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. starkreactor

    I Thought Exp: Gen Relativity & Time Dilation

    I am trying to understand a thought experiment I just posed, which is: if an observer is traveling near the speed of light, and sends out two photons; one in the direction of travel and one in the opposite direction, how does general relativity account for time dilation? The photon "in front"...
  2. phoenix-anna

    B First Experimental Confirmation of GR

    Trying to understand how Einstein predicted the appearance of stars during the solar eclipse around the turn of the 20th century (the first experimental confirmation of General Relativity, I believe). My impression, which is not physically correct, is that the sun attracts the photons as they...
  3. Charlie K Trout

    I Explore Alternative Interpretations of Special Relativity

    A few years ago, a nuclear engineering professor explained to the class that the widely accepted theory between the relationship between space, time, and velocity may be wrong. At the time, I didn't think much about it since he had a lot of out-there beliefs and he seemed to be a bit of a...
  4. G

    Relativity David Morin's special relativity for the enthusiastic beginner

    1.8 Exercises page 49 I can't find the solutions to the questions. Where are they?
  5. G

    Textbook: "Special Relativity For the Enthusiastic Beginner" question....

    Special Relativity: For the Enthusiastic Beginner on page 49 1.8 Exercises I can't find the answers to the questions? I apologize if this is a dumb question.
  6. Martian2020

    B General Relativity and the curvature of space: more space or less than flat?

    General relativity. Curvature of spacetime: ok. time dilation: ok. What about space? Curvature is intrinsic and given by complex equations. But could we definitely say is there more space between 2 points along curved space through the star than would be through flat space (no star there) or...
  7. F

    I Momentum Conservation in Special Relativity: mwu

  8. robphy

    B Einstein's Universe (1979) - Nigel Calder, Peter Ustinov

    In contributing to the thread https://www.physicsforums.com/threads/i-need-recommendations-to-read-about-physics.996304/ , I see that the video that greatly influenced me to study relativity is now available online on YouTube (made available by the film distributor). In 7th-grade, after...
  9. I

    B Doubt about the relativity of simultaneity

    (I hope my English is understood). Hello, I have a question regarding the relativity of simultaneity. The mean temperature of the cosmic microwave background tells us how big the universe is with respect to the recombination epoch. If I now measure that the mean temperature of the cosmic...
  10. D.S.Beyer

    B Lagrangian Point in General Relativity

    Is there a relationship between the Lagrangian ‘hill diagram’ and the spacetime curvature embedment graphs? The Lagrangian map shows effective potential, which deals with centrifugal force. As centrifugal force is a fictitious force (and gravity is as well), I would assume the underlying...
  11. LCSphysicist

    Special relativity — Observer measuring the velocity of a passing rocket

    I am a little confused with the text above. Actually, all is ok until 2.51, i think i am missing something. What is this? Another way to define gamma? (The doubt is not about the raising indices, neither about "n00 = -1", is just where does the equation came from. That is, the equation is easy...
  12. SamRoss

    I Gravitons Replacing General Relativity: Can it Work?

    Einstein's theory describes gravity as a curvature of spacetime. As such, everything is affected by it. This includes light, which has no mass, as was made clear for the first time during the famous 1919 solar eclipse. In the standard model, the cause of gravity is supposed to be gravitons...
  13. R

    I Special Relativity: Intuition & Quantities Explained

    Hi, I'm taking an introduction course to Special Relativity and encountered a fairly simple problem: Dirac travels to alfa centauri, which is 4.37 lightyears away. He stays there one Earth year and then travels back, and when he comes back he has aged 5 years. At what speed did he travel...
  14. Amrator

    Relativity: Force and a Collision

    I don't know if I did this correctly. ##\int Fdx = \int dE## ##F \Delta x = \Delta E## ##Fx = Mc^2 - (mc^2 + mc^2)## ##M = \frac{Fx + 2mc^2}{c^2}## ##M## is the mass of the resulting particle. ##2mc^2## is the total energy before the collision. The issue is I'm assuming that the resulting...
  15. ThisTimeTomorrow

    I Logic question on relativity- no math

    Checking my understanding. Can it be said that it is the overlap of the reach and effect of each of the 4 forces, from each respective point of origin possible within the universe, that gives us universal general relativity? Like the most intricate gear set ever? Could it then also be said...
  16. J

