Physics Question #699

Kjell, a 13 year old male from Edmonton asks on March 9, 2002,

Does light exert force?

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The answer

Donald J. Barry answered on March 17, 2002

Almost all physicists believe the answer to this question is yes, and in fact laboratory measurements dating from the 1960's forward have measured the force exerted by light. They are remarkably tricky because the pressure is so vanishingly small: typically a mirror is suspended by a very fine quartz thread in a vacuum, and light is alternately applied to one side of the mirror at the frequency with which this "spin-pendulum" arrangement likes to swing. An oscillation of the mirror rotation can be built up, and the rate at which this swing builds up lets one determine the force in the beam of light.

The theory behind this was developed by Einstein in 1906, and the laboratory confirmation generally satisfies the vast majority of scientists. However, there is one noted dissident, Cornell theorist Thomas Gold, who steadfastly criticizes the laboratory experiments and maintains that it is possible that light behaves in another manner which would not exert a force. His argument is subtle, but it is rejected, I must remind you, by essentially all working physicists.

Perhaps someone will redo this experiment with modern techniques, producing an answer of sufficient precision to really confirm the theory, and dismiss Gold's claims. I'm not aware of anyone currently at work doing this.

[Editor: A quick search also yielded this more tangible way of getting a feeling for the force of light:

"Sunlight exerts a very gentle force. The power of sunlight in space at Earth's distance from the sun is between 1.3-1.4 kilowatts per square meter. When you divide 1.4 kilowatts by the speed of light, about 300 million meters per second, the result is very small. A square mirror 1 kilometer on a side would only feel about 9 Newtons or 2 pounds of force."

This was from a website about solar sails, a way to power spacecraft with nothing but the light from stars.]

Ian Keith Affleck answered on March 28, 2002

When photons reflect off a mirror they exert a force on it like baseballs exert a force when they bounce off a wall. The force exerted by light is normally very small though. If all the light from a 100 watt flashlight was focussed on a mirror and reflecting straight back, then the force exerted on the mirror is only 200 watts divided by the speed of light which is about 1 millionth of a Newton. For comparison, if the mirror weighs 1 kilogram then the downward force exerted on it by gravity is about 10 Newtons, 10 million times larger.

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