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

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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 ...
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2answers
544 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, $x$ ...
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1answer
113 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 ...
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1answer
272 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 ...
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1answer
489 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?
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1answer
197 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 ...
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1answer
262 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 ...
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1answer
308 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 ...
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3answers
921 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 $c$. ...
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3answers
13k views

How is light affected by gravity?

Light is clearly affected by gravity, just think about a black hole, but light supposedly has no mass and gravity only affects objects with mass. On the other hand, if light does have mass then ...
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3answers
4k 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?
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2answers
888 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 ...
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3answers
280 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 ...
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4answers
187 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 ...
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2answers
479 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 ...
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1answer
101 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 ...
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2answers
384 views

Why doesnt gravity speed up light?

We know that gravity speeds up a body; for instance, a meteor which enters the earth gets constantly accelerated up by earth's gravity. And from relativity we know that light bends near a massive ...
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3answers
133 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 ...
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1answer
310 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 ...
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4answers
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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 ...
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5answers
572 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 ...
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1answer
174 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 ...
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2answers
227 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 ...
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1answer
696 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?
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1answer
59 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 ...
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2answers
58 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 + ...
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2answers
501 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 ...
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1answer
210 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 ...
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1answer
610 views

Ephemeris calculations: Light time correction of the moon

I am currently trying to calculate apparent positions from raw JPL data. I've got it pretty much figured out, but there is one thing that's bugging me: Has the light time correction of the moon to be ...
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2answers
234 views

Superluminal particles with causality

What kind of CLASSICAL theories would allow to true (non-apparent) superluminal particles (beyond speed of light, BSOL) agreeing with causality to exist? I mean, are causal superluminal classical ...
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0answers
97 views

Current state of the existence of absolute space debate [closed]

Samir Okasha writes about the controversy concerning the existence of absolute space in Philosophy of Science: A Very Short Introduction (2002): One of the intriguing things about the ...
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2answers
247 views

Gravitation not instantaneous = non elliptic orbits?

When I studied physics some time ago my teacher explained that if we consider the gravitational atraction not instantaneous, such as the General Relativity says, the planets would be attracted towards ...
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3answers
3k 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 $$t = \frac{t_0}{\sqrt{1-v^2/c^2}}$$ ...
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2answers
778 views

Gravitational Time Dilation with big masses

From Wikipedia I read that the formula for calculating the time dilation caused by a mass is $t_0 = t_f{\sqrt{1-\frac{2GM}{rc^2}}}$ where: $t_0$ is the proper time between events A and B for a ...
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1answer
229 views

From a particle's point of view is his temperature absolute zero

Since the vibration of the particles is what cause temperature and a particle from its own point of view is not moving, is its temperature 0°K from its own point of view? Is there a thing like ...
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2answers
138 views

Relativity conclusions from Michelson-Morley Experiment

This is what my textbook says about the Michelson-Morley experiment: "This invariance of the speed of light between inertial reference frames means that there must be some relativity principle that ...
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3answers
491 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 ...
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1answer
249 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 ...
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420 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 ...
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1answer
238 views

Will a relativistic rocket be destroyed by incident photons?

I have read, that as a rocket approaches light speed, objects in front of it become blue shifted and also intensity of light increases. Does this mean that at some point any rocket will be destroyed, ...
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3answers
244 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 ...
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2answers
237 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?
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1answer
365 views

Is time the property of an object?

I don't know if the title makes much sense, but hopefully it will become clear with the text. Temperature is not a property of a point in the three dimensions, but actually of the object occupying ...
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1answer
492 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 arbitrarily separates spacetime into a singularity and a time track.
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1answer
427 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 ...
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2answers
140 views

Speed cannot be greater than speed of light. How about acceleration? Does it have limit?

We know that $c$ is the largest speed in the universe. How about the acceleration, does it has limit?
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2answers
3k views

If I travel close to the speed of light and come back, why is everyone else dead, and not me? [duplicate]

Consider the following scenario: I get in a spaceship, and travel really close to the speed of light for a while, and then come back. A lot of time has passed on the Earth, but since I was traveling ...
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2answers
193 views

Light & Observer moving perpendicular to each other

Light is the yellow arrow. Observer is the black arrow. Observer is moving at a constant speed of v, w.r.t to a Galilean frame of reference. Now from the point of view of the observer (O), how ...
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2answers
66 views

Is spacetime absolute?

As I understand it Newton's Laws imply that space is relative, as the laws of physics are the same in all inertial frames and as such there is no way, even in principle, to distinguish a frame that is ...
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80 views

space contraction near lightspeed

I read a lot of the Time-Life Science books when I was a kid (well, mostly the pictures and captions). One of the things that always stuck in my mind was an explanation of the twin paradox, using ...