    Special Relativity - relative motion of two bars

    Hi, I'm stucked in this exercise. I don't know how to proceed. Any hints I appreciate it.Two bars of the same proper length L, moves in the same way on the x axis. In the referential of one of the bars the time interval between the events, when the extremity right of one bar and the extremity...
  17. S

    Compton Effect with the electron initially moving

    Hello! I do not understand how to prove the exercise. I have searched all over but I have found no hints on how to get started. Can anybody help me?
  18. LCSphysicist

    Relativity problem -- A person walking in a train that is traveling at 3c/5

    "(a) How much time does this take in the ground frame? (b) What is the person’s speed with respect to the ground? (c) How much time elapses on the person’s watch?" I solved it, but i am with a doubt yet wrt to the letter b. We can find that the train length in the ground frame is 4L/5, so, why...
  19. F

    I Special Relativity & Non-Instantaneous Force Equations

    In classical mechanics, the gravitational force is described by the equation: F = Gm1m2/r^2 What would this equation - or other similar equations - look like in special relativity? This equation cannot be correct because it implies that the force acts instantaneously.
  20. LCSphysicist

    Confused about relativity and a train

    We can find the difference of time to light reach both clocks, it should be Lv/c², what i am confused is why the rear clock show a higher reading of the front clock. Ok, the light take longer time to travel and reach the rear clock, and here is the thing! What i am interpreting is: When the...
  21. Amaterasu21

    I How Does Relativity of Simultaneity Clash w/Thermodynamics?

    In special relativity, observers can disagree on the order of events - if Alice thinks events A, B and C are simultaneous, Bob can think A happened before B which happened before C, and Carlos thinks C happened before B which happened before A - provided A, B and C are not causally connected, of...
  22. F

    I Acceleration in Special Relativity

    Can special relativity handle acceleration? I believe the answer is yes, but I don’t recall dealing with any acceleration problems when I took SR. I remember using the time dilation, length contraction and Lorentz transformation equations. These equations all assume constant motion iirc.
  23. F

    Calculate a specific boost and rotation

    Let's begin with the first point. a.I) Apply a generic boost in the y-z plane (take advantage of the arbitrariness in deciding the alignment of the y and z axes). \begin{equation*} B_{yz} = \begin{pmatrix} \gamma & 0 & -\gamma v_y & -\gamma v_z \\ 0 & 1 & 0 & 0 \\ -\gamma v_y & 0 &...
  24. S

    Can Flying Ships Defy Inertia and Make Instant 90-Degree Turns?

    I haven't gone to movie theaters in 6 months. So I have to be content with online movies, and reading sci fi models, and maybe writing a short story or two. If a flying ship could travel thousands of miles per second, it can't suddenly turn 90 degrees because of inertia, the occupants would be...
  25. T

    A Alternative theories to General Relativity

    Hey there.Have you read about the alternative theories proposed by other scientists to General Relativity?So far General Relativity still stays the most accurate.But if we could generalise it?Perhaps try with some new maths like complex differentiable manifolds or Lie groups or another...
  26. AHSAN MUJTABA

    General Relativity: How many Christoffel symbols?

    Actually I know there would be some permutations used here. I know how to calculate the symbols but estimating is quite a new thing to me
  27. Boltzman Oscillation

    I Particle Perspective: How Relativity Affects Us

    Some background, I am an undergraduate electrical engineering student with a knack for physics. I plan to attend graduate school for physics but for the meanwhile I've only taken an undergraduate course in QM mechanics, which used griffith's book, and a modern physics course, which covered some...
  28. thequantumcat

    I Special Relativity and Velocity

    I can't seem to wrap my head around it: if an object is moving at speed v in frame S, and its observed to move at speed v' in S', what is the relationship?
  29. F

    I Why Does Special Relativity Not Contradict General Philosophy?

