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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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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1answer
48 views

what is the meaning of infinite mass?

IF a particle/object is moving at speed of light (v=c), using relativistic mass equation we get 0 in denominator i.e. m = infinite. So what is the meaning of that? At v= 0.99c as per the arithmetic ...
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50 views

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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Compton Scattering problem verification [on hold]

A photon emergre in an angle $\theta=\frac{\pi}{4}$ in a Compton scattering the kinetic energy of the photon is $K_i=2K_f$. I must find $K_i$ in $MeV$. $\lambda_f -\lambda_i= \lambda_c$. Then: ...
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1answer
33 views

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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41 views

Is it accurate to say that nothing can travel faster than c in a GR context, where more space can be created?

Years ago, my brother and I had an argument where I was trying to convince him that nothing could travel faster than the speed of light. I was pursuing this in the context of Special Relativity. My ...
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1answer
25 views

Special Relativity and the Light Clock and which direction was the laser pointing?

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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2answers
113 views

A classical explanation of why there is a speed limit in the universe [on hold]

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 - (1) Why there is a certain speed of ...
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1answer
82 views

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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2answers
50 views

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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0answers
41 views

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 ...
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3answers
57 views

Twins paradox question [duplicate]

In the twins paradox, the twin that travels away from earth comes back to find that his (her) twin has aged much more due to relativity's effect on time. Why can't we posit that it is not the one twin ...
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32 views

Representations of spinors

I'm trying to understand spinors but I haven't found a good explanation of how to derive them. I'm generally interested. I read that there is a representation of SO(3) as spinors, and also that there ...
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1answer
48 views

Is there any description of a mechanism of curving of space-time fabric

Most of us know about famous line - "mass tells space how to curve and space tells mass how to move". There is quantitative description of the curving and its effects in Einstein's theory. Mass and ...
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34 views

What is conformal symmetry physically?

I'm reading a paper by t'Hooft http://arxiv.org/abs/1410.6675. There is an argument in the paper that I could not understand: "Now that system, described by Maxwell’s equations, does have conformal ...
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42 views

Identifying Lorentz Covariant Equations

Statement: $\phi , A^{\mu}, T^{\mu \nu}$ are a Lorentz scalar, vector, and tensor. Which of the following equations are Lorentz covariant. a. $\phi = A_{0}$ b. $\phi = A^{\mu}A_{\mu}$ c. $\phi = ...
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1answer
48 views

Cold Solid Matter at the End of a Black Hole's Acceleration Disk?

Is it possible that black holes magnetic field are rapidly cooling matter using a type of magneto caloric effect and cold fusion creating new matter only possible with a black hole's gravity? A type ...
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1answer
84 views

What am I doing wrong while proving time dilation using Minkowski space-time diagram?

Consider two events A and C. They have same value of x' and time interval between them is $\triangle \tau$. This is the proper time interval between the events. ...
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1answer
99 views

Why does time stop its flow? [duplicate]

According to the "list verse" time gets to a pause when someone or something travels at the speed of light. But a question still lies in my mind, if time always travels forward and does not stop at ...
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2answers
61 views

When does travelling at a fraction of the speed of light appear to the traveler as if they're going the speed of light? [closed]

If a spaceship is travelling to Proxima Centari, there is a fraction of the speed of light that would make the spaceship appear to reach Proxima Centauri in four years' time. What fraction of the ...
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2answers
56 views

Conversion of energy to matter? [closed]

Now, this answer has sort of been asked before. But I haven't really found the answer that I'm looking for. Exactly what is the mechanism by which energy can be converted into matter? By ...
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0answers
23 views

Decay of lepton into meson and neutrino using four-momentum

I'm learning about how to use four-momentums and am working through a problem involving the decay of a charged lepton into a charged meson and neutrino. The equation would be: $L^+ \to M^+ + \nu$ ...
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0answers
23 views

Which date the twin paradox was formulated first time? [migrated]

Any research about real measurements with faster moving than me systems - like GPS for example - leads to a mix of different sideway phenomenons like different gravitational potential on earth and in ...
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30 views

If humanity was to send a probe to Proxima Centauri, how long would the travel last?

[Feb 3 2016] (I am including the date I asked this as a reference, since technology evolves with time, this question's answer will change in time...) If humanity was to send a probe to the nearest ...
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1answer
55 views

How do I know which observer is running the time faster or slower

Ok, I'm not a physicist, so I don't know if my question is silly (probably yes), but there is something in special relativity that I can't understand and I would really like it to be clarified. If ...
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127 views

Cold Fusion and Black Holes [closed]

How does a magnetic field heat and Cool objects? Would an Atom rapidly cooled with a magnetic field release photons? Can solid magnets be used in the same manner? Einsteins cold fusion formula is ...
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1answer
55 views

What makes things move in a universe with blocktime? [closed]

In a universe with blocktime the past, present and future all exist at the same time. But what makes it appear that for example my body is going from one movement ...
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0answers
50 views

Where other frame observer will put new stick [closed]

my question is with relativity, if we draw two circle of same radii and fill the perimeter with stick (very small so it cover whole perimeter) now if we rotate a stick with veey high speed ...
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0answers
32 views

Time Travel Plausible? [closed]

