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. Omega0

    B Is a Spin 2 Particle the Key to Understanding Gravitation in Quantum Mechanics?

    I have some questions regarding the expected exchange particles for gravitation. From my understanding the following was valid: We can linearize the equations of GTR for weak fields "Quantum mechanics" (Schrödinger, Dirac equations) are linear Those linear equations allow eigenstates and...
  2. S

    I Explore Spacetimes, Metrics & Symmetries in Relativity Theory

    I was discussing this paper with a couple of physicists colleagues of mine (https://arxiv.org/abs/2011.12970) In the paper, the authors describe "spacetimes without symmetries". When I mentioned that, one of my friends said that no spacetime predicted or included in the theory of relativity...
  3. Erland

    I Understanding Einstein's second postulate of special relativity

    Einstein's two postulates of special relativity reads, according to Wikipedia: The postulates are most often formulated similarly to this. But in my opnion, the second postulate shouldn't be formulated as above, because then one misses the point. This is particular true for the second...
  4. A

    B Relativity of Measures: A vs B Frame & Light Source S

    A frame “A” of three perpendicular axis (x, y, z) and an identical frame “B”, with an observer at rest in each, are moving toward each other at 1/2c on their common x axis. Centered between A and B on x is a light source S. At the time t1 when A, B and S coincide on x, S emits a light flash...
  5. ka_reem13

    AP French Relativity chapter 3 problem

  6. C

    B A thought experiment of the relativity of light

    A light source in uniform motion emits a flash of light. A spherically symmetric wavefront propagates from a central point, the source, or the “origin” of emission. The wave front remains at c relative to the origin as measured by all observers. How fast do these origins move? Am I missing...
  7. bhobba

    Relativity Lenny Susskind Has Done It Again - General Relativity

    This time with General Relativity: https://www.amazon.com/dp/1541601777/?tag=pfamazon01-20 I got a copy as soon as I noticed it. And it is good - as all his books are. Notice - number one best seller. Lenny deserves a medal. There is a genuine thirst for science beyond banal...
  8. Arman777

    GTRPy - A python package for General Relativity Related Calculations

    Hello, everyone I am now working on this project quite a while now and I just wanted to share it with this forum, which I was a member for a long time. I am working on a python application about GR and I believe I managed to create a very user-friendly layout. It's called GTRPy, and it allows...
  9. P

    A Popular Pseudoscience Video on General Relativity: Analysis & Criticism

    Here is the video: [link deleted by moderators] His basic idea is to take the spacetime interval and add a 5th term for the 5th dimension he is describing so it looks like: $$\Delta S^2 = c^2\Delta t^2 + c^2\Delta w^2 - \Delta x^2 - \Delta y^2 - \Delta z^2 $$ where w is the difference in time...
  10. James1238765

    I Modeling the Earth and Sun (2 body orbits) using general relativity?

    Modeling the time evolution of the sun and earth orbiting each other using ##F = \frac{GMm}{r^2}## is straightforward. However, it appears that modeling the time evolution of the same 2 body system using general relativity seems to be a hard/intractable problem? There was in depth discussion by...
  11. S

    I Can time be another basis vector under Galilean relativity?

    I refer to the video of this page, where there is a description of Galilean relativity that is meant to be an introduction to SR, making the comprehension of the latter easier as a smooth evolution from the former. All the series is in my opinion excellent, but I think that this aspect is...
  12. EclogiteFacies

    I Special Relativity philosophy and anxiety

    Hello all you clever people, I was wondering if you guys could answer a question for me regarding special relativity and the none existence of time. At least in the sense that the block universe people believe. So I'm writing as if the block universe is the correct interpretation What does...
  13. H

    B Relativity & Gravity: Resolving the Discrepancy

    I listened to and quite liked Sabine Hossenfelder's recent vid on relativity. Though one thing struck me as wrong. "Gravity is not a force," so it's effects does not cause true acceleration, so it has no effect on time. OK, but I'm pretty sure that satellites in different orbits have...
  14. L

    I Exploring Special Relativity: Patterns, Conjectures and Questions

    I have previously studied special relativity, but only at an introductory level. So I decided to explore the subject more in detail later by thinking and working things out on my own, in addition to doing research online. In particular, I seem to have noticed some intriguing patterns between...
  15. Ahmed1029

    I Charge conservation and special relativity

    If conservation of charge gets violated in future experiments, what would be the implications on relativity? I have some faint idea that this will cause photons to have non-zero rest mass, but does this affect special relativity at all? Also, does special relativity make conservation of charge...
  16. Ahmed1029

    I Is this just a typo in Schutz' book on General Relativity?

    I'm wondering is I'm missing something, or this should be " a non-zero component"?
  17. J

    Is Doubly Special Relativity Connected to Variable Light Speed?

