What is Relativity: Definition and 997 Discussions

The theory of relativity usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity, proposed and published in 1905 and 1915, respectively. Special relativity applies to all physical phenomena in the absence of gravity. General relativity explains the law of gravitation and its relation to other forces of nature. It applies to the cosmological and astrophysical realm, including astronomy.The theory transformed theoretical physics and astronomy during the 20th century, superseding a 200-year-old theory of mechanics created primarily by Isaac Newton. It introduced concepts including spacetime as a unified entity of space and time, relativity of simultaneity, kinematic and gravitational time dilation, and length contraction. In the field of physics, relativity improved the science of elementary particles and their fundamental interactions, along with ushering in the nuclear age. With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.

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  1. N

    I Relativity of Simultaneity: Analyzing Wavelengths

    Am i right in thinking that relativistic simultaneity explanations tend not to mention the fact that one of the ‘simultaneous’ events may well be red shifted and the other blue shifted and by analysing wavelengths you could presumably work out whether the event was in reality simultanous or not.
  2. V

    Understanding geometry problem in relativity

    Homework Statement It is not ordinary problem, it is connected to it. I don't understand a figure and how is described. The problem is here http://www.pmaweb.caltech.edu/Courses/ph136/yr2011/1102.2.K.pdf How did equation 2.4 from figure 2.3 arise? Why disappear ##\Delta t^2## from the right...
  3. A

    A Speed of Gravitational Waves confirmed by Experiment?

    It seems that with the 2016 LIGO and VIRGO confirmation of the gravitational chirp that we have also experimental confirmation that the speed is indeed the speed of light. True? https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves#Direct_observation
  4. mpolo

    B Special Relativity: Time Dilation Explained

    I am trying to get this idea of Time dilation understood. If there exists only two objects in a Universe and one object is stationary and the other object is moving at 99 % the speed of light. Their clocks were both synchronized when both objects were stationary relative to each other. Then the...
  5. V

    Special relativity, gradient of velocity

    Homework Statement I have noticed that in some calculations they use $$ \vec{\nabla}\cdot\vec{u}=\frac{1}{V}\frac{dV}{d\tau}$$. I would like to derive it. Homework Equations ##\vec{u}=(\frac{dt}{d\tau},\frac{dx}{d\tau},\frac{dy}{d\tau},\frac{dz}{d\tau})## ##\vec{A}\cdot\vec{B}=A^{\mu}B_{\mu}##...
  6. T

    Special Relativity - Rocket problem (particle mechanics)

    Problem statement: A rocket propels itself rectilinearly by giving portions of its mass a constant (backward) velocity ## u ## relative to its instantaneous rest frame. It continues to do so until it attains a velocity ## v ## relative to its initial rest frame. Prove that the ratio of the...
  7. R

    B Special relativity and expansion of the Universe, A paradox

    Consider two bodies A &B are moving apart with a velocity V due to the expansion of space. According to an observer in A the body B is moving away and an observer in B feels the body A is moving apart. Can some one answer in which body the time dilates and why?. ( I am specifying once again that...
  8. davidge

    I Spin 0 Particles & General Relativity | Carrolls Notes

    In Carrolls notes on General Relativity, it is said that the general formula for finding the spin number of a particle is $$\frac{360°}{\theta}$$ where ##\theta## is the angle of rotation, after which the state of the field describing the particle returns to its original state polarization. He...
  9. Ibix

    Nailing the Physics Forums: Relativity, QM & BTSM

    I think he's pretty much nailed the relativity, QM and BTSM forums: https://xkcd.com/1861/ or https://m.xkcd.com/1861/ for mobile users.
  10. davidge

    I General Relativity as a Theory of Gravity Explained

    Suppose there is a charged particle far enough of any mass so that there is no gravitational interaction between the particle and any other body. The trajectory of the particle in space-time would appear to us like this (we are at the origin of our coordinate system). Consider that at...
  11. A

    I Why special relativity is unsuitable to describe gravity

    I am trying to understand why the special relativity is not suitable for describing the gravity. Consider a counterexample assuming it is the suitable and the space-time containing a gravitational mass is flat. Then one could describe the acceleration of a test particle from his inertial frame...
  12. davidge

    Self-learning Special and General Relativity

    (I don't know if this is the right place to post it, but I think the "textbooks" section is'nt. So I'm going to put it here.) I have been self studying S&G relativity for almost eight months, mostly from Weinberg's book on S&G relativity, but also from papers I occasionally find on web and from...
  13. SJay16

    Courses Relativity an Quanta? What can I expect?

