Modern physics teaches that light waves, like sound or water waves, bunch up AT THE EMITTER:
View: https://www.youtube.com/watch?v=xsVxC_NR64M
Stephen Hawking, "A Brief History of Time", Chapter 3: "Now imagine a source of light at a constant distance from us, such as a star, emitting waves of light at a constant wavelength. Obviously the wavelength of the waves we receive will be the same as the wavelength at which they are emitted (the gravitational field of the galaxy will not be large enough to have a significant effect). Suppose now that the source starts moving toward us. When the source emits the next wave crest it will be nearer to us, so the distance between wave crests will be smaller than when the star was stationary." http://www.fisica.net/relatividade/stephen_hawking_a_brief_history_of_time.pdf
The wavelength-variation-at-the-emitter scenario clearly violates the principle of relativity. The traveler could measure the wavelength variations inside his spaceship - so he would know his spaceship's speed without looking outside.
Theoretical physicists see no problem in teaching the absurd wavelength-variation-at-the-emitter scenario because they know the physics community neither understands nor cares about the principle-of-relativity violation. In some situations, however, they are forced to answer the provocative question and then they say: There are wavelength variations, but the emitter does not see them; only the receiver/observer does.
In Einstein's relativity, any absurdity can be "explained" by introducing another absurdity. No red lines at all.
Stephen Hawking, "A Brief History of Time", Chapter 3: "Now imagine a source of light at a constant distance from us, such as a star, emitting waves of light at a constant wavelength. Obviously the wavelength of the waves we receive will be the same as the wavelength at which they are emitted (the gravitational field of the galaxy will not be large enough to have a significant effect). Suppose now that the source starts moving toward us. When the source emits the next wave crest it will be nearer to us, so the distance between wave crests will be smaller than when the star was stationary." http://www.fisica.net/relatividade/stephen_hawking_a_brief_history_of_time.pdf
The wavelength-variation-at-the-emitter scenario clearly violates the principle of relativity. The traveler could measure the wavelength variations inside his spaceship - so he would know his spaceship's speed without looking outside.
Theoretical physicists see no problem in teaching the absurd wavelength-variation-at-the-emitter scenario because they know the physics community neither understands nor cares about the principle-of-relativity violation. In some situations, however, they are forced to answer the provocative question and then they say: There are wavelength variations, but the emitter does not see them; only the receiver/observer does.
In Einstein's relativity, any absurdity can be "explained" by introducing another absurdity. No red lines at all.