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Questions tagged [observers]

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2answers
122 views

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 ...
1
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4answers
164 views

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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1answer
158 views

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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3answers
140 views
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1answer
100 views

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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4answers
252 views

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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5answers
370 views

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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4answers
124 views

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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0answers
77 views

Confusion about expression for length contraction of rod [closed]

My physics book states that in the construction in the figure, if the event C has coordinates x'= 0 and t' = l in the primed frame, then event C has x -coordinate in the unprimed frame: $$x_C = \...
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0answers
50 views

How can two people on opposite sides of the Earth both be accelerated? [duplicate]

As I understand it, then when I am standing on Earth I feel a force on my feet and is accelerated through spacetime. But what if a person is standing on the other side of Earth. That person is also ...
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7answers
902 views

Is the second postulate of Einstein's special relativity an axiom? [closed]

I'm trying to grasp Einstein's special relativity theory and can't seem to find a clear answer as to why Einstein concluded that the speed of light is constant to observers in different inertial ...
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1answer
127 views

Doubt on relativity

I have a doubt: if in Classical physics the maximum speed is limited to the speed of light, will simultaneity be relative even in Classical Physics? Also, two observers in separate inertial frames, ...
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2answers
241 views

The role of the “observer” in quantum theory?

I've been reading about different ideas of getting the observer out of quantum theory. One Sean Carroll seems to like is Everett's many worlds concept. I had a thought that I need someone here to ...
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2answers
428 views

How can a comoving observer be “privileged”?

Yesterday I went to my GR (undergraduate) lecture, and the professor exposed the concept of comoving frame of reference. Well, he said that in this particular frame an observer is a privileged ...
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2answers
120 views

What will the observer observe?

Consider the case where observer $A$ is at rest, and observer $ B$ is moving with speed $\frac{c}{2}$ (where $c$ is the speed of light) propagating a wave with wave speed $c$. So my question is what ...
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0answers
36 views

Does a black hole evaporate by the time an observer reaches the singularity? [duplicate]

Assume you are an invincible observer who falls into a blackhole... Your time runs slower than the universe outside, as you get closer and closer to the singularity, the ratio between your time and ...
9
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4answers
2k views

Is time sped up when orbiting a black hole? Why? What does that mean?

I was watching a very interesting short documentary in which the author said that while it takes 8 minutes in space to loop around the black hole, an observer from Earth observes that it takes 16 ...
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2answers
112 views

Mistake in simple calculation in special relativity [closed]

I am just learning a little bit of special relativity on my own and am obviously still quite confused. I would appreciate it if someone could point out the error in the following calculation: Let A ...
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2answers
154 views

Can someone explain Einstein's light clock?

The explanations for time dilation that I have seen all use the thought experiment of a photon bouncing between two parallel mirrors, which are themselves moving in a direction perpendicular to the ...
1
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1answer
443 views

Does one age quicker at higher altitudes?

See title: Does one age quicker at higher altitudes? A few years ago I heard that you would age slightly faster (or slower) at higher altitudes. Is this true? What is the theory or evidence for ...
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2answers
73 views

If The Flash ran a 100m race at 99.9% the speed of light, in what time would he finish the race according to himself and a stationary observer? [closed]

According to the time = distance/velocity formula it would take him 0.33 microseconds to complete. t = 100/(0.999 * 299 792 458) = 100/299 492 665.54 = 0.000000334 seconds (0.33 microseconds) My ...
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3answers
141 views

Doubt of speed of light

I have a question about how an observer sees the speed of light or the distances when at rest. Suppose two events occur simultaneously in $A$ and $B$. If someone in $X$ is watching, they will see ...
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2answers
83 views

Is my interpretation of these observers correct in General Relativity?

