The union of special (SR) and general (GR) relativity. Use this tag if both SR and GR apply.

learn more… | top users | synonyms

6
votes
3answers
1k views

Cooling of atomic clocks

I have read of cooling atoms to one-quarter of one-thousandth of a degree above absolutely zero. According to Physicist William Phillips, this improves the measurement of the ticking frequency. I have ...
6
votes
1answer
1k views

Is this the correct method for deriving E=mc^2?

So lately I tried to deriving Einstein's $$ E = mc^2$$ which later I found from this website : http://www.emc2-explained.info/Emc2/Derive.htm They use calculus based method, so I tried another way. ...
6
votes
3answers
522 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
4answers
597 views

What is the intuition behind the Lorentz factor from Special Relativity

Time Dilation = $1/\sqrt{ 1-v^2/c^2}$ but why? How do you get to that conclusion? I know you use Pythagoras'theorem and my current understand goes like this: $$vt^2+ct^2=cT^2$$ then you take the ...
6
votes
1answer
428 views

Can we actually have null curves in Minkowski space?

I know that this sounds really stupid but, when I think of the Minkowski space I cannot imagine a null curve, only null lines. For me, the only possible way to have one is to change the basis of the ...
6
votes
2answers
611 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
1k views

Doesn't Warp theory violate causality?

I have heard many physicists (ex:- Michio Kaku) saying "Warp speed" from Star Trek doesn't violate any known physical laws. But doesn't it violate causality? Say, we make warp drive possible and ...
6
votes
3answers
404 views

Is my grandmother alive for some alien who is located on some place and is moving with some speed?

(I am science fiction writer. I had a goal in my novel to argue that time travel to past will not be available even in future. I asked here how to argue that. I collected answers and analyzed them and ...
6
votes
2answers
918 views

Wormholes, time travel, and time dilation

I believe I understand the "wormhole time machine" now, but still have one question regarding what happens if you enter the stationary mouth. A wormhole is created and (of course) has two mouths, $x$ ...
6
votes
2answers
629 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
1answer
290 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 ...
6
votes
1answer
92 views

Pauli exclusion principle and relativity

I'm somewhat curious about how the Pauli Exclusion Principle functions when relativistic time becomes a significant factor. Just to clear things up, my (possibly poor) understanding goes like this. ...
6
votes
2answers
1k views

Feynman's proof for Liénard-Wiechert's potential of a moving charge

Feynman's proof utilizes a geometrical and fundamental integration argument. I like it, except this bit: What makes me unconfortable somehow is that in (c) we are counting in some of the charge we ...
6
votes
1answer
242 views

Null Coordinates

I have a very basic question: what are the advantages of writing a metric in the null coordinates? Which extra insight do they provide? I've looked in Caroll's "Spacetime and Geometry" and Wald's "...
5
votes
1answer
619 views

Doesn't dating the universe violate the concept of spacetime's inseparability?

It would seem that measuring an age of the universe from the big bang requires separating spacetime into a 3D coordinate system and a time track. I fail to understand why it is appropriate to take ...
5
votes
2answers
2k views

Can light affect gravity? [closed]

I have a mentor at work, who wrote a paper in the past regarding light and relativity. I am an aerospace engineer by training and a system administrator/programmer by trade, so I know a little bit ...
5
votes
6answers
379 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 ...
5
votes
2answers
169 views

According to relativity theory, what is the force that two electrons moving radially apart exert on each other?

According to relativity theory, what is the most general expression for the force that two electrons moving radially apart exert on each other? I am looking for a function $$F(r(t)),$$ where $F$ is ...
5
votes
4answers
3k views

Space-like and time-like: where do the names come from?

Space-like separated events are events that, in a well-chosen reference frame, can take place at the same time but never happen at the same location. On the other hand for time-like events, one can ...
5
votes
1answer
131 views

Is there still mystery about spin crisis?

The gluon is a vector boson; like the photon, it has a spin of 1. If this is true and if we know the spin of protons and neutrons then why the news SPIN COMES FROM GLUONS? is telling that, we aren'...
5
votes
1answer
291 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
503 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
211 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
2answers
65 views

Acceleration in relativistic particle [closed]

Suppose that a relativistic particle of mass at rest $m_0$ and charge $q$ moves under the influence $F$. Show that the acceleration $a$ of the particle can be expressed as follows: $$a = \frac1mF + \...
5
votes
1answer
444 views

Relationship between the continuity equation and the wave equation

What exactly is the relationship between the continuity equation and the wave equation? Suppose $J^\mu$ is a contravariant vector that satisfies the continuity equation $\partial_\mu J^\mu=0$. Let $J^...
4
votes
3answers
999 views

Chasing a photon [duplicate]

According to this article, the Theory of Special Relativity holds that if you were chasing a stream of light at half the speed of light, $c/2$, the light's speed relative to you would still be $c$. ...
4
votes
2answers
828 views

Nature of spacetime 4-vector and tangent space?

