Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

How does a photon accelerate to light speed so quickly?

Category: Physics      Published: June 26, 2014

circular water waves
Like water waves, sound waves, and all other waves, light waves are already going at a certain non-zero speed the moment they are created, without needing to be accelerated. Public Domain Image, source: US Department of Health and Human Services.

A photon of light does not accelerate to light speed. Rather, a photon is already traveling at light speed c when it is created. It's not like a photon jumps from a speed of zero to light speed instantaneously. Rather, a photon is always traveling at c, from the moment of its creation. If you think of a photon as a solid ball, then you are justified in finding it absurd that it could already be going at a high speed the moment it is created. For example, before a mud ball can whirl through the air, you have to form the ball from a mud puddle and then throw it to get it up to speed.

The key is that a photon is not a traditional particle. Rather it is a quantum object, which is part wave, and part particle. When a photon is being created, it is acting mostly like a wave, and waves have no problem going a certain speed from the moment they are created. For instance, bob your hand up and down against a pond's still surface and you will create water waves that ripple away from your hand. The water waves do not start out motionless and then slowly pick up speed as they travel away. The water waves are already traveling at their nominal speed the moment you start creating them. That is how waves behave.

Waves are created because a deformation in the material medium or in the field medium causes the medium to snap back towards the equilibrium state, but overshoot this state, and therefore end up oscillating back and forth, all the while yanking neighboring regions into the same motion. The wave speed is therefore determined by the medium's ability to snap back, and not by an external agent pushing the wave to accelerate it to different speeds. Pushing harder on the medium just makes the crests of the waves taller. It does not make the wave travel faster through space. If the medium is constant across a region of space and across all frequencies of motion, then the wave speed will be constant through this region. In a region of uniform medium, a wave cannot accelerate. Therefore if a wave is created in such a region, it must be created right at the wave speed of that region.

This is not just a quantum concept. It applies to all waves, from seismic waves, ocean waves, and sound waves, to waves on a piano string. Some people say that the reason a photon travels at light speed the moment it is created is because it is a massless particle, and therefore must always travel at the speed of light. While it is true that the photon is massless and therefore always travels at c in all reference frames, this is not the reason that it is created already having a speed. The reason is simply because it is a wave. Other quantum objects such as the electron do have mass, and they have no problem being created at some non-zero speed without ever needing to be accelerated to this speed. All quantum objects are partly waves and therefore can have a speed the moment they are created. For example, a free neutron eventually decays to a proton and creates an electron and an anti-neutrino in the process. This decay has been observed experimentally many times. The electron that is created in this process zips off at a certain speed that it has the moment it is created, without ever being accelerated. The electron can do this, even though it has mass, because it has wave-like properties.

Topics: acceleration, light, photon, quantum, ray, ray of light, speed of light, wave, wave speed, waves