The special theory of relativity describes the motion and dynamics of objects moving at significant fractions of the speed of light.
-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 ...
8
votes
2answers
243 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
$$\phi(x)\phi^\dagger(y)\ne0.$$
...
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 ...
-4
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.
1
vote
0answers
149 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, ...
13
votes
4answers
454 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 ...
2
votes
3answers
270 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 ...
1
vote
1answer
88 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 ...
8
votes
3answers
323 views
Where's the energy in a boosted capacitor?
Suppose I look at a parallel plate capacitor in its rest frame and calculate the electrostatic energy, $E$.
Next, I look at the same capacitor in a primed frame boosted in the direction perpendicular ...
1
vote
1answer
94 views
Does special relativity unify the two phenomena at the base of Faraday's flux law (was Feynman wrong in this case)?
Consider Faraday's flux law for the EMF generated in a conductor loop:
$$ \varepsilon = - \frac{d \phi}{dt},$$
where $\varepsilon$ is the EMF, and $\phi$ is the magnetic flux through the loop.
...
2
votes
3answers
119 views
Correct expression for D'Alembert operator in $c=1$ units
In QFT texts with $c=1$ units (most of them), D'Alembert operator is written as:
$$\Box ={\partial^2 \over \partial t^2} - \nabla^2$$
For pedagogical purposes, however, some texts don't set $c=1$, ...
2
votes
1answer
103 views
Is there a stationary frame of reference?
My understanding of relativity is at the beginner level, so please bear with me to help a beginner to understand. Something which I don't understand is consider if you travel close to the speed of ...
4
votes
3answers
159 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 ...
0
votes
0answers
27 views
Run with speed of light with a mirror in hand [duplicate]
Possible Duplicate:
Reflection At Speed of Light
Imagine you are able to run with the speed of light holding a mirror in your hand.
Now will you be able to see yourself in the mirror?
0
votes
3answers
207 views
Relativistic basic question - four vector, Lorentz matrix
I have heard relativistics only very compressed during my student time. Now I looked up the definitions again and a question comes into my mind:
A contravariant vector is transformed like this: ...
0
votes
0answers
83 views
Alternate derivation of kinetic mass increase in special relativity, from Maudlin
I've recently come across a derivation, which I've not seen before, of mass increase in special relativity. It seems to make sense, but I get tripped-up on an intermediate step, and I can't seem to ...
2
votes
2answers
52 views
Is it part of special relativity that mass possessing energy is more dense?
I was reading http://www.edge.org/3rd_culture/hillis/hillis_p2.html and it says that a charged battery weighs more than a dead one or a rotating object weighs more than a stationary one (i.e. mass ...
1
vote
1answer
67 views
particle accelerator in space
I'm attempting to learn special relativity and i'm having trouble calculating velocity and momentum for each part of the system after interactions.
I wanted to know how fast a linear accelerator and ...
4
votes
2answers
110 views
Where might hertz per dioptre actually be useful?
I once came across the strange, artificial unit "hertz per dioptre", which is dimensionally equivalent to "metres per second". Could this unit, by some stretch of the imagination, be used in some ...
2
votes
1answer
84 views
Graviton and photons interaction
If one believes in the theory of gravitons then by viewing a black hole you see gravitons affect photons. This in turn leads to the conclusion that force carrier's mass equivalences allow them to be ...
1
vote
1answer
299 views
Special Relativity: Transforming Maxwell's equations
I'm working through Einstein's original 1905 paper*, and I'm having trouble with the section on the transformation of Maxwell's equations from rest to moving frame.
The paper proceeds as follows:
...
0
votes
1answer
106 views
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?
2
votes
1answer
81 views
Relativity exposition from the “naïve” viewpoint
A naïve observational viewpoint doesn't automatically try to correct for signal delays. In other words, the time co-ordinate is retarded.
For example:
In popular expositions of relativity you'll ...
0
votes
1answer
39 views
reltivistic action is negative or positive number? [duplicate]
Possible Duplicate:
Why lagrangian is negative number?
In the special relativistic action for a massive point particle,
$$\int_{t_i}^{t_f}\mathcal {L}dt,$$
where the Lagrangian
$$\mathcal ...
0
votes
0answers
38 views
How to calculate the scatterng angle of a particle collision [closed]
z0 decays to electron + positron pair as below. The momentum of z0 is 150 Gev/c and its mass is 90 Gev. Find the scattering angle of the electron
1
vote
2answers
192 views
Why lagrangian is negative number?
In the special relativistic action for a massive point particle,
$$\int_{t_i}^{t_f}\mathcal {L}dt,$$
why is the Lagrangian
$$\mathcal {L}=-E_o\gamma^{-1}$$
a negative number?
0
votes
1answer
97 views
Confusion about time shift in special relativity
I have never really found a way to comfortably comprehend the idea of time shift even though I know its not the hard part of relativity theory. In that light, can someone point out what is wrong or ...
1
vote
1answer
90 views
what is use of relativistic action?
this is relativistic action:
$$S=\int_C \mathcal {L}dt$$
where the $\mathcal{L}$ is $-m_oc^2\gamma^{-1}$
what is use of relativistic action!?
5
votes
1answer
225 views
What is the speed of time
When we measure the speed of a moving element we do it with the help of a reference frame. Now if we need to measure the speed of time, is it possible? Is time really has speed?
Thanks in advance.
