special relativity

Relativistic Beaming

Speed of light is a constant, but the path is not.

the galaxy M87

introduction

A light source which produces an isotropic distribution of light, even in all direction, when stationary will not produce an isotropic distribution when moving. The moving light source will appear to concentrate light in the direction of motion. The faster the source moves the more the light gets beamed in the direction of travel.

The effect can be seen indirectly in the jets produced by Black Holes in the centres of galaxies, as in the image of galaxy M87

animation

The animation shows a 2 dimensional distribution of light about a light source. As the light source moves faster the distribution of light is seen to concentrate towards the direction of travel. This is called relativistic beaming.

While studying the animation be mindful of the following statements.

  • The grid is used to help define the path of the photons. Note each photon passes specific locations in the grid. Everyone must measure the photons passing the same locations in the grid irrespective of how the spacecraft is moving.
  • The spacecraft and grid undergo length contraction .
  • The velocity of the spacecraft has no effect on the speed of the photons.
  • All the photons travel a c. Hence when radiating from a point they all travel the same distance and so the distribution of photons always lies on a circle.
  • An observer in front of the spacecraft will observer the light getting brighter with increasing velocity, because the photons concentrate in the direction of motion.
  • An observer behind the spacecraft will observer the light getting fainter with increasing velocity, because the photons concentrate in the direction of motion. This may explain why only on beam is seen in the M87 jet.

animation controls

The velocity can be controlled by clicking the radio buttons.

description

Relativistic beaming is an aberration effect.

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