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Questions tagged [relativity]

The union of special (SR) and general (GR) relativity. Use this tag if both SR and GR apply.

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Derivation of Maxwell's Equations using the Energy-Momentum tensor [duplicate]

If the energy momentum tensor is related to the EM field tensor by $$ T^{\mu v}=F^{\mu \sigma}F^v_\sigma-\frac{1}{4}\eta^{\mu v}F^{\sigma \tau}F_{\sigma \tau} $$ Is it possible to derive Maxwell's ...
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Apparent temperature of the universe at relative speeds

Suppose you are in a vessel traveling at relativistic or near relativistic speed relative to your local galaxy--whatever is physically reasonable, and won't get rapidly torn apart by high energy ...
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Speed of light in vacuum in non-inertial frame [duplicate]

Does the speed of light remains $c$ also in non-inertial frames? Or it changes from frame to frame?
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How to do calculation in relativity of simultaneity and Lorentz contractions [on hold]

I read the beautiful explanation given by Prof. Fabri concerning the calculation metod in relativity of simultaneity. At some point in the explanation Fabri says: (Nothing will be said about their ...
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Examples of fallacies regarding physical phenomena ( laymen's, student's, philosopher's or even past physicists fallacies)? [closed]

Apparently some fallacies can be made in physics, there are posts on this subject on this site : separation fallacy , equal transit time fallacy, thermodynamics fallacy. So this question may not be ...
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Question about Relativity and Proper Reference Frames [duplicate]

If your in a space ship moving at relativistic speeds in reference to an observer on Earth, time would have to be going slower on the space ship and faster on Earth. However, wouldn't the Earth be ...
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2answers
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Perception of time [duplicate]

A video I came across in youtube recently had mentioned how Einstein perceived time. It said he thought of all moments, the present, past and future to exist together and how it is not like one ...
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Why is time 'negative' in a proper distance in Relativity? [duplicate]

so my question is as follows: the proper distance between two events $a$ and $b$, $S_{ab}$ is given by $$S_{ab}^2 = -[x_0(b)-x_0(a)]^2 +[x_1(b)-x_1(a)]^2 +[x_2(b)-x_2(a)]^2 +[x_3(b)-x_3(a)]^2.$$ ...
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Why do we perceive time?

Since everything (at least what we observe) is travelling through the time in the same direction and speed why do we even perceive the time? Of course, there is the time dilatation, but it is ...
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The Expansion of universe [duplicate]

if the space itself is expanding at considerable rate, that means we are also expanding along with it(as we are the part of the universe). so why can't we feel ourselves expanding or detect it?
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Rindler observers are at rest with respect to each other?

I'm studying a chapter about Rindler coordinates right now. In this they say that two Rindler observers at $x = 1/a_1$ and $x= 1/a_2$ will both have speed $v =0$ at $\tau = 0$ compared to the inertial ...
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The role of Lorentz transformation

I will assume that spacetime is flat four dimensional manifold equipped with a Lorentzian metric and define, Physical systems: any object that is capable of causing a response ( measurement) in the ...
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Is the gravitational waves velocity also $c$, regardless the adopted inertial frame, and the source movement conditions? [duplicate]

The second postulate of STR was made exclusively for light (electromagnetic waves)? If gravitational waves also travel at velocity $c$, then are they obliged to fulfill this second postulate? Any ...
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1answer
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The relationship between time, relativity and entropy

I came across a discussion about the nature of time and whether or not time is an illusion on a physics forum. I'm not so much interested in the philosophical issues regarding time, but the following, ...
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1answer
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Dual space and Metric tensor

So I know that the dual space is the set of all linear transformations that map a vector from a vector space to the field of the space itself (the real number line, complex, quaternions). From YouTube ...
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Why do photons get sucked into black holes? [duplicate]

As far as I am concerned photons don't have a mass so how it gets sucked into black holes or is it because of black holes huge mass bends time can some one explain why and what forces or what ever ...
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2answers
59 views

