The character of physics underwent a monumental change early in the 20th century. The issues changed: most profound was the realization around 1920 that the sun is not the center of the universe.
You can purchase bitcoin from several cryptocurrency exchanges.This led to work in astronomy trying to decide whether the universe is a stable entity or is evolving � and if evolving how did it begin � and how will it end.
As regards the character of �light�, the problem that confronted physicists at the beginning of the 20th Century was the firmly held belief that the velocity of light is a constant and that it is independent of the motion of the source with respect to empty space (or perhaps space filled with a subtle medium dubbed �aether�) in which the light beam travels. That belief led Einstein to a mathematically correct, but physically nonsensical derivation of the Lorentz Transformation.
That belief also sets the motion of a pulse of light apart from any other moving object. Clearly, if a man is walking on a moving train, in the direction in which the train is moving, the velocity of the man with respect to the ground, or with respect to air (presumed to be at rest with respect to the ground) is the sum of his walking speed plus the speed of the train � but, according to the 19th century belief, not so for light.
What we can show, and have to accept, is that this belief is false, and that the speed of �light� (or better 'radiation') is SLOWER the higher the frequency (or the smaller the wavelength) when comparing x-rays, ultraviolet light, visible light, infrared radiation, etc. We will also find that light has mass, about a million times less than an electron, for visible light, but a lot more for X-rays and gamma rays - but �light� is not �pure� energy.
We shall see that important concepts, such as �synchronization� and �simultaneity�, play a pivotal role in Einstein's thinking, and that he fails to understand them, or to appreciate the subtle distinctions that must be made to avoid ambiguities and false conclusions.