    Hello. This is my first post on this site. anyone here familiar with "Doubly Special Relativity"? an ARXIV paper was linked in the OP. Some follow up links or other advice would be appreciated. Kowalski-Glikman, J., "Introduction to Doubly Special Relativity", (2004). arXiv:hep-th/0405273...
  18. D

    I Metrics of GR Solutions: Comp List, Hist & Viability

    Does anyone know of a comprehensive list of solutions to GR, their developmental history, and the viability for serving as a practical model for the observable universe?
  19. Ahmed1029

    I How is photon momentum compatible with special relativity?

    In relativity, momentum of a body is given by ##p=mv/\sqrt{1-v^2/c^2}##, but if mass is exactly zero and velocity is exactly ##c##, how is the photon momentum even defined? I don't think this problem can be resolved by simply stating the other formula relating energy to momentum, since it was...
  20. K

    Relativity Calculation: Two bodies traveling at relativistic speeds

    Body 1 travels 48 light years from point A to point B at 82% of light speed. Body 2 leaves point A 33 years after Body 1 and travels the 48 light years to point B at 99.99995% of light speed. What I think I know: Body 1 takes 58 years to reach point B according to an earth observer. Body 1...
  21. P

    A Time Travel, General Relativity & Information Paradoxes

    General relativity permits some exact solutions that allow for time travel. Some of these exact solutions describe universes that contain closed timlike curves, or world lines that lead back to the same point in spacetime. I wondered if these solutions also permits Causal loops? Such as the one...
  22. N

    I Do AEST (Absolute Euclidean Spacetime) models work?

    I was reading a paper by J.M.C Montanus which was published in <low quality journal reference removed> in which he claims under AEST the new gravitational dynamics and electrodynamics are reformulated in close correspondence with classical physics, and subsequently leads to the correct...
  23. AndrzejB

    Can Special Relativity Explain the Alice and Bob Spin Measurement Paradox?

    Alice measures the spin, also Bob measures. If we assume that the signals (from Alice to Bob) were sent, they had to be with the speed e.g. 10 c But in another frame of reference Bob first measures, next Alice. Measurement of Alice was cause and was sent back in time, or measurement of Bob was...
  24. A

    B Principle of Relativity: Classical Physics Example

    hi everyone "The principle of relativity: The laws of physics are the same in all inertial reference frames." Is in classical physics The laws of physics aren't the same in all inertial reference frames!? Give an example in classical physics Thanks
  25. racerunner

    B Muon Tomography Evidence for Special Relativity?

    Muons are a popular way to provide evidence for Special Relativity. But, does Muon Tomography provide evidence for SR? Can you calibrate your muon detectors without reference to SR? Is there any need to refer to SR when interpreting the data? I tutored a high school student who argued the...
  26. anm_ol

    Relativity Free Books & Lectures on Special Theory of Relativity

    I recently viewed some online free lecture series on Special theory of Relativity. I think I have an understanding of the basics so far, but would like some books for problems on special relativity. (Preferably solutions or at least answer keys included). It would be a great help if they...
  27. Ahmed1029

    I Instantaneous coordinates of an event in space (special relativity)

    In relativity, no signal travels faster than light, and hence if something happened away from me, I will only know about it after some time. This means I cannot measure instantly the position and time of something as it happens; this would violate special relativity. I however imagine that I...
  28. DmitryS

    B A question about relativity of simultaneity

    Hello. I'm new here and very much afraid of breaking rules. I would gladly post this question in the Homework section, because it's homework, but my question doesn't fit the template, it's a theory question. I hoped to find it in Relativity FAQ's, but it's not there. I can tell you I grasped...
  29. BadgerBadger92

    I Difference Between Lorentz Transformation & Special Relativity

    Sorry for the extra question. Just have a lot of questions lately and I know some people around here are annoyed with that.
  30. M

    Relativity Resources to start understanding relativity

    Hi everyone, I want to learn relativity but right now I don't know where to start. I have been doing Newtonian mechanics for quite some time from resnik and Irodov and want to go further. Which book should I now read?
  31. H

    I Def Proper Time GR: Half or Integral Along Path?

    In texts on General Relativity, the proper time ##d\tau^2 = -ds^2## (with an appropriate choice of metric signature) is commonly said that the time measured by a timelike observer traveling along a path is given by the integral of ##d\tau## along this path. Of course it's possible to construct a...
  32. Feynstein100

    What does G mean in general relativity?

