# 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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### Are all conserved scalars proportional to relativistic mass? [closed]

I have read in Rindler's relativity book that all scalars depend only on the magnitude of velocity of a particle and that are conserved are proportional to relativistic mass. ...
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### What happens when an open string start to stretch in a full Neumann boundary scenario?

I'm only reading about string theory. I'm an undergraduate. I see that for a full Neumann boundary condition the ends of a relativist string travel at the speed of light. If they travel in opposite ...
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### Special relativity-measuring a lorentz contraction as reference frame changes

Ok so, in the context of special relativity alone, If you were to have a train moving at a speed near the speed of light (say 0.7c), and containing two people, who, with their watches synchronised in ...
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### Special relativity and black hole paradox [duplicate]

Today a friend of mine posed me a paradox involving black-holes, one that I couldn't solve. Suppose we have planet, with a density such that it is almost to the point of turning into a black hole, ...
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### Historical vs modern presentation of special relativity

I have noticed that historical or brief introductions of special relativity will discuss it in terms of inertial frames and postulates: Principle of Relativity - (from Einstein's 1905 paper) "the ...
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### Lorentz invariance, energy-momentum conservation & the locality of interactions

I have been reading these notes ("Minkowski Spacetime: A Hundred Years Later", by Vesselin Petkov) 1, in which the author states (in the middle of the text on page 137) that "The only Lorentz ...
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### Do all predictions of special relativity follow from the Lorentz transformations? [closed]

Is there a proof that all observable predictions of special relativity follow from the Lorentz transformations? I have edited my question with concrete examples, so that it can be understood more ...
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### Unitary Lorentz transformation on quantized Dirac spinor

I am stuck again on page 59 of Peskin and Schroeder. In particular, I do not know how they get equation (3.110). Let me first give some background in the way that I understand it (but I might be ...
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### Causality and how it fits in with relativity

I was talking to my teacher the other day about Einstein's spacetime and there's one thing he couldn't explain about the nature of Cause. I may be being stupid or just unable to comprehend, thanks for ...
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### Is there a most efficient speed to travel in space with reference to time dilation? [closed]

I was thinking hypothetically, a ship traveling to a distant star system with something like food wants to get to the system as fast as possible. The trick is, we want to travel as fast as possible in ...
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### Particle accelerators reaching light speed [duplicate]

I just have a question in mind that's stuck in my head, and it's why particle accelerators cannot accelerate any particle to the speed of light. I'm assuming it involves Einstien's theory of ...
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### Is it possible to go faster than the speed of light in vacuum?

If it is possible for particles to go faster than the speed of light in certain events, would it be possible to create a situation in which the barrier of the speed of light can be crossed?
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### When spinning a pole with uneven mass at relativistic speeds will its center of gravity change?

Scenario: You have a pole that is a meter long and it is balanced when you rest it on your finger at the 10 cm mark due to the uneven weight distribution. (center of gravity at 10 cm mark) You also ...
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### Stripped twin paradox [closed]

Consider an empty space, except for Alice and Bob. Suppose Alice sees Bob move away and then come back again. Suppose Bob sees Alice move away and then come back again. Suppose either of them will ...
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### Covariant and contravariant 4-vector in special relativity

I've just learned about contra- and covariant vector in the context of special relativity (in electrodynamic) and I'm struggling with some concept. From what I found, an intuitive definition of ...
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### How to construct fields from from unitary representation of the Poincaré group?

I want to construct fields from unitary representation of the Poincaré group but I do not know how. In Weinberg book he proposed that the Hamiltonian should be of certain kind and from that he derived ...
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### 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 ...
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### What is the maximum time dilation between two objects, if one is standing still and the other is moving at $c$?

What is the maximum ratio in the rate of change in time in reference to object $A$ which is standing still and object $B$ which is moving at the speed of light?
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### Construct fields from from unitary representation of Poincaré group

I am trying to understand how construct fields from unitary representation of Poincaré group and the reasoning that Weinberg give in his book is the cluster decomposition principle and Lorentz ...
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### Time dilation and symmetry in special relativity

Trying to grasp special relativity concepts, I thought in the following experiment. Imagine Alice took a trip in a spaceship to another star. Now, she is returning close to light speed. When she ...
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### Objective velocity (Relativity theory) [closed]

Suppose that I have a velocity of (approximately) $c$. Then I decelerate with $c$ per hour for exactly one hour. Then I would know that I would be standing still in the end, because the decelleration ...
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### At relativistic speeds, how is an object's increased mass created and distributed?

This question says that, at relativistic speeds, an object's increased mass will result in increased weight or gravitational force. But if we have increased mass and corresponding gravitational ...
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### Does the speed of light differ for two frames moving at different speeds relative to an non moving observer?

Wikipedia says that: The speed of light in a vacuum is the same for all observers regardless of the motion of the light source. How can this be true and and returning to my question, if there is ...
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### Does time dilation affect the speed of a moving object?