3
votes
3answers
175 views
Proof for $p=\gamma_Pmu$
As I'm reading about Relativistic Momentum, my book states the following:
$$p=m \frac{\Delta x}{\Delta t}=m\frac{\Delta x}{\sqrt{(1-u^2/c^2)}\Delta t}=\frac{mu}{\sqrt{1-u^2/c^2}}=\gamma_Pmu$$
...
1
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 ...
3
votes
1answer
51 views
Single electron non-perturbing detector
I am designing an experiment where I need to trigger the release of an electron by a radioactive source (Sr-90).
The easy way to do it is to use a thin scintillator right after the source collimator. ...
0
votes
1answer
33 views
Speed of light and lorentzian factors [duplicate]
Possible Duplicate:
How can a photon have no mass and still travel at the speed of light?
If light travels at the speed of light, and anything with rest mass will experience relativistic ...
0
votes
3answers
130 views
where the proper time is invariant why $d\tau$ is not zero?
where the proper time is invariant why change (differential) in proper time $d\tau$ is not zero?
$\Delta \tau=\tau_f-\tau_i$
as i know.
$d(invariant)=0$
note to comment:
action
$S=-m_oc^2\int_C d\tau$
...
0
votes
1answer
78 views
Can the zeroth-component of a 4-velocity be negative?
Is it allowed to have the zeroth-component of a four-velocity be negative? I presume the answer is yes, but I just want to make sure. Many thanks.
This is referring to $V^0$ for a curved space ...
0
votes
0answers
31 views
time oscillation
Can there be a situation, where the time dilation is oscillating, in the sense that one observer notes that the time for the other frame is moving faster then slower, etc...? Can this oscillation be ...
1
vote
1answer
171 views
Does the increase of (relativistic) mass, while flying near speed of light, has any impact on astronauts? [duplicate]
Possible Duplicate:
Would travelling at relativistic speeds have any impact on human biology?
I am asking myself this question for a few day. What is the answer on: Does the increase of ...
3
votes
0answers
138 views
Should the Jacobian be negative in $\mathrm{d}^4 x$?
In page 24 of Srednicki's QFT textbook, he says that $\mathrm{d}^4x$ is a Lorentz scalar. I understand that the determinant of a Lorentz matrix is always $\pm 1$. So in an improper Lorentz ...
0
votes
2answers
59 views
Why does isotropy principle require existence of inertial transformation when axes are reversed?
Assuming one spatial and one termporal dimension, let's assume an intertial transformation $A(v)$ as follows:
$$
\begin{pmatrix}
t' \\
x' \\
\end{pmatrix} = A(v)
\begin{pmatrix}
t \\
x \\
...
5
votes
2answers
312 views
Does quantum mechanics allow faster than light (FTL) travel?
Let's suppose I initially have a particle with a nice and narrow wave function[1] (I will leave these unnormed):
$$e^{-\frac{x^2}{a}}$$
where $a$ is some small number (to make it narrow). Let's also ...
5
votes
1answer
214 views
Is there any uncertainty between mass and proper length or time?
I was trying to naively draw a parallel between special relativity and the Heisenberg uncertainty principle. I try to understand uncertainty principle as a consequence of 4-position and 4-momentum ...
1
vote
2answers
99 views
What is 'past null infinity'?
For example, in the sentence "there is no incoming radiation at past null infinity".
3
votes
1answer
180 views
Can the overall sign of the Minkowski metric be changed?
If we take the Minkowski metric, $\eta_{\mu\nu}=(1,-1,-1,-1)$, instead of the usual $(-1,1,1,1)$, does this change the form of the Lorentz Transform? I think the standard Lorentz Transform looks like: ...
0
votes
5answers
258 views
How a spacecraft travelling near light speed avoid asteroids?
How would a spacecraft traveling near light speed avoid a (relatively dense group of) asteroids? Or suppose such spacecraft is designed, how would the physics work for steering it inside such a ...
3
votes
1answer
240 views
How is the speed of light constant in all directions for all observers?
Please imagine the following thought-experiment:
Order of Events:
Pulse - A single pulse of light is emitted from the light towards the mirror
Reflect - The pulse hits the mirror and is ...
2
votes
2answers
201 views
D'Alembert operator and special relativity
We are currently covering special relativity in the theoretical physics lectures where we defined:
$$
\mathrm ds^2 := \mathrm dt^2 - \mathrm dx^2 - \mathrm dy^2 - \mathrm dz^2
$$
In Road to Reality, ...
0
votes
1answer
125 views
Why does weak equivalence principle say gravity is equivalent to acceleration?
I am told that the weak equivalent principle, that $m_i=m_g$ (inertial and gravitational masses are equivalent) is equivalent to the statement that in a small system you can't tell whether you are in ...
-1
votes
1answer
73 views
Speed astronauts measure moving at the speed of light
Lets suppose a spaceship travels with v = 0.9c relative to the Earth. The time inside the spaceship would pass slower than on Earth. Would the astronauts measure a different speed (that means, a ...
0
votes
1answer
135 views
Does photon possesses no time to cover any arbitrary distance?
Photon travel 8 minutes (with speed $c$) from the sun to reach the earth. Any particle (or space-ship) with velocity $0.99 c$ covers the same distance (93 millions km) within less than 2 minutes ...
3
votes
1answer
96 views
What's wrong with this reactionless drive?
I think this is analogues for the Woodward effect, but macroscopic:
We assume a spacecraft consisting of a broomstick, a donut and lots of gear for storing and transfering mechanical energy.
Take the ...
