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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In QFT, why do fermions have to anticommute in order to insure causality?

I have seen this question and I believe I understand the answer to it. However, AFAIK, only for bosons the causality condition is a vanishing commutator. For fermions we expect the anticommutator $[\...
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If something is not moving in space, is it moving on the time axis at the speed of light? [duplicate]

I heard this theory yesterday: If something is not moving in space, then it is moving on the time axis at the speed of light. I realize that in essence there is no object which can be considered as "...
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4answers
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Does inertia increase with speed? [duplicate]

I have heard that when the speed of the object increase, the mass of the object also increase. (Why does an object with higher speed gain more (relativistic) mass?) So inertia which is related to ...
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1answer
2k views

Does the increase of (relativistic) mass, while flying near speed of light, has any impact on astronauts? [duplicate]

Possible Duplicate: Would travelling at relativistic speeds have any impact on human biology? I am asking myself this question for a few days. What is the correct answer on: Does the increase ...
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2answers
157 views

Have scientists ever experimentally proved the Andromeda paradox-like events? [closed]

We all know that a light year is of the order of 1016 metres and all the objects in the space are beyond a light year. So the Andromeda paradox should give considerable difference in time while ...
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Energy-Momentum Tensor under Lorentz Transformation

In relativity, the symmetric energy-momentum tensor is given by $$ T^{ij}, $$ where $T^{00}$ is the energy density and $\frac{1}{c}T^{10}$ is the momentum density. Thus: $$ \left(\frac{1}{c}T^{00}dV, ...
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Is the light in a Lorentzclock going slower when you travel with it with 0.9c? [closed]

In a Lorentzclock the light is bounced between the two mirrors with his speed of appr. 300.000km/s. Now when things speed up their time goes slower. But the light is always c, so in the Lorentsclock ...
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3answers
64 views

Rotation matrix for aligning x-axis in an arbitrary direction

I want to align the x-axis of my coordinate system, with an arbitrary direction in space $\hat{n}$. About which axis should I rotate? Ceratinty rotation about x-axis or $\hat{n}$-axis will not serve ...
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2answers
92 views

Wouldn't a photon disappear because of length contraction? [duplicate]

I was experimenting with the formula for length contraction, when I realized that anything traveling at the speed of light shrinks out of existence. This is the formula for length contraction: $$T=T'\...
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2answers
46 views

Space-Time Geometry that shortens time intervals?

So we've all heard of the concept of time dilation and length contraction (from both general and special relativity). Suppose we work with a metric of a black hole, and person A is far, far away and ...
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Is there a limit in angular speed in special relativity? [duplicate]

I was thinking in the following problem: Suppose I have a bar of lenght $l$. If I spin it with constant angular velocity $\omega$, according to the special theory of relativity, is there a limit ...
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Why moving charges cause magnetic field (module and direction)?

Why an constant electric current in a wire produces a magnetic field, that circles that wire? I know that this question was posted before. However, all answers talk about Maxwell equations, axioms, ...
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Synchronization of clocks using material waves

I am not a physicist by training, but I have studied special relativity enough to understand why simultaneity is a relative concept, and so why if you synchronize clocks in one inertial frame, an ...
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1answer
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Is it possible to introduce 4-velocity very early on?

I was wondering if it would be possible to teach new physics students about 4-velocity straight away at the beginning of their studies. Keeping the regular notion of the three dimensional velocity $\...
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2answers
225 views

Presentism: doesn't everything exist at the same moment? [closed]

The Help Center recommends I 'fix' this question: (original question) "It seems self-evident that everything exist in the Now. Notwithstanding time-dilation and different rates of the passage of ...
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1answer
80 views

Inference of relativistic time dilation wrong?

One popular example of relativistic time dilation uses the idea of a Light clock where time is measured in terms of cycles the light between the two mirrors (which are at distance L from each other) ...
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1answer
131 views

Relativistic action is negative or positive number? [duplicate]

Possible Duplicate: Why lagrangian is negative number? In the special relativistic action for a massive point particle, $$\int_{t_i}^{t_f}\mathcal {L}dt,$$ where the Lagrangian $$\mathcal {L}...
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In a moving light clock, does the velocity of the clock add to the velocity of the light?

