All Questions
188
questions
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2
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100
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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 ...
0
votes
3
answers
130
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 ...
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0
votes
1
answer
116
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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 ...
1
vote
2
answers
72
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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 ...
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1
vote
1
answer
117
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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 ...
2
votes
3
answers
240
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Relativity, Lorentz Transforms and the time it takes to perceive
I've been working a bit at understanding relativity a bit more, and understanding Lorentz transformations. I want to make sure I understand the meaning of a Lorentz transofmration, and when to use it ...
- 160
9
votes
5
answers
2k
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A bar that appears straight in one reference frame appears bent in another?
Is there a relativistic effect at play that explains the following thought experiment, that I am unaware of?
The apparatus depicted in the image has two straight bars in the middle emitting light from ...
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1
answer
178
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Can we consider that the photons that were not and will never be detected live in a zero-dimensional space? [closed]
According to Einstein’s theory of relativity, any particle traveling at the speed of light experiences no passage of time. It follows that if a photon travels through space then within any two points ...
- 115
0
votes
2
answers
86
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Time desync with light delay: can we still "look into the future"?
I'm reasonably familiar with special relativity and its effect such as time desynchronization, but I'm having trouble understanding how these effects come into play when we also consider the time for ...
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0
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0
answers
26
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Question on Example 5.9 of Prof. Hartle Gravity textbook
I'm reading the Gravity Hartle book (ed.2003) and I'm having trouble with the question in the last part of Example 5.9 - Frequency Measured by an Accelerating Observer. More specifically the problem ...
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1
vote
1
answer
145
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How to calculate positions when switching reference frames in a Minkowski spacetime diagram
The below Minkowski spacetime diagram includes three worldlines, where B is the observer and has a rest frame. A and C both have a velocity of 0.71c.
I then created a second diagram where worldline A ...
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1
vote
4
answers
165
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Is constant acceleration the same relative to other objects at all velocities?
If you are accelerating at a constant rate of 10 $m/s^2$, will you see things around you move at 10 $m/s$ faster every second, even when approaching the speed of light?
a) If not: How do you calculate ...
- 171
0
votes
2
answers
67
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Question about length contractions [duplicate]
So I'm kind of confused about length contractions, whenever I think about length contractions I think about the faster an object goes it will be able to impair more photons which would create an ...
1
vote
2
answers
132
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Do events very far away happen in a different timeline?
I am not sure how to ask this question in a concise manner so I am sure somebody out there explained it but I cannot seem to find it.
So I recently watched some videos explaining that $c$ not only ...
- 119
8
votes
1
answer
357
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The synchronized clocks on earth's surface: at which observer's rate are they beating?
From what I understand, the time rates (I'm not speaking about absolute times) of all clocks on earth's surface are synchronized. This means that, say, a mobile phone's clock is generally not beating ...
- 3,753
0
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2
answers
119
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Time in the Standard Model of Cosmology
Beyond a formal preference for background independence, what is stopping us from setting cosmological time as a de facto universal timeline, analogous to newtonian absolute time? General relativity ...
- 1
0
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6
answers
173
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If time runs slow for moving train, how can it cover same distance in less time than someone on the ground unless the track decreases in its view?
According to special relativity,
Δt=γ*Δt' ... (1)
Where,
Δt is the time the train takes to completely pass by me according to my watch,
Δt' is the time I should see has passed for someone on the train....
1
vote
1
answer
90
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What does hypersurface of simultaneity exactly mean?
HSS - "Hyper Surface of Simultaneity"
Listening to different sources online I understood that HSS for a observer represents the points that are at same moment of time.
Consider a 1d world. ...
-2
votes
4
answers
163
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Relativity of simultaneity in flat spacetime
In Einstein’s (1917) thought experiment on the relativity of simultaneity…
The lightening flashes at A and B are both equal distance from the center of the train and the center of the platform.
The ...
