The union of special (SR) and general (GR) relativity. Use this tag if both SR and GR apply.
19
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4answers
393 views
Paradox?: What is the form of radiation experienced by a harmonically accelerated observer?
Theory predicts that uniform acceleration leads to experiencing thermal radiation (so called Fulling Davies Unruh radiation), associated with the appearance of an event horizon. For non uniform but ...
13
votes
5answers
472 views
Why can't you escape a black hole?
I understand that the event horizon of a black hole forms at the radius from the singularity where the escape velocity is c. But it's also true that you don't have to go escape velocity to escape an ...
12
votes
2answers
128 views
Cascade in relativistic turbulence
The Kolmogorov theory of turbulence indicates an energy cascade in turbulence. Is there a corresponding version of relativistic fluid?
11
votes
4answers
380 views
Is there an easy way to show that $x^2-t^2=1/g^2$ for a (relativistic) body undergoing acceleration g?
A professor asked me about the (c=1) equation:
$$x^2 - t^2 = 1/g^2$$
which I used in a paper. Or with $c$:
$$x^2 - (ct)^2 = c^4/g^2.$$
I told him that it was the exact equation of motion for a ...
11
votes
2answers
612 views
A simple pendulum moving at a relativistic speed - how does the period change?
I've been pondering the precise mechanism of time dilation for the example of a simple pendulum in two different situations:
The observer and ground are at rest in one frame of reference; the ...
11
votes
2answers
194 views
What are the consequences of relativistic angular velocities?
If I take a rod of some radius $r$ and length $L$, and I spin this rod with angular velocity $\omega$. How would the geometry of the rod appear to an observer as one converges to $c$? What are the ...
9
votes
4answers
612 views
Does the future already exist? If so, which one?
In the NOVA Fabric of the Cosmos program, Brian Greene explains a theory in which there is no "now", or more specifically, now is relative. He describes an alien riding a bicycle on a far off planet ...
9
votes
4answers
1k views
Is time travel possible? [duplicate]
Time travel -- often featured in movies, books, or facetiously in conversation. There are also theories treating time as simply another dimension, which to the layperson might imply forward and ...
9
votes
1answer
553 views
Cross-section in relativistic limit: Fermi's golden rule still valid?
In order to calculate the cross-section of an interaction process the following formula is often used for first approximations:
$$
\sigma = \frac {2\pi} {\hbar\,v_i} \left| ...
8
votes
3answers
528 views
Is there a relativistic (quantum) thermodynamics?
Does a relativistic version of quantum thermodynamics exist? I.e. in a non-inertial frame of reference, can I, an external observer, calculate quantities like magnetisation within the non-inertial ...
8
votes
3answers
570 views
Special Relativistic approximation to GR
Some time ago I was talking to a professor in college about some of the fundamental aspects and origin of General Relativity. I was surprised to learn, in fact, that a pretty good approximation to GR ...
7
votes
6answers
817 views
Quaternions and 4-vectors
I recently realised that quaternions could be used to write intervals or norms of vectors in special relativity:
$(t,ix,jy,kz)^2 = t^2 + (ix)^2 + (jy)^2 + (kz)^2 = t^2 - x^2 - y^2 - z^2$
Is it ...
7
votes
6answers
2k views
Is time travel possible? Is it possible to go back in time?
I read somewhere that according to relativity, black holes and other space related stuff it is possible to jump into past.
Is it possible for anything to go back in time either continuously or by ...
7
votes
4answers
607 views
Special Relativistic Time Dilation — A computer in a very fast centrifuge
Ok, I've stumbled onto what I think is a bit of a paradox.
First off, say you had some computer in a very fast(near light speed) centrifuge. You provide power to this computer via a metal plate on ...
7
votes
6answers
260 views
In coordinate-free relativity, how do we define a vector?
Relativity can be developed without coordinates: Laurent 1994 (SR), Winitzski 2007 (GR).
I would normally define a vector by its transformation properties: it's something whose components change ...
7
votes
2answers
414 views
Mechanism by which electric and magnetic fields interrelate
I read that force due to electric field on some particle in one reference frame can exhibit itself as force due to magnetic field in some other reference frame and that electric and magnetic fields ...
6
votes
7answers
1k views
How does Newtonian gravitation conflict with special relativity?
In the Wikipedia article Classical Field Theory (Gravitation), it says
After Newtonian gravitation was found to be inconsistent with special relativity, . . .