Currently going through the class Space, Time and Einstein from worldscienceu. On module Time in Motion an example is given of 2 light clocks, one moving and one stationary. The point is made that ...
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Does a interstellar spacecraft traveling at relativistic velocity require continous thrust to maintain velocity?

Assuming completely empty space, does a spacecraft traveling at 0.5 C require continuous thrust to avoid deceleration? If the spacecraft is traveling at 0.5 C, does it's relativistic mass act upon ...
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How long does it take to travel 36 light years with tolerable acceleration and deceleration?

The recent discovery of HD85512b only 36 light years from Earth has promising attributes to harbor life. Assuming we want to travel there, we cannot instantaneously jump to light speed, (StarTrek ...
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102 views

In relativity, perpendicular motion does not show contraction… Isn't the whole concept lost?

I read in HC Verma's Concept of Physics that a body moving in direction perpendicular to length, doesn't show Length Contraction. And Length Contraction, they said, is to maintain the velocity of ...
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1answer
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Can anyone clarify this example of Relativity of Simultaneity [closed]

While doing this course: http://www.worldscienceu.com/ I couldn't understand this example http://www.youtube.com/watch?v=53oAxycVhhg. (full example here https://www.youtube.com/watch?v=Iiugtmt18W4) ...
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Expanding Universe [closed]

So an expanding universe has the metric $$ds^2 = g_{\mu \nu} dx^\mu dx^\nu = -dt^2 + a(t) ( dr^2+r^2 d\Omega^2)$$ where $d\Omega^2 = d\theta^2 + \sin^2\theta d\varphi^2$ as the usual spherical ...
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Can sound waves be used as simple explanation of relativity effects in STR?

There are so many similarities (Doppler Effect, independence of wave velocity from source speed etc..). Try moving in your car with music and ask you friend outside record it while you moving towards ...
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Why under Lorentz transformations the Higgs boson is a scalar field and under $SU(2)$ it is a doublet?

I am a bit confused about this difference. My understanding is that when we build a $G$-bundle, where $G$ is a gauge group, we have a representation $\rho:G\to GL(V)$ that acts on the fibers of the $G$...
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Conservation of linear momentum (classical mechanics and special relativity)

How did Newton deduce the law of conservation of linear momentum? Can it be derived only by Newton's laws, or does it follow from practical experiments? If the law of conservation of linear momentum ...
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Information, entanglement and EPR

I'm currently trying to understand why superluminal information transport is not possible. Therefore, I would like to get some help concerning the definition of information or the general setting. ...
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167 views

When particle number can change in quantum physics?

Let me write a paragraph from D.Tong lecture notes on QFT-chapter2 when he is talking about non-relativistic limit of scalar quantum field theory : A related fact is that the conserved charge $Q=\...
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4answers
679 views

How can we show that the speed of light is really constant in all reference frames?

I had a debate with a friend who cannot believe that the speed of light is constant. He said something like: so what if in the Michelson-experiment the moving apparatus simply added a constant ...
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2answers
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Composition of Lorentz Transformations

If a particle is moving in the $x$-direction with velocity $c/2$, then the Lorentz transformation $\Lambda = \begin{pmatrix}\gamma & -\beta \gamma & 0 & 0 \\ -\beta \gamma & \gamma &...
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Where are all the slow neutrinos?

The conventional way physicists describe neutrinos is that they have a very small amount of mass which entails they are traveling close to the speed of light. Here's a Wikipedia quote which is also ...
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What's the max speed a man-made satellite can travel in space before its circuitry stopped working?

Assume it could withstand extreme temperatures in either direction. I'm mainly curious what would happen to its circuits as it approached the speed of light.
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Four Vectors in SR and QFT

I'm covering both special relativity and quantum field theory in the summer. I'm currently using Spacetime Physics by Taylor and Wheeler to cover SR. Since I'm covering SR on the side with QFT, I'm ...
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Why Do Glueballs Have Mass, When Individual Gluons Are Massless?

