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

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Which one is inertial frame?

I'd like to ask a question which made me confused. I think that, e.g., a man falling to earth could think himself as stationary and the Earth as accelerating towards him. But I, as an observer, I ...
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1answer
127 views

Null Coordinates

I have a very basic question: what are the advantages of writing a metric in the null coordinates? Which extra insight do they provide? I've looked in Caroll's "Spacetime and Geometry" and Wald's ...
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32 views

What would an relativistically expanding sphere look like for an external observer?

Consider a reflecting sphere, whose radius is increasing at constant rate V close to light speed. What would the collision look like for a remote observer stationary relative to the centre of the ...
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1answer
72 views

Possibility of simulating quantum physics and relativity [closed]

I must be clear; this is not a 'specific' question, but rather a hypothetical one. As of now, computer physics simulations are becoming more and more advanced, and I am programming one now. I was ...
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1answer
145 views

Is there any relationship between the $E=mc^2$ equation and the $a_n=\kappa v^2$ formula for the normal component to acceleration?

To clarify, I know very little about physics and don't pretend to have any insight whatsoever into relativity beyond what has entered the popular imagination; my knowledge is more or less at the level ...
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0answers
94 views

Will relativistic glider cause lower gravitational force in Newtonian gravitation?

In the articles The relativistic glider and The relativistic glider revisited, the relativistic glider is proposed where a quasi-rigid body can slow down its fall under gravity without any reaction ...
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1answer
51 views

Relativistic Induced Magnetic Field in particle's frame

Suppose there is a constant magnetic field: $\vec{\mathbf{B}} = B \hat{z}$. A charged particle orbits that magnetic field perpendicular to the magnetic field, and induces a magnetic field in the ...
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4answers
170 views

Why are we still teaching Newtonian physics if it's “false”? [duplicate]

Why are we teaching an outdated theory when the math in general relativity isn't that hard? I really don't see a reason why we should teach a highly oversimplified theory in our schools. For me, it's ...
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0answers
35 views

Does a body curve spacetime at higher velocities? [duplicate]

Suppose we have two objects where the distance over time decreases. Now, as I understand it, general relativity says that we can observe the Universe from the perspective of both objects an get a ...
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0answers
15 views

Can I choose any reference frame? [duplicate]

I'm a horrible noob at anything past mechanics, so please bear with me. According to Einstein's theory of relativity (at least as it is known in "popular science" (lowercase)), mass-full objects ...
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0answers
25 views

Relativity question [duplicate]

If a rocket is traveling away from earth, why does time pass slower for the rocket if from the rocket's perspective, earth is going away from it?
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2answers
74 views

For a rocket travelling to a planet, which reference frame measures the dilated time?

I'm getting into some confusion with relativity. Consider a rocket and the Earth as two observers, and then the rocket travels to a distant star. Is the proper time taken measured in the frame of ...
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2answers
201 views

“Entangled photons never show interference in the total pattern without coincidence count” implies FTL

In my previous question, the most defended objection to the gedankenexperiment was that "Entangled photons never show interference in the total pattern without coincidence count". Here I show another ...
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26 views

Using Special Relativity in Uniform Circular Motion [duplicate]

Can one utilize the formulas of Special Relativity in uniform circular motion? The radial component of the object does accelerate, but sometimes we are just interested in its tangential speed. Here ...
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1answer
139 views

Question about time dilation and relativity [duplicate]

Take this scenario for example: I am on a rocket ship that will take a trip into space and back at 90% the speed of light. And my friend "Bob" stays on Earth. As soon as I leave Bob and I both start ...
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0answers
48 views

Two-Observer Spacetime Diagram

What is the significant in a two-observer spacetime diagram's primed axes? More specifically, the area enclosed by them. Obviously, if an event occurs within the axes, it is simple to find its ...
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1answer
43 views

The Higgs field and spacetime intervals

The Higgs field imparts a rest mass to certain fundamental particles, but it also seems to do something more interesting. Particles that don't couple to the Higgs field, i.e. the rest-massless, are ...
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1answer
45 views

Higgs field and the null geodesic

How does coupling to the Higgs field take a particle with a spacetime interval of zero and give it a non-zero value, i.e, how does it move it off the null geodesic? [of course, the Higgs field is ...
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3answers
113 views

Is it possible to express various nonlinear motions as straight lines in transformed spacetime?

