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

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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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67 views

Need help with relativistic dynamics

I understand the concept, but I'm having a hard time applying the consequences of conservation (energy/momentum). For example: A proton with kinetic energy 437 MeV hits a proton at rest elastically ...
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0answers
22 views

Period of Transmission of signals in different frames

A radar transmitter (T) is fixed to a system $S_{2}$ which is moving to the right with speed v relative to system $S_{1}$. A timer in $S_{2}$, having a period $\tau_{0}$ (measured in $S_{2}$) causes ...
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7answers
2k views

Inertial Frames of Reference - Inertial vs. Accelerated Frames

According to Robert Resnick's book "Introduction to Special Relativity", a line states the following as the definition of an inertial frame of reference: "We define an inertial system as a frame of ...
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1answer
129 views

Could light travel more slowly than the “universal speed limit”? Could this imply quantization of spacetime?

One description of relativistic effects that I've heard/read goes something like this: Everything moves through spacetime at a constant speed. An object's direction of travel through spacetime can ...
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1answer
417 views

How do photons experience time? [duplicate]

I know that as velocity approaches the speed of light the time dilation shoots to infinity as shown below. 1)So I want to know how time is perceived from the point of view of the photon? 2)Since ...
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1answer
191 views

Deriving infinitesimal time dilation for arbitrary motion from Lorentz transformations

I'm trying to derive the infinitesimal time dilation relation $dt = \gamma d\tau$, where $\tau$ is the proper time, $t$ the coordinate time, and $\gamma = (1-v(t)^2/c^2)^{-1/2}$ the time dependent ...
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3answers
225 views

Black hole accretion of dark energy

Dark energy physically can be interpreted as either a fluid with positive mass but pressure the negative of its density (pressure has units of energy/volume, and energy is mass), or a property of ...
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3answers
479 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 ...
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0answers
62 views

A Mathematician Who Wants to Learn Particle Physics [duplicate]

Possible Duplicate: Book recommendations I'm a grad student in pure math, wrapping up a thesis in Lie theory. After years of talking to mathematicians and physicists, I've decided that it's ...
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2answers
392 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 ...
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2answers
218 views

Photons in a gravitational field

I have been really staring for a while in a MP-Beiser book and I totally disagree with a statement he does there. On a page 85 he states that photons act as they have a mass $m$. He derives this by ...
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4answers
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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 ...
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6answers
221 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 ...
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1answer
67 views

Are branes in 4D-spacetime moving, or are they static?

Given that a worldline, worldsheet, worldvolume, are representation in a 4D-spacetime of a point particle, a string or a brane, respectively, I was wondering if those objects necessarily have to be ...
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1answer
285 views

Degrees of freedom in the infinite momentum frame

Lenny Susskind explains in this video at about 40min, as an extended object (for example a relativistic string) is boosted to the infinite momentum frame (sometimes called light cone frame), it has no ...
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1answer
233 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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2answers
573 views

Am I faster than my shadow?

I guess that everything that happens to me (or I do), happens before my shadow "records" the information. Is that right? Is it the same for any observer?
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2answers
191 views

Cascade in relativistic turbulence

The Kolmogorov theory of turbulence indicates an energy cascade in turbulence. Is there a corresponding version of relativistic fluid?
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2answers
360 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 ...
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1answer
164 views

Why is energy-momentum 4-vector so much easier to explore/observe than spacetime 4-vector

I have read that spacetime 4-vector is quite difficult to observe/explore and that energy-momentum 4-vector is much more appropriate for CERN etc. Why is that? Could anyone give me a brief ...
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2answers
325 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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4answers
279 views

Is there a default notation for 4-vectors while handwriting?

In printed paper 3-vectors can be denoted bold italic while 4-vectors can be denote just bold. While handwriting 3-vectors are denoted by arrows above letters. Is there a similar way to denote ...
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1answer
322 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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1answer
202 views

How to relate photon's higher frequency to time dilation?