The question of possibly achieving time travel has been exhaustively deliberated over and over again but in the modern era of new discovery and technological advancment, can we reexamine/rethink its ...
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1answer
52 views

Special Relativity problem - proper time interval

The supernova 1987A explosion in the Large Magellanic Cloud 170 000 light years from Earth produced a burst of anti-neutrinos ν ̄e which were observed in terrestrial detectors. If the anti-neutrinos ...
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0answers
36 views

Is any photon really moving? [duplicate]

I read that when something moves at the speed of light, length contracts at the direction of motion. Now consider a photon traveling in the z axis. Since it is travelling at the speed of light, the z ...
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1answer
61 views

Small particle rotating around neutron star

First let me say I have read the article at "users.ictp.it › lns003 › Ruoff › Ruoff.ps.gz" about a small particles orbiting a neutron star and causing excitation of different modes of gravitational ...
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2answers
135 views

Confusion with Weinberg's QFT book, volume 1, chapter 3: time translation and Heisenberg picture

Sorry if this is a naive question, but I am new to QFT. In the treatment of scattering in section 3.1 of The quantum theory of fields, vol.1, Weinberg first presented the general transformation rule ...
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2answers
76 views

Help understanding why can't we measure absolute velocity?

(This is probably a clone of some other question, but I haven't been able to find a satisfactory answer, so I hope this is okay.) So, I know that in special relativity, there's no such thing as a ...
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1answer
56 views

Minkowski's dot product

I am trying to deduce the Minkowski's dot product for two dimentional space: $$g=x^1y^1-c^2t_xt_y$$ If $f$ denote the Lorentz's transformation for two dimentional case: $$\begin{array}{rcll} ...
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1answer
42 views

Are there certain reference frames where proper length of proper time are supposed to be measured?

Let's say we have two frames: Bob on Earth and Alice in a rocketship. Let's say Alice is traveling at a relativistic speed away from Bob. With that in mind. Is there a certain frame where we should ...
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4answers
275 views

Up or Down? A Question about Relativity

The standard text discussion of the equivalence principle and the bending of light rays goes like this. We, as observers watch a glass elevator ascending. A light beam is fired from left to right. The ...
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0answers
55 views

QFT: Ground State Momentum - Normalisation of States

In my notes I have, $$ \left\langle \mathbf{p} \left| \mathbf{q} \right.\right\rangle = \left\langle 0 \left| {a(\mathbf{p})}\ {a(\mathbf{q})}^{\dagger} \right| 0 \right\rangle $$ I am not sure how ...
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1answer
51 views

Visual pictures of Lorentz transformations

How can I visualize Lorentz transformations? I know it's four dimensional, but without a visual picture in my mind I can't understand any of this. For example how do: Time reflection Space ...
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3answers
282 views

Why isn't a Photon justified in concluding that it is at rest and everything else is moving past it at c? [duplicate]

Why isn't a photon moving at c not justified in concluding that it is at rest and everything else is moving past it at c since relativity postulates that the laws of physics are invariant (i.e. ...
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0answers
16 views

Show that an isolated photon cannot undergo pair production [duplicate]

I need to see the mathematical details as how the conservation of four momentum is violated.
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1answer
94 views

What does it look like for a ball falling to the event horizon observed by distant static observer?

Here is the picture used in susskind&Lindesay's book ''An Introduction to Black Holes, Information and String Theory Revolution'' I understand very well that the ball will be contracted at the ...
3
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1answer
62 views

Most efficient speed to travel at, considering the effects of time dilation; according to Special Relativity

I understand that this question has recently been asked, and yet not answered to the greatest and most understandable extent, but I was wondering whether travelling at the speed of light is really the ...
1
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1answer
51 views

Implication of Relativity [duplicate]

According to relativity, no useful matter can travel faster than light. Suppose that your are a light beam travelling. Now you have a parallel light beam travelling beside you. Suppose you look at the ...
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0answers
80 views

Classical Fermion and Grassmann number

In the theory of relativistic wave equations, we derive the Dirac equation and Klein-Gordon equation by using representation theory of Poincare algebra. For example, in this paper ...
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4answers
108 views

Physics newb: Why can't I plug $E=mc^{2}$ into $W=mg$?

Starting with $E=mc^{2}$, I solved for $m$ and plugged the result into $W=mg$. Which was very confusing, as objects at rest had no weight: $$W = (e/c²)\cdot g \implies W = (0/c²)\cdot g \implies W ...
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0answers
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Why can't we use a simple stick to communicate faster than light? [duplicate]

Assuming I and a partner are each holding an end of a very light lightyear long stick, could I transmit morse code faster than light to my partner by tapping or wiggling that stick? Wouldn't he feel ...
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1answer
50 views

What is the difference between the $Spin(3,1)$ group and the $SO(3,1)$ group?

What is the difference between the $Spin(3,1)$ group and the $SO(3,1)$ group?
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2answers
81 views

Speed of light invariance (once again)

The answer to this very good question seems to be favored by a large amount of users. Yet it seems to imply that the constancy of the speed of light and its finiteness stems from the underlying ...
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3answers
101 views

Observing a point 13.82b ly away, 1b years ago

I understand that we can observe far away galaxies to get an idea how the universe looked like in the past. Assuming the universe is 13.82b years old, would it be correct to say that if we looked at ...