Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 23/03/2016 16:37:02

Title: What would happen if you switched on a laser pointer in spac
Post by: thedoc on 23/03/2016 16:37:02

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?
Asked by Allan Ruell


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Title: Re: What would happen if you switched on a laser pointer in space?
Post by: alancalverd on 08/12/2015 14:40:09

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.

Title: Re: What would happen if you switched on a laser pointer in space?
Post by: guest39538 on 08/12/2015 14:48:28

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.

Title: Re: What would happen if you switched on a laser pointer in space?
Post by: evan_au on 08/12/2015 21:20:29

A Solar Sail (http://en.wikipedia.org/wiki/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!

Title: Re: What would happen if you switched on a laser pointer in space?
Post by: lightarrow on 09/12/2015 07:48:48

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

Title: Re: What would happen if you switched on a laser pointer in space?
Post by: Colin2B on 09/12/2015 14:01:21

Quote from: Thebox on 08/12/2015 14:48:28

There is a fault in the question was floating in space and not moving in any direction

I think the questioner, like everyone else, was assuming an inertial reference frame.