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Questions tagged [observers]

For questions related to observers in relativity or quantum mechanics.

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About time dilation [closed]

first of all i am really sorry for my bad english let start the question if i understand correctly if i move with speed that close to speed of light i will get time dilation effect so if i focus ...
sretthaphong thongdi's user avatar
-4 votes
1 answer
90 views

In SR, why do we claim length contraction rather than faster than $c$ travel in the rocket frame? [closed]

In special relativity, a rocket traveling at .5c will take .86 years (in the astronauts frame) to travel 1 light year (in a stationary observer frame). There are two possible ways to look at this ...
foolishmuse's user avatar
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0 answers
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Does two clocks moving toward each other at the same speed, then decelerating at the same rate coming to a stop create a paradox? [duplicate]

Note I have asked this question previously but upon reflection I realized I was not specific enough, leading to confusion in the answers. I have decided to ask the question again instead of editing my ...
Shannon T's user avatar
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3 votes
2 answers
644 views

When do we talk about spaghettification or pancakification in black holes?

So I've been doing some research for a while now, and yesterday came across the video of PBS space time talking about what happens to quantum information in a black hole. In the thought experiment ...
Anais-Ellie Gucek's user avatar
1 vote
2 answers
522 views

Please help me with this paradox [closed]

Physicists believe that some galaxies are moving away from us at faster than the speed of light. A galaxy that is moving away from us at faster than the speed of light would be moving backwards in ...
Cecilia's user avatar
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2 answers
52 views

When it comes to getting closer to the Schwarzschild radius, how is discrete a limit?

From Keeton (2014) in Principles of Astrophysics: Using Gravity and Stellar Physics to Explore the Cosmos, Gravitational time dilation near a large, slowly rotating, nearly spherical body, such as the ...
olivierlambert's user avatar
0 votes
2 answers
97 views

Thought experiment circumventing finite speed of light via relativity of simultaneity - what's wrong?

I just watched this video regarding block universe: https://www.youtube.com/watch?v=wwSzpaTHyS8&t=676s and it provoked the following thought experiment: Let's assume two observers, O1 and O2, at ...
Uffe Poul Hansen's user avatar
0 votes
3 answers
127 views

Can you tell who is moving through time? [closed]

In relativity, there is no way to tell if you are moving through space. So, if you were inside of a box, there would be no way for you to tell if you were moving or not. However, can you know who is ...
John W's user avatar
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-1 votes
4 answers
168 views

How is "Many Worlds" supposed to work?

Let's say Anne and Bob are spacelike separated and we carry out the usual entangled photons experiment. Each has a polarising analyser and the entangled photons are prepared in such a way that they ...
KDP's user avatar
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1 answer
67 views

If velocity is relative, why does time dilate for some observers and not others? [duplicate]

I'm trying to get my head around relativity and time dilation. What I can't figure out is how time can dilate for fast-moving observers if all velocity is relative. Here's a scenario: Alice and Bob ...
Justin Morgan's user avatar
2 votes
0 answers
66 views

Is there a formula relating the rate/speed a black hole event horizon will grow, to the density of the medium surrounding it?

I am thinking about the average density of the space around the black hole, not the density immediately adjacent to the event horizon which might be different. It will probably be best to model the ...
John Hobson's user avatar
5 votes
3 answers
640 views

Special relativity and accelerating twins

Imagine two twins synchronise their clocks and then twin A quickly accelerates to velocity v. After a time T twin B quickly accelerates to 2v and catches up with twin A. Which one will be younger? How ...
Moji Ghadimi's user avatar
0 votes
2 answers
176 views

How do we know that a black hole radius is not significantly contracted for a stationary outside observer?

It is my understanding that just as special relativity contracts length with velocity general relativity contracts length with gravity. Would this mean the radius of a BH is smaller than it would ...
Joe's user avatar
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1 vote
0 answers
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Radiation from a stationary charge relative to earth for a free falling observer

It is my understanding that if we have a charge at rest on earth, a free falling observer will see it radiating, as the charge is in an accelerated frame of reference. This observer can in principle ...
Pato Galmarini's user avatar
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1 answer
53 views

Length contraction doesn't affect the perpendicular direction...but it seems as if it can

A is a stationary observer watching B who is moving relative to A. Both of them hold two identical light clocks and each shoots light rays to estimate the lengths of their clocks. A's light ray will ...
I am Einstein's user avatar
1 vote
0 answers
47 views

What is tangential and radial velocity of light in Schwarzschild metric at distance $r$ for observer that is infinitely far away?