Let the Schwarzschild spacetime be given with coordinates $(t,r,\theta,\phi)$. Change coordinates to Kruskal-Szekeres $(T,X,\theta,\phi)$ so that the line element becomes $$ds^2 = \dfrac{32 M^3}{r}e^{...
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1answer
109 views

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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4answers
123 views

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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1answer
67 views

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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1answer
34 views

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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0answers
60 views

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?
3
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2answers
122 views

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 ...
7
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5answers
1k views

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 ...
0
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2answers
62 views

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 ...
1
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1answer
109 views

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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1answer
2k views

Does light really travel more slowly near a massive body?

It is a routine problem for beginners in general relativity to calculate the coordinate velocity of light for the Schwarzschild metric. Starting from the metric: $$ ds^2 = -\left(1-\frac{r_s}{r}\...
4
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0answers
121 views

Role of the observer in Gödel's universe

I am here to clarify myself about the role of the observer in Gödel's solution (1949) of Einstein's field equations. The Universe we are dealing with is anisotropic, since the axis of rotation ...
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2answers
64 views

Distance of an observer from a parabolic light source

Suppose there is point source at the focal point of parabolic mirror. The observer's eyes are on the axis of the parabola. What does the observer see as he starts to move away from the light source ...
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0answers
30 views

Special relativity time dilation - what will the outcome be? [duplicate]

if observer 1 sees observer 2 moving toward him at near the speed of light, observer 1 will say that his clock runs faster then observer's 2 clock, but observer 2 will see observer 1 toward him at ...
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4answers
845 views

How can a mechanical clock tick slower due to time dilation?

I was about to watch Interstellar for the third time when I decided to do some research on its phenomena. Time dilation is the one that struck me most. I have read and read about it and understand -- ...
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0answers
29 views

Behavior of a gravitation field of an object falling into a black hole [duplicate]

A thought experiment: Take an object that is massive enough so we can detect it's position in space by measuring its gravitation field (for example, by sending almost tangential laser beams and ...
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1answer
173 views

Is length contraction in Special Relativity the same as the Doppler Effect?

In my further reading of Special Relativity, the idea of length contraction when travelling at the speed of light is such that the length gets "squished" in the direction of travel. This immediately ...
0
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1answer
87 views

Do the comoving observers agree on the cosmic time?

Is it guaranteed that there exists a universal time on which all comoving observers will agree? Special relativity teaches us that inertial observers in relative motion do not agree on the time they ...
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0answers
219 views

Why is the double slit experiment highly regarded as mysterious?

I read in Wikipedia: The need for the "observer" to be conscious has been rejected by mainstream science as a misconception rooted in a poor understanding of the quantum wave function ψ and the ...
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1answer
211 views

Will 2 spaceship flying at opposite direction with the speed of 0.51c be able to see each other?

I understand the relativistic addition formula which means the sum is around 0.8c, but is it safe to conclude that they will be able to see each other?
1
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1answer
114 views

If “time is relative”, what does “the universe is 13 billion years old” mean? [duplicate]

Physcists say that the universe is approx. 13 billion years old. However, the amount of time that has elapsed since the big bang differs per object, dwpending on the trajectory that that object has ...
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2answers
422 views

Derivative of proper time w.r.t. time?

On page 86 of Theoretical Minimum (part 3 on special relativity) by Susskind, he writes: $$\frac{d\tau}{dt}=\sqrt{1-v^2}$$ Where $v$ is the velocity of a moving reference frame relative to the ...
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1answer
77 views

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 ...
0
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4answers
170 views

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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2answers
163 views

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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1answer
153 views

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?
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3answers
302 views

Einstein's train experiment with clocks

Referring to Einstein's train hit by lightning bolts thought experiment. What if, in the middle of the train, together with the moving observer there is a stopwatch, with 2 displays. One display ...
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2answers
181 views

Young and Freedman Proof: No Observer Can Travel at the Speed of Light

I have been reading the section on Relativity in the ninth edition of University Physics by Young and Freedman. They include the following proof that no observer can move at the speed of light. ...