The special theory of relativity describes the motion and dynamics of objects moving at significant fractions of the speed of light.

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Help with relativistic notation (Derivative of Lagrangian)

I am trying to learn QFT, but I haven't taken a course in general relativity so the relativistic notation stuff is taking me a bit to get used to. I do not understand how to do the following. For a ...
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Speed of light, observed speed while travelling at the speed of light

I was watching Discovery channel the other night, they were telling that time slows down when you travel at a higher speed. This means there is a difference between the actual speed you travel at, and ...
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Why time is considered a dimension?

Why is time considered to be a dimension? And the other 7 (except the 3 dimensions of space, and the dimension of time) dimensions that string theory suggests, why can't they be realized?
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319 views

Doppler shift for a uniformly accelerating observer

This was given in textbook as an example. An observer on a spaceship with a four velocity $u$ is approaching from $x = +\infty$ a star at rest in the reference frame $S$ while undergoing constant ...
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1answer
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Translation Transformation on the Invariant Interval (Spacetime) [closed]

So we know that the invariant interval in a two-dimensional spacetime in special relativity is given by $$ s = -c^2t^2 + x^2 = -c^{'2}t^{'2} + x^{'2}$$ So this scalar should hold true in all ...
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Help Me Gain an Intuitive Understanding of Lorentz Contraction

I'm having a hard time getting an intuitive understanding of Lorentz Contraction. I understand what it is by definition but I don't 'get it.' I'm not a physicist, just an amateur, so sorry if this ...
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4answers
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Could the Heisenberg uncertainty principle be derived from the speed of light limit? [closed]

Background: As I was thinking about an alternative approach to the question: "why is there a finite speed of light, and why its magnitude corresponds to c?" –ultimately, I was trying to understand ...
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2answers
49 views

Regarding relativity of simultaneity

I recently started studying Special Relativity an my book discusses the following: Say I have synchronized two separated clocks in a reference frame S, if then an observer in another reference frame ...
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1answer
63 views

how long will take for an astronal travel in near lightspeed in his point of view

I always had a question about relativity. Suppose an astronaut in orbit Saturn just must reach earth fast as possible. Your ship can travel at any speed between 0 and the speed of light. the distance ...
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533 views

What is the highest speed time dilation has been tested?

What is the highest speed time dilation has been tested? How close to the Special Relativity prediction did it get?
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Geometric definition of the Lorentz inner product

In Euclidean space one can define the dot product as projecting one vector to the other and multiply the length of the projected vector with the length of the other vector. This definition doesn't ...
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Maximum speed of a rocket with a potential of relativistic speeds

Ultimately, the factor limiting the maximum speed of a rocket is: the amount of fuel it carries the speed of ejection of the gases the mass of the rocket the length of the rocket ...
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0answers
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How do I form the four-momentum quantum operator?

I am trying to form the four-momentum quantum operator. These are the steps I have taken so far: The 3-momentum operator is given by $ \hat{p}_{i} = -i\partial_{i} $. This is covariant because it is ...
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2answers
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Understanding the formula in an exercise

I'm stuck for quite some time now on this issue. I have the following question. Spaceship with velocity of $0.5c$ passes above two points $A$ and $B$ the distance between $A$ and $B$ respect to the ...
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1answer
30 views

Special Relativity and Composition Law for Velocities with respect to an observer watching two objects moving away from a central point

So I understand that special relativity is all about the frame of reference and there is a lot to do with time dilation and how space-time is warped at velocities near $c$. So my question is what ...
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1answer
47 views

Why does length contraction seem to conflict with invariance of intervals?

Suppose we have two simultaneous events, $A$ and $B$, separated by a distance $L$ (the simultaneity is in frame of reference $S$). Now suppose we have a second frame of reference $S'$ moving with ...
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Do tachyons move faster than light?

I am trying to understand whether or not tachyons travel faster than light. The linked Wikipedia page shows some seemingly contradictory statements, and they are confusing. For instance, the first ...
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142 views

Is distance always 0 relative to an object moving at speed of light $c$?

