# Tagged Questions

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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### Gravitational waves induce changes in the $h_{00}$ (time) component of the metric?

I'm rather stumped by a subtle point regarding metric perturbations of GW. I'm well aware the GW are able to produce changes in the flat space metric, They are transverse and have planes of ...
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### Time in spaceships near the speed of light

I know that the time on a moving object moves slowly compared to a stationery one, but if someone on a spaceship that moves at V=0.5c and a person is driving it. Will he notice that the clock on his ...
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### General Relativity as a Special Relativistic Field Theory

In this question, I want to consider only the classical case. I have seen the statement that general relativity can be considered as a spin-2 field living on a Minkowski background. In that case, you ...
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### Representing 1+1 Minkowski space as a surface in 3D Euclidean space

In 1+1 Minkowski space the distance between two points is given by$$(x_1 -x_2)^2 -(t_1 - t_2)^2.$$ This is different from the Euclidean distance. But is it possible to come up with a 2D surface ...
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### Exercise prob. in Lorentz invariance [closed]

How we do prove that $\langle p\vert q\rangle$ is an invariant by a boost along $z$ axis knowing that: $$\langle p\vert q\rangle=(2\pi)^3\cdot2\cdot E\cdot\delta(p-q)$$ where $\delta(x)$ is the Dirac ...
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### Relativistic Particles [closed]

The question: An electron is moving at $0.6c$ in the lab frame. For an observer moving at $0.8c$ in the same direction as the electron's motion, what is the electron's kinetic energy (in MeV)? My ...
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### A definition for relativistic energy without quantum mechanics

What are the minimal postulates to define energy in special relativity? Let us assume that momentum has not been defined yet (it can be defined from the energy in fact). I expect we are looking for ...
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### Relativistic electrodynamics between two point charges moving in perpendicular directions [closed]

Let there be two point charges, positive or negative, having velocities in directions perpendicular to each other. I need to evaluate the total interactive force on those (Lorentz force). I do it by ...
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### How to use Special Relativity in simple numericals? [closed]

So I was recently thinking about what an electron's velocity would be after it was accelerated through a potential difference of 1 V. Doing simple calculations like just dividing its energy (which ...
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### Feynman Lectures on Physics, Michelson Morley question about angle of light

My question is similar, if not identical, to this one, but I don't find the answer satisfying, given the context of experiment. First, here is an outline my understanding of the motivation behind the ...
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### Why is the SR behavior of mass and a clock different? [closed]

A frame contains a mass and a clock, an an adjacent frame contains an identical mass and clock. One frame travels and returns. The special relativity equations for mass and time are identical in form....
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### Why did Einstein allow unphysical objects in his 1906 paper on $E=mc^2$?

In the first part of this paper, Einstein considers a "rigid cylinder", "massless cavity", as well as a "massless carrier mechanism" that is used to transport the massless cavity. These hypothetical ...
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### Transformation of coordinates in relativity

I'm doing some self studying through a textbook. Here I will post the question exactly, and show what I have done to solve the problem. I'm completely convinced there is an error in the text, and I ...
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### Light at the Speed of Light [duplicate]

Since light particles travel at the speed of light, we know it's possible for something to travel at that speed. However, since all things are relative, and if time does stop at the speed of light, ...
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### Special relativity - Nokton theory [closed]

Is this theory described in this link Nokton theory respects special relativity conditions.
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### Effects of FTL travel on time [closed]

The twin paradox explains, using relativity theory, what happens to a person travelling close to the speed of light. When he returns to Earth, he finds his twin brother much older because the ...
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### Why invariance holds for the electromagnetic waves but not for the mechanical waves?

Why the electromagnetic wave equation provides a wave with constant speed in all reference frames, but the mechanical elastic wave equation (from linear elasticity) does not? The electromagnetic wave ...
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### Trouble in understanding spacetime

I have a problem in understanding spacetime. What i understood from the conversion of time to distance is that the interval between any two events is always the same for any observer. But how is that ...
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### Explanation for a much simpler version of the twin paradox?

I have seen the classical twin paradox before. It uses a twin stationary on Earth and the other traveling away and back. I have seen many contradictory solutions for it, some use general relativity, ...
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### Confusion with Thomas precession

Suppose an inertial frame $S^\prime$ is moving with a relative velocity $\textbf{v}=v\hat{n}$ w.r.t another intertial frame S with their axes parallel and $\hat{n}$ is an arbitrary direction. In that ...
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### How is it that we can send signals faster than the speed of light?

I came across this while reading The Feynmann Lectures on Physics (Vol.1,Ch.7)- According to Newton, the gravitational effect is instantaneous, that is, if we were to move a mass, we would at once ...
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### Twin paradox on hypertorus [duplicate]

I will not describe the twin paradox again. But let's suppose we have two twins, one stationary and the other moving with uniform velocity $c/2$ for instance. And let's suppose that they live in a ...
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### Does sound waves pick up the speed of its source?

