Is simultaneity relative?

Question

The question comes from an earlier post: How can moving observer explain non-simultaneity?

which uses the classic scenario of "passenger on train passes platform when 2 equidistant explosions go off."

Most people here (and textbooks I have seen so far) argue the two observers (Bob and Alice) represent equally valid frames of reference. That's the main idea of relativity, after all.

But there is a key feature which breaks the simple relativity: the doppler-effect

Let Alice detonate the 2 bombs in her (and platform's) rest-frame.

Then ONLY Alice is capable of observing the two explosions without a doppler-induced difference.

In a sense, she represents the true rest-frame of the two simultaneous events. Bob can never see non-doppler-shifted lights.

Switch it around, so two bombs go off on the train, and the same statement becomes true for Bob. Now only he can see non-doppler shifted lights, and it's a physically detectable situation.

It suggests simultaneity comes with a rest-frame, just like there is rest-mass, rest-length etc.

This could be an easy way to distinguish:

• 'true' simultaneity - where a rest-frame exists, from which the two events can be observed with no (or equal) doppler-shift

from

• 'artificial' simultaneity - where no such rest-frame exists. In the frames where the events appear simultaneous they always have opposite (unequal) doppler-shifts caused by the observer's speed relative to the events

Edit: Both Doppler and how it works is completely within mainstream physics. The use of a "rest-frame" for simultaneity works perfectly fine with special relativity, just like rest-length and rest-mass.

I think some people missed that point, and that's why this post got some agitated responses.

Answer 1

Notwithstanding the fact that several people have tried to explain simultaneity to you in response to other questions you have posted on the subject, you are stubbornly sticking to your own misunderstanding of the facts. The Doppler effect has nothing to do with the relativity of simultaneity per se. The equations that determine times of events in different reference frames contain neither the frequency nor the wavelength of light, which should prove to you that the effect is independent of both. You can, if you wish, easily conduct a thought experiment in which two events are simultaneous in the frame in which the wavelength of light is shifted by the Doppler effect, and not simultaneous in the frame in which there is no Doppler effect, which should be another proof. I suggest you go back to the answers you have been given, and work on them until you understand.