As I understand it, when an object is traveling at the speed of light, relative to itself all travel is instantaneous and the distance is zero. If a photon traveling from the sun was aligned with the ...
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2answers
140 views

Can the constancy of the speed of light in vacuum be derived from a deeper theory?

Einstein's second postulate of special relativity is: The speed of light in vacuum takes the same value ($c$) for any observer in an inertial frame of reference. I know there is a lot of ...
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1answer
99 views

Particle here at a given time, in another galaxy a second later… Really?

I read "The Quantum Universe (Cox & Forshaw)" that a particle can be measured at a given position at a given time, and in another galaxy one second later. The probability of such event may be ...
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Hidden character in EPR paradox [duplicate]

I am a beginner in Quantum Mechanics so i am pretty new to the EPR paradox although i have heard about it a long time ago but finally studying in detail. And came across a doubt: Why the hidden ...
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1answer
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Minimum gravity required to suck light? [closed]

I was studying about gravitational forces of black holes when I came to a thought of what is the minimum gravity required to capture light in vacuum?BY capturing I mean the inability of light to ...
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2answers
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What do physicists mean by “information”?

On the question why certain velocities (i.e. phase velocity) can be greater than the speed of light, people will say something like: since no matter or "information" is transferred, therefore the ...
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1answer
33 views

Is light send from the top of a ship to deck different when the ship is moving or not?

If you drop a ball from the top downwards to the deck it will follow a rather straigth line because the ball gets also a force horizontaly because the ship is moving. That is the cause in history why ...
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1answer
113 views

Does antimatter travel faster than light

I have read in the answers here that an electron traveling backwards in time would behave as a positron. I also read in another there that antimatter is matter traveling backwards in time As far as I ...
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3answers
86 views

When calc. time elapsed for an inertial reference frame during travel between two points, are length contraction and time dilation taken into account? [closed]

When calculating the amount of time elapsed for an inertial reference frame over the course of its travel at constant velocity between two points, are the effects of both length contraction and time ...
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Direction of the lightbeam in SRT thought experiments [duplicate]

Extending on the question about the time dilation on Wikipedia, namely this diagram in the accepted answer: Image credit: John D. Norton The thing that I don't understand is, why does the light ...
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What is the proper way to explain the twin paradox?

The paradox in the twin paradox is that the situation appears symmetrical so each twin should think the other has aged less, which is of course impossible. There are a thousand explanations out there ...
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Poynting vector from 1st term in Lienard-Wiechert field

I start with 1st (non-radiative) term from Lienard-Wiechert fields: $$ \vec{E} = q (1-v^2) \frac{\vec{R_{t'}} - \vec{v}R_{t'}}{(R_{t'} - \vec{v}\vec{R_{t'}})^3} $$ $$ \vec{H} = - q (1-v^2) ...
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1answer
162 views

Does Inertial time dilation demonstrate that Time is not a dimension? [duplicate]

If time is a dimension and 'now' simply an expression of your position with respect to that dimension, the progress of any object along that dimension should remain in step with all other objects. By ...
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101 views

Objects travelling relatively to each other faster than light?

When we say that something is travelling a certain speed, it's really travelling that speed relative to the Earth. When saying the speed of anything, it is, for the most part, relative to something ...
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2answers
96 views

A twin-like paradox, with no acceleration and no 3rd reference point. What would happen?

The following is a variation on the twin paradox, with no acceleration and no 3rd reference point (i.e. described purely in terms of relative motion). Imagine two observers initially located right ...
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1answer
147 views

Does the rest mass energy include the potential energy of the particle?

The potential energy (as far as I have studied - that is, mainly classical physics) depends on the reference level, since its absolute value cannot be calculated. It can therefore be negative as well. ...
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Is this a fundamentally relativistic phenomenon?