I don't see how Newtonian ...
6
votes
3answers
305 views
How are accelerated reference frames non-symmetrical?
As in, if I'm accelerating away from the Earth, then does the Earth also appear to be accelerating away from me at the same rate? Or is there something to "break" this type of symmetry?
My question ...
6
votes
8answers
721 views
Reactionless Drives
According to the third law of motion, you van't have an mass move in a particular direction unless there is a proportional opposite mass/acceleration ratio in the opposite direction.
No-one has been ...
6
votes
2answers
190 views
Blue-shifting as opposed to violet-shifting
A recent XKCD comic implies that the sky is blue as opposed to violet due to human physiology, and that animals more sensitive to shorter wavelengths will perceive the Earth's sky as the shortest ...
6
votes
2answers
303 views
Do Lorentz Boosts in the same direction form a group?
I know that two consecutive Lorentz Boosts in different directions produce a rotation and therefore Lorentz Boosts don't form a group. But, my intuition tells me that, Lorentz Boosts in the same ...
6
votes
4answers
604 views
Help Me Gain an Intuitive Understanding of Lorentz Contraction
I'm having a hard time getting an intuitive understanding of Lorentz Contraction. I understand what it is by definition but I don't 'get it.' I'm not a physicist, just an amateur, so sorry if this ...
6
votes
1answer
328 views
Decomposing geodetic/de Sitter effect into Thomas precession and spatial curvature
According to Rindler the geodetic effect can be considered as consisting of Thomas precession combined with the effect of moving through curved space.
Wolfgang Rindler (2006) Relativity: special, ...
6
votes
2answers
154 views
Do the energy levels of electron orbitals change relativistically?
When an electron emits a photon from changing energy levels, the frequency of the photon depends on the difference between the energy levels.
But if someone is moving with respect to the atom, the ...
6
votes
3answers
302 views
Getting back out of an Alcubierre warp bubble
Does the theory on paper provide a way for hypothetical travelers to get back out of the bubble that has gotten them close to their distant destination by compressing all the space in front of them ...
6
votes
1answer
87 views
Is there a relativity-compatible thermodynamics?
I am just wondering that laws in thermodynamics are not Lorentz invariant, it only involves the $T^{00}$ component. Tolman gave a formalism in his book. For example, the first law is replaced by the ...
5
votes
2answers
603 views
Entering a black hole, jumping into another universe---with questions
I'm quite familiar with SR, but I have very limited understanding in GR, singularities, and black holes. My friend, which is well-read and is interested in general physics, said that we can "jump" ...
5
votes
4answers
803 views
Why are four-vectors needed in the Dirac equation, when there are 4 linearly independent 2D matrices?
I was taught that for the Dirac-equation to "work", you need matrices of the following form:
$Tr(\alpha^i) = 0$.
Eigenvalues +1 or -1
2 previous points together: equal number of negative and ...
5
votes
2answers
480 views
Is this a quaternion Lorentz Boost?
The quaternion Lorentz boost $v'=hvh^*+ 1/2( (hhv)^*-(h^*h^*v)^*)$ where $h$ is $(\cosh(x),\sinh(x),0,0)$ was derived by substituting the hyperbolic sine and cosine for the sine and cosine in the ...
5
votes
2answers
199 views
Why absoluteness of time implies galilean transformations?
In Landau course, vol.1 Mechanics, one finds the statement: "...the absoluteness of time necessarily implies that the ordinary law of composition of velocities is applicable to all phenomena."
I ...
5
votes
1answer
150 views
Confused about indices of the Ricci tensor
In an intro to GR book the Ricci tensor is given as:
$$R_{\mu\nu}=\partial_{\lambda}\Gamma_{\mu \nu}^{\lambda}-\Gamma_{\lambda \sigma}^{\lambda}\Gamma_{\mu \nu}^{\sigma}-[\partial_{\nu}\Gamma_{\mu ...
5
votes
1answer
373 views
Length contraction in rotating frame?
If an observer were to rotate around a point at near light speeds, what sort of length contraction would he observe the universe undergo?
5
votes
1answer
116 views
Is space curvature relative?
If I have an object that is of some length $l$ moving at a relativistic velocity $v$ for some reference frame in a 1D universe, then length contraction states that $l=\gamma\times l'$. But at the ...
5
votes
1answer
192 views
Are objects in a gravitational well shortened?
