Relativity and Light

The hardest thing to explain when it comes to relativity is the fact that light travels at a fixed speed in all directions no matter how fast we travel relative to the source.

If we are on board a space ship and we turn on the light inside the spaceship, it flows equally fast in all directions. If we let the light shine out in front of us so that an observer farther up can see it, the light does not travel at light speed + spaceship speed. It travels at light speed only. To compensate for the extra speed of the spaceship, the light is shifted to towards the blue end of the spectrum.

Conversely, light going out to the rear of the spaceship is shifted towards the red end of the spectrum.

In my blog post on a mirror moving through space I argued that the mirror would actually pick up energy from light coming from behind, and yield energy to light coming from the front. Reflected light wold be blue shifted to the front and red shifted to the back.

If we accelerate in speed towards a fixed light source, we will find the light source increasingly blue shifted. The photons coming at us blue shift as they travel through space. How can that be?

If we are growing in size as we speed up relative to the oncoming photons, shouldn't the result be a red shift? After all, the photons are smaller relative to us.

The answer may lie in the total energy of the system. The photons may be relatively smaller than us as we grow in response to an increase in energy. However, on impact with our eyes and other sensors, our instruments' energy have to be added to the energy of the photons. The photons are therefore measured and perceived to be blue-shifted.

Looking at light coming from behind, the effect is doubled. Not only are we growing larger relative to the photons, we subtract our energy from them as we register them with our eyes and instruments.

What then about light that is emitted from our spacecraft. How does it speed up on leaving the spaceship to the back, and slow down as it leaves the spaceship to the front? The observers at either end do not measure the light speed to be different. Nor does the astronaut. How can this be true all at the same time?

The answer must lie in the way the speed of light is measured. The outside observers measure the speed of light from the moment of emission until the moment of arrival. So does the astronaut.

For the astronaut, the speed of the light emitted inside the spaceship is the same whichever direction he measures it. For the observer at the back, the speed of light is measured from the source at the moment of creation. The same is true for the observer to the front.

Photons are energized instantaneously. They do not get any extra push or pull. They get the energy of the source, plus or minus the energy of the added speed relative to observers.

Local observers see no energy addition or subtraction. Outside observers see such additions and subtractions.

But what about photons that travel through the spaceship from one outside observer to another? Would not such a photons appear to travel at a different speed to a local observer inside the spaceship?

The answer is that this cannot be observed directly. The local observer has to detect the photon entering and leaving the spaceship. On entry, the time and energy is recorded. The photon is thus "consumed". An equivalent photon has to be produced. It travels to the next detector where time and energy is recorded. However, this is now a local photon. All that can be recorded regarding photons inside the spaceship must be locally produced!

When a photon of the same energy as the one detected on entry into the spaceship is produced to send onward to the other observer, it is red-shifted or blue-shifted in whatever way is required. However, this shift is canceled out completely as the outside observer registers the photon as being no different from the photon that was let straight through the spaceship without measuring.

All that may be detected at the outside is a phase shift due to the change in speed locally during the measurements. The photon that was let through without any measurements arrives at a different time than the photon that was measured, replaced, measured again, and replaced again for onward communication.