The usual explanation for photon's higher frequency in lower altitudes (higher gravity), when the photon is going downward towards a massive body, is that gravitational potential energy is converted ...
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2answers
3k views

If nothing in the universe can travel faster than light, how come light can't escape a black hole? [duplicate]

Possible Duplicate: How does gravity escape a black hole? If nothing in the universe can travel faster than light, how come light can't escape a black hole? I mean, Einstein's relativity ...
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1answer
381 views

Why are we talking about space curvature as if we know what space is? [closed]

1) Why are we talking about space curvature as if we know what space is? Every question about gravity seems to evoke an answer involving "space curvature" which seems like an undefined placeholder ...
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1answer
176 views

Light orbiting a massive body [duplicate]

Possible Duplicate: Can a photon be made to orbit a known (or undiscovered theoretical) body? How massive would a black hole have to be for light to orbit it at 1km away from the ...
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1answer
159 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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2answers
104 views

Causal reconnection

Can causally disconnected regions join up again? For example the universe is expanding faster than light creating cosmological horizon, but what if something causes the expansion to slow down and ...
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2answers
181 views

Is a compact universe consistent with the results of (for example) the Michelson-Morley experiment?

If the universe is compact then there is a twin paradox that is resolvable only by selecting a preferred inertial reference frame (arXiv). I was under the impression that the lack of a preferred ...
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1answer
53 views

Derivation of superluminality in astronomy

Can somebody provide derivation of the relativistic explanation of (apparent) superluminality in astronomy? The derivation on wikipedia seems to be a bit confusing.
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3answers
6k 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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1answer
73 views

At what rate does radiation exposure occur at high-c speeds?

If a future astronaut travelled to Alpha Centauri at a significant percentage of light-speed? Apart from increased blue shifted radiation from their direction of travel - how would they experience ...
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1answer
239 views

What is the speed for an object that travel close to the speed of light?

I have some questions regarding Einsteins theory of Relativity that should be fairly easy to answer. Lets say we make an experiment where we have a rocket (with an astronaut inside) that travels very ...
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7answers
448 views

Relation between coordinates and frames of reference

I always get a little uneasy that all the theories I can think of (at least since Newton) are constructed in a way such that they would be true in heaven and on earth ... but we can never go ...
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1answer
237 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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3answers
215 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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0answers
165 views

How do I extend the Lorentz transformation metric to dimensions>4?

How do I extend the general Lorentz transformation matrix (not just a boost along an axis, but in directions where the dx1/dt, dx2/dt, dx3/dt, components are all not zero. For eg. as on the Wikipedia ...
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2answers
372 views

How do I adjust the kinematic equations to avoid reaching speeds faster than light?

I like some 'science' in my 'science fiction', so I started crunching out the kinematic equations for some of the scenarios my characters are getting involved in, and ran smack dab into an issue. ...
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2answers
219 views

What if a normal digital clock used to measure time in twin paradox?

Last night I was watching a youtube video about twin paradox. It still continues to baffle me :). In that video, the speaker conceptualizes a clock that measures time by reflections of light. If we ...
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1answer
81 views

Does relativity apply to something experiencing Centripetal forces?

If I tie a rope to a clock and spin it over my head, would it lose synchronization with a clock sitting at my feet?
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3answers
2k 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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5answers
453 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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2answers
306 views

length contraction question

we know from eintein's theory of relativity that lets say, a ruler is travelling to a speed if light, then we can say that the ruler (from our view as observers) has shorten. but why, lets say we have ...
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2answers
181 views

Using Einstein's Relativity: Who is younger?

Suppose we have a person A and a person B. Person B travels very close to speed of light and never returns. He's constant in speed. Then, we can say two things: B is younger than A. A is younger ...
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1answer
128 views

reletivistic mass as seen by different observers

Imagine there are two observers $A$ and $B$ and a particle $P$. $A$ and $B$ are at about the same point, $P$ is some distance away. From $A$'s point of view, $P$ has velocity $V$ and $B$ has ...
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3answers
171 views

When a high speed neutrino just misses an old neutron star, why isn't it trapped?

Suppose a neutrino is seen travelling so fast that its Lorentz gamma factor is 100,000. It races past an old, no longer active neutron star, narrowly missing it. As far as the neutrino is concerned, ...
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4answers
503 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 ...
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2answers
203 views

Finding wave-fuctions of a Dirac particle for given 4-momentum and spin 4-vector

I've been reading through various materials on relativistic quantum mechanics, but I find the lack of simple examples disturbing. I'm acquainted with the general form the solutions to the Dirac ...