I am trying to understand simple examples of space-time curvature. Assume for the moment that $c$ is infinite (classical curvature due to Newton's laws). Also, I will only consider 1+1-dimensional ...
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2answers
88 views

Do bad clocks measure proper time?

It has been claimed several times e.g. here, and here, also here, or (outside PSE) there that Clocks measure proper time. Or equivalenty: proper time is just the time that would be ...
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1answer
90 views

Gravitational Time Dilation Formula? [closed]

What is the equation for calculating the dilation of time at different levels of gravitational force?
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3answers
133 views

Is acceleration caused by curvature or space or time or both?

I'm trying to get a hold of the idea of gravity in general relativity and spacetime. I've seen plenty of demonstrations of the rubber mat analogy to describe gravity and spacetime curvature. Is this ...
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7answers
498 views

“Reality” of length contraction in SR

I was in argument with someone who claims that length contraction is not "real" but only "apparent", that the measurement of a solid rod in its rest reference frame is the "real length" of the rod and ...
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0answers
76 views

Books on special and general relativity? [duplicate]

I have idea about all the theories related to space and time and stuff. But I wanna dig deep into it and understand all the theories like special and general relativity to begin with for now. Can ...
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4answers
127 views

Twin paradox where the twins start at different locations

Suppose we have this scenario with twins A and B: 1) Instead of the twins starting at the same location, let's say the twins start out some distance apart, in the same reference frame. 2) The ...
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1answer
69 views

What are the two dimensions of relativity that are added to string theory?

Based on the Ramanujam's modular functions, somehow these magic numbers 10 and 26 spacetime dimensions appear in string theory. The dimensions can be viewed as 8 + 2 and 24 + 2. The number 2 is added ...
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1answer
229 views

From a particle's point of view is his temperature absolute zero

Since the vibration of the particles is what cause temperature and a particle from its own point of view is not moving, is its temperature 0°K from its own point of view? Is there a thing like ...
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4answers
291 views

If Julian Barbour is Correct, is the Speed of Light Special?

According to this article in Discover Magazine, Albert Einstein's Theory of Relativity is wrong, because it didn't fully live up to the ideas of Einstein's idol, Ernst Mach. Mach proposed a truly ...
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2answers
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Is space-time a property of mass?

Imagine looking at a quasar millions of light years away. From your perspective, a photon emitted by the quasar has spent millions of years travelling through space, and its trajectory has been ...
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2answers
113 views

Time dilation in outer space vs in a gravitational field? [duplicate]

I have been wondering how could aging process differ for a person moving close to velocity of light or for a person close to a very strong gravitational field compared to a person who is in a slow ...
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0answers
25 views

Can a characterization of “inertial motion” be expressed in terms of interval ratios?

Consider a "material point" $A$ having taken part in the two distinct events $\varepsilon_{A J}$, and $\varepsilon_{A Q}$, as well as in additional events "inbetween", such as any event ...
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1answer
39 views

Is there a rigorous, explicitly geometric, general characterization for whether a given clock had been “good”, or not?

In MTW, "Gravitation", there is a brief section (p. 26 and Fig. 1.9) discussing that clocks may be characterized, trial by trial, as having been "good" in the trial under consideration, or having been ...
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1answer
74 views

Can we calculate L-S coupling without Dirac equation?

It is known that there exists an orbital and spin angular momentum coupling for an electron moving in the atom. And the Hamiltonian can be directly derived using Dirac equation. I want to use a ...
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1answer
91 views

If all energy is removed from an object, what will it stop moving relative to?

