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

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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 ...
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Gravitoelectromagnetism or graviton thory to derive equivelent formulas to relativity theory [on hold]

Is it possible to change the definitions of an event happening (which currently in relativistic theory is defined by the moment you see* something - the time light took to travel to you) such that ...
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3answers
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Which is more fundamental,''constancy of speed of light'' or ''space-time''?

My friend asked me,''which is more basic or fundamental? constancy of speed of light which makes space-time behave dynamically OR dynamical behavior of space-time that makes light to travel with ...
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General Relativity - Four Velocity Derivative Question

I am trying to get my head around a small point used in a book I am reading about General Relativity. The book states that because $u_au^a = c^2$ it follows that $u_a \nabla_b u^a = 0 $ The first ...
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22 views

Does relativity in any way “disprove” Newton? [duplicate]

I was reading a science magazine and it claimed that Einstein's theory of relativity disproves all of Newtonian physics. I was highly skeptical because all we have been learning at school is Newtonian ...
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Are there computerized ontologies for Relativity and (Experimental) Physics? How to contribute?

As machines and automation can generally assist humanity in physical activities, and even exceed mere human capabilities, so, increasingly, also in tasks of reasoning. A basic component of the latter ...
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What exactly is meant by “locating points in spacetime”, in the RT?

In this (presently quite popular) answer to another recent question of mine concerning foundations of the (Einstein's) theory of relativity, it is asserted that "Any observer can construct a ...
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Is Einstein's characterization of “time” as “the position of the little hand of my watch” definitive and binding in the RT?

In Einstein's very first publication dealing with the Theory of Relativity, effectively as a preamble to all subsequent thought-experimental considerations and descriptions, Einsteín put the ...
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Clocks in moving inertial frames [closed]

A clock $C$ is at rest at the spatial origin of an inertial frame $S$. A second clock $C'$ is at rest at the spatial origin of an inertial frame $S'$ moving with constant speed $u$ relative to $S$. ...
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What would a nuclear explosion look like at relativistic speeds where time dilation and length contraction become significant?

Suppose a rocket carrying a nuclear weapon is moving past our planet at very near the speed of light while a stationary observer watches from here on Earth. As the rocket passes directly overhead, an ...
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2answers
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Are the words “coincident” and “simultaneous” considered synonymous? Else, please explain the difference

In discussions, experimental and thought-experimental descriptions (especially concerning the Theory of Relativity) the words "coincident" and "simultaneous" appear, notably for instance in the ...
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What exactly is meant by saying that two events had been “simultaneous in an inertial frame”?

In order to address my question based on a concrete example setup let the following two separate events be given: participants $A$ and $J$ encountering each other in passing (additional participants ...
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What is the speed of gravity? [duplicate]

Hypothetically, let`s say that one day our Sun is suddenly removed from the galaxy. For us, on earth, would take up to 8.3 minutes to notice that the sun was removed, since the distance from the earth ...
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4answers
94 views

What is the proper time interval for simultaneous events?

Let us take a rod of length L and the two ends A and B have two photon guns. A----------------------B Let the end A fire a pulse (event ...
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17 views

Is there any evidence for the increase of mass due to time dilation also causing a strengthening in said mass' gravity? [duplicate]

Logically it make sense that gravity would strengthen with an increase of mass but I want to know if there is evidence for this.
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43 views

What is the force between two points in space due to expansion of space between them?

Is there any way(equations?) to calculate the force between two points due to the expansion of the space between them? (To clearify the question I should add that the reason we donot expand while the ...
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1answer
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Special relativity, electromagnetic fields, charge and Q

Is it true, were Coulomb's constant k to be several orders of magnitude smaller, that there would be no (or increasingly negligible) magnetic fields generated by moving charges? The reason being the ...
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In special relativity, is it correct to say the speed of a body B relative to A is the same as measured by every observer?

Here, two objects A and B are moving relative to each other. I use the Einstein velocity addition formula $v = {v_1 + v_2\over 1 + {v_1 v_2\over c^2}}$ to calculate the relative speed between A and ...
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2answers
96 views

How can a photon have energy when its mass is zero? [duplicate]

How can a photon have energy when its mass is zero? According to Einstein's equation $E = mc^2$ energy depends on $mass*c^2$ Light has zero mass so the energy would be zero too but solar cells use ...
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31 views

How can we represent the 3 space coordinates uniquely by one point that can be put on a single axis?

In the Space-Time diagram (in the rest frame) we often take the Space axis as the horizontal axis and the time axis as the axis perpendicular to it as in the given figure. While there are other ...
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71 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. ...
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3answers
181 views

How can light travel at a speed greater than the speed of light

I was reading John Rennie's answer here, Can light travel faster? I now know that the sentence ''the speed of light is always constant in vacuum'' is just an oversimplification, for example light ...
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1answer
60 views

Question about basic formalism of GR and the metric tensor

I really don't know much about GR, but I've come across a few rough sketches of its formalism in my DG books. I'm trying to piece it together to get a very basic intuition of what spacetime is in GR. ...
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2answers
36 views

Space: a vacuous void or an entity?