Imagine you are traveling at 99% of the speed of light. Will the speed of the moving object slow down for both objects and for the resting observer or either one?
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### Confusion about reading of clock as seen from different inertial systems

Suppose a clock, located at point $x'$ in the inertial frame $S'$, registers two events $t_1'$ and $t_2'$. Let $\Delta t'=t_2'-t_1'$. The same two events will be registered by two different ...
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### Can the single object accelerate? (Relativity theory) [closed]

Suppose space is empty except for one single object. According to relativity theory it doesn't matter whether we assign a constant velocity to it or not; in both scenarios the same is predicted. The ...
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### Poincare group representation and complete set

In Weinberg's book of Qft, chapter 2 of volume 1, he uses the eigenstates of the four-momentum to construct the unitary irreducible representations of the Poincare group. My question is, since ...
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### Is this argument valid as to why we can (not can't) reach the speed of light?Please do explain [closed]

I have already asked this question on quora, with difficulty but I have not yet obtained a "works for me" answer. Please discuss with me about this.:) Q:Why can we not touch the speed of light? ...
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### Generation of back emf with relativistic approach to EM Waves

I learnt that Magnetic force around a current carrying conductor is nothing but relativistic electrostatic force observed by a charge particle from its frame of reference. But I am not able to ...
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### Minkowski space-time

Suppose we have the vector space $\mathbb{R}^4$ and the Lorentz's transformation $f:\mathbb{R}^4\to\mathbb{R}^4$. Consider a inner product $g$ given by: $$g(x,y)=x^1y^1+x^2y^2+x^3y^3-c^2t^1t^2$$ for ...
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### Proving the conservation of 4-momentum for a particle collision $A+B\to C+D$

Let me say that particle A hits particle B and two particles come out - C and D; In system S I can write: $$p_A^μ+p_B^μ=p_C^μ+p_D^μ;\tag{1}$$ here $p_N^μ$ is the 4-momentum. Using the Lorentz ...
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### How to calculate the thrust of a rocket with relativistic exhaust

The general thrust equation is $F = \frac{dm}{dt}\cdot v$, where $m$ is propellant's mass and $v$ is the exhaust velocity, is the equation right? What if the propellant is highly relativistic? One ...
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### Relativistic drift velocity of electrons in a superconductor?

Is there a formula for the effective speed of electron currents inside superconductors? The formula for normal conductors is: $$V = \frac{I}{nAq}$$ I wonder if there are any changes to this ...
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### Why is rapidity additive?

With the rapidity $\phi$ defined so that $\frac{v}{c}=\tanh{\phi}$, say you have 3 parallel moving reference frames $S$, $S'$ and $S''$ with a constant but different velocity/rapidity. If the ...
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### Is all of a black hole's mass situated arbitrarily close to the event horizon?

Forgive me if I'm thinking about black holes in completely the wrong way, but since time dilation increases to arbitrarily large amounts the closer you get to the event horizon of a black hole, ...
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### Does hotter radioactive substance have longer half life?

Sorry to have a newbie question! But I want to ask, if it is possible to change the half life of radioactive substance by heating it, my hypothesis is: When substance becomes hotter, the kinetic ...
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### Why is $|\Lambda^0_0| \ge 1$ for a Lorentz transformation?

I'm taking a course of QFT and in the notes that the professor gave us he says that for a Lorentz transformation $\Lambda^\mu_\nu$ we have $|\Lambda^0_0| \ge 1$. He doesn't give further information ...
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### Killing field in Minkowski space-time

If we look at the killing equation for a vector field $X$ in $\mathbb{R}^{(p,q)}$ (or on an open subset thereof) in coordinates with constant diagonal pseudo-metric we get: ...
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### Why are one-particle states called representations of Poincaré group?

The one-particle states in the Hilbert space of a quantized relativistic field theory are said to form representations of the Poincaré group. Why is that? I mean, popular texts in QFT do not ...
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### Can an event occur before or after the other one depending on what frame of reference we are in?

Two buses moving relative to each other with 30% the speed of light (4 light-min apart). Now by some mysterious way bus 1 comes to the knowledge that after 3 min-bus 2 time bus 2 is going to explode. ...
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### 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 ...
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### Understanding Tensor-operations, covariance, contravariance, … in the context of Special Relativity

I'm currently learning about special relativity but I'm having a really hard time grasping the Tensor-operations. Let's take the Minkowski scalar product of 2 four-vectors: \pmb U . \pmb V = ...
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### Rest mass energy of electron

For electron's energy why we some time use the word "REST MASS ENERGY" why we use this word as electron is always spinning? ...
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### Clarification for the mutual time dilation for two observers in their inertial frame of reference

I am trying to understand the idea under the theory of special relativity recently. The time dilation predicted by special relativity baffles me. I used to think that I understand the idea underlies ...
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### Special Relativity and symmetry of time dilation

I stumbled across a paradox in special relativity for which I have found no answer. In the twin paradox, the loss of symmetry in time dilation is usually explained by the reversal of movement of the ...
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### relativity, light, transformations [duplicate]

is the light itself a reference frame in special relativity? Can we write the special relativity transformations between a photon and other reference frames? If we can't then what other transformation ...
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### Is photon massless? [duplicate]

if photon is massless then in E2=m2c4+p2c2 is m=0 ? and p=mv1-mv2 shouldnt be zero too. if both the statements are true then does photon has no energy?
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### Relativity of simultaneity subtle example [closed]

imagine three persons, Person A is standing next to a device these device is an AND gate if two photons(one from right and one from left) reaches the AND gate at the same time the device explodes and ...