    I'll be entering my sophomore year as a physics major and all the classes seem really cool and standard; however, there is one class I'm curious about. It's called "Relativity and Quanta" and it's description is as follows; "Special relativity: Lorentz transformation, dynamics, and conservation...
  14. Andy_K

    I Special Relativity Course at World Science University by Brian Greene

    Dear All, Just wanted to share this excellent free Special Relativity online course I came across by Brian Greene, which I believe is one of the best around. Not sure if it's been shared before here. Special Relativity — World Science U, by Brian Greene...
  15. Hiero

    How the surface area of a cone changes; special relativity

    Homework Statement A cone has half angle θ0 and lateral surface area S0 in the frame in which the cone is at rest. If someone moves at relative speed β=v/c along the cones symmetry axis, what surface area will they see for the cone? Homework Equations I believe the lateral surface area of a...
  16. K

    I Simultaneity hyperplanes "curved" while stationary?

    I found this spacetime diagram at https://en.wikipedia.org/wiki/Proper_acceleration with an accelerating traveler, resulting in "curved" hyperplanes of simultaneity. Notice how the simultaneity planes are angled "normally" (The faster the steeper the slope and no slope when stationary) but...
  17. Pushoam

    How does relativity affect the observation of moving objects?

    Homework Statement A.P. French q.no. 4.3 Homework Equations t = L\c, (1) L0 = ϒ L The Attempt at a Solution The observer's frame is that frame w.r.t. which the meter stick is moving with speed 0.8c. The observer sees the mid-point at (x,y) at a time to(measured from the clock situated at...
  18. binbagsss

    General Relativity geodesics, killing vector, conserved quantities

    Homework Statement Homework EquationsThe Attempt at a Solution [/B] Let ##k^u## denote the KVF. We have that along a geodesic ##K=k^uV_u## is constant , where ##V^u ## is the tangent vector to some affinely parameterised geodesic. ##k^u=\delta^u_i## , ##V^u=(\dot{t},\vec{\dot{x}})## so...
  19. binbagsss

    General relativity, geodesic, KVF, chain rule covariant derivatives

    Homework Statement To show that ##K=V^uK_u## is conserved along an affinely parameterised geodesic with ##V^u## the tangent vector to some affinely parameterised geodesic and ##K_u## a killing vector field satisfying ##\nabla_a K_b+\nabla_b K_a=0## Homework Equations see above The Attempt at...
  20. TheQuestionGuy14

    B How to travel back in time according to General Relativity?

    Is there any way to travel back in time in reality according to GR? Let me know!
  21. binbagsss

    General relativity, geodesic question

    Homework Statement question attached Homework EquationsThe Attempt at a Solution Attempt : Check if ##V^{\alpha}\nabla_{\alpha}V^u=0## Since Minkowski space, connection tensors/christoffel symbols are zero so this reduces to: ##V^{\alpha}\partial_{\alpha}V^u=0## where...
  22. Arman777

    I Solving General Relativity Equations

    Is it really hard to find a solution of a GR equation maybe two planet system ? Or It could be a different system.I just wonder 1-How much its difficult ( Like can a person calculate those solutions) 2- Whats the boundries (Like we can solve 2 planet system but not 5 etc ? ) 3- Can...
  23. Hiero

    B Special relativity implies the space cannot be "closed"?

    I think special relativity would disallow our universe from having the structure similar to an ant on a sphere. What I mean is that it can't be possible to travel in a constant direction and to come back to your original location. Suppose there is an observer S on a planet and an observer S'...
  24. jk22

    B Temperature & Relativity: Can Plasma Fusion Test Limits?