If you somehow removed all energy from an object, what would it stop moving relative to? According to relativity, an object's position is not relative to the universe, just reference points, and, as ...
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How do tachyons violate causality?

Something moving faster than light should have imaginary mass, like photons have zero mass and thus travel at $c$. I have read this article of physicist E. C. George Sudarshan. He said taking mass ...
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1answer
113 views

Doesn't the second postulate of special relativity imply the existence of “luminiferous ether”? [closed]

Einstein expressed his Second Postulate in On the Electrodynamics of Moving Bodies as: light is always propagated in empty space with a definite velocity $c$ which is independent of the state of ...
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2answers
201 views

Why does “closing speed” $>c$ not violate relativity's second postulate? [closed]

Why does closing speed $>c$ of a particle and photon or photon and photon not violate relativity's second postulate? Einstein expressed his Second Postulate in On the Electrodynamics of ...
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22 views

Thomas Precession of an orbiting electron

So I am working through a problem on Thomas Precession. The set up is that we're given at lab time zero, the initial spin of an electron in circular orbit is: $$ ...
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1answer
133 views

Why is it legitimate to only observe motion, but call it time, such that concepts like “the relativity of simultaneity” are seen as possibly genuine? [duplicate]

My question is asking for evaluation of a new concept or paradigm within the framework of the current mainstream physics understanding of Relativity. Specifically, that concept or paradigm being that, ...
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3answers
292 views

Where in, Einstein’s “On the Electrodynamics of Moving Bodies” (Relativity), is “Time” reasonably shown to exist, or why is it a valid assumption? [closed]

Given the widespread acceptance that “Einstein’s Relativity” reasonably proves the existence of Space-‘Time’, and thus “Time”... Can any member of the Physics StackExchange please show precisely ...
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2answers
51 views

Time Dilation, Multiple Earth's?

I was reading about time dilation and suddenly a question raised in my mind. (This might be a misconception but please do correct me if I am wrong) I read that if we travel near to the speed of ...
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2answers
503 views

Feynman's proof for Liénard-Wiechert's potential of a moving charge

Feynman's proof utilizes a geometrical and fundamental integration argument. I like it, except this bit: What makes me unconfortable somehow is that in (c) we are counting in some of the charge we ...
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4answers
98 views

Time dilation if earth was stationary

Being on earth, we are constantly flying through space at incredible speeds. We revolve around a sun in a system that revolves around the center of the galaxy that is hurling itself through space. ...
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0answers
49 views

Doppler effect for the electrons emitted from ions moving at a particular velocity

If I have ions moving in X axis (along both directions; positive X axis and negative X axis) and if I irradiate laser along the positive X axis, electron will be ejected from the ions (if I am using a ...
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1answer
132 views

Photons inside a box

One of my friends told me that the definition of mass is the amount of matter. I told him that mass is not the amount of matter, because when we heat an object, the mass of the object increases. I ...
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1answer
72 views

What is the definition of inertial mass?

What is the definition of inertial mass? I can see two options, either it's the coefficient associated with the object being accelerated in Newton's 2nd Law, or it's the coefficient relating momentum ...
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1answer
91 views

Perpetual Mobile and Gravitation

I have fundamental question about what is called the “law of conservation of energy”. We all hear about the tidal power stations which using the tidal power. The source of the tidal power came from ...
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2answers
545 views

Wormholes, time travel, and time dilation

I believe I understand the "wormhole time machine" now, but still have one question regarding what happens if you enter the stationary mouth. A wormhole is created and (of course) has two mouths, $x$ ...
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1answer
27 views

Can we measure a mass increase for a system with more energy?

This question is about whether the mass of a particle increases with its speed. Here is my related question from which this question spawned: Why is there a controversy on whether mass increases with ...
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3answers
182 views

Why does time slow down the closer you are to a mass?

When ever i look this up all I get is sites saying how its because general relativity says "-" why does it do it though? it is because there is more motion near gravity than further away? Or is it ...