An entry level confusion about spacetime. I understand that a 4-vector describes a point or event in spacetime. But I've also read (Bertschinger, 1999) that re spacetime "we are discussing tangent ...
4
votes
3answers
5k views

Difference between timelike and spacelike vectors

Other than one having a positive invariant scalar product and the other a negative one, what are the actual physical differences between these vectors?
4
votes
2answers
706 views

How will the Twin Paradox become, for Time Dilation, if no acceleration was ever involved?

I think one catch in Twin Paradox was about the big acceleration that can turn back the traveling twin from light speed outward bound, to become light speed inward bound. What if there is strictly no ...
4
votes
2answers
499 views

If a photon has no mass, how can it be attracted by the Sun?

I read that the photon doesn't have mass, but my teacher says that the photon has mass because the sun can attract it (like in the experiments to prove the theory of relativity). I think that there ...
4
votes
3answers
316 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 ...
4
votes
4answers
205 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
2answers
615 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 equations ...
4
votes
3answers
266 views

Relativity and predictability

My old theoretical physics professor used to say that (already) (special?) relativity shows that "the future is not predictable". Any ideas how this should be interpreted?
4
votes
1answer
156 views

Relativistic Time dilation and travel to exoplanets

NASA recently discovered the first habitable exoplanet with the Kepler space telescope. Recently I was on this site: http://www.cthreepo.com/lab/math1/ With the Long Relativistic Journeys ...
4
votes
1answer
559 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 ...
4
votes
2answers
89 views

Plane wave shift in a differential operator

Does anyone can help me to prove the following equation \begin{equation} e^{-i\vec{k}\cdot\vec{x}}f(\partial_{\mu})e^{i\vec{k}\cdot\vec{x}} = f(\partial_{\mu}+ik_{\mu}) \end{equation} Where $\vec{k}\...
4
votes
3answers
190 views

Is acceleration caused by curvature or space or time or both?

I'm trying to get a hold of the idea of gravity in general relativity and spacetime. I've seen plenty of demonstrations of the rubber mat analogy to describe gravity and spacetime curvature. Is this ...
4
votes
1answer
450 views

Question about gravity in 1 dimension

Consider 2 pointmasses in 1D space. More specific ; object $A$ with mass $m_a$ and object $B$ with mass $m_b$. $A$ and $B$ are a distance $d$ away from eachother. Let these pointmasses $A,B$ have no ...
4
votes
4answers
2k views

Why acceleration is not relative in General Relativity?

I was thinking of it, If I say: "I'm moving at a velocity $v_1$ relative to a reference frame $M$ then the acceleration will be the derivative of $v_1$ relative to the reference frame $M$." In other ...
4
votes
5answers
625 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
1answer
89 views

If Speed is Relative can Absolute Acceleration be Real

This is , I suppose not a good question , but I think I am missing something which confuses me in this question. So my question is - Velocity is relative. So suppose a car is moving at a speed 200 ...
4
votes
1answer
87 views

Are all diffusion-like processes described as wave-like in relativity-compatible formulations?

Citing from Wikipedia's article on relativistic heat conduction: For most of the last century, it was recognized that Fourier equation (and its more general Fick's law of diffusion) is in ...
4
votes
1answer
221 views

How does Spacetime Curvature increase the velocity of particles falling towards the earth?

Two particles fall side by side, towards the earth. The horizontal distance between them is 10m. As they advance nearer and nearer to the earth's surface, the horizontal distance decreases, from 10m ...
4
votes
2answers
282 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 ...
4
votes
1answer
898 views

How much time has passed for Voyager I since it left the Earth, 34 years ago?

34 years have passed since Voyager I took off and it's just crossing the solar system, being approximately at 16.4 light-hours away. How much time have passed for itself, though?
4
votes
2answers
631 views

Introduction to relativity books [duplicate]

Possible Duplicate: Getting started general relativity I am an engineer who loves to read science fiction books especially when there's more science than fiction but usually I see that I lack ...
4
votes
1answer
56 views

Would a star be observed to burn brighter traveling at relativistic speeds?

Would a star, similar to our own, be observed to "burn" differently at relativistic speeds? I am only thinking this because the observed relativistic mass would change.
4
votes
1answer
65 views

Long rod in an expanding universe

Suppose you have a metal rod, of vast length, floating in space. From it's center-of-mass reference frame it is assumed to be at rest with respect to the CMB. The rod will have some degree of ...
4
votes
1answer
291 views

How is relativistic N-body simulation possible without knowing the entire histories of each particle?

In n-body simulation you need to know the positions of the particles in order to calculate the force between them. The new velocity of each particle can then be calculated given a simulation timestep ...