# Featured Can a space vessel generate its own photon wind?

1. Feb 26, 2017

### EnumaElish

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?

2. Feb 26, 2017

### Astronuc

Staff Emeritus
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?

3. Feb 26, 2017

### jbriggs444

Is the sail reflective? And what is the desired direction of travel?

4. Feb 26, 2017

### EnumaElish

Let's say the sail is 100% reflective and the direction is "any which way."

5. Feb 26, 2017

### jbriggs444

What do you think? And why?

6. Feb 26, 2017

### ZapperZ

Staff Emeritus
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.

7. Feb 26, 2017

Staff Emeritus
It does work. It just works no better than shining a flashlight out the back.

Last edited: Feb 28, 2017
8. Feb 26, 2017

### ZapperZ

Staff Emeritus
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.

9. Feb 26, 2017

### A.T.

Of course it works, but is in reality (non-ideal reflectivity) less efficient that just shining the light back.

10. Feb 26, 2017

Staff Emeritus
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.

11. Feb 26, 2017

### ZapperZ

Staff Emeritus
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.

12. Feb 26, 2017

### A.T.

No, they don't cancel.

13. Feb 26, 2017

### ZapperZ

Staff Emeritus
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.

14. Feb 26, 2017

### jbriggs444

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.

Edit: Or, at least it was, back in the days when flashlights used incandescent filaments.

15. Feb 26, 2017

### A.T.

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:

16. Feb 26, 2017

### ZapperZ

Staff Emeritus
But then, as Vanadium stated, why not just simply shoot it out the back and get the same effect?

Zz.

17. Feb 26, 2017

### EnumaElish

For the record: yes, this is as accurate a representation, as any, of what I meant.

18. Feb 26, 2017

### EnumaElish

Okay thanks. Back to the sketchbook

19. Feb 26, 2017

### EnumaElish

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.

20. Feb 26, 2017

### pixel

Some modern flashlight with LED sources also use parabolic reflectors.