Bob is in a gravitational potential well, he moves a long vertical stick up and down a distance of 1 meters. Alice observes the upper end of the stick, at upper location.
There is the phenomenon of ...
4
votes
5answers
513 views
Time Dilation - How does it know which Frame of Reference to age slower?
Okay, I'm asking a question similar to this one here: Time Dilation - what happens when you bring the observers back together?. Specifically, I am curious about a specific angle on the second part of ...
4
votes
2answers
910 views
How to deduce the theorem of addition of velocities?
Lorentz contraction and time dilatation can be deduced without Lorentz transformation. Can you deduce also the theorem of addition of velocities
$$w~=~\dfrac{u+v}{1+uv/c^2}$$
without Lorentz ...
4
votes
2answers
686 views
How can time be relative?
I don't understand how time can be relative to different observers, and I think my confusion is around how I understand what time is.
I have always been told (and thought) that time is basically a ...
4
votes
3answers
795 views
Is acceleration relative?
A while back in my Dynamics & Relativity lectures my lecturer mentioned that an object need not be accelerating relative to anything - he said it makes sense for an object to just be accelerating. ...
4
votes
4answers
127 views
When we talk about speeds in relativity theory, where are they measured?
I recently asked a question here about if the direction we travel matters in relativity theory:
Does it matter in which direction I travel in relativity theory?
After I got answers and making more ...
4
votes
6answers
197 views
Why are distant galaxies not actually tiny bits of matter?
Distant galaxies are said to be moving away from the Milky Way (and us) at speeds approaching the speed of light. Since Special Relativity tells us that any object moving away from us at a velocity of ...
4
votes
5answers
285 views
Does a interstellar spacecraft traveling at relativistic velocity require continous thrust to maintain velocity?
Assuming completely empty space, does a spacecraft traveling at 0.5 C require continuous thrust to avoid deceleration?
If the spacecraft is traveling at 0.5 C, does it's relativistic mass act upon ...
4
votes
2answers
82 views
Mirror image at relativistic speeds
Imagine moving parallel to the surface of a very large flat mirror at relativistic speeds. What would be the effect of viewing yourself in the mirror? At non-relativistic speeds your image would be ...
3
votes
3answers
777 views
How to calculate time dilation in approaching speed of light
If a spaceship travels close to the speed of light (say, at 0.9c), how do I calculate the time as the spaceship pilot experience it? I thought the formula was
$$t = \frac{t_0}{\sqrt{1-v^2/c^2}}$$
...
3
votes
3answers
332 views
Time Dilation - what happens when you bring the observers back together?
I'm having trouble getting my head around the time dilation paradox.
Observer A and B are at the same "depth" in a gravity well. Observer B then descends into the well. A will observe B's time as ...
3
votes
3answers
158 views
How does relativity explain gravity, without assuming gravity [duplicate]
I have seen the "objects pull down on space-time" explanations, but they assume a "pull down" force themselves. Could anyone explain the space-time explanation without assuming gravity in the first ...
3
votes
3answers
315 views
Why do clocks measure arc-length?
Apologies in advance for the long question.
My understanding is that in GR, massive observers move along timelike curves $x^\mu(\lambda)$, and if an observer moves from point $x^\mu(\lambda_a)$ to ...
3
votes
1answer
197 views
Which kinds of Physics laws do and don't comply with the principle of relativity?
In Physics, the principle of relativity is the requirement that the equations describing the laws of physics have the same form in all admissible frames of reference.
However, according to this and ...
3
votes
1answer
179 views
Is a preferred reference frame of the universe the old aether?
About two years ago I posted a question about a symmetrical twin paradox: Here.
Recently a new answer was posted and an intense discussion ensued: Here.
One of the points discussed concerns a ...
3
votes
3answers
146 views
Having trouble seeing the similarity between these two energy-momentum tensors
Leonard Suskind gives the following formulation of the energy-momentum tensor in his Stanford lectures on GR (#10, I believe):
$$T_{\mu \nu}=\partial_{\mu}\phi \partial_{\nu}\phi-\frac{1}{2}g_{\mu ...
3
votes
2answers
169 views
Relativistic Hamiltonian Formulations [duplicate]
Possible Duplicate:
Hamiltonian mechanics and special relativity?
The Hamiltonian formulation is beautifully symmetric. It's a shame that the explicit time derivatives in Hamilton's ...
