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

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If all energy is removed from an object, what will it stop moving relative to?

If you somehow removed all energy from an object, what would it stop moving relative to? According to relativity, an object's position is not relative to the universe, just reference points, and, as ...
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3answers
3k views

How to derive addition of velocities without the Lorentz transformation?

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 ...
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2answers
210 views

Why does “closing speed” $>c$ not violate relativity's second postulate? [closed]

Why does closing speed $>c$ of a particle and photon or photon and photon not violate relativity's second postulate? Einstein expressed his Second Postulate in On the Electrodynamics of ...
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1answer
120 views

Doesn't the second postulate of special relativity imply the existence of “luminiferous ether”? [closed]

Einstein expressed his Second Postulate in On the Electrodynamics of Moving Bodies as: light is always propagated in empty space with a definite velocity $c$ which is independent of the state of ...
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23 views

Thomas Precession of an orbiting electron

So I am working through a problem on Thomas Precession. The set up is that we're given at lab time zero, the initial spin of an electron in circular orbit is: $$ ...
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3answers
9k views

What is the speed of time? [closed]

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? Does time actually have a speed?
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1answer
137 views

Why is it legitimate to only observe motion, but call it time, such that concepts like “the relativity of simultaneity” are seen as possibly genuine? [duplicate]

My question is asking for evaluation of a new concept or paradigm within the framework of the current mainstream physics understanding of Relativity. Specifically, that concept or paradigm being that, ...
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2answers
55 views

Time Dilation, Multiple Earth's?

I was reading about time dilation and suddenly a question raised in my mind. (This might be a misconception but please do correct me if I am wrong) I read that if we travel near to the speed of ...
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4answers
106 views

Time dilation if earth was stationary

Being on earth, we are constantly flying through space at incredible speeds. We revolve around a sun in a system that revolves around the center of the galaxy that is hurling itself through space. ...
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53 views

Doppler effect for the electrons emitted from ions moving at a particular velocity

If I have ions moving in X axis (along both directions; positive X axis and negative X axis) and if I irradiate laser along the positive X axis, electron will be ejected from the ions (if I am using a ...
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1answer
138 views

Photons inside a box

One of my friends told me that the definition of mass is the amount of matter. I told him that mass is not the amount of matter, because when we heat an object, the mass of the object increases. I ...
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2answers
403 views

Can a scalar field model gravity? How accurate would be the results? Are there any difficulties with such a model?

Newtonian gravity can be described by the equation: $$ \nabla^2 \phi = 4 \pi \rho G $$ where $\rho$ is the mass density, $\phi$ is the gravitational potential, and G is the universal gravitational ...
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1answer
120 views

Does Gravity Warp Coulomb's force?

Does gravity effect the "sphericalness" of the Coulomb force? For example, is the field 1 meter above a charge weaker than the field 1 meter below the charge? Could a 0.014% difference in the field ...
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1answer
87 views

What is the definition of inertial mass?

What is the definition of inertial mass? I can see two options, either it's the coefficient associated with the object being accelerated in Newton's 2nd Law, or it's the coefficient relating momentum ...
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2answers
619 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
31 views

Can we measure a mass increase for a system with more energy?

This question is about whether the mass of a particle increases with its speed. Here is my related question from which this question spawned: Why is there a controversy on whether mass increases with ...
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2answers
447 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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0answers
61 views

What is the physical meaning of the norm of the electromagnetic four-potential?

In SI units this would be $\frac{1}{c^2}\phi^2 - A_x^2 - A_y^2 - A_z^2$. Is this just not a physically meaningful quantity at all because it's not gauge invariant?
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2answers
168 views

Circular motion and time dilation

Einsteins' time paradox was shown when two atomic clocks were synchronized and one was sent in a super sonic plane and the other kept at rest. When the two where then checked it was shown that yes ...
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0answers
29 views

Are photons travelling in future? Are they lifespan 0 (or close to 0)? [duplicate]

We know that if we are going closer to speed of light, we are travelling in future. Since the photons are travelling at the speed of light, that means that they are travelling into future? Also, if ...
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0answers
138 views

At near light speeds, are small objects in your path a problem?

A mental exercise: A space ship is accelerating toward me from far away. As it approaches me, I can observe its acceleration. This high speed ship keeps accelerating faster toward the speed of light. ...
3
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1answer
223 views

an example where changing the frame of reference of an observer changes the outcome of events!

consider two identical charges moving with uniform velocity. There will be a magnetic force of attraction between them as two currents in the same direction attract each other. If I sit on one of the ...
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2answers
92 views

Experimental Verification of No Special Frames of Reference

Certainly, there have been numerous tests of both Special Relativity and General Relativity. Given all the phenomena and behavior stipulated by Relativity, one could perhaps divide these phenomena ...
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2answers
417 views

Gravity vs. Velocity Time Dilation [closed]

Can some one give me the unified formula for Gravity Time Dilation and Velocity Time Dilation.
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2answers
79 views

De facto superluminal travel?

