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Questions tagged [observers]

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2
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4answers
109 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-...
0
votes
1answer
66 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 ...
0
votes
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 ...
0
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0answers
59 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
votes
2answers
116 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 ...
6
votes
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
votes
2answers
61 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
vote
1answer
107 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 ...
19
votes
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
votes
0answers
98 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 ...
1
vote
2answers
59 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 ...
0
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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 ...
5
votes
4answers
682 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 -- ...
0
votes
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 ...
-2
votes
1answer
129 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
votes
1answer
71 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 ...
1
vote
0answers
199 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 ...
-1
votes
1answer
154 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
vote
1answer
108 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 ...
1
vote
2answers
313 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 ...
-4
votes
1answer
76 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
votes
4answers
161 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 ...
1
vote
2answers
144 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 ...
3
votes
1answer
142 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?
2
votes
2answers
220 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 ...
1
vote
2answers
166 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. ...
-6
votes
1answer
173 views

What would happen if we put Schrödinger inside the box and the cat opened the box door? [closed]

Schrödinger's cat is a thought experiment, sometimes described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen ...
0
votes
2answers
157 views

Solving the ladder paradox: Minkowski distance leads to a contradiction

Consider the ladder paradox; Let the length of the barn in its fixed frame be $80m$, and consider the ladder's reference frame which is moving toward the barn with a speed $v$, and in ladder's frame ...
0
votes
1answer
119 views

Question on Painlevé Gullstrand coordinates

A known set of coordinates used for the Schwarzschild metric is the Painlevé-Gullstrand coordinates. They consist in performing a change from coordinate time $t$ to the proper time $T$ of radially ...
-1
votes
3answers
128 views

Special Relativity-Simultaneity

If two events are simultaneous in one reference frame, are they simultaneous in all reference frames? Please provide thorough qualitative explanation If blue light is going at speed $c$, with red ...
1
vote
1answer
253 views

Length contraction in GR

In general relativity, $\sqrt{1-v_{e}^2}$ (with $v_e$ the escape velocity as a fraction of the speed of light) is the factor by which time is dilated in a gravitational field. Is it also the factor by ...
2
votes
2answers
511 views

Determining the rest frame

Sorry if this has been asked before in some way, I did try looking through the special relativity tag and didn't really see a good answer to my question. I am having trouble determining which frame ...
0
votes
1answer
88 views

What if you pointed a laser away from a black hole while falling into it?

Often this is asked if it was pointed towards a black hole. so I thought, if a scientist decided to sacrifice himself and jumped in a black hole, released some dry-ice mist and pointed a laser pointer ...
5
votes
6answers
503 views

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, ...
0
votes
1answer
65 views

Observer independent quantities in special relativity

I have been thinking about what things about a point particle do all observers agree about? And I thought trajectories of particles must be the same for all observers, right? But, clearly it is not. ...
0
votes
2answers
110 views

Length contraction of a train

Suppose a train of length $3x~\rm m$ and a tunnel of length $x~\rm m$. When the velocity of train is $282842712.5~\rm m/s$ (from length contraction equation), then its length contracts to $x~\rm m$. ...
1
vote
0answers
76 views

Indeterminacy in Quantum Mechanics

Heisenberg's uncertainty principle tells us that there is a lower limit on the accuracy with which we can determine both the momentum and the position of a particle simultaneously. $\Delta$$x$ $\...
1
vote
2answers
102 views

Is it possible to change the frequency of the light by magnetic fields?

I do not recall where, but I have read someting about the light approaching a blackhole. In the writing, it was said that an object would gain kinetical energy due to the blackhole's gravity, and ...
3
votes
1answer
327 views

Derivation of Relativity of Simultaneity

Suppose you have a train moving forward relative to an inertial observer at velocity $v$. Suppose you have a clock 1 at the front of the train and a clock 2 at the back, and, in the frame of the train,...
1
vote
0answers
44 views

Thoughts on Black Holes [duplicate]

Given that time slows to a stand-still at the event horizon of a Black Hole, how long would it take a black hole to form, from the perspective of an outside observer? It would seem, as a massive star ...
0
votes
3answers
135 views

Inside a box moving with constant velocity

Suppose You were put into a box with all the instruments you might require. The box moves with constant velocity on the surface of the earth. We are asked to detect inside the box whether we are ...
-4
votes
4answers
312 views

How Lorentz transformation is connected to reality and physics?

I don't understand whats the meaning of Lorentz transformation in Special relativity. In physics, the Lorentz transformations (or transformation) are coordinate transformations between two ...
80
votes
8answers
9k views

How can time dilation be symmetric?

Suppose we have two twins travelling away from each other, each twin moving at some speed $v$: Twin $A$ observes twin $B$’s time to be dilated so his clock runs faster than twin $B$’s clock. But twin ...
1
vote
1answer
76 views

How does a distant observer see matters that “form” an initial black hole?

I did see Can black holes form in a finite amount of time? but it does not seem to discuss how a distant observer would see evolution of collapsing matters that form a black hole. Does it view these ...
-1
votes
3answers
176 views

Schrödinger's cat question

what would happen if a CC Camera or videocorder is put along with Schrödinger's cat inside the sealed box? Would it influence the outcome?
0
votes
3answers
207 views

Why does gamma show up in the relativistic velocity equations in the $y$ and $z$ terms?

I understand the addition of velocities in relativity in terms of the $x$ direction is $$u_x=\frac{v+u_x'}{1+vu_x'/c^2}$$ What I don't understand is why, when calculating for the $y$ and $z$ ...
0
votes
1answer
66 views

What (if any) significance exists in the analogy between the speed of light limit and the Schwartzchild radius of a black hole

I have always been intrigued by what seems to be very similar characteristic of the phenomena of traveling very, very fast, (close to the speed of light , and the Schwartzchild radius. AS a body gets ...
2
votes
1answer
95 views

In special relativity, is there a prescription to find the coordinate transformation to a general (for instance accelerated) observer? And in GR?

Suppose we are in an inertial frame, so that the metric in our coordinates is just the Minkowski metric, \begin{align} \text{d}s^2 = -\text{d}t^2 + \text{d}x^2 + \text{d}y^2 + \text{d}z^2. \end{align} ...
0
votes
1answer
66 views

Time travel (Velocity and Mass)

I learned that if I move on a high velocity, if my watch shows 12:00 and a my home clock shows 12:00 before the trip, when I come back, my watch will be like 12:05 while the clock will be 12:10. I ...
1
vote
5answers
163 views

Two objects are thrown into a black hole. The first crosses the event horizon at time's end, so when does the second one cross?

An observer throws an object towards a black hole, and then an arbitrary amount of time later, throws a second object towards the black hole. Disregarding Hawking radiation and assuming the Black Hole ...