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. 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...
  2. 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...
  3. 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...
  4. 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...
  5. 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...
  6. 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...
  7. 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...
  8. 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...
  9. 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...
  10. 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...
  11. 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...
  12. 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.
  13. Question69

    I Alexander Vilenkin and energy of a closed universe

    For reference:
  14. 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?!
  15. 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...
  16. 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...
  17. 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...
  18. 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...
  19. A

    I Ideal Fluids & General Relativity: Why Needed?

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

    I Constant Velocity Motion in Relativity: Radiation Effects?

    Does relativity imply that everything that goes at a constant velocity must not emit radiation of any kind?
  21. A

    I General Relativity: Is it Local?

    Spacetime is a differential manifold and at each point is attached a Minkowski spacetime. There the laws of physics are the usual ones without gravity. Gravity is the curvature of spacetime. To define the concept of curvature do we need to evaluate at least one neighborhood of point P? Is...
  22. C

    B Robert Wald's General Relativity: Energy-Momentum Relation

    Hello, this is my first thread. Robert Wald, in General Relativity, equation (4.2.8) says : E = – pa va where E is the energy of a particle, pa the energy-momentum 4-vector and va the 4-velocity of the particle. How can I see this is compatible with the common energy-momentum-relation E2 – p2 =...
  23. K

    I Length contraction in General Relativity

    In GR, a free falling object when viewed by a distant observer appears to be length contracted and slows down as it approaches the event horizon of a black hole. The length contraction piece, however, seems counterintuitive. I would have thought that the leading edge of the object would...
  24. S

    A Does Spacetime Absorb Energy in General Relativity?

    Some physicists prefer to explain the problem of conservation of energy in General Relativity by considering the gravitational potential energy of the universe that would cancel all the other energies and therefore the energy in the universe would be conserved this way. However, many other...
  25. A

    Mathematical prerequisites for general relativity

    What mathematical topics do I need to know to start studying general relativity? From which textbooks can I learn them? I don't currently know anything about differential geometry. I know calculus, linear algebra, mathematical methods of physics (the necessary topics for quantum mechanics) and...
  26. G

    B General Relativity: Action & Reaction in Gravitation

    Hi, In general relativity, gravitation is not anymore a Force but a deformation of space time. I would like to know what's becomes the 3 law of Newton for gravity that action equal reaction ? When a apple fall on the earth, does "the force" is exactly the same as the one applied on Earth ...
  27. Diracobama2181

    Special Relativity Muon problem

    i) The muon reaches the ground ii) To a ground observer, the decay time is dilated $$\Delta t_d=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}\Delta\tau_d>\Delta \tau_d$$ The time for the muon to reach the ground is $$\Delta t_g=\frac{10 km}{0.999c}< \Delta t_d$$ which is why it reaches the ground...
  28. Jamestein Newton

    I Discussing General Relativity - Beliefs & Ideas

    Following are not strictly physics questions. But cool questions to discuss. We need to add dark energy to our cosmological model if we strictly follow GR. This lead to some beliefs that GR is an effective theory. (1)Do you believe that GR is the fundamental theory? If GR is an effective...
  29. Aerodyn

    I Understanding Gravity with GR: Beginner's Guide by Aerodyn

    Hi everybody. I recently started to learn GR from a very begginer level. I would like to share with you some lines of discussion, to understand your approach to some specific topics which for me are key to better understand the whole story. If a put an accelerometer on the floor it reads 9.8...
  30. L

    A Special Relativity: A^μ_ν Differences Explained

    Can someone explain me difference between A^{\mu}_{\hspace{0.2cm} \nu} A^{\hspace{0.2cm} \mu}_{\nu} and A^{\mu}_{\nu}?
  31. Tertius

    I Computing Volume in General Relativity: Use of Tensor & Friedmann Eqns

    When we compute the stress energy momentum tensor ## T_{\mu\nu} ##, it has units of energy density. If, therefore, we know the total energy ##E## of the system described by ## T_{\mu\nu} ##, can we compute the volume of the system from ## V = E/T_{00}##? If it holds, I would assume this would...
  32. thedubdude

    B Special Relativity violation via Quantum Mechanics?

    We know that both momentum and position can not be known precisely simultaneously. The more precisely momentum is known means position is more uncertain. In fact, as I understand quantum mechanics, position probability never extends to 0% anywhere in the universe (except at infinity) for any...
  33. wnvl2

    I Relativity paradox: rocket landing in a cylinder

    A rocket has length L with a separate head on top. The rocket lands in a cilinder on Earth with height L with speed v. From the point of view of the rocket, the cylinder undergoes a Lorentz contraction. The rocket will therefore collide with the bottom of the cilinder and damage it. From the...
  34. phyz2

    I Klein Gordon Invariance in General Relativity

    Hello! I'm starting to study curved QFT and am slightly confused about the invariance of the Klein Gordon Lagrangian under a linear diffeomorphism. This is $$L=\sqrt{-g}\left(g^{\mu\nu}\partial_\mu \phi \partial_\nu \phi-\frac{m^2}{2}\phi^2\right),$$ I don't see how ##g^{\mu\nu}\to...
  35. U