I am a chemist and, until recently, a senior Research Manager with a major chemical company. Please excuse the potentially naive ramblings of an idiot in what follows, but I do not have the ...
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58 views

Magnetic force between two point charges

I tried to derive the magnetic force between two point-charges for iterative computation. Starting out with Lorentz force and Biot–Savart law for a point charge. $$ \vec F = q_2( - \Delta \vec{v} ...
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In $1$-dimensional space, how would the gravity generated by an electron affect a photon moving away from the electron if the photon can’t slow down?

Suppose we had a universe obeying the same physical laws as our own. But it had only one spatial dimension (represented by the $x$ axis) and it was totally empty. There are just two point-like ...
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60 views

Could gravitational waves and electromagnetic waves/light be the same thing? [duplicate]

They travel at the same speed, why? Maybe they are the same thing, but seen from different perspectives.
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1answer
53 views

Boyles law & relativity

In relativity when an object approaches the speed of light, $c$,its volume decreases due to length contraction. As a thought experiment, consider a rectangular metal box travelling at a relativistic ...
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69 views

Special relativity and Lorentz transformation

I recently learned about special relativity and different reference frames. The conversion from two inertial reference frames is given by Lorentz transformations: $$x' = ...
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21 views

Weizsäcker–Williams approximation

I'm having some trouble understanding the Weizsäcker-Williams approximation What I think it is is the following: I have a charged particle at high speed, close to the speed of light, at this speed ...
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91 views

Why invariance is important?

The concept of invariance seems to have a great importance. Indeed, the fact that the laws of Electrodynamics are not invariant in every inertial reference frame led to the theory of Special ...
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2answers
78 views

Acceleration to lightspeed using gravity

Suppose we have a very advanced spaceship that (aside from colliding with planets or entering a blackhole) is indestructable, and can go very fast, but is incapable of moving at lightspeed on it's ...
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4answers
98 views

What is the mass of a sphere?

A solid sphere of mass M is rotating along an axis. We can consider it as a collection of large number of point masses, every point mass is moving with respect to center of mass with velocity which ...
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Relativity: stationary vs moving

In the book The Elegant Universe, Greene describes a situation in which there are two space travelers, George and Gracie, moving in relationship towards each other at a constant velocity with no other ...
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Is the intuition of “absolute simultaneity” necessarily wrong?

I read about Einstein’s theories of relativity with all the interesting stuff like time dilation, mass increase and whatnot. And also about the relativity or conventionality of simultaneity. The ...
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Do photons exert gravity? [duplicate]

I read about GR today and came across the Bonnor beam. As I understand it, two parallel light beams (in a flat space) will in fact creep – slowly – towards each other. As Wikipedia puts it: On the ...
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1answer
53 views

Relativistic addition of velocities in the y-direction? [closed]

I've been having some difficulty with the problem described below, please have a look: Two particles with identical masses (m) undergo a collision. Before the collision they move with velocities ...
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5answers
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Light's oscillation in time

Electromagnetic waves have electric (and magnetic) fields that oscillate spatially and with time. But light, moving at the universal speed limit, is a "space-like" object according to relativity since ...
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Time and speed - Theory of relativity - 'felt time'

Assuming time is relative according to the objects current speed, means that a clock within a space shuttle which has 30% of light speed is 'ticking' slower than a clock staying on planet earth just ...
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Simultaneous observations in relativity

Consider an object which responds to two different signals A and B: it responds to signal A with blue color and signal B with red color. Also, it responds only to the signal that reached first. In ...
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Michelson & Morley

I KNOW this is a common question in it's topic, but not in this form.. I'm reading a book which explains the significance of certain aspects of experiments.. This section is called "null ...
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1answer
29 views

Orbital variations [closed]

My question relates to relativity and the warping of spacetime. If the geodesics approaching the star are coplanar with the ecliptic how can polar planetary orbits form?
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1answer
56 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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1answer
82 views

Laws of physics in curved space-time

The speed of light is not constant everywhere in a gravitational field. Suppose there is a region of space-time which is curved due to gravity such that the speed of light or any electromagnetic ...
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27 views

How can I calculate time dilation as viewed from an accelerated frame?

Imagine I am speeding past you at near the speed of light, at a constant velocity. It is very simple for me to calculate what I will see looking at you. According to the Lorentz transformations from ...
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30 views

What would we see if a camera orbiting close to light speed beamed down images of us on earth?

If a camera was sent up in a satellite, put in orbit at close to light speed, and instructed to beam down images to earth, what would we see if we watched its video feed? (Ignoring the fact that ...
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1answer
44 views

What are the differences between special and general relativity? [duplicate]

What are the differences between special relativity and general relativity? I am looking for a naive, non-mathematical explanation.
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1answer
76 views

What is the speed of a Photon? [duplicate]

How fast does a Photon travel through space? And is gravity able to influence a Photon's path?
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1answer
118 views

Could there be a time dilation of 2,000,000x between Earth and other parts of the universe?

Don't ask me why, but you may be able to guess why I would ask such a question. The question is, what kind of conditions would have to be present in a universe in order for billions of years pass in ...
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Laws of physics for a point in cyclical 2D motion

A follow up question to the question in What does the statement "the laws of physics are invariant" mean? and Formal definition of an observer? Which laws of physics are they talking about ...