    I hear that philosophy refuse to "relativity concepts"(e.g relativity moral).Relativity theory says about the relativity of spacetime.I do not understand why special relativity does not contradict with general philosophy?(I know the theory Idea Form of Plato)
  30. Data Base Erased

    I Beginner question about tensor index manipulation

    For instance, using the vector ##A^\alpha e_\alpha##: ##g_{\mu \nu} e^\mu \otimes e^\nu (A^\alpha e_\alpha) = g_{\mu \nu} (e^\mu, A^\alpha e_\alpha) e^\nu ## ##g_{\mu \nu} e^\mu \otimes e^\nu (A^\alpha e_\alpha) = A^\alpha g_{\mu \nu} \delta_\alpha^\mu e^\nu = A^\mu g_{\mu \nu} e^\nu = A_\nu...
  31. fingerprintregion

    Relativity: Relativistic Kinematics and Dynamics

    I’ve read Beiser’s Modern Physics Chapter 1 and I am able to grasp the general theories but not so much when applying it to problems. I major in Chemistry and would really appreciate any head start/help/suggestions. For number 1 I thought of using L = L0 sqrt 1-v^2/c^2 but can’t seem to find the...
  32. S

    Courses Statistical Physics vs QFT vs General relativity

    Good day, I'm starting my master in physics, and it's time for me to choose my courses. I will take one or two of the following three courses, which are: Statistical Physics, QFT and General relativity. Now, I'm finding it very hard to decide as on the one hand, I'm interested in QFT and...
  33. V

    I Momentary Co-Moving Reference Frame in SR

    In SR, for the momentary co-moving reference frame, U (the velocity four vector) takes the form (1,0,0,0). I'm wondering whether the basis vectors associated with this velocity are zero or if the coefficients in front of the basis vectors are zero. In classical mechanics we would say that the...
  34. I

    B Fundamentals of Lightspeed: Questions & Answers

    Hey there, I'm aware this is a bit of a stupid question, and I think that I understand the principle fundamentally, however, my intuition is still having a little trouble catching up, and I'm trying to figure out if it is because of an important detail that I have missed/misinterpreted. I think...
  35. entropy1

    I Which clock was slower in special relativity?

    I don't know much about the math of SR, but this is what's bothering me: if a moving clock B ticks slower than the stationary one I have (A), then from the viewpoint of B, my clock (A) is ticking slower. So if we turn around and meet each other in the middle, which clock was slower than which...
  36. S

    B Light Clock & Length Contraction in GR

    Hi, can i use a light clock made out of mirrors a distance appart to measure whether there is length contraction in different regions of spacetime? If the clock speeds up then the distance between the mirrors decreased. If the clock slows down the distance between mirrors increased.
  37. Haorong Wu

    Relativity How are David Tong's Lectures on General Relativity?

    Hi. I have tried David Tong's note on QFT. I think it works well for me and lead me into QFT. Now I am confident to read Peskin's book. Now I am trying to learning GR. I planned to try David Tong's lectures on GR first and then read Sean Carroll's book. But I am not sure this plan now. I got...
  38. M

    Length of a Wave Train in Special Relativity

    I am confused about how to find the length of a wave train emitted within a time interval T and that is moving with speed c relative to a moving frame that is itself moving with velocity v. Apparently the answer is that the wave train's length is cT - vT, but I tried to plug in variables into...
  39. M

    Special Relativity Math Thing (Fresnel's Drag Coefficient)

    I am wondering if there is a typo in my textbook. Please see the attachment. The textbook says "...keeping only the lowest term in x = v/c." I am wondering if it should be "x = v/(nc)," as I circled in blue on the left side. It is a binomial expansion of the denominator. Shouldn't x be v/(nc)...
  40. F

    I Understanding the Stress-Energy Tensor in Special Relativity

    Hello, I try to understand how to get the last relation below ##(3)## ( from stress energy tensor in special relativity - Wikipedia ). I understand how to get equation ##(1)## but I don't grasp how to make appear the gradient operator in the dot product and the divergence operator in the...
  41. V

    Which course should I take? (General/Special Relativity or Astrophysics)

    I'm about to be a senior in the physics program. I can choose General/Special Relativity or Astrophysics. The astrophysics course is not as intensive - the GR/SR course requires mechanics and is more math-savvy. I've been doing some modeling with Lagrangian mechanics for star systems etc. and...
  42. docnet

    B Special Relativity for Beginners: Clock Comparison

    Hi, I have no education in physics beyond the intro undergrad level. How do the clocks compare in the following scenarios? In the beginning, two clocks are placed next to each other and synchronized. Then they are moved in opposite directions, traveling an equal distance from the beginning with...
  43. BruceAW

    I Apparent Violations of Principle of Relativity