# Tagged Questions

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

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### What is the velocity of the photon through the fourth dimension x4? [on hold]

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 ...
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### What is the name of matter with $w = - \frac{1}{3}$?

What kind of "matter" is described by the following thermodynamic relation ? $$\tag{1} p = -\, \frac{1}{3} \; \rho,$$ Where $p$ and $\rho$ are the pressure and density ...
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### Regarding off-topic questions, what's wrong with my question? [migrated]

On the page http://physics.stackexchange.com/questions/266602 I have asked a question Has the special theory of relativity been disproved theoretically i.e. logically? with the content: Here is the ...
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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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### Was Einstein familiar with the Michelson-Gale experiment?

Was Einstein familiar with the Michelson-Gale experiment (related to Sagnac's experiment)? If so, did Einstein explain the results of the Michelson-Gale experiment with his relativity theory? If so, ...
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### General and special relativity books [duplicate]

What are some good books on the exact unabridged theory? Please keep in mind, though, that I need a book with digestible mathematics. Many books are available with the full theory but they have higher ...
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### Photon pair production at relativistic speeds?

This is probably an obvious question, but I don't see it answered anywhere. Imagine we have an object in the Universe that is traveling, relative to the Milkyway at about .99999999999999999999999999c ...
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### Which planet will have a faster clock? [closed]

This is a complication to the twin paradox. Eddy stays on Earth. George goes flying quickly through space then he turns around and heads back toward Earth. As George is coasting back toward Earth ...
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### Is measure relative velocity the same for both observer n particle

A particle is moving at velocity v. A stationary observer tries to measure its velocity. From the observer reference frame, he will measure a shorter distance travel as compared to what the particle ...
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### Need clarity about relativity/magnetism explanation

Please bear with me - this is not a duplicate question.... From what limited knowledge I have gathered, it seems that when two wires in parallel are carrying equal current, the electrons moving in ...
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### Is there a fundamental picture for 'the speed of light'?

I am curious about the profound reason for the concept of 'the speed of light'. My considerations are: (1) It's related with the structure of spacetime (for example in Penrose's twistor plan),so ...
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### If Speed is Relative can Absolute Acceleration be Real

This is , I suppose not a good question , but I think I am missing something which confuses me in this question. So my question is - Velocity is relative. So suppose a car is moving at a speed 200 ...
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### What is physical meaning of Lorentz boost?

Physical meaning of spatial part of Lorentz transform is, obviously, rotations in R-space. Does anybody have a nice physical interpretation of boosts though? I can understand to some extent off-...
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### Symmetry, Space-Time interval, and Coordinate Time

I had the following question I was working from a book: Question: A physics professor runs across the hallway covering 120 ns of distance in 150 ns of time as measured in the frame of the earth....
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### The speed of light and sound in materials

Obviously, nothing moves faster than the speed of light in a vacuum. Light propagates through materials slower than in a vacuum, and that rate is often related to the specific frequency. Is there a ...
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### Is this the correct method for deriving E=mc^2?

So lately I tried to deriving Einstein's $$E = mc^2$$ which later I found from this website : http://www.emc2-explained.info/Emc2/Derive.htm They use calculus based method, so I tried another way. ...
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### Homogenity and Isotropicity of space

In school it is given that law of conservation of momentum is a result of homogeneity of space and law of conservation of angular momentum is a result of isotropicity of space but what is isotropicity ...
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### How to prepare mental skills to understand relativity theory? [closed]

Upto the high- school, I had to admit and write-on-exam the strange, counter-intuitive statements of relativity theory; those I could not believe. Now, I'm not a physics or math-student, (and indeed ...
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### Relativity on a universal scale

Imagine there was a clock on a planet the same size as earth, travelling at the same speed through space, and that this planet was at the most distant part of the universe from earth. If we had a ...
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### Relativity theory - lower end of falling to Earth object moves slower or faster than higher end?

According to relativity, time slows close to gravitational field (I prefer to say the processes with which we measure Time slows). (https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment) - "...
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### According to relativity theory, what is the force that two electrons moving radially apart exert on each other?