What i have learned from Schwarzschild metric is that the time dilation at radius $r$ compared to distant observers time is: $$ \frac{\tau_r }{t_\infty } = \sqrt{1 - \frac{r_s}{r}} $$ and i have heard ...
Sami M's user avatar
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2 answers
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States of entangled particles after no/partial/full measurement

I'm getting contradictory information from the internet concerning entangled particles, measurement, and state knowledge that I'm hoping can be cleared up with a simple setup. Say we have a source of ...
user401228's user avatar
-1 votes
2 answers
111 views

How Fast Must a Man Run to See Rain Falling horizontally?

The fact that tan 90° is undefined indicates a key issue in the problem. Does this mean it is impossible to observe rain falling at 90 degree? When rain is observed to be falling at 90° with the y-...
Dron Bhattacharya's user avatar
0 votes
0 answers
50 views

Would observation affect the results of a double-slit experiment if the observation isn't recorded?

Given that, when an electron passes through the double slit experiment without observing the slit it goes through, it produces an interference pattern, and that, when you do observe which slit it goes ...
Sofia's user avatar
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0 votes
1 answer
113 views

Time required to reach Black Hole's Event Horizon from outsider perspective?

Let's imagine a pair of particles that is entangled. One (call it $P_1$) is released and then falls to a black hole from a distant $x_0$, (for example $x_0=5r_s$) and velocity $v_0(=1/2c)$, while the ...
Nhat Nguyen's user avatar
4 votes
3 answers
1k views

Is the meter relative to time?

Is the meter relative when we are near the speed of light? I was reading a physics book and I found that the meter is the length that light travels for an amount of time, so since time is relative ...
Angel Echavarria's user avatar
2 votes
4 answers
1k views

Time Dilation when they meet

I am starting over because what I was asking was unclear. I have read many articles, such as How can time dilation be symmetric? and others suggested by people here and understand the overall concepts ...
DMacks's user avatar
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2 votes
2 answers
769 views

Is there a different way to measure speed, other than with time?

A second on Earth is different from a second on the Sun and a second next to a black hole. Therefore speed, as measured in meters per second, is different in each of these locations. Is there an ...
foolishmuse's user avatar
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5 votes
4 answers
761 views

Relativity of Time from an Observer Perspective

We have established that time is relative and goes slower for objects in motion and we have experimentally proven with the ISS. A thought experiment: What if the ISS was moving at much higher speeds, ...
Niccola Tartaglia's user avatar
1 vote
0 answers
90 views

Special Theory of Relativity Contradiction [duplicate]

I’m struggling with an apparent contradiction of special theory of relativity (STR) that I cannot resolve. Imagine two twins flying by each other in space along the same straight line but in the ...
Becarev's user avatar
  • 11
1 vote
2 answers
73 views

Image of an electron

Suppose we are trying to see the image of an electron with a microscope. Is it possible? And if it's possible then what do we see? A point at a time, or a blurry spot, or something else?
Users's user avatar
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1 vote
0 answers
37 views

How does the light from distant stars change for an observer at the center of the collapsing or falling sphere?

At the center of a spherically symmetric thin solid static shell lies a point observer. For this observer, distant stars appear violet shifted slightly more $\frac{{G \cdot M}}{{{c^2} \cdot r}}$ ($\...
Imyaf's user avatar
  • 201
1 vote
0 answers
47 views

Photon and Observer effect

We cannot determine the position and momentum of a particle simultaneously with certainty . The product of uncertainty of them is greater than or equal to reduce planck's constant . The reason for ...
S K's user avatar
  • 61
1 vote
2 answers
70 views

Train example of special relativity

My friend Eric is at the center of the train. The train is moving forward. The front end and back end of the train flash a light at the same time. From Eric's perspective, both light arrives at him at ...
Hai's user avatar
  • 13
4 votes
2 answers
626 views

GR contribution to time dilation when both clocks are falling freely

When reading simplified explanations of time dilation experienced by satellites, such as those used for the GPS and other satellite navigation systems, the time dilation is often presented as having ...
Jeppe Stig Nielsen's user avatar
3 votes
2 answers
169 views

Schutz description of Galilean invariance of interval

In B. Schutz's textbook "A First Course in General Relativity", there is a sentence on page 172 discussing Galilean relativity and how the distance between events is invariant in coordinate ...
nickodel's user avatar
  • 137
1 vote
1 answer
63 views

How does the metric transition from a falling dust-like sphere to the Schwarzschild metric?