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-1
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2
answers
116
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Does special relativity imply that there's such a thing as absolute time, or base time?
If time measured by one observer moving at a greater velocity than another observer is observed to be passing more slowly, does this imply that there's such a thing as "absolute time" or &...
1
vote
0
answers
106
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Energy is the time component of 4-momentum in SR: Proof as per R. Wald's book
This is an excerpt fom R. Wald's book on General Relativity (page 61). I'm not able to understand how he deduces that $E$ must be the time component of $p^a$ with only the assertions made before this ...
- 107
0
votes
2
answers
119
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Do all observers see the same curved spacetime -- or would that be a (prohibited) preferred inertial frame? [duplicate]
My question -- pardon if not asked in the most incisive scientific prose
Do all observers see the same curved-space time?
Let me clarify:
Given that objects traveling at different relative speeds will ...
0
votes
1
answer
67
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Can we observe a more recent space?
The space we are viewing now is their distant past as their light has only reached our eye or telescope after travelling a long distance at the speed of light. However, for lights that are still on ...
3
votes
1
answer
106
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How does the second postulate of special relativity imply invariance of spacetime interval?
I have trouble understanding why the second postulate of Special Relativity implies the invariance of spacetime interval $\Delta s^2=-c^2\Delta t^2+\Delta x^2+\Delta y^2+\Delta z^2$.
Suppose we have ...
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votes
1
answer
99
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Would time dilation increase the distance between objects? [closed]
Two objects are moving towards a gravity well. They are at $x$ distance from each other and moving at a fixed speed. The gravity well is massive enough for the objects to experience significant time ...
- 141
3
votes
4
answers
731
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Intuitive Derivation of Length Contraction in Special Relativity via Thought Experiment
I am trying to intuitively derive length contraction in special relativity using a thought experiment, without relying on Lorentz transformations. My aim is to obtain a derivation similar to how time ...
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0
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3
answers
176
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How do I make spacetime diagram consistent with length contraction? [closed]
Say the Earth and a distant galaxy is a distance $l$ away from each other. There is also a spaceship starting its journey at the Earth with a speed $u$ towards the galaxy.
I tried to draw a spacetime ...
- 111
0
votes
0
answers
40
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Twin paradox symmetry in time dilation during the first leg [duplicate]
I was watching this Ted Ed video on Youtube about the twin paradox and found the explanation with the spacetime graph a bit confusing. At 3:00 in the video, they show a graph and explain how bursts ...
0
votes
2
answers
75
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General Relativistic version of the Lorentz factor
In curved spacetime, the Lorentz factor is different than that in flat spacetime.
Is there any expression that gives the Lorentz factor for any arbitrary metric tensor?
user355398
2
votes
1
answer
651
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Relativity of Simultaneity - Is there ever a Preferential Frame of Reference? [closed]
A regular description of the paradox involves a train travelling through a tunnel which is fitted with doors at either end which can be raised and lowered. In their respective rest frames the length ...
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votes
3
answers
213
views
Does proper time have two different values or meanings? [closed]
I'm trying to intuitively understand proper time which is defined as the time that is always measured in a moving observer’s or particle's rest frame and is given by the equation
$\Delta\tau = \sqrt{1 ...
- 335
2
votes
5
answers
355
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Special Relativity and Hypersurface of Simultaneity?
In Special Relativity when I move backwards and forwards my hypersurface of simultaneity moves too.
Does time really oscillate backwards and forwards as I move around? Or is it just the application of ...
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2
votes
2
answers
97
views
Is it possible for a Ricci-flat manifold to have a negative energy measured by distant observers?
Supposing that there is a stationary sphere made of regular matters, the external spacetime geometry can be described by the Schwarzschild metric, which describes a spacetime with vanishing Ricci ...
- 1,455
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3
answers
97
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Is there a "micro" explanation for why time slows down in relativity?
I know the explanation involving a clock based on reflected light. But why would every physical/chemical process be slower? Not that the following is a very good idea, but could it be something like, ...