I googled the speed of sound and found that it only depends on the medium (just like the speed of light but with different parameters). I can't see how it doesn't pick up the speed of its source! I ...
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### Boldly taken postulates

Einstein's Special theory of relativity was a landmark in physics, which totally changed our viewpoint of observing physical phenomena. While proposing his new theory, Einstein assumed two simple but ...
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### Twin paradox with horns [closed]

I am trying to figure out if my conception of time dilation is correct. In order to avoid the acceleration-deceleration complexity in the traditional twin paradox statement, I came up with my own ...
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### Geometrically deriving Lorentz transformation from Minkowski diagram

How can we derive Lorentz transformation from a Minkowski diagram (like below image) by using only geometry theorems such as sines theorem and Pythagoras theorem?
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### Are there (interesting) Poincare-invariant QFTs with non-invariant Lagrangian densities?

In all QFTs I know, the Lagrangian density is completely invariant under the Poincare group, $$\mathcal L \to \mathcal L.$$ On the other hand, the action would be invariant even if the Lagrangian ...
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### Working with indices of tensors in special relativity

I'm trying to understand tensor notation and working with indices in special relativity. I use a book for this purpose in which $\eta_{\mu\nu}=\eta^{\mu\nu}$ is used for the metric tensor and a vector ...
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### Can the value of $c$ be calculated from Relativity? [duplicate]

I have three related questions about what theories or equations can calculate and what is an input parameter/constant to the theories. This question is not about parameter units (as another similar ...
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### Theory of relativity paradox? [duplicate]

I have seen the classical twin paradox before. It uses a twin stationary on Earth and the other traveling away and back. I have seen many contradictory solutions for it, some use general relativity, ...
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### Special Relativity - Observing events multiple times [duplicate]

At t = -1, you observe an event to your left. At t = 0, you accelerate to 0.5c (to your right). Might you then observe that same event (to your left) again? For example, say you observe an event x = -...
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### Special relativity and uniform circular motion

Suppose we have two observers A and B. A is in some inertial frame. B is undergoing uniform circular motion with some speed $v$ as observed by A. They each have a clock. My concern is with what A ...
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### Speed bigger than light? [duplicate]

We know, according relativity theory, no information or matter can travel with velocity bigger than light. However, consider the following situation: a completely empty water hose is hypothetically ...
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### Relativity gedanken experiment

Lets consider the following thought experiment: A spaceship is in circular orbit around Earth traveling at 99% of $c$ (the orbital distance is chosen in such a way that inside the ship there are no ...
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### Rest mass of a moving particle that decys into two phtons

A particle of rest mass M is moving along the positive x direction. It decays into two photons having energies 1 GeV and 0.82 GeV, and these go in different directions making angles of 45 and 60 ...
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### What does it mean to say that something is “relativistic for an electron”?

I want to understand a concept better. I did a homework problem where I solved it all the way, then checked my answer with a solution set. My answer was different, so I followed the solution set from ...
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### Linkal causality in special relativity

I have a slight variation of the barn / ladder paradox, where there is a ladder too long to fit into a barn at rest, but when moving at a sufficient speed, it is length contracted with regards to the ...
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### Exercise 18b in Schutz's First course in GR

The question is as follows: Show that a timelike vector and a non-zero null vector cannot be orthogonal. So we have a timelike vector $\vec{A}$, s.t $\vec{A}^2<0$; and a non-zero null vector, ...
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### Heisenberg Uncertainty Relation and Relativity

It's well known that in QM the position and momentum of a particle cannot be known simultaneously. It's also well known that in special relativity the idea of simultaneity loses its meaning. Would it ...
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### Can speed of light be $c$ in air or other medium?

I know that the speed of light in a vacuum is $c\sim 3\ 10^8\ \mathrm{m/s}$, but I also know that speed of light in a medium (e.g., air) is less than that in vacuum. Special relativity says that ...
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### What happens to a body if it rotates extremely fast?

I am thinking on a object, e.g. ball or planet that starts rotating with increasing speed. Let's assume that his speed get's closer to the speed of light, what happens to this object? There are ...
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### What is time, does it flow, and if so what defines its direction?

This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address ...
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### Does the Lorentz transformation necessary follow from the two postulates of relativity?

The two postulates of special relativity are: The choice of what inertial frame to use is arbitrary: all laws of physics are invariant. (the principle of relativity) The metric (\Delta s)^2 ...
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### Special Relativity and the Light Clock and which direction was the laser pointing? [closed]

If I were to make a light clock, I would have some sort of laser pen attached to the top of one of two mirrors. That laser pen would be perpendicular to the surface of either/both mirrors. I would ...
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### A classical explanation of why there is a speed limit in the universe [closed]

The question is - is there any flaw in this explanation, or it is the right explanation in general language? Let me break the explanation into two parts - Why there is a certain speed of ...
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### Can a particle have an imaginary mass?

Now, I'm not very involved in the physical sciences, beyond some high-school classes - though I did listen to some college lectures for physics. I was reading "Do Tachyons Exist?" by John Baez, which ...
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### Is the energy of a collision of two photons the same as one photon hitting a 'wall'?

Special relativity says that the speed of ligt is always the same. So when a photon meets another photon he will encouter it with the speed of light and also the other one. But in case of a wall, the ...
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### Are objects like $a^{\mu \nu} a_{\mu \nu} b^{\mu \nu}$ consistent with Einstein summation?

I'm familiar with Einstein' summation notation and I understand objects like $a^{\mu \nu} a_{\mu \nu}$ just fine. But I'm wondering why I've never come across objects like this: \$a^{\mu \nu} a_{\mu \...