    Do we know if experiments with plasma fusion could test relativity and its speed limit because of the maximal speed ? Like $$3/2kT=1/2mc^2$$ hence for example with hydrogen the maximal temperature should be around $$10^{12}K $$. I read that they achieved 8*10^7 in wendelstein. Or should it be...
  25. R

    I Are special relativity rules encoded in the Dirac equation?

    This may seem like a stupid question, but i can't get my head around this so please bear with me. I just looked at the derivation of Dirac equation and my question is: do the solutions for a free particle obey special relativity? because if yes why? I mean I thought using E2=(mc2)2+(pc)2 would...
  26. Umaxo

    I Mass/energy in general relativity

    Hi, i got little confused after this conversation in https://www.physicsforums.com/threads/some-questions-about-light-and-relativity.918094/page-2 There was also this conversation: So i want to clear the confusion. As was pointed out to me, in STR mass is defined as m2=E2 - p2, which...
  27. S

    B Relativity’s Effect on Consciousness

    So there will be a few hypothetical in the original story here, but in the end ill wrap it up with what I believe are facts. My question, how does Einstien’s theory of relativity interact with consciousness? Let’s say I have a twin, he becomes a farmer and I become an astronaut. I go to space...
  28. T

    B Some questions about light and relativity

    I'm trying to study on light velocity and relativity. And I have a few questions I hope someone can make clear to me. Please try to not use math but just explain in layman's terms if possible. I know the light is measured at the same speed regardless of the observer. Why? More to my...
  29. Hiero

    B Special relativity; mixed up time and space due to motion

    I just want to be sure I understand this correctly: Suppose there's a long spaceship moving in uniform motion relative to me (an inertial frame). Now someone in the front* of the spaceship calls someone (on the phone) in the back of the spaceship. *(this long length of the ship is oriented...
  30. C

    B General Relativity Aces a Test at Supermassive Black Hole

    General relativity passes test at Milky Way’s central black hole by Ken Croswell For the first time, astronomers use stars orbiting a supermassive black hole to test Einstein's general theory of relativity, finding no sign of a fifth fundamental force. Links: John Batchelor Show...
  31. V

    I Meaning of Constants of Motion in General Relativity

    Hello, I have a mess in interpretation of constants in description of movement in GR. First of all I define Lagrangian ##l=1/2g_{\mu\nu}u^{\mu}u^{\nu}##, and I would like to talk about axial smyetric spacetime (for example Kerr black hole) ##l(r,\theta)##. l is independent from ##t## and...
  32. M

    A How to obtain components of the metric tensor?

    In coordinates given by x^\mu = (ct,x,y,z) the line element is given (ds)^2 = g_{00} (cdt)^2 + 2g_{oi}(cdt\;dx^i) + g_{ij}dx^idx^j, where the g_{\mu\nu} are the components of the metric tensor and latin indices run from 1-3. In the first post-Newtonian approximation the space time metric is...
  33. B

    I Rest Energy in special relativity

    I was reading about Rest Energy and came across this line: "In special relativity, however, the energy of a body at rest is determined to be mc2. Thus, each body of rest mass m possesses mc2 of “rest energy,” which potentially is available for conversion to other forms of energy. The...
  34. G

    B Creating a Physics-Based Video Game: A Journey to Learn Relativity

    Hi, I'm trying to make a physics based game to teach my progeny about the universe. My first step will be to show how bodies gravitate around other bodies, and then model a solar system, a galaxy... simple things. I could do it using Newton's laws, but I want to include things like black holes...
  35. M

    B Is General Relativity Incomplete Without Dark Matter and Dark Energy?

    Dark matter and dark energy are fudge factors to solve problems in general relativity such as unexplained mass and drag. Is this correct or am I missing something?
  36. P

    Does the first Maxwell equation work also for moving charges?

    My professor said that the first two maxwell equation's are about static fields Another fast question: Do a single charge that is moVing generate a magnetic field? I saw a demonstration that explains the magnetic field as an electric field using only special relativity, there is a free charge...
  37. Pushoam

    I Is the Principle of Relativity different for different theories?

