Question of the Week Podcast

Question of the Week episode

Sun, 23rd Oct 2016

Can light exert a force to move an object?

A torch (c) Dave Ansell

Why don't you get thrown backwards when you switch on your torch? Kerstin Göpfrich made her way to the Nanophotonics Centre in Cambridge to find out from Dr. Anna Lombardi. The answer to Matt's question may blow you away...

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Because they dont have any rest mass they dont have any inertia.
There is interestingly a reaction due to their momentum, but in some materials it is absorbed by the material's crystal lattice and it is extremely small and you wouldnt feel it. Colin2B, Tue, 11th Oct 2016


If we take a very bright light source - like the Sun.

The pressure of reflected sunlight at the distance of the Earth is 9 uPa (microPascals).
- Atmospheric pressure at sea level is 100 kPa (kiloPascals).
- So air pressure is about 11 billion times higher than sunlight pressure.

The intensity of sunlight is about 1kW/m2.
- The intensity of a handheld torch may be 1W/mm2.
- If you reduce this sunlight pressure to the light-emitting area of a hand-held torch, the force is 9 x 10-3 Newtons, or 1 milligram force on your hand.

Note that the pressure of reflected light is twice the pressure of emitted light, so the pressure from a torch is even lower.

No wonder you can't feel any pressure from  your torch!

However, some people have seriously discussed the use of Solar sails to accelerate space probes (and have even tried it on a test satellite in orbit). In a weightless environment, even a small pressure can build up a significant speed, if you are in no hurry. evan_au, Tue, 11th Oct 2016

Ah, you do, there is a 'recoil' when measuring interactions with 'invariant mass'. But if you mean what the 'original recoil' of a 'original photon' not 'interacting' was? I don't know.
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the real point of this question is whether you think of 'photons' as 'propagating', or as 'properties' of a 'field' to me. Another very interesting point about it is that the explanation doesn't involve 'action and reaction' as much nowadays as it involves a slightly different property/principle 'conservation of momentum'. What conservation laws does with this universe depends on from where you look at it. Globally defined, over a universe it might make one think of it as a 'proof' of a 'container of sorts'. Locally defined it could be about different frames of reference interacting though, although telling me nothing about what makes them being able to interact in this fashion, aka, build a 'universe'.
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Not entirely true. Accepting conservation laws they must hold any which way, locally as well as globally, and they are 'properties' to me, of 'SpaceTime'. Not 'touch ables' at all, just as 'spin' can be seen as in QM, its becoming a interesting universe, isn't it?

yor_on, Fri, 4th Nov 2016

I would say that bosons like light travel on a single direction and posses little momentum. Fermions can be compared to a planetary system and travel in a sort of complex spiral motion also at much higher frequency thus containing more momentum. This explains for example clocks ticking rates reduced with speed.
This is only a speculation - personal opinion. Nilak, Wed, 16th Nov 2016

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