Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 25/10/2016 12:31:13
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How can photons travel at the speed of light instantaneously without producing a force in the opposite direction? Why don't I get thrown backwards when I switch on my torch?
Asked by Matt
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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.
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Why don't I get thrown backwards when I switch on my torch?
If we take a very bright light source - like the Sun.
The pressure of reflected sunlight (https://en.wikipedia.org/wiki/Radiation_pressure#Pressures_of_absorption_and_reflection) 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 (https://en.wikipedia.org/wiki/Solar_sail) 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.
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We discussed this question on our show
Kerstin Geopfrich put this question to Dr Anna Lombardi from the University of Cambridge...
Kerstin - To shed light on this question I dug out my torch and made my way to the Nanophotonics Centre in Cambridge where I met with Dr Anna Lombardi.
Anna - can light exert a force to move a physical object?
Anna - Yes, definitely. According the Newton’s second law of motion, a force is the mass of an object times its acceleration. But light is weird, it travels at a constant speed, the speed of light and it never accelerates. In addition, light is made of photons which don’t have any mass. The crucial point is that while light doesn’t accelerate, and doesn’t have mass, it does carry momentum and momentum, as a form of energy, can be transferred. By transferring their momentum, photons are able to exert a force on an object. Physicists refer to it as an optical force. The higher the frequency of the light, the larger its momentum and, therefore, a stronger force it can exert. This means that blue light will push you stronger than red light.
Kerstin - The theory tell us that light does have a little bit of a push but I certainly cannot feel it when I switch on my torch. What’s the point of all the theory then?
Anna - While the push of light is so tiny that you don’t feel it in everyday life, we can observe it at the nanoscale in the world of the infinitely small. Arthur Ashkin, a scientist working at the Bell Labs in the seventies demonstrated that nanometer and micron sized particles can be accelerated, trapped and manipulated by radiation pressure of a highly focused laser beam.
Nowadays, scientists use light quite literally like optical tweezers to manipulate objects from cells to single atoms
Kirsten - Does that mean that we just need a superpower torch to move the big stuff?
Anna - If as a light source we don’t limit ourselves to a simple torch, but we consider the Sun, then the radiation pressure exerted is strong enough to push spacecrafts and even asteroids from their path.
Kirsten - So you’d better know the math when you plan your next mission to space. But Matt, I think we’re safe to turn the torch back on. Thanks for your question and thank you Anna for the answer.
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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?
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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.