Relativity Definition and 998 Threads

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. J

    I Relativistic Reference Frames and the Big Bang?

    Peeling this out into its own thread for clarity: How is time dilation of extreme reference frames (photons, black holes, intergalactic space-time) taken into account in Big Bang cosmology? Since from the POV of a singularity or a photon, their clocks have effectively stopped and any lower...
  2. A

    I Paradox: Rocket ship moving in a circle

    Say there is a circular fence that has a diameter of 10 meters, and a rocket ship that is normally 20 meters goes very quickly so that its relativistic length is 1m from the position of an observer standing at rest with relation to the fence. The rocket ship starts to go in a circle inside the...
  3. Zack K

    B Satellite Clocks: General & Special Relativity Time Variations

    Something that crossed my mind recently; I know that satellites have to adjust their clock due to their relativistic time variations in relation to us. I was wondering do they adjust their times in accordance to general relativity or special relativity or both? Or is their speed so insignificant...
  4. K

    I Allowed transformations in General Relativity

    Does General Relativity allow for transformations which are not isometries of the metric?
  5. L

    Other Which math skills to learn for understanding relativity?

    Summarized: Which mathematical fields should you focus on if your goal is to gain a better understanding of relativity and related subjects? (My apologies if there are already other threads asking this question, but 'math' and 'relativity' aren't really useful search terms on this forum.) Hi...
  6. F

    Illustration about the "relativity" of light

    I just found this on the internet. It reminded me of a joke I once made: https://www.physicsforums.com/threads/science-jokes-p2.847743/page-8#post-5603278 It's a nice illustration about the "relativity" of light - kudos https://www.linkedin.com/in/james-o-donoghue-astronomer/
  7. Orest

    B Solve FTL Problem with Faraday Cage and Relativity Theory

    Is it possible to solve a problem of overcoming of speed of light with solid objects using faraday cage Theory with equivalent charge?
  8. Zack K

    Distance of a Point Charge: Solving for Initial Distance Using Relativity

    Homework Statement You make repeated measurements of the electric field ##\vec E## due to a distant charge, and you find it is constant in magnitude and direction. At time ##t=0## your partner moves the charge. The electric field doesn't change for a while, but at time ##t=24## ns you observe a...
  9. Luke Tan

    I Varying The Relativistic Action

    In his book, Landau mentioned varying the relativistic lagrangian However, I do not understand how he got from varying the integral of ds to varying only the contravariant components. Would the general procedure not be varying $$\delta S = -mc\delta\int_a^b\frac{dx_idx^i}{\sqrt{ds}}$$ and...
  10. FourEyedRaven

    Relativity Raychaudhuri - General Relativity, Astrophysics, & Cosmology

    Hello. The book "General Relativity, Astrophysics, and Cosmology", by Raychaudhuri, et al., looks like a good compact exposition of GR, astrophysics and cosmology, which is exactly the kind of book I'm looking for. What do you think of this book? Thank you in advance.
  11. N

    I Gravity in General Relativity: Explaining Acceleration

    Greetings: I can understand that an object's trajectory curves about a greater mass (e. g., satellite in Earth's orbit). The spacetime is curved via the great mass and the moving object simply follows the curvature. My problem is this: Why does a stationary object at a short distance from...
  12. D

    I Orbital Simulator with general relativity

    Is it possible to calculate with precision the orbits of the Earth, Moon and Sun with general relativity instead of Newtonian Physics? How would this work? Would the software have to record past positions of each body and have the Earth be attracted to the old sun's position 8 minutes ago...
  13. L

    I Does Special Relativity Deal with Non-Inertial Frames?

    In several special relativity textbooks, I have read that special relativity only deals with observations made in inertial frames, and that it makes no predictions about observations made in non-inertial frames, and that only general relativity deals with non-inertial frames through the...
  14. m4r35n357

    I Solve General Relativity "Paradox" w/ Automatic Diff. ODE Solver

    After making a couple of comments on this StackExchange question, and pointing yet again to this article, a thought occurred to me. I have been working on an Automatic Differentiation based ODE solver and equation analyzer, mentioned in this thread. Why not use it to solve equation (7) in the...
  15. starstruck_

    B Understanding Special Relativity: 2nd Year SR Course Explained

    Hey everyone! So, I just finished my 2nd year intro to SR course, we spent most of the term on Taylor expansions for relativistic corrections vs concepts. I’ve watched a crash course video and a documentary on SR and I do somewhat understand the thought experiment involved with respect to...
  16. K

    I Einstein & Relativity: Metric, Tensor Fields, Scalar Product

    I was reading a document from the time of Einstein where the metric is referred to as "the fundamental tensor". That made me wonder if it's possible to derive all concepts of tensor field, scalar product, connectivity etc. starting from the requirement that the metric is invariant under change...
  17. T

    Are there job opportunities in general relativity for theoretical researchers?

