Science Questions

What would happen if you switched on a laser pointer in space?

Tue, 22nd Mar 2016

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Allan Ruell asked:

If a laser pointer was floating in space and not moving in any direction, would the laser pointer move if the light was switched on? Maybe even a nano meter or movement? Has this been tested?


We ask Dave Ansel for the answer to Allan's question... Argon-ion and He-Ne laser beams

Dave - You certainly can. In fact, itís absolutely tiny, but you will get pushed backwards very, very, slowly.  In fact, theyíre planning to use the same principle to sail around the solar system.  So, if you build yourself a very, very, very large mirror and then the sunlight shines on it, then the light is coming towards it in one direction, itís reflected back again and that means itís changed itís direction and your mirror will get pushed away.  The force is a tiny micro-newtons (I mean mili-newtons), but because you donít need to carry fuel to do that you can keep your rocket engine going continuously as you fly round the solar system.  It means you can actually travel round the solar system, especially if youíre going a long way, far quicker than you couldÖ

Chris - Because over time, it will build up, and build up, and build up with no losses because thereís almost nothing to hit you and slow you down again.

Dave - Itís been suggested itís one of the ways you could get to another solar system would be to have one of these solar sails and a very, very, very large laser.  Point at the solar sail and then sail on it all the way out to another star.



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Light has momentum, and momentum is conserved, so the pointer would move backwards. You can indeed measure "radiation pressure" when a light beam strikes an object. alancalverd, Tue, 8th Dec 2015

There is a fault in the question was floating in space and not moving in any direction from release there is nothing to stop it moving it will already be in motion the test would have to be a specific vector with the light facing the direction the laser pointer  was travelling and then maybe we could see if turning the light on had any effect on velocity.
Thebox, Tue, 8th Dec 2015

A Solar Sail is a promising way of using light pressure to move or position spacecraft in the inner solar system.

However, with solar radiation reaching 1kW/m2 at the distance of Earth's orbit, and an area in excess of 100m/100yards on a side, the solar sail will reflect over 10,000,000 Watts of radiation, far more than the paltry 0.001 Watts put out by an "eye-safe" laser pointer.

Solar Sails actually get up to twice the thrust you may expect, as reflection causes a change in the sign (direction) of the momentum, which transfers more momentum than you can get from just emitting or absorbing photons.

Another advantage of a Solar Sail is that you don't need to replace the batteries!

evan_au, Tue, 8th Dec 2015

let's put some numbers.
beam's power: P = 3 mW
pointer's mass: m = 10 g
batteries time of continuously work: t = 1000 s
(these values are invented, I believe they are reasonable).

The force on the pointer (assuming its line of action passes through the pointer's centre of mass so it can only translate) due to the beam's momentum is:
F = P/c
so the acceleration is:
a = P/(m*c) =~ 10-9 m/s2
The final speed acquired is then: v = a*t = 10-6 m/s.

Let's compute the time it would need to cover 1 astronomical unit, that is ~ 1.5*1011 m:
1.5*1011/(10-6*365*86400) =~ 4.8*109 years.

lightarrow, Wed, 9th Dec 2015

I think the questioner, like everyone else, was assuming an inertial reference frame. Colin2B, Wed, 9th Dec 2015

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