    The images have been taken from the section : Relativity according to Galileo and Newton, page no.66, special relativity , A.P.French,1968What I understood is: According to the first paragraph, Laws of transformation are needed so that a theory which describes a phenomenon w.r.t. one reference...
  38. Wrichik Basu

    Relativity Book Recommendations in General Relativity

    I have read Special Relativity from Resnick and Halliday's book Fundamentals of Physics. Now I want to read general relativity. I tried reading Einstein's book "Principles of Relativity", but sad to say, many things went tangentially above my head because I couldn't follow many equations, as...
  39. bachir1994

    I Electromagnetism and relativity

    Hello everyone, I read on WIKIPEDIA, that: In 1953 Einstein wrote to the Cleveland Society of Physics on the occasion of the commemoration of Michelson-Morley's experience that "what brought me more and less Directly to the theory of special relativity was the conviction that the electromotive...
  40. A

    B Muon Time Dilation: Earth vs Muon Perspective

    From the reference frame of the earth, the distance between the surface of the Earth and the muon is longer, but the muon survives because time for the muon is slowed down. From the reference frame of the muon, the time experienced by the muon is not slowed down but the muon survives because...
  41. D

    Momentum Energy and relativity question

    Homework Statement A particle of mass m is moving along the positive x direction with momentum p and energy E It collides with a particle of the same mass at rest to form a new particle of mass M. Show that M^2 = 2m/c^2 * (E + mc^2) 2. Homework Equations E^2= p^2c^2 + m^2c^4 The Attempt...
  42. D

    Relativity With velocity of objects moving in different fram

    Homework Statement i)A police spaceship P is chasing another spaceship A. Both ships have velocities βP = βA = 3c/5 as measured along the x-axis in the Solar System reference frame O. The police ship is a distance L = 1 light-second (i.e. the distance traveled by light in one second) behind...
  43. J

    Exploring the Boundless World of EE and Science: A Scientist's Journey

    EE, and life-long science explorer and tinkerer. I look forward to reading the forum, and hopefully contributing at some point. This looks like a great forum! If we weren't limited to 5 tags here, these would have been mine: physics (duh!), relativity, nuclear, antennas, time, radio...
  44. i_hate_math

    I Relativity and Eternalism: Proving the Possibility of 'Seeing the Future

    Eternalism is the view that many moments of time exists and each of these moment are equally privileged/unprivileged. I am trying to back up this view with SR, so I remember watching Brian Greene's documentary "the B-series of time" or something, where he used the spacetime loaf to illustrate...
  45. L

    Other Quantum Information and General Relativity

    Last year I've finished the undergraduate course in Mathematical-Physics and Mathematics and this year I've started on graduate school on Physics in order to obtain a master's degree. What I'm really interested are two main topics: general relativity and quantum field theory. I also like...
  46. Peter Martin

    I Special Relativity vs Newtonian Gravity: Understanding Their Compatibility

    In his little book for the layman, Seven Brief Lessons on Physics author Carlo Rovelli states: What is it about SR which is incompatible with gravity-as-a-force?
  47. davidge

    I Relativity Matter & Vacuum Energy: Stress-Energy Tensor

    Matter is responsible for the stress-energy tensor. What is included in "matter"? Does vacuum energy contributes somewhat to the stress-energy tensor? Would it be correct to include all forms of energy in the stress-energy tensor, through the mass-energy equivalence?
  48. X

    I Relativity Paradox w/ Charged Spheres

    There is a similar thought experiment I imagined to help me begin to understand the Bell's spaceship paradox: Consider two positively charged spheres, placed side-by-side inside a frame S' with a string stretched to the point where it balances the repulsive electrostatic force between them. For...
  49. C

    Which light turned on first and how much later did the second one turn on?

    Its been a few years since I have done any special relativity so I am a bit rusty, need help with either my working/understanding or if correct, making sense of my answer. This is not CW, just a question from a past exam paper that I am using as preparation. 1. Homework Statement Street light...
  50. binbagsss

    General Relativity - FRW Metric - FRW Equations show that ...

    Homework Statement Homework Equations see above The Attempt at a Solution Using the conservation equation for ##p=0## I find: ##\rho =\frac{ \rho_0}{a^3}##; (I am told this is ##\geq0## , is ##a\geq0## so here I can conclude that ##\rho_0 \geq =0 ## or not?) Plugging this and ##p=0## into...
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