    Are there a lot of theoretical research jobs in general relativity or would that be too hard to find?
  18. The black vegetable

    I Solve General Relativity Equation: Steps & Tips

    Maybe I'm just being stupid, missing something simple ? How do you get from the top two equations to the second equation? Many thanks
  19. B

    I Acceleration vs Gravity in General Relativity

    Hi, It is usually claimed that a person in an accelerating elevator with an acceleration equals to the gravity of the earth; this person cannot make any experiment that makes him know whether he is in the elevator or on the surface of earth. However, if this person project two light beams...
  20. M

    B Special Relativity & Acceleration: Unequal Inertial Frames

    Given that one or both inertial frames must have been subject to acceleration at some point; resulting in an imbalance of application between the two inertial frames, why does the consequential effect of general relativity not feature in calculations. Surely, as such, neither observer's...
  21. S

    I Escape Velocity: Newtonian vs Relativistic

    Hi, Do we obtain the same escape velocity equation: Ve = sqrt(2GM/r) using both Newtonian and Relativistic approach?
  22. D

    I Favorite Visualization of General Relativity?

    I am collecting ways to visualize the curvature of spacetime -- and movement of objects affected by gravity -- as per Einstein's GR. Alternatives to the bowling ball / trampoline image so often used in the popular press. Images that show the similarities / differences between Newton & Einstein...
  23. J

    Relativity Thoughts on General Relativity by Hobson?

    Background: I'm about to graduate with a mathematics degree(in the US), hence I have exposure to multivariable calculus, real analysis, abstract algebra, complex variables etc (no differential geometry however). I have also taken a class on QM(using Griffiths) and I saw some SR during a Modern...
  24. astroman707

    Physics Is gravitational wave research a smart direction to go in?

    Is it a fair prediction to state that in the next several years or so, globally, there will be major investments into gravitational wave research, and many more ‘LIGOs’ being developed? Is it a good idea to venture into that area of physics?
  25. K

    I Displacement vector in general relativity

    Is there a sensible way of defining a displacement vector in a general manifold? That is, the displacement vector being the difference between position vector at two different points... the problem is that these two different points have, in general, different tangent vector spaces. Never the...
  26. N

    I Measuring Star Radii with General Relativity

    Hi, I'm wondering if it is possible to measure radii of stars using general relativity, given its explanation of gravity as curvature of space. Thank you
  27. N

    Best books for special relativity?

    Hello everyone, I'm studying physics as an undergraduate and I'm currently taking the course special relativity. But sadly I'm not too big a fan of the book we are using for the course. So I was wondering what are the best books you know of regarding special relativity for undergraduates.
  28. I

    I Is Mathematics a Paradigm of Truth in General Relativity?

    I have little exposure to mathematics beyond high school algebra and physics, but I have become increasingly interested in physics, particularly its most towering achievements of modern times : Quantum Physics, as well as A. Einstein’s Special and General Relativity, the latter of a...
  29. E

    B What would the universe be like with less mass and energy?

    So I have sort of a conceptual question about the big bang and gravity. Imagine yourself in a universe, in which existed about the number of particles/energy in a 3X3 metre room at any given moment. This universe has the same laws of physics, constants and is identical in every way to our...
  30. R

    I Relativity and Electromagnetism

    Hello, There is a common setup used when describing the intimate relationship between electricity and magnetism. I have a question about the setup. Setup: There is some long current-carrying wire. Outside of that wire, there is some test charge. In the first situation, the test charge is...
  31. A

    I Visual effect in special relativity

    I have an exercise: A distant camera snaps a photograph of a speeding bullet (speed v) with length b in its rest frame. Behind the bullet and parallel to its path is a meter stick, at rest with respect to the camera. The direction to the camera is an angle $\alpha$ from the direction of the...
  32. Jonathan Stanley

    B Special Relativity Self-Contradiction?

    From his original thesis, Einstein said light speed is always constant (c). There is very clear evidence for this. Every ray of light moves in the “stationary coordinate system” with the same velocity c, the velocity being independent of the condition whether this ray of light is emitted by a...
  33. Mr C Odd

    Time dilation in a magnetic field

    Hi all. I was wondering if time is dilated whilst traveling in a stable magnetic field that is generated by the object travelling, and if so, does this vary if you reduce or intensify the magnetic field? Also, what happens if the object is generating two opposing magnetic fields, would...
  34. Amanuel

    How to calculate the energy in gamma photons

    how do I calculate the energy of to photons moving in the opposite direction. starting from the same origin.
  35. E

    B Momentum in special relativity

    Although I thought that I understand special relativity enough, I cannot now clearly answer on the following question: What is the most direct derivation, why momentum in special relativity is ##p=\gamma m v##, where ##v## is velocity of the rocket? Let us assume that Lorentz equations are...
  36. C

    I Mass/Density Increase in Motion: Relativistic or Observable?

