A photon is a quantum of electromagnetic energy. The energy E of a photon is given by Planck's equation as E=hf, where h is Planck's constant and f is the frequency of the radiation. The velocity of the photon c=fl, where l is the wavelength of the radiation. The alternating electric and magnetic vectors of the photon lie in the plane transverse to the direction of propagation, are at right angles to each other, and are in phase with each other. In the course of traveling a distance of one wavelength the electric and magnetic vectors of the photon alternate one complete cycle. When the photon's energy is converted to another form, the photon no longer exists. A photon exists only while it travels at the velocity of light (c). The value of c depends on the medium through which the photon travels. When not otherwise specified c is taken as the velocity of light in a vacuum (space).

When a wave propagates in water, peaks and valleys of the water propagate away from the source that provided the energy for the waves. A wave in water is a propagating disturbance of the water, that is, of the medium supporting the wave motion. The wavelength is the distance between successive similar positions of the water, such as peaks or valleys. It is the water that does the waving.

In order to explain the wavelike phenomena of light discovered in the 18th and 19th centuries, scientists assumed that some kind of supporting medium had to do the waving. That medium, called the ether, was supposedly at rest with respect to the "fixed" stars. The famous Michelson-Morley Experiment, begun in 1881, tried to detect the orbital velocity of the earth around the sun relative to that stationary ether. Their optical equipment was accurate and sensitive enough to detect one-tenth the known velocity. They could not detect any motion of the earth relative to the ether. The ether idea was subsequently abandoned.

Thus, the photon is not thought of as a wave supported by a medium. A photon does not exist as successive peaks and valleys. Its electric and magnetic vectors alternate periodically as it travels; but, it is not extended in the direction of that travel. The time for one period is 1 / f. The distance through which it travels during one period is called its wavelength; but, at no instant during its travel through space does a photon have a wavelength. A photon is not a wave.

Its wavelength only appears as a result of its interaction with matter. A 1.0 megacycle photon is not a 100 meters long thing moving through space. A photon has only its velocity, its period, and the oscillation of its electric and magnetic vectors. (I have omitted the photon's polarization since it is not relevant to the present line of inquiry.) A photon, while it is travelling, does not have a wavelength to be stretched by the expansion of space.

Considering all the above, and since the velocity of light in space is constant, the only property of a photon that can affect the distance traveled by a photon during one period is the time elapsed during one period. To produce a red shift of the wavelength of the observed (no longer moving) photon, the time duration of its period would had to have been increased prior to its observation. Since a photon is not extended in the direction of its propagation and since wavelength is not intrinsic to a photon, how can the expansion of space increase the time of a photon's period during its travel through space?