# Questions tagged [observers]

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646 questions
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### Does a graviton in vacuum have a rest frame?

I have read these questions: Does a photon in vacuum have a rest frame? Based on dmckee's answer, the answer is no to a photon's rest frame. In the modern view each particle has one and only one ...
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### Relative motion and time

When someone reaches to a speed which is close to the speed of light with respect to earth, will he see the things actually moving faster than when he is in low speeds?
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### Lorentz Transformation Exercise confusion

So there is this very simple situation in one of my exercices: In the earth's frame of reference a tree is at the origin and a pole is at $x=20$km. Lightning strikes at both the tree and the pole ...
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### Can I speed up time by slowing down universe? [closed]

baseline facts: ...
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### In Algebraic QFT, is the state observer dependent?

In the usual approach to QFT presented e.g., in Weinberg's book, the state of a system is dependent on the observer. Quoting this book, in page 109 we have: Notice how this definition is framed. To ...
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### Does time really slow down in relativity, or is it an illusion caused by the Doppler shifting of light? [duplicate]

Does time really dilate, or is it only how we perceive time, which is in the form of light waves. When an object moves away from another object at very high speeds, the light takes longer to reach it ...
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### Does an expanding event horizon “swallow” nearby objects?

In a view of a remote observer, an object falling into a black hole is "hanging" at the horizon (slowly falling with a deceleration). Around this moment, the event horizon expands for some reason that ...
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### What is the proof that the speed of light in vacuum is independent of relative motion? [duplicate]

Most people know that the speed of light in vacuum is independent of relative motion but I want to know the proof of it.
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### How fast would i have to travel for time to appear frozen?

i have to make a school project about something that interests me. so i thought about this question. if anyone could answer me, that would be nice :) (sorry if my English is bad) also im wondering ...
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### Time of flashlight crossing the event horizon as seen from an external observer

This is a follow-up to my previous question: Can something (again) ever fall through the event horizon? Consider the following thought experiment: I am again on my rocket at height $h$ over a black-...
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### Does general relativity say (or assume) that there is a universe-wide present all observers and objects live in?

I understand that in general relativity all observers agree on what it is they see (looking at the same object or event) when they account for the effects on their observations of the gravitational ...
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### Reference when measuring the speed of fast moving objects

Let's assume an object is moving at a speed of V=50% of the speed of light, so the time in the frame of this moving object becomes slower by an amount of X. when we say time becomes slower by, it ...
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### Is it the same time, does time pass at the same pace everywhere in the universe? [duplicate]

According to big bang cosmology, the universe has a beginning, a definite age. Does this mean that it is the same time, that time passes at the same pace everywhere?
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### Does event horizon expand instantly or does it take time for large black holes?

When something falls into a black hole, its mass increases and its event horizon expands a little to reflect the new mass. In case of very large supermassive black holes, with event horizon spanning ...
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### What if two twins flew off in opposite directions and were reunited in a perfectly symmetric way, would they have aged same? [duplicate]

If No, well this can't be, as there is perfect symmetry, you can't tell one from the other. If Yes, they had relative velocities all along, then their times must have dilated and somehow they must ...
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### Twin paradox comparing with length manipulation

when you are measuring a length of a subject in a moving frame you get length contracted but after bringing the subject to rest frame you get the proper length. Which I understand the relativistic ...
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### Could we construct an experiment to measure the speed of light greater than c?

As I understood we were not able but then i read John Rennie's question and answer here: Does light really travel more slowly near a massive body? And got me a little bit curious. He says: We ...
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### Observer problem in Relativity [closed]

Which clock or observer conclude that time dilation has occurred? Is it the stationary clock/observer or the moving one? (Please bear in mind that each observer/clock independently concludes that one ...
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### In the twin paradox, are some events lost to the observer turning around?

From what I understand of the twin paradox, the time difference in time can be accounted for when the twin on a rocket ship turns around/switches reference frames. Does that imply that, events that ...
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### Clarification of proper time and inertial frame of reference

“Standing beside railroad tracks, we are suddenly startled by a relativistic boxcar traveling past us as shown in the figure. Inside, a well-equipped hobo fires a laser pulse from the front of the ...
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### At what moment will matter falling into a black hole affect its size?

Does the mass of matter falling into a black hole affect the size of an event horizon the moment it passes through it, or when it has been incorporated into the singularity?