From Wikipedia Glueballs Glueballs are predicted by quantum chromodynamics to be massive, notwithstanding the fact that gluons themselves have zero rest mass in the Standard Model. Glueballs with ...
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1answer
153 views

Invariance in Euclidean and Minkowski spaces

Consider Wick's rotation from Minkowski to Euclidean space in QFT. What is the connection between O(4) invariance in Euclidean space and Lorentz invariance in Minkowski space? If we define a quantity ...
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1answer
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In Einstein's “relativity of simultaneity” thought experiment, would not the passenger on the train see a dimmer signal? [closed]

This is the updated, more precise question--is this a paradox?: Suppose a rocket traveling close to the velocity of light which emits a single photon from its midpoint at point A, illustrated ...
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299 views

How is time not a constant?

I am a visual person, so it's hard to imagine the information I keep getting, but shouldn't time be a constant? If you were traveling at the speed of light and your able to cover $299{,}792{,}458$ ...
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Why did Einstein retain the hypothesis of length contraction?

Why did Einstein retain the hypothesis of length contraction, which Lorentz introduced to save his æther theory, yet Einstein had no need to accept length contraction because he rejected the æther?
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Relativistic effects

When are relativistic effects justifiably negligible? (I know that that is true for 'small velocities', but how small is 'small enough'?) 0.1c, 0.01c, etc.? And how does one properly justify that? I ...
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1answer
111 views

What does it look like for a ball falling to the event horizon observed by distant static observer?

Here is the picture used in susskind&Lindesay's book ''An Introduction to Black Holes, Information and String Theory Revolution'' I understand very well that the ball will be contracted at the ...
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1answer
140 views

Does speed in space really matter?

This is about validating the science from a science fiction novel. My understanding is that speed is all relative. So while I am in a car on the freeway my speed relative to another passenger is ...
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1answer
37 views

What exactly were the “asymmetries which do not appear to be inherent in the phenomena” in Einstein's 1905 SR paper?

The first ¶ of Einstein's "On the Electrodynamics of Moving Bodies" says (my emphases): It is known that Maxwell's electrodynamics—as usually understood at the present time—when applied to moving ...
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1answer
354 views

Time-reversed twin paradox [closed]

This started with wondering about the nature of certain physical quantities under time-reversal - chiefly, that acceleration retains its magnitude and direction at a given time regardless of the '...
2
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1answer
75 views

What is the meaning of a (1/2, 1/2) representation?

A spin-1 representation is equivalently a (1/2, 1/2) representation of the Lorentz group. Does this mean we are summing together two irreducible representations labelled by the 'quantum number' a 1/2 ...
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43 views

Space travel and time dilation

I want to know how long it takes, from an observer in a spaceship moving at constant velocity $v=\beta c$, to cover a distance $d$. From a stationnary observer outside, the spaceship goes at a ...
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0answers
122 views

Why does the clock work slowly at higher speed? [duplicate]

I know nothing about relativity but I cannot accept that there is a phenomenon called time dilation. However I have no problem with it because of mathematics behind it. I have no problem if time is ...
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1answer
39 views

Why we assign a rotation transformation and not any other when we derive Lorentz factor

I can't truly understand why when we derive Lorentz factor we should assume that the transformation of coordinates is rotational one. In the book "Relativity demystified" they say that: "In some ...
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1answer
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What happens in this collision? [closed]

First, I describe a simple contraption. Second, I describe a simple collision. Third, I replace a box in the collision with the contraption and ask whether, "What happens after the collision?"-i.e., "...
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1answer
115 views

What is the matrix representation of the momentum operator (generator of translations) that is used in the commutators of the Poincaré Group?

So the commutators of the Poincareé group are given by \begin{eqnarray} [J_{i},P_{j}]=i\epsilon_{ijk}P_{k}, \quad [J_{i},J_{j}]=i\epsilon_{ijk}J_{k}, \quad [J_{i},K_{j}]=i\epsilon_{ijk}K_{k}, \quad [...