It seems that the limitation to subluminal travel can be practically circumvented (so to speak...not breaking any physical laws here) if we consider the viewpoint of the traveler, not some outside ...
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1answer
93 views

Significance of angle in relativistic mass equation

I was playing with $$M=\frac{m_0}{\sqrt{1-v^2/c^2}}$$ and when I put $v=c\sin\theta$, I got $$M=m_0\sec\theta.$$ And this new equation is appearing so beautiful and attracting to me that I was ...
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4answers
393 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 ...
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2answers
103 views

Consider relativity. If a stick accelerates from a stationary state, how will it look like during the acceleration?

I mean how it will be shortened in the sight of an observer in a stationary coordinate system. It seems I was not clear enough. If the stick becomes shorter, then while this is happening, the ...
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0answers
58 views

How is force felt at relativistic speeds?

Hypothetically, should a force feel the same as its measured force at relativistic speeds? Obviously a human body would not survive the force of a rotation at say, 0.6 the speed of light, but it’s ...
5
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1answer
321 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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0answers
99 views

Must length equal distance? [closed]

In his book The Elegant Universe, Brian Greene points out that when a foot ruler is laid out by an observer on a spinning “Tornado” ride (a disk) that a “A shorter ruler means that he will have to lay ...
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0answers
39 views

Are signals exchanged between a rotating frame and a central frame assymetric?

This question pertains to EM signals exchanged between an inertial central clock and an orbiting clock i.e. non-inertial frame. (I edited out the variables connected with gravity and separation ...
2
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1answer
61 views

The Great Atomic Clock of Kansas

It is said that astronauts on the ISS experience time slightly slower than the rest of us. (I believe that the lesser gravitational field would increase their rate of time, so that affect must be ...
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1answer
122 views

Does spin-0 or spin-2 describe massive or massless particles?

spin-0 is massive or massless? How does we separate the massive and massless degrees of freedom for spin-2? What is the partially massive?
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1answer
142 views

Killing Equation, trouble with tensor algebra

I'm attempting to follow a proof that the commutator of two Killing vectors is itself a Killing vector. The source that I've posted is from my course notes. I've highlighted the part I'm stuck on. ...
2
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1answer
66 views

Distance from redshift

I am looking for a exact derivation of a relation between redshift $z$ and distance $d$. What I know is the definition ...
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0answers
81 views

Gravitational atraction of fast object [duplicate]

Let's imagine a asteroid that travels with 0.99999999999999999c. (I know it's impossible). Anyway... Relativistic mass of such object would be almost equal to earth's stationary mass. Now let's ...
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3answers
2k views

Would computers accelerated to high speeds compute “faster” from our point of view?

I woke up to this thought yesterday: Lets say Computers A and B have exactly the same specifications and at time T both are set to process an algorith that would normally take exactly 1 year and ...
5
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1answer
117 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 ...
2
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1answer
240 views

Relativity of simultaneity and the second law of thermodynamics? [closed]

Has the connection been noted between these two? I have been studying special relativity and in learning Minkowski diagrams I have noticed that as v approaches c, the relativity of simultaneity is so ...
2
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1answer
52 views

Relativistic Effects Observed with Time Travel

Disclaimer: My domain knowledge on these topics is pretty minimal. I'm a "physics fan". From what I understand about relativity. If there are two identical objects, A and B, and A is stationary and B ...
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2answers
214 views

Do photons feel gravity of approaching objects only?

I have read that photons while travelling near massive objects such as the sun experience gravitational pull which is why we see some stars at different positions than they are when seen towards the ...
2
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1answer
55 views

what if the time is zero in one reference frame

Consider measuring the length of an object in another frame of reference. Of course this should happen at the same instance in the frame of reference the measurement takes place in. but using Lorentz ...
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1answer
49 views

Cheating to communicate beyond the horizons in expanding universe?

Let's say the Hubble constant is constant in the time interval we consider. So subjective "horizon" for any object has the same radius R. Let's say Bob and Charlie are just within the horizon of ...
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2answers
951 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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1answer
77 views

A question about a relation between time-like world lines

Given any Lorentzian manifold containing three distinct time-like world lines $L$, $A$ and $B$ such that $L$ and $A$ have exactly one common coincidence event, $\mathcal{E}_{AL}$, $L$ and $B$ have ...
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1answer
73 views

Question regarding FTL information transfer between two inertial frames

Why can’t information travel faster than the speed of light, if the two endpoints to and from which the information is being sent are moving relatively to each other, as long as the information ...
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2answers
202 views

Can relativistic momentum (photons) be used as propulsion for 'free' after the initial generation?

In discussing this question about propelling a spacecraft with photons and their relativistic momentum, the author asked that I restate my comment as another question. If photons can really be used ...
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1answer
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Homogeneity of space

How do I verify homogeneity and isotropy of space, for example for hyperbolic space? My idea is to verify that Lorentz transformations in 4-vectors can move any desired point on the hyperboloid into ...
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57 views

Conditions that the coordinate must satisfy in order to become local inertial

Consider the coordinate transformation $$ \tilde x^a=x^a+\frac{1}{2}\Gamma^a_{bc}x^bx^c $$ I have shown that at the origin $O=(0,0,0,0)$, $$ \frac{\partial\tilde g_{ab}}{\partial\tilde x^c}=0 $$ ...