    B Special Relativity and the Measurement Problem of QM

    Given what we know about special relativity and its implication for time and the observer, could this in any way be linked to why the isolated processes of QM are exhibiting everything happening at once and then collapsing to classical physics when bigger objects interact - the measurement...
  36. robphy

    Insights Introducing Relativity on Rotated Graph Paper: A Graphical Motivation

    Continue reading...
  37. G

    A Understand (k,l) Tensors in Gen. Relativity

    In both Wald and Carroll, a type (k,l) tensor has k dual vectors and l vectors, yet a (1,0) tensor is a vector and a (0,1) tensor is a dual vector. I must be missing something simple. Please explain.
  38. M

    B Is the relativity of simultaneity more than just a fudge factor?

    <Moderator’s note: a reference to an article published in a predatory journal has been removed. References on PF should be from the professional scientific literature or from other sources consistent with the professional scientific literature> I read through many posting on various threads in...
  39. P

    A Sidney Coleman's Lectures on Relativity: New Book by CUP

    In case you haven't heard, there is a new textbook on relativity to be published by Cambridge UP in 2022. It is compiled and edited by Coleman's three students Griffiths, Derbes, and Sohn who took Physics 210 relativity course at Harvard in the late 60's when Sidney was teaching it. The book...
  40. Istiak

    Find out velocity of an observer (relativity)

    Initial observer is at rest. So ##x\prime=0##, and according to question they are 10 meter apart. So lorentz transformation becomes ##vt=x## ##v=\frac{x}{t}## ##=\frac{10 \\ \mathrm m}{13\times10^{-9} \mathrm s}## But I don't get the expected answer. I believe if I had took ##\beta c## instead...
  41. G

    Special relativity - measure of a rod and simultaneity

    Hi, I´m trying to solve a special relativity problem, and I think I need some help. There are two inertial frames of reference, ##O## and ##O'##, the last one moving with relative velocity ##v## in the ##x## direction. There's a rod with length ##L'## fixed to frame ##O'##, such that front end...
  42. C

    I Special Relativity: Grokking for a Female Neurobiologist in her 60s

    I'm a female neurobiologist in my 60s with a lifelong passion for physics (but my math was not strong enough). I have a special interest in special relativity and a decent grasp of the basics. I can solve Lorenz calculations 'til the cows come home'. I don't need help with homework of any type...
  43. Istiak

    Find second object's velocity in relativity

    > A particle of mass M at rest decays into two particles of masses m1 and m2 traveling in opposite directions at velocity v1 and v2 respectively. Express v2 in terms of v1, m1, m2, and M. Since both objects are from a single object that's why I took relativistic mass of both objects are same. I...
  44. robphy

    I Learn Physics with Eigenchris: Relativity & Tensors

    This set of videos by eigenchris (separate playlists on Relativity and on Tensors) also looks interesting and can help anyone interested in learning about these topics. A while back I watched some of them and thought they could be helpful. I like his presentation of one-forms. (I've been...
  45. robphy

    I Ta-Pei Cheng's "Relativity & Cosmology" Course on YouTube

    I was browsing YouTube (again) and stumbled upon this set of lectures by Ta-Pei Cheng. (I've seen the cover of the textbooks... but I'm not familiar with the author-lecturer or the details of the texts... but this looks interesting.) http://www.umsl.edu/~chengt/...
  46. R3ap3r42

    Special relativity and Lorentz Transformation Exercise

    Summary:: Special relativity and Lorentz Transformations - I got this problem from a first-semester course at university. I have been struggling for a few days and decided to get some help. A rocket sets out from x = x' = 0 at t = t' = 0 and moves with speed u in the negative x'-direction, as...
  47. robphy

    I Andrzej Dragan's "Course on Relativity" Videos | YouTube Playlists

    Andrzej Dragan ( https://www.fuw.edu.pl/~dragan/ ) has a "Course on relativity" on YouTube. https://www.youtube.com/c/relaTVty/playlists I'm not familiar with Dragan... but the videos display some flair and personality. (...
  48. alexandrinushka

    B Any plausible explanation for dynamical length contraction?

    I am new here, so pardon my ignorance. First of all, I am aware of the impossibility to distinguish experimentally between SR (Special Relativity) and LET (Lorentz Ether Theory). I know there is a PF policy article on LET and the Block Universe. I must admit though that LET is more appealing...
  49. The Bill

    Relativity Opinions on Rovelli's General Relativity: The Essentials?

    I recently noticed that "General Relativity: The Essentials" by Carlo Rovelli has been published. Based on the description, table of contents, and the Amazon reviews, it seems to me that it might be a spiritual successor to Dirac's "General Theory of Relativity." Is that an accurate assessment...
  50. jaketodd

    I Can Thinking Fast Slow Down Time in Relativity?

    In a stressful situation, when you're thinking rapidly, time really does seem to slow down. Like "that was the longest 30 seconds of my life." Since thinking is made up of the firing of action potentials in neurons, could those events, in the brain, work with relativity to slow down personal...
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