    In Newtonian mechanics, G is simply a proportionality constant or the force with which two bodies of unit mass attract each other. However, GR doesn't treat gravity as a force. So how is G defined in GR? Is it a property of spacetime or just some useless mathematical artefact? What does G...
  33. IXWELL

    B General Relativity & Grav. Time Dilation Qn

    hello I'm korean high school student and sorry for my poor English. I saw ## t_0=t_f\sqrt{1 -\frac{ 2GM}{rc^2}} ## in wikipedia. does ## \sqrt{1 -\frac{ 2GM}{rc^2}} ## of this equation have name like lorentz factor ## \frac{1}{\sqrt{1 -\frac{v^2}{c^2}}} ##of ## t=\frac{t_0}{\sqrt{1...
  34. A

    Light beam fired in a moving train (relativity)

    1) really does not make sense to me. It is not clear to me how light could be reflected in multiple directions if the source is not a tilted mirror or another object with specific properties. I think the thought of the "point" P confuses me. Further, the fact that light travels in the opposite...
  35. Delta2

    I Entanglement and FTL signaling in professional scientific literature

    According to professional scientific literature and to our best understanding, are there any suggestions that entanglement might imply some sort of faster than light signaling between the entangled particles? I know that according to relativity nothing can travel faster than light, but what...
  36. H

    A Loop quantum gravity and General relativity

    Hi PFs, I am reading this paper written by carlo Rovelli: https://arxiv.org/abs/1010.1939 there are many things that i fail to understand, but i would like to begin with a simple thing. Rovelli write that: It is locally Lorentz invariant at each vertex, in the sense that the vertex amplitude...
  37. Narasoma

    I Gravity Wave Speed: Deriving Constancy from Maxwell Eqs.

    We can derive the constancy of the speed of light from Maxwell equations. My questions are: 1. Why it is then need to postulate it when we can obtain it from Maxwell equations? 2. It is stated in many books that gravity wave also propagates with the same speed, c. How do we conclude that? Is...
  38. R

    B Calculating Time Dilation w/ Light Clock - Implications & Mistakes?

    In this picture it shows a light clock. Let's use the moving light clock example. Am I essentially calculating the b component of moving clock. Assume the moving frame is the B frame. Assume the stationary frame is the A frame https://simple.wikipedia.org/wiki/Light_clock Or essentially the b...
  39. Tan Tixuan

    A How to take non-relativistic limit of the following Lagrangian

    In https://arxiv.org/pdf/1709.07852.pdf, it is claimed in equation (1) and (2) that when we take non-relativistic limit, the following Lagrangian (the interaction part) $$L=g \partial_{\mu} a \bar{\psi} \gamma^{\mu}\gamma^5\psi$$ will yield the following Hamiltonian $$H=-g\vec{\nabla} a \cdot...
  40. G

    B What does relativity say about wormholes?

    I would like to think this is a legitimate question in the fold of Relativity theory. It originally started with the idea that "If someone were to wormhole themselves 2000 or so light years away and turn their sights back on the Earth, they could possibly get a glimpse of the Crucifixion?" On...
  41. Thytanium

    A Special Relativity Fiber Bundle: R^3 & Lorentz Metric

    In the space-time of special relativity considered as fiber bundle, could it be stated that the base space is time and the fibers are space ##R^3## related to each other by the Lorentz metric as a connection and in this case would there be parallelism, and in this case: how would this fiber...
  42. Delta2

    I Earth's orbit not perfect ellipse

    Listen to the following arguments: Earth's orbit isn't perfect ellipse because classically there is the gravitational field of moon and possibly of Mars and Venus which affect it According to general relativity isn't perfect ellipse because there is the curvature of space time which doesn't...
  43. Adwit

    From General Relativity to Dark Energy

    If we insert the values from (2.9), (2.10), (2.11) into (2.5) & (2.6) how can we get (2.13) & (2.14) ?? I need to see the calculations step by step.
  44. Question69

    I Alexander Vilenkin and energy of a closed universe

    For reference:
  45. D

    B General Relativity & Ether: Clarifying a Contradiction?

    Hope this question can be quickly clarified: There was a statement that the General Relativity can be interpreted by speaking of an ether whose state varies from point to point. Is this correct?!
  46. A

    A Generalize Special Relativity for Flat Spacetime

    I am looking for generalizations of special relativity for flat spacetime. Of course, most well known generlaization of SR is general relativty. There are many other generalizations of SR for curved spacetime. All what I found is for curved spacetime. Are any more or less successfull attempts to...
  47. J

    B Gravitational Time Dilation on Mountaintops: New Member Questions

    New member here; just a physics hobbyist. There is probably a simple answer to this question but I could not find it. We know time flows faster on mountaintops relative to sea level due to gravitational time dilation. Over millions of years, wouldn't there be a cumulative effect making the...
  48. LCSphysicist

    Relativity General relativity with focus on action?

    I am having a class of general relativity. It seems that the professor will follow an approach which consist of achieve the action, and variate it to get the equations of motion (indeed, that's how we already got the geodesic equation, the dynamics of a particle in electromagnetism, the equation...
  49. J

    I What is the Meaning of Lagrangian in Special Relativity?

    According to @vanhees71 and his notes at https://itp.uni-frankfurt.de/~hees/pf-faq/srt.pdf under certain conditions one can choose ##\tau## as the parameter to parametrize the Lagrangian in special relativity. For instance if we have, $$A[x^{\mu}]=\int d\lambda...
  50. A

    I Ideal Fluids & General Relativity: Why Needed?

    Why in general relativity do we need the physics of perfect fluids?
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