- 2,238
0
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0
answers
27
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Is it correct to say that acceleration slows the frequency of an oscillator?
My question is based on differential aging or differential timekeeping due to, 1) increased speed, and 2) proximity to center of gravitational field. As far as I know, both involve acceleration, and ...
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0
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2
answers
159
views
Observers in General Relativity: do transformations happen between two different observers or between two local coordinates both of the same observer?
In my understanding of general relativity, I am a little confused. Could someone please clarify this:
Given the definition of a manifold with its collection of subsets(patches) and mapping functions, ...
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votes
1
answer
209
views
What is the definition of an asymptotic observer?
I can't find the definition of it in Wikipedia or in books on my bookshelf.
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vote
5
answers
497
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Is relativity of simultaneity an "observer issue"?
There are some threads about this, but some answers seem to disagree.
First, this is what Einstein said on this matter:
The light rays emitted by the flashes of lightning A and B would reach him ...
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votes
2
answers
148
views
How meaningful is the notion of 'now' here on Earth? [closed]
So special relativity and the lack of a now moment is causing me a bit of grief.
My friends and families notion of "now" doesn't ever line up with mine; but how much of a difference does ...
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votes
1
answer
103
views
Are the distance of things different as we change the reference frames? [duplicate]
x' = γ(x - vt)
We know the time that has passed from each references are different. And it influences the speed of things in each station and bus, but I think that ...
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0
votes
0
answers
39
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Is length contraction actually get things smaller or is it just how one reference frame look at things? [duplicate]
I want to check something..
when we nearly reach the speed of light, we'd look the length of things differently. Although we know that the speed of light is the same for all reference frames. Is it ...
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0
votes
1
answer
120
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Aging when travelling at relativistic speeds
I have a twin. If I acquired a craft that travels at 10% of light speed and wanted to go to Proxima Centauri, it seems now like it would take me 42 years to get there. Would I be 42 years older when I ...
- 740
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1
answer
81
views
Does the ladder really fit inside the barn?
Does the ladder really fit inside the barn?
If we could take a photo, would we see the ladder inside the barn?
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vote
0
answers
170
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Rindler Observers
In the process of transition from STR to GR, I'm trying to understand what Rindler observers actually are. Here is how one of the questions from our assignment defines them:
If the distance between B ...
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votes
3
answers
203
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Twins Paradox: Back on Earth, same moment? (and always) [closed]
Supposing twin A, having been on a well-known circular near-light-speed journey, returns to Earth a younger man than twin B.
Let's say A took off at 3pm and upon return has aged 2 hours while B has ...
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votes
2
answers
92
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Beginner question special relativity: How many clocks does each observer use when measuring simultaneity?
I am reading a bit about special relativity and saw this picture in a book:
If I understand correctly, the author is using it to demonstrate that when we consider observer Alice at rest, she will ...
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1
vote
3
answers
129
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Exact meaning of Lorentz transformations [closed]
This is my first year in Physics and we have been shown about S.R very superficially, just having a bunch of equations to apply. However, I feel rather confused about results such as time dilation, ...
0
votes
2
answers
221
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If a spaceship is heading towards the Earth at a relativistic velocity, is the distance between earth and the spaceship the same in both frames?
Recap: A spaceship is heading towards the earth at a relativistic velocity, if the earth in its frame of reference measures some distance between itself and the spaceship, would the spaceship in its ...
0
votes
2
answers
63
views
Does a person in relativistic circular orbit have the same age as a stationary person at the centre? [duplicate]
Consider a circular orbit whereby a spaceship travels around near the speed of light.
Say the radius of this orbit is such that the angular velocity is low.
An observer is placed at the center of the ...
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votes
4
answers
184
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Spaceships with relativistic instruments
I wonder if it is possible, for future spaceships, to have somekind of instruments on board, that will tell the pilot what his true speed through spacetime is and how much time dilation he is ...
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