnumaElish said:
I don't understand (another one) the impetus or origin of this question. Why won't photons follow Newton's 3rd Law, which is a manifestation of the conservation of momentum? Haven't this been shown very clearly already via the Compton effect?
Zz.
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It's okay guys.
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If the initial source of light had a wide beam then using the reflector could improve the efficiency by directing the momentum of the beam more accurately in one direction. (Any off axis components would tend to cancel, losing energy but not gaining momentum for the ship).ZapperZ said:
That balloon / sail system would probably perform much worse than the balloon on its own because the sail has a poorly designed profile with lots of sideways spread of the ejecta. A suitably designed 'chute' at the front could work well though, I think. But aerodynamics is not an unintuitive subject. (As Colin Chapman, designer of sports cars, has remarked.)
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I think everyone is getting off track. Momentum is conserved. Newton's third law still holds for massless particles and in relativity, as long as the proper form for momentum is used.
DaveC426913
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That's adorable!EnumaElish said:
You thought that a question - posted on a physics forum with a quarter million nerds - would stop being discussed once the question was answered?
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Sow the wind and reap the whirlwind!DaveC426913 said:That's adorable!
You thought that a question - posted on a physics forum with a quarter million nerds - would stop being discussed once the question was answered?
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I think it has been established that it will work, but not any better than shining a light out the back.
In the same way, throwing rubber balls at a (hard) sail will work as long as they bounce rearward so the net momentum of the balls is rearward.
Consequently, the way momentum conservation works (in this application) is the same for light as for matter.
The fan and sail do not usually work, because the sail redirects the wind from the fan to the sides rather than to the rear.
In the same way, throwing rubber balls at a (hard) sail will work as long as they bounce rearward so the net momentum of the balls is rearward.
Consequently, the way momentum conservation works (in this application) is the same for light as for matter.
The fan and sail do not usually work, because the sail redirects the wind from the fan to the sides rather than to the rear.