# Questions tagged [observers]

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### What does the accelerated twin see in the twin paradox?

I am referring to the usual twin "paradox" where one twin remains on Earth while the other one takes a round trip journey on a rocket. I understand how the experience of the twins is not symmetrical, ...
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### Einstein's train, considering the thunder

From Einstein's "easy" explanation https://www.bartleby.com/173/9.html (Yes, I have reviewed 8 other answers to similar questions. Please bear with me.) M is on the platform, T (aka M') is on the ...
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### What is exactly meant by “time is relative”?

Let's say we are traveling to the galaxy X - which will take 4 years for us to reach. However, it will take 100 years for the observer on the earth. How come is it possible? How 4 years can be ...
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### Basic Special Relativity Question [on hold]

I am trying to understand the implications of c being relative to the frame of reference. Is the following analysis correct? N is on a slow train moving 3 m/s and M on the platform next to N both ...
16 views

### Which frame to consider for time dilation? [duplicate]

Clearly for constant motion we cannot define absolute rest or motion. So ,when dealing with time dilation problems for example twin paradox , how do we define which frame is in motion? If we ...
2k views

### Two Trains Relative Motion [closed]

I am confused about finding the solution to this relative motion problem: You are on a train traveling east at speed of 20 m/s with respect to the ground. Your friend is sitting on another train ...
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### Would an observer going faster than the speed of light in a vacuum be able to observe photons that they emit? [closed]

If I could exceed the speed of light in a vacuum, would I be able to see photons that I previously emitted? Would this be theoretically possible?
567 views

### Is an event horizon absolute to all observers?

Recently I had discussion whether the event horizon of a black hole is absolute or relative to different (outside) observers. Does someone just 1m above the horizon (disregarding effects of tidal ...