Outside a static spherically symmetric body in vacuum lies the Schwarzschild metric. However, when observing a falling sphere from a point near the sphere, it no longer appears spherical. Its opposite ...
Imyaf's user avatar
  • 201
1 vote
1 answer
78 views

Imagine a universe with only 2 objects: [closed]

a single electron and a detector for magnetic fields. If the electron is moving at a constant velocity, will the detector ever register a magnetic field?
Adonay Gandolffi's user avatar
2 votes
1 answer
67 views

Borde-Guth-Vilenkin (BGV) Theorem equation for the motion free falling observer

In the derivation of the BGV theorem paper: https://arxiv.org/abs/2307.10958 There is the following relation (numbered (8)): $\dfrac{d}{d s}\left[a^2(t) \dfrac{d \mathbf{x}}{d s}\right]=0$ It's very ...
Vincent ISOZ's user avatar
-4 votes
1 answer
115 views

Absolute rest in special relativity

I am new to special relativity and with still little experience in Physics. I would like to know why a privileged frame of reference that determines absolute rest is not possible. I want you to ...
Luis's user avatar
  • 13
0 votes
0 answers
13 views

Can relativity apply to temperature? [duplicate]

Given a stationary observer and a moving one. One would see the particle in a system of gas moving faster than the other would. Does that mean the particles have more kinetic energy and thus higher ...
Ali's user avatar
  • 1
1 vote
6 answers
118 views

Question regarding time dilation

if people in semi-light-speed ships age slower, does the world appeared to those people also 'age slower'? Because it is the external world which is moving at semi-light-speed to the people in the ...
Icer's user avatar
  • 11
3 votes
3 answers
353 views

One-way Tachyonic anti-telephone

When you have length contraction in special relativity $$L' = L/\gamma$$ the interpretation is that $L'$ is the length of an object with rest-length $L$ moving with respect to an observer at rest. ...
Dr. user44690's user avatar
2 votes
2 answers
406 views

Observer Effect

The standard explanation for the observer effect is that a small amount of energy from the observer causes the wave function of lets say an electron to break down and for it to develop determinate ...
Paul Hayman's user avatar
2 votes
7 answers
2k views

Does quantum mechanics respect the principle of relativity?

The principle of relativity says that all observers see the same laws of physics. It is, to my knowledge, the underlying principle behind General Relativity; put alternatively, General Relativity is ...
Allure's user avatar
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0 votes
3 answers
122 views

Relativity and variable time - there is an alternate formulation where time is always linear. Has it been studied?

Lorentz, Einstein et. al. assume time is the variable which changes in a gravity well or as speed approaches $c$. That's the commonly accepted model. For nearly 50 years I've wondered if anyone else ...
BobT's user avatar
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1 vote
0 answers
49 views

In the reference frame of an observer an arbitrarily large distance away, can event horizons form in finite time? [closed]

There are similar questions answered already but the answers disagree. I understand that simultaneity/time is local in GR and that in a given region of space, an event horizon and singularity can form ...
K340's user avatar
  • 11
0 votes
1 answer
39 views

Inertial and a gravitational component of two observers

Started reading the book "How Einstein found his field equations" by Janssen and Renn and I am already blocked on chapter 1. What do the authors means after the "split" below?: &...
gianpaolo's user avatar
  • 147
0 votes
0 answers
57 views

Time dilatation of a free falling observer [duplicate]

I have two questions about time dilation near a black hole. (I question) The relation $d\tau^2 = (1-\frac{r_s}{r}) dt^2$ between the proper time $d\tau$ of an observer near a B.H. and the time dt ...
Ghilele's user avatar
  • 21
0 votes
1 answer
137 views

Confusion regarding Special Relativity and Relativity of Velocity? [duplicate]

Suppose that there are two points in an empty space A and B and observers at A and B having clocks: If A were to be traveling near speed of light, when clock at A passes 1 hour, observer at A ...
CuriousCrypto's user avatar
0 votes
1 answer
44 views

Relativity - how do you calculate the duration of this trip from various reference frames?

Let's have following scenario: A spaceship departs from the Earth and constantly accelerates in a straight line for 60 000 000 seconds at 1g (10 m/s^2), then immediately starts decelerating at 1g for ...
Tondo PX's user avatar
1 vote
2 answers
168 views

If you have two clocks moving toward each other and they both stop when they meet does it point to a paradox?

Say they both decelerate at the same rate also. If clock A sees clock B as slower than itself and clock B see clock A as slower than itself due to relativity isn't this a paradox because when they ...
Shannon T's user avatar
  • 361
1 vote
1 answer
108 views

How would a spacecraft travelling near light speed say 0.9c compensate for time dilation in radio communication from spacecraft to earth?

For a spacecraft travelling at 0.9 c for 5 seconds, only 5 seconds would have passed for an observer on Aircraft, while 26.31 second would pass for a stationary observer watching from Earth. In a ...
adarshsrivastva's user avatar
0 votes
0 answers
74 views

Perceiving the rainbows

Famous Astrophysicist Dr Neil DeGrasse Tyson, explains that whenever we observe a rainbow it appears exactly semi-circle to us. If another person is observing the same rainbow from a slightly ...
Shamant Desai's user avatar
0 votes
3 answers
96 views

Question regarding aparent position and velocity of an object

A time lag is always involved while observing objects. Due to this reason we can never observe the correct position of an object at a particular time. We will see it where it was some times ago. Is ...
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