Can light exert a force to move an object?

24 October 2016

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Question

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?

Answer

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.

Comments

Take two Sources of light one is more powerful than the other and the two beams of light at each other does the more powerful light push the weaker light backwards?

Instead light waves interact with one another through a process called "interference". 

When two wave meet they superpose - meaning that they merge - and at that point in space the waves add their relative displacements together.

To make this clear, let's envisage two waves, A and B, which are both the same size, wiggling up and down as they travel and they meet at a certain point.

First, lets imagine that A is going "up" at this point, and wave B is also going up.

A added to B makes a much bigger wave with a displacement equal to A + B; so if A and B were both the same height, we'd get a wave at that point that was twice the height of A and B individually. Here you'd see a brighter spot.

Now let's imagine that A is still going up but B is going down. This time the two cancel out because A is equal but opposite to B, so at the point where they meet there is no net displacement and you'd see a dark spot.

But, these observations refer only to the point where the two waves meet. After they have interacted in this way, the waves continue in their original directions, un-changed.

When you place a torch in complete dark toom in upright position,turn it on and you see dust particles move as they were already moving in random direction but after few seconds they start moving in this direction of light.

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