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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+100

trouble reconciling doppler shift for energy and momentum of light

[See late addition at bottom for what currently remains unresolved in this question.] I have seen the assertion in a couple of places that if you trap light in a reflective resonator, or any other ...
0
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0answers
77 views

How does the wave function change upon a boost? [on hold]

I was recently thinking about Lorentz invariance description of a particle. In special relativity is based on the invariance of $$ \text{constant} = c^2t^2 - x^2 - y^2 - z^2$$ I was wondering if ...
6
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3answers
300 views

Lorentz-invariance of step function

I was reading about the Lorentz invariant integration measure $\int \frac{d^3k}{2E_K}$, and ways to prove that this was Lorentz invariant. Many of the proofs I have read use the step function (or ...
3
votes
3answers
131 views

What happens to forces when a charge travels at near c?

Suppose 3 electrons are travelling at $c - 1$mm at 1 cm distance The charges are at rest relative to one another in their frame, but what happens to fundamental forces Fe and Fg? What Fe does A get ...
8
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2answers
349 views

Is it possible that all “spontaneous nuclear decay” is actually “slow neutrino” induced?

This thought was inspired by a comment from the current leading answer, by @Sentry, to the question Where are all the slow neutrinos? This [slow-neutrino induced nuclear decay] will still be an ...
11
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12answers
1k 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 ...
9
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5answers
816 views

With respect to what can't we travel at the speed of light? [closed]

According to theory of relativity the speed of light in vacuum is ultimate. But since objects move relative to each other, with respect to what can't we travel at the speed of light?
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1answer
75 views

How fast does matter grow more massive when accelerating? [on hold]

I am wondering since it is understood that mass of matter increases as its velocity increases, how fast does this process occur if it is a physical process? For instance, if a mass initially at rest, ...
2
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3answers
81 views

Length Contraction in Relativity

Let's suppose we have a one dimensional rod made of elementary point particles, in contact with each other placed along the x-axis.If the rod is moving along x-axis then we know (because of relativity)...
-2
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1answer
44 views

Minimum and Maximum Speeds of a Mass [on hold]

I understand that it is impossible for a object with rest mass to travel at $c$ (the speed of light) due to time limitations. I also understand that the speed of an object is calculated using a frame ...
37
votes
10answers
33k views

How can a photon have no mass and still travel at the speed of light?

I've read a number of the helpful Q&As on photons that mention the mass/mass-less issue. Do I understand correctly that the idea of mass-less (a rest mass of 0) may be just a convention to make ...
0
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1answer
52 views

Action at a Distance [duplicate]

Since photons move at $c$, do they experience time or distance? If they don't, doesn't this explain action at a distance? From the point of view of the photons, there is no time, so the action at a ...
29
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6answers
4k views

Does the speed of light in vacuum define the universal speed limit?

Is light the thing causing the universal speed limit to be $299\,792\,458\,\mathrm{m/s}$? So the universal speed limit would be different if light travelled faster or slower? Or, is $299\,792\,458\,\...
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1answer
40 views

special relativity question [on hold]

How can I find the velocity of an object given the time elapsed of trip from the frame of reference of the object and the distance from the frame of reference of Earth.
2
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3answers
81 views

Plane waves in special relativity

I don't understand how there can be plane waves that by definition are spread through all of space if nothing can travel faster than light. Wouldn't every wave have to spread over time with at most ...
2
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2answers
437 views

How do I describe the order of events in spacetime?

In Newtonian mechanics, we have 3-space and time. Time allows us to order events. For example, I hold a ball in the air at time $t_0$. Then I drop it at some later time $t_1 > t_0$. Finally it hits ...
0
votes
2answers
90 views

As electron is accelerated nearly to the speed of light what would occur? [closed]

What would occur if an electron at rest was accelerated nearly to the speed of light? Would the photons emitted by the electron approach a gamma wave photon as it approached the speed of light, or ...
-4
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0answers
37 views

Parenthetical tensor notation [on hold]

Just out of curiosity, what does it mean to be a type $(n,m)$ tensor? For instance in the context of special and general relativity, the Minkowski metric $\eta$ is considered a type $(0,2)$ tensor. I ...
2
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3answers
88 views

Bell inequality violations evidence for 1935 EPR claims?