This question was inspired by some silliness in other threads but is independent of that silliness. Say that a train car sitting on a track is accelerated uniformly along its length if each point on ...
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1answer
83 views

How is the Lorentz group related to spin? [closed]

I've been reading about the agebra of the Lorentz group. It is given by, $$G\equiv SO(1,3) ~\cong~ SU(2)\times SU^*(2)$$ Now, representations of this group $G$ as labelled by $(j,j')$ where $j$ is ...
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48 views

Ether (LET) vs electromagnetic field in the double slit experiment

According to this answer, Lorentz Ether Theory (LET) is experimentally indistinguishable from SRT, because the Ether is immobile and practically redundant. However, on quantum scales, photons are ...
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A train on a track--relativity paradox

While answering a question over on Worldbuilding.SE I found myself looking at a situation that I can't figure out: You have a train track of length L that makes a very large circle. You have a train ...
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When a body accelerates, it gains (relativistic) mass; after stopping, is the (relativistic) mass different from before it started accelerating?

When a body accelerates, it gains (relativistic) mass $m$ according to the relation $$m=\frac{m_0}{\sqrt{1-(v/c)^2}},$$ where $m_0$ is the (rest) mass. But after it stops is the gained (relativistic) ...
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2answers
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Does the existence of “gravitational waves” (assuming they exist) imply that time exists as a 4th dimension in the universe? [closed]

I'm new to thinking about special and general relativity and I have no formal training as a physicist. However, I've been doing a bit of thinking about spacetime recently. I was wondering if ...
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2answers
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Is kinetic energy a relative quantity? Will it make inconsistent equations when applying it to the conservation of energy equations?

If the velocity is a relative quantity, will it make inconsistent equations when applying it to the conservation of energy equations? For example: In the train moving at $V$ relative to ground, ...
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3answers
64 views

Can the passage of time be measured in the absence of motion? [duplicate]

All of the ways I have heard of to measure the passage of time involve measuring some sort of motion (e.g. vibrations of a cesium atom, movement of the hands on a clock, etc.). Can the passage of ...
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1answer
54 views

Relativity of simultaneity: two light sources

A. Suppose a moving train. The train has two wheels(front and back) and each wheel is connected to light source inside the train. The light source is triggered(light is emitted) when train passes ...
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2answers
135 views

Lorentz contraction explanation correct? [closed]

I was having a heated SE chat debate with someone re relativity. One of us believes the explanation above is correct, while the other believes it is not. Who is correct, and, if the explanation is ...
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1answer
74 views

Would forcing a photon to travel under $c$ even after it leaves a medium break Relativity? [duplicate]

So, I stumbled onto this article which really blew my mind (http://www.bbc.com/news/uk-scotland-glasgow-west-30944584). To sum it up, these researchers set up some kind of material that slowed a ...
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1answer
58 views

Does travelling towards something at relativistic speed cause it to appear to speed up?

If one were to travel towards a giant TV floating in space, at a fraction of the speed of light, would a video playing on the TV appear to play faster? I'm guessing the answer is no. From what I ...
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5answers
316 views

How is it possible for the wavelength of light to change in a medium?

So my physics class has just finished a long unit on optics while at the same time I've been trying to teach myself relativity. I admit my understanding is probably rudimentary, but I figured all the ...
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In relativistic electrodynamics , why doesn't the Lorentz force not change its formulation ? And only the electric and magnetic fields transform ?

My question is that when we want to find the Lorentz force acted on a particle moving in an electric and magnetic field , the equation is invariant in any two inertial relativistic frames. Why is that ...
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High speed and low speed photons

Looking at the discovery of the neutron, and I came across this page: http://www-outreach.phy.cam.ac.uk/camphy/neutron/neutron3_1.htm The animation on the left, talks about low energy photons and ...
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1answer
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Lorentz Invariants and Time dilation/Distance contraction?

If distances contract in direction of motion and time dilates at high speeds, why are the rest mass $m_0$ and proper time $t_0$ called "invariant" under Lorentz transformations. Since depending on ...
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Mass has the same value in all inertial reference frames?

Is mass the same in all inertial frames? If it is, why is that? If not, can you also explain?