Gravitation in relativity vs classical physics and the 1919 eclipse experiment

I am trying to understand the issue of light bending in relativity vs classical physics. I will clearly describe my understanding/assumptions in case I am wrong on those. In relativity, unlike ...
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Penrose diagram and coordinate transformation

I am looking at the Minkowski line element in spherical coordinates $$ \mathrm{d}s^2 = - \mathrm{d}t^2 + \mathrm{d}r^2 + r^2 \mathrm{d}\theta^2 + r^2 \sin^2(\theta) \mathrm{d}\phi^2$$ and want to ...
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Clarification on Phase wave derivation in De Broglie's paper

De Broglie's paper refers to his theorem A periodic phenomenon is seen by a stationary observer to exhibit the frequency $ν_1=h^{-1}m_0c^2\sqrt{1-\beta^2}$ that appears constantly in phase ...
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51 views

How to show the metric is unchanged under an orthogonal transform?

Say we have the transform such that $x^i \rightarrow (x')^{i'}=M^{i'}_i(x)$ where $M$ is an orthogonal rotation matrix I've been asked to show that a general metric $g_{ij}(x)$ invariant under the ...
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2answers
145 views

Newton's First Law and things that are very old [closed]

Recently I've been revisiting physics text books, and books by Feynman, and others. A curious thought has arisen, while I was reading about Mach's principle, and it seems to gnaw on my mind. I hope ...
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Is there a temporal difference between planets due to the sun's gravitational field?

since the Sun generates a gravitational field it also generates gravitational time dilatation. Hence, time further from the Sun should pass quicker than in its proximity. Can we, therefore, say that ...
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2answers
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Given the Lagrangian of a system, is there a way to extract the total energy?

If an object of mass $m$ is under the action of a conservative force and there are no constraints on the system, can $E=K+U$ be obtained? If yes, I am more interested if the answer could be ...
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Do the fundamental forces change in an object at rest vs the same object moving at relativistic speeds?

Say this object is a magnet, would it have the same magnetic strength if it were traveling at.9c, less, more? Similarly, would gravitational attraction be the same? Would the other fundamental ...
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1answer
66 views

Keeping Time constant instead of speed of light in “Special Theory of Relativity” [closed]

I was reading special theory of relativity and I figured out that, basically this whole theory is revolving around three main factors. Motion(speed) Time Space (length) The result of Michelson–...
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1answer
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How do we know who is moving? [duplicate]

If I am in a free float environment and there is another object at rest relative to me, and then I start to accelerate towards this object, do I know I am the one who is moving, since I am the one who ...
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Calculate EM field of accelerated charge

When calculating EM field of a constant speed moving charge, we can take the EM field of a rest charge (Coulomb's equation) as starting point then apply Lorentz transformation to get the result. When ...
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Is magnetism caused by effect of relativity on interaction between moving charges?

I have heard that cause of magnetism is easily amswered by relativity ? Is it true ? If then how can we explain cause of Fleming's left and right hand rules ?
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Why are number changing interactions inelastic?

Inelastic processes are those in which the kinetic energy of the initial particles and final particles are not the same. Is this true also for number changing processes such as pair creation or pair ...
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1answer
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Terminology: can I use the world “comoving” to describe a reference frame in which a certain object is at rest?

Here a question about terminology. Suppose I have a particle that is moving at velocity $\beta$ in the observer (or laboratory) frame. Now, is it appropriate / legitimate to describe the frame in ...
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If you could bottle a photon would it have mass? [duplicate]

Ok. A photon has no rest mass for the good reason that it has no rest energy. But what about a standing wave? There is fair belief that particles are standing waves, so perhaps nothing rests. So. ...
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Non-existence of double time-derivative of fields in the Lagrangian and violation of equal footing of space and time

In classical field theory, we consider the Lagrangians with single time-derivative of fields whereas double derivative of the field w.r.t. space is allowed sometimes. I understand that the reason of ...
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Would not the oscillation of light slow down the wave? [duplicate]

Would not the oscillation of the light wave slow down the wave? Does light travel at "c" of its net position over time, or does it travel at "c" on the wave?
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Confusion about the twin paradox in general relativity?

