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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3answers
480 views
Maxwell equations invariant under Lorentz transformation but not Galilean transformations
Why Maxwell equations are not invariant under Galilean transformations, but invariant under Lorentz transformations? What is the deep physical meaning behind it?
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5answers
503 views
About the Ether Theory acceptance
Why was the Ether Theory refused by Modern Physics? If you please explain me, I just wanted to understand it more.
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2answers
207 views
Causality and Quantum Field Theory
I have a problem with proof of causality in Peskin & Schroeder, An Introduction to QFT, page 28. To avoid confusion I use three vectors notation, rewriting the Eq. (2.53) for $y=0$ as follows:
...
2
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2answers
106 views
Why is there no time dilation for frequency of a wave?
Since frequency of a wave is a function of time, then for a particular ray of the wave, why would the frequency remain the same when observed from a moving reference frame? The frequency should change ...
9
votes
3answers
340 views
How does a photon experience space and time?
To an an external observer it appears that time has stopped for photon. But this relation is reflexive, so for an observer travelling with the photon it appears the universe has stopped everywhere.
...
5
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4answers
621 views
What was Albert Einstein's proof for $E=mc^2$?
Most people know the famous equation:
$$E=mc^2$$
What were his steps of thinking for this equation that helped us discover so much about our world?
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votes
4answers
753 views
Double light speed
Let's say we have 2 participles facing each other and traveling at speed of light
Let's say I'm sitting on #1 participle so in my point of view #2 participle's speed is c+c=2c, double light speed? ...
10
votes
9answers
3k views
Accelerating particles to speeds infinitesimally close to the speed of light?
I'm in a freshmen level physics class now, so I don't know much, but something I heard today intrigued me. My TA was talking about how at the research facility he worked at, they were able to ...
3
votes
2answers
127 views
When traveling close to the speed of light
I hope this isn't a stupid question for you guys but here we go.
As you travel close to the speed of light, it is to my understanding you gain mass.. does this also apply when the brain sends ...
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votes
1answer
63 views
How would time duration and space distance change when we move in the speed of light? [closed]
It is possible to figure out how time duration and space distance change when we are close to the speed of light - but I am not sure what happens to them if we travel in the speed of light.
9
votes
3answers
893 views
Does a photon have a rest frame?
Quite a few of the questions given on this site mention a photon having a rest frame such as it having a zero mass in its rest frame. I find this contradictory since photons must travel at the seed of ...
1
vote
0answers
28 views
Why can't we accelerate to the speed of light? [duplicate]
Why can't we accelerate to the speed of light? It's just a speed and nothing else. Universe also crossed the speed of light at the time of big bang. Is this is just a interpretation or there is any ...
4
votes
2answers
88 views
What are the uses of proper length as a parameter?
Proper time is used to parameterize the world line of a moving particle in a way which is Lorentz invariant, which is elegant and powerful. Since space and time are usually treated on the same ...
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vote
2answers
103 views
Does the mass of a falling body decrease?
Let's say a body with m=2kg falls from 100 meters. Obviously it's speed would be far lower than the speed of light so the change in mass (if it exists) would be very tiny. However, I know that if the ...
12
votes
3answers
310 views
Are gravitomagnetic monopoles hypothesized?
My understanding is that gravitomagnetism is essentially the same relativistic effect as magnetism. If so, why is it that I've heard so much about magnetic monopoles, but never gravitomagnetic ...
1
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2answers
81 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?
4
votes
4answers
314 views
Can a photon get emitted without a receiver?
It is generally agreed upon that electromagnetic waves from an emitter does not have to connect to a receiver, but how can we be sure this is a fact? The problem is that we can never observe non ...
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0answers
34 views
Special relativity: Related speed [closed]
If a sheep is moving in a velocity of 2mph related to a train, and the train is moving in a velocity of 2mph related to the ground what is the velocity of the SHEEP related to the ground.Is there an ...
2
votes
2answers
107 views
Stroboscope-and-telegraph problem
Narrative:
Consider, in a suitably flat region, two straight lines which don't necessarily intersect. Let vector $\mathbf{x}$ point along one line, and vector $\mathbf{y}$ point along the other. Let ...