    Is the increase in mass/density of an object in motion purely relativistic? What I mean is if you were in a spaceship approaching the speed of light would you be able to measure a change in your mass/density or would the change only be observable to someone at rest. I'm aware of the Lorentz...
  37. M

    Physics How is it to work in numerical relativity?

    the problems/challenges that you have to face daily are mostly related to code issues with the physics itself? Is there room to improve our knowledge of fundamental physics while working on it? Do you enjoy doing it? why? I'm asking this because I'm considering working on numerical relativity...
  38. P

    Relative velocity in special relativity

    Homework Statement Imagine we are observing two aeroplaes from the ground and let their velocities be ##\mathbf{u}## and ##\mathbf{v}## respectively. Assume that the first plane has radar equipment permitting a measurement of the speed of the other plane relative to itself. The velocity so...
  39. R

    When Will We Arrive? Calculating Time Differences in Special Relativity

    Homework Statement : You are the first astronaut aboard a ship to travel to Alpha Centauri. Coincidentally, a scientist working on an outer space station post is one of your former classmate. Your ship, with your former classmate onboard, leaves the space station traveling at constant velocity...
  40. MeAndMyLucidLife

    How Does the Relative Speed of Photons Change in Different Scenarios?

    Homework Statement What is the speed of a photon with respect to another photon if: the two photons are going in the same direction. they are going in opposite direction? 2. The attempt at a solution I think the answer to the first question should be zero and to the second one be 2xC; C⇒speed...
  41. D

    Other What books to get before studying Quantum Mechanics and General Relativity?

    Hi. I want to learn - amateurishly - Quantum Mechanics, and General Relativity, but my experience with Physics is very small. I want to ask, what should I learn - what books should I read - before I start to learn those theories? Sorry for my english.
  42. Sandeep T S

    A General Relativity Book Recommendations

    Any one please refer some text for general theory of relativity, Also share some link of PDF about GTR
  43. V

    B Special relativity - frame of reference

    This must be a basic question. :) Bob and Alice have the same age. So in special relativity Bob leaves Alice and travels at very high speed and when it returns is younger than Alice. Bob's time is dilated and his space is contracted from Alice frame of reference. But now, if I take Bob's frame...
  44. N

    I Relativity of Simultaneity: Observers Perceiving 3 Events

    In looking at this light cone diagram, I'm focusing on the part in purple. I'm interested to know if mathematics reveals the exact potential difference in time perceived by observers of three events as they shift between their different planes of simultaneity. This makes more sense using this...
  45. Cryo

    A Electromagnetism for Media in Arbitrary Motion

    Hello To develop one interesting idea I need to be able to do calculations on (1) scattering of light from bodies in arbitrary motion, possibly at relativistic speeds; (2) Propagation of light in electromagnetic media that are in arbitrary motion (possibly relativistic). For example, I would...
  46. P

    A Is a volume element in relativity represented by a vector?

    Can we say that a volume element can be represented by a vector, or is there some hidden complication that makes this inadvisable? For some background, the stress-energy tensor has been described as the density of energy and momentum, in for instance MTW. So if one says that the represention...
  47. P

    I GPS relativity error - really?

    There are many popscience articles about relativity and how relativity is essential for GPS and in many is mentioned something like this: If GPS wasnt corrected for time dilation predicted by GRT, there would be an error of 12 kilometers in GPS positioning per day. But when I checked how GPS...
  48. FallenApple

    What is the Relativity of Wrong?

    I read the essay. It's a very interesting account of scientific progress. What type of scientific paradigm is this? It seems to have a very close feel to Bayesian Epistemology and/or Inference Quote from Wiki https://chem.tufts.edu/answersinscience/relativityofwrong.htm
  49. S

    B Using SR & Curved Coordinates for Time Calculation

    Someone told me that I don't need the whole mechanics of GR to be able to calculate the proper time in an accelerated frame of reference. I can just use SR but with curved coordinates and then integrate for time. But he didn't give me a reference where I could find the formula to do this. How do...
  50. M

    B Help with constant acceleration under special relativity

    Hi, I am trying to wrap my brain around special relativity equations but I'm struggling with the math. I am a computer programmer comfortable with the algebra and but never studied calculus or physics. Say I am already traveling at velocity "v" where "v" is approaching the speed of light. I...
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