The twin paradox states that if you have one twin on earth and one twin is sent in a round trip journey to space in a rocket close to the speed of light, when the twin sent in the rocket returns to ...
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1answer
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Electromagnetic Mass [closed]

Planck introduced a variant/correction term of Einstein’s mass-energy equivalence of the form $E_0 = mc^2 + PV$ The pressure is related fundamentally to a number of terms related to the temperature ...
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1answer
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Time dilation inside a black hole

A clock is sent towards a black hole. My understanding is that because of time dilation, the clock will appear to tick more and more slowly to an observer on Earth, to the point where the time ...
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Different clock rates near a black hole

We have two TVs on Earth that 2 users watch living in different time zones. For them, the TV shows happen at the same hour, they receive signal from the same source. they agree because their clocks ...
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Misner space - closed timelike curves

Could someone give me a good resource to read about closed timelike curves in Misner space? Somewhere where they are actually derived and some comments are made for the simple 2D Misner space?
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7answers
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If the Earth is moving, then how is it possible to slow down light? [closed]

Given the following: Earth is spinning on its axis and also rotating around the sun. Light has no mass and is not affected by our moving atmosphere When we slow down light as is done here to a mere ...
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2answers
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On Distant Lanterns and Relative Speed [closed]

It is said, that by measuring distant sources, we can gather information about the past state of the universe. The further we measure, the faster things appear to recede, which brings me to my ...
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108 views

The chain rule and velocity transformation in relativity

From elementary calculus, we have that the chain rule occurs when we differentiate a function like $f(y(x)) \equiv f(x)$: $$\frac{\mathrm{d}}{\mathrm{dx}}[f] = \frac{\mathrm{d}}{\mathrm{dx}}[f(y(x))] ...
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Lattice gauge theory under a Lorentz transformation

Taking a grid of 'evenly-spaced' space-time points. e.g. at integer values of (x,y,z,t). Now do a Lorentz boost on this grid. We end up with a grid of points which are much closer together. It is ...
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2answers
107 views

Is a finite lightspeed necessary for cause and effect to exist? [duplicate]

In this question I asked in somewhat unclear (unclear enough to be closed) fashion about the relationship between an infinite speed of light and the existence of cause and effect. In thís question, I ...
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Thought experiment - travelling one hour at the speed of light [duplicate]

I'm new to physics and I've been debating a thought experiment with my friends. I think something similar has been answered on this site but to be honest I don't understand the responses - if anyone ...
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1answer
46 views

Einstein's Theory of Relativity Time Dilation

My friend claims that this proves that time moves faster on a mountain than at sea level. He says that since both of these people start and end at the same angular position and are both traveling at ...
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2answers
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Relativity Confusion

In the time dilation formula $\Delta t=\gamma\Delta t_0$, I am confused about what $\Delta t$ and $\Delta t_0$ measure. If we have two people, Tom and Bob, moving at constant velocity relative to ...
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139 views

Time dilation in a gravitational field

A friend and I were talking about relativity and we had different opinions about this situation: If I choose a frame of reference on the surface of a planet, would time "run" differently between the ...
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1answer
77 views

Toy almost everywhere flat metric

I was looking into this paper from 2003 by Krasnikov - “The quantum inequalities do not forbid spacetime shortcuts” - and apart from the fact that I find it very scetchy without many details and hard ...
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It is said that an object cannot exceed the speed of light, because high speed makes the object closer to infinite mass, but speed relative to what?

That is, the speed of an object, when getting closer to speed of light, makes the mass of the object closer to infinite, but isn’t speed relative? So a man on earth feels a metal ball on the table has ...
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Question (mostly) About Relativity

Before I get into the actual question, I'd like to say that my only knowledge on Einstein's Theory of Relativity (both Special and General) have been obtained through about an hours worth of videos ...