Is it possible that Bell inequality tests provide experimental evidence in support for the EPR claims in their 1935 paper titled "Can Quantum-Mechanical Description of Physical Reality Be Considered ...
-2
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1answer
47 views

Time dilation of a body A with respect to B [duplicate]

Let there be two bodies a and b. Let a be on Earth and b in space with velocity $(√3/2)c$. Then let the time interval which has passed (on the earth) be twice that of the time interval of which passed ...
2
votes
1answer
106 views

Does the factor $k$ in Hooke's law change in special relativity? [closed]

let's say an object is tied to a spring and it's oscillating. But this system is moving in 0.5c. Then, the period of this oscillation increases. But the problem is $k$. I think the $k$ increases ...
-3
votes
1answer
50 views

Constancy of the speed of light in the hypotenuse of the triangle in the Michelson and Morley experiment [closed]

In Michelson and Morley experiment, Michelson calculated that the total time traveled by light with respect to the stationary observer standing at ether field was $2L/c$. Here, Michelson followed ...
3
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1answer
67 views

$(1/2, 0)$ representation of the Lorentz Group $SO(1,3)$

Let us consider the $(j, j') = \left(\frac{1}{2}, 0\right)$ representation of $SO(1, 3)\cong SU(2) \otimes SU(2)$. $j = \frac{1}{2}$ corresponds to $SU(2)$ generated by $$ \tag{1} N_i^+ = \frac{1}{...
0
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1answer
57 views

Can we work out how fast we are traveling by looking at our mass?

If we (the galaxy) were traveling close to the speed of light; relativity says we would need proportionally more energy to go faster. Given that relative to the cosmic microwave background, the Local ...
0
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1answer
63 views

Is there energy output when mass moves between two spacetimes [closed]

Is there energy output when mass $m$ moves between two spacetimes? Say, it starts in a flat spacetime and then falls into a black hole (other examples don't come to mind, but this doesn't mean they ...
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0answers
32 views

The “Other” mechanical clock appears to click slower from both observers point of view. How? [duplicate]

In professor Shankar's Yale University lecture Lorentz transformation video, https://www.youtube.com/watch?v=202fU9qIVK4 , at the time period between 49:00 to 53:00, he describes light clocks and how ...
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5answers
922 views

Why Sun light can reach us if Time is dilating?

I understand that if something is moving with constant speed in respect to an observer, the time of the moving one runs slower, so the more your speed is, the more your time ticks slower. Which ...
6
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2answers
244 views

Performing Wick Rotation to get Euclidean action of scalar field

I'm working with the signature $(+,-,-,-)$ and with a Minkowski space-stime Lagrangian $$ \mathcal{L}_M = \Psi^\dagger\left(i\partial_0 + \frac{\nabla^2}{2m}\right)\Psi $$ The Minkowski action is $$ ...
3
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4answers
6k views

Relativistic Doppler effect derivation

This is about a step in a derivation of the expression for the relativistic Doppler effect. Consider a source receding from an observer at a velocity $v$ along the line joining the two. Light is ...
3
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1answer
80 views

Why does the Klein-Gordon propagator $D(x)$ depend on the sign of $x^0$?

In A. Zee's Quantum filed theory in a nutshell, it says the Klein-Gordon propagator depend on the sign of $x^0$. Here $x=(x^0,x^1,x^2,x^3)$. $$D(x)=-i\int \frac{d^3k}{(2\pi)^32\omega_k}[e^{-i(\...
2
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2answers
345 views

Watching a live stream in spaceship traveling away from earth close to speed of light

My name is Dennis. This might sound just straight up ignorant of basic concepts of physics. I was having a conversation with my friend in regards to streaming a live event on earth to a spaceship ...
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1answer
118 views

What is the velocity of the photon through the fourth dimension x4? [closed]

Photons are real, physical objects. The fourth dimension is a real, physical entity. Therefore, photons must have a relationship with the fourth dimension. They must have some velocity relative to ...
3
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0answers
82 views

Question on the Derivation of Lorentz Invariance of Electric Charge

On Zwiebach's "A First Course in String Theory" textbook, there is a problem 8.2 that derives the invariance of electric charge given a few assumptions: 1. Conservation of Maxwell 4-current $j = (c\...
2
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1answer
148 views

Does Special Relativity require a “ruler postulate” analogous to the “clock postulate”?

It's fairly well known that the clock postulate is needed in Special Relativity when dealing with accelerated clocks, so does something analogous exist when dealing with accelerated spatial ...
2
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1answer
326 views

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 $[\...
4
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3answers
1k views

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 "...
2
votes
4answers
8k views

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 ...
1
vote
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 ...
0
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2answers
155 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 ...
5
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4answers
964 views

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, ...
-4
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1answer
76 views

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 ...
1
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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 ...
3
votes
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'\...
1
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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 ...
1
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0answers
37 views

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 ...
0
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1answer
1k views

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, ...
0
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2answers
61 views

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
48 views

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
224 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 ...
2
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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) ...