According to relativity theory, what is the most general expression for the force that two electrons moving radially apart exert on each other? I am looking for a function $$F(r(t)),$$ where $F$ is ...
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### If I have a huge analog clock, what happens to the seconds hand if I build it longer than 2.864.788 kms? [duplicate]

Suppose I'm able to build a huge analog clock, with a seconds hand very long. I want have hand end running at light speed (300,000 km/s). Needle then has to be 300,000km*60s/(2pi) = about 2,864,788km (...
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### How does rotation of a mass affect the geometry of space? [closed]

I understand (or rather, I have been told, but do not understand) that a large mass can distort space by its rotation. While I am somewhat familiar with the concept of a mass distorting space and ...
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### Expansion of the Universe and Relativistic Mass

First let's say that all velocities are relative to the centroid of mass of the Universe. Given that every body in the universe is accelerating apart from every other, and mass increases with velocity ...
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### How do I draw the Minkowski diagram of a event in two inertial systems when t=t'=0?

An Introduction to Mechanics" by David Kleppner & Robert Kolenkow, 1st Edition, 12.1 Assume that v= 0.6c . Find the coordinates in S' of the following events: a). x=4 [m], t= 0[s] b). x=4 [m], ...
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### An unknow atom has the shown energy levels

In an excersice i found, a supposed atom called fictitious (Fi) has the following energy levels: Then i´m asked: A) The energies of the emitted photons after a gas of Fi is bombarded with ...
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### Merging black holes makes them less dense, so

According to What is exactly the density of a black hole and how can it be calculated? (more specifically, John's answer here made me think: if you merge a whole load of chunks of an element heavier ...
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### How can time be a dimension when it is relative? [closed]

I understand that by definition dimension is defined by just another coordinate to pin-point something in space-time. Therefore we need to know not only where but when. At the same time, this somehow ...
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### Fastest Speed of Sound

Obviously, sound (like every other causal phenomena) may not travel faster than the speed of light. I know that materials with a high bulk modulus and low density will typically have faster speeds of ...
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### Usage of tensors in physics [closed]

As I understand it, tensors are multi-linear maps that map vectors (and dual vectors) to real (or complex) numbers, but I'm hoping to gain some intuition as to why they are useful in physics. Is it ...
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### If we actually conducted a twin paradox experiment using a rocket launched from earth

Does the equivalent acceleration / deceleration required to reach whatever speed then come back and rest in the original frame of reference zero out the time dilation effects for each party? So if we ...
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### Does Heat Cause Time Dilation?

Since heat is defined as the movement of molecules, and because of relativity time slows for faster moving objects, would a hot object be in a slower time frame then a cooler object, because the hot ...
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### Frame dragging VS Mach's principle: rotating body in an empty universe

What I understand about Mach's principle VS modern physics: According to classical physics, there are ways to distinguish weather a body is rotating or not. For example if it is rotating, the ...
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### According to Einstein & Brian Greene, does the photon remain stationary in the fourth dimension? [duplicate]

According to Einstein and Brian Greene, does it logically follow that the photon remains stationary in the fourth dimension? In An Elegant Universe, Brian Greene writes: “Einstein found that ...
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### Would a particle the size of a neutron, if it had enough mass, collapse into a blackhole?

For example, a neutron is a particle that occupies a certain volume. If you pack enough mass into that volume, it would collapse into a black hole (I assume there is not enough mass now). At least if ...
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### Trouble understanding spacetime and invariant interval

First, how is the invariant interval useful? How can it help us understand things around us in the universe? Second, I know that they changed time into space or better say SPACETIME in order to ...
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### Is the time for light to go a certain distance the same amount of time sped up through relativity?

If you were to stand on the tower of my Lemmon and had an atomic clock that was exactly synchronized with one at tucson, (6734 feet difference) for 24 hours, it was shown to be roughly a 20 nano ...
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### How to show invariance using the Maxwell tensor?

I want to show the invariance of $E^2-c^2B^2$ under the Lorentz transformations. The obvious way to do this is to show that $$E^2-c^2B^2=E'^2-c^2B'^2,$$ where $E'$ and $B'$ are the Lorentz ...
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### Are all diffusion-like processes described as wave-like in relativity-compatible formulations?

Citing from Wikipedia's article on relativistic heat conduction: For most of the last century, it was recognized that Fourier equation (and its more general Fick's law of diffusion) is in ...