2
votes
1answer
113 views
Life of a photon [duplicate]
I am a student of class 12th and as far as i know when anything reaches about 99.99% of the speed of light it starts traveling in time or time for it slows down so that it don't breaks the speed ...
2
votes
3answers
94 views
Please explain this statement about Lorentz transformations
I'm reading Sternberg's Group Theory and Physics. I have a question about chapter 1.2 Homeomorphisms.
Background:
A Lorentz Metric is defined as $||{\bf x}||^2=x_0^2-x_1^2-x_2^2-x_3^2$
And a ...
1
vote
1answer
90 views
Faraday's Law and Galilean Invariance
In Jackson's text he says that Faraday law is actually:
$$
\oint_{\partial \Sigma} \mathbf{E} \cdot \mathrm{d}\boldsymbol{\ell} = -k\iint_{\Sigma} \frac{\partial \mathbf B}{\partial t} \cdot ...
2
votes
3answers
182 views
If time slows down when you move faster, does this give you an “effective speed” different from your proper speed?
Suppose you're travelling at 0.9c toward the sun, and you tag past the Earth and start a clock. Would Lorentz contraction/time dilation cause you to get to the sun faster than (about) 9 minutes ...
5
votes
1answer
146 views
Impulse from absorbing a photon? Is there an increase in rest mass?
I'm going through A P French's special relativity. In one chapter (6) the following is set up:
Suppose that a stationary particle of mass $M_0$ is struck by a photon of energy $Q$, which is ...
5
votes
3answers
137 views
How can we know, today, that there's something from 100 light-years from here?
In my understanding, to take a picture of something that is 100 light-years from here, our "camera" would have to travel 100 years at light speed, take the picture, send to us, and 100 years later we ...
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votes
1answer
114 views
Lorentz transformation problem
In the equation (1.18) they omitted the translation vector $a^\mu$, but why?
0
votes
1answer
24 views
Measuring life through radiation
Dramatic title, I know. But it's shorter than Measuring a person's effective mass through radiation and comparing it to their weighed mass and I figured this would get people's attention.
I just ...
13
votes
4answers
453 views
Thought experiment regarding an object approaching a mirror
Here's a thought experiment I came up with in class today when my mind drifted (I however highly doubt I'm the first to think about this since it is pretty rudimentary) :
Let's say superman ...
3
votes
2answers
125 views
Metric coefficients in rotating coordinates
Let $(t,x,y,z)$ be the standard coordinates on $\mathbb{R}^4$ and consider the Minkowski metric
$$ds^2 = -dt^2+dx^2+dy^2+dz^2.$$
I am trying to compute the metric coefficients under the change of ...
4
votes
1answer
111 views
Mass of a compressed spring
I was wondering if:
The mass of a compressed spring is greater than that of an uncompressed spring
The mass of a body on the surface of the Moon is greater than that on Earth
according to the ...
3
votes
1answer
139 views
Lorentz transformation of classical Klein–Gordon field
I'm trying to see that the invariance of the Klein–Gordon field implies that the Fourier coefficients $a(\mathbf{k})$ transform like scalars:
$a'(\Lambda\mathbf{k})=a(\mathbf{k}).$
Starting from the ...
12
votes
9answers
444 views
How to explain (pedagogically) why there is 4 spacetime dimensions while we see only the 3 spatial dimesions?
I have been asked this question by a student, but I was able and in the same time incapable to give a good answer for this without equations, so do you have ideas how one can explain this in a simple ...
2
votes
0answers
75 views
Car parking special-relavity puzzle
Hi I read the following puzzle from an old text book long time ago. However it doesn't provide the answer. So what is the solution?
Let's suppose a car is going to park to a garage and the garage is ...
1
vote
1answer
51 views
What is movement through time? [duplicate]
In general, when I think of movement through space, I think of this:
$$\frac{dx}{dt}$$
But in special relativity, we also have a concept of relative duration, which means that $t$ must have a rate ...
3
votes
3answers
308 views
Why do clocks measure arc-length?
Apologies in advance for the long question.
My understanding is that in GR, massive observers move along timelike curves $x^\mu(\lambda)$, and if an observer moves from point $x^\mu(\lambda_a)$ to ...
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votes
0answers
183 views
Is there an easy derivation of relativistic velocities addition and relativistic doppler shift?
Is there an easy derivation of relativistic velocities addition and relativistic doppler shift?
I'm looking for some easy derivation of the above. Like this ...
3
votes
1answer
78 views
How would I perceive a purple ball when traveling at relativistic speeds
I have a 'thought experiment' which I'd like to ask about.
I was thinking what speed I would have to drive towards a crossing to see a red light as a green light - pretty easy, using the doppler ...
0
votes
1answer
99 views
Photons, no energy? [duplicate]
I am only 14, so I don't know much about physics, and I would appreciate help on this topic.
A photon, is said to have 0 mass. However, they carry energy, as electromagnetic radiation. Albert ...
8
votes
2answers
242 views
In QFT, why does a vanishing commutator ensure causality?
In relativistic quantum field theories (QFT),
$$[\phi(x),\phi^\dagger(y)] = 0 \;\;\mathrm{if}\;\; (x-y)^2<0$$
On the other hand, even for space-like separation
$$\phi(x)\phi^\dagger(y)\ne0.$$
...
0
votes
1answer
153 views
The real meaning of time dilation
Is this true or false: If A and B have clocks and are traveling at relative velocity to each other, then to B it APPEARS that A's clock moving slower, but A sees his own clock moving at normal speed.
...
1
vote
1answer
100 views
Why does only one twin travel in the twin paradox?
The wikipedia page repeatedly says that the twin travelling in space is the only one which travels, and also is the only one which faces acceleration and deceleration. So it does not age, while the ...
-2
votes
2answers
106 views
If we are travelling with the speed of light, can we see what's behind us? [closed]
If we are travelling with the speed of light, can we see whats behind us(like if we are moving away from earth can we able see the earth)? And how we see the things that we are approaching with speed ...
1
vote
1answer
85 views
Special relativity, acceleration and being in two places at once (How to resolve this paradox?):
Imagine I am on Earth I have a clock which measures time $t$. When my Earth clock reads time $t$, then when I look at a moving spacecraft's clock I see the time $t'$. Let us assume time is measured ...
1
vote
0answers
148 views
Does Mansuripur's Paradox involve fictitious charges?
Mansuripur's Paradox involves a magnet moving at relativistic speeds in an external electric field.
Additional: thanks to Retarded Potential, who found the original paper.
If I understand correctly, ...
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votes
1answer
87 views
A riddle: What is the inertial frame of a clock? [closed]
Parts of the clock must be moving for the clock to tick.
5
votes
3answers
536 views
Trouble with the Lorentz law of force: Incompatibility with special relativity and momentum conservation?
In Physical Review Letters, there was a paper recently published:
Masud Mansuripur, Trouble with the Lorentz Law of Force: Incompatibility with Special Relativity and Momentum Conservation, Phys. ...
2
votes
3answers
267 views
Is time dilation an illusion?
It is said that we can verify time dilation by flying a very accurate clock on a fast jet or spaceship and prove that it registers less time than the clocks on earth. However, the clocks on earth ...
2
votes
2answers
218 views
A relative time dilation paradox.
Let us assume that there are two astronauts A and B who are floating in space. A sees B passing by and vice versa. A sends signals to B every minute. According to A since B is moving his clock will be ...
4
votes
3answers
158 views
Setting up a local-coordinate system in space-time using only a single clock and light beams
I have a question to ask about the operationalist view of space-time. I am a mathematician who happens to be interested in physics, so if anyone thinks that my question is a silly or vague one, please ...
1
vote
1answer
84 views
Why does 'proper length' exist as a notion?
$$\text{proper time}= \tau= \sqrt{dt^2-d\mathbf{s}^2}$$
$$\text{proper length}= L= \sqrt{-dt^2+d\mathbf{s}^2}$$
What tangible benefit is brought about by calling $i \tau$ 'proper length' (applying ...
