Ask Me Help Desk - Speed of light, beaten?

I made an article for our school magazine about a theoritical way of beating the speed of light (c). It is turning a light source at high speed in a vacuum. The tip of it (where the light falls) will moves faster than light. Our physics teacher says it is not going to work because new particles of light are being produced, not the same one. Then I proposed turning a 6km rod at 500,000 rpm (the fastest motor on earth is 1,000,000 rpm) with mobile supports for the rods. But he said no. I said it to be kept as an only theoritical way but he said everything can be done theoritically. He said that my article is like building a bus faster than light. He rejected my idea. Is my idea pointless? Or is it good? Is it worth sth or it deserves to be thrown away? I am a student of class 9 and could show ALL necessary calcularions. Truly speaking, I invented the idea by myself.

You can build your rotating device and try your experiment because there is nothing impossible about it. The thing though is that the result will not be as you expect because of the nature of light. You are considering a beam of light a rigid continuation of your rotating rod -- this is something one cannot assume. Calculating the actual pattern of the light spot propagation is a cool project. The limit on the speed of light gives you a nice effect no one of your schoolmate would expect.

I get it. Thanks! But How do I do the experiments? I mean I can set up the apparatus but what values of motor should I use? Should I use rods? The values in my eqn are impossible for me to get.

I guess this is the type of an experiment you can completely do on paper. Choose the angular velocity of light source and the distance to the light spot such that their product exceeds the speed of light.

Let me propose a clearer example. Imagine that you had a hollow sphere that is 50,000 Km in radius, and you built a light house apparatus at the center that rotates at 1 RPM, shining a beam of light on the inside wall of the sphere. Since the circumference of the sphere is a bit more that 300,000 Km, the spot of light travels around the shell of the sphere at a bit more than 300,000Km/s, or in other words at a bit more than the speed of light. So yes - it's possible for "something" to travel faster than light, BUT this does not violate the principle because of two factors: first the spot of light is not a physical thing, but rather a manisfestation of where some photons happen to be at some point in time, but not the same photons. Second, it's impossible to convey any information with this technique - you can't code a message and send it at faster the the speed of light.

Another similar example is to consider a very large pair of scissors and the point that represents the intesection of the two blades of the scissors. If you close the scissors at a steady rate the point of intersection between the blades accelerates as the blades rotate toward each other. If you had a large enough pair of scissors that intersection point would travel at great sped, even faster than light. But again - that point has no mass and can convey no information, so it's not a violation.

As for the idea of a rotating rod - the problem is that the rod has mass, which means the motor that drive the apparatus must provide torque to accelerate the rotation of the rod. As the tip of the rod approaches relativistic speeds its mass increases, and in fact approaches infinity as the tip approaches the speed of light. Eventually the motor reaches its limit in terms of how much power it can produce to accelerate the rod just a bit more. To actually reach the speed of light would require a motor that can produce infinite torque, which is impossible. Hence the tip of the rod can not be driven at the speed of light (or higher).

Hope this helps.

harum, yes I hv done every calculation. I wrote formulae and proved and as well. I took random high values to show that this actually works.

ebaines, I get it. But I donot know what torque is. Thus I did not get why motor needs infinite torque. Ps Tell me frankly if you liked my project.

Quote:

Originally Posted by Yusf

ebaines, I get it. But I donot know what torque is. Thus I did not get why motor needs infinite torque. Ps Tell me frankly if you liked my project.

Torque is a measure of how much turning force a motor can produce, in units of force times moment arm length. It's related to power: the horsepower of a car's engine is proportional to the torque it produces times its RPM. So another way of stating the point I was trying to make is that it would take an engine with infinite horsepower to be able to turn the rod at the speed of light. This is impossible, so no - your experiment won't work. But to put some numbers on it - if you had a rod of length 1 meter you would need to spin it at about 3x10^9 RPM to approach the range where relativistic effects occur. No, you won't find such a motor - this speed is about 1,000 times faster than the fastest motor ever devised, which can spin at about 1 million RPM.

Quote:

Originally Posted by Yusf

harum, yes I hv done every calculation. I wrote formulae and proved and as well. I took random high values to show that this actually works.

Try to think now about what would be seen by an observer witnessing your experiment by being elevated above the surface on which the tip of the light beam slides. Will he see a faster than the speed of light motion of the light spot? How should you convince him that one spot is actually moving faster than light?

At what speed of the light tip on the surface will the nearby observer see a strip of light instead of a moving spot?
Curiously, at some speeds, the spot moving towards the observer would appear to him as moving away from him.

And, of course, at no value of angular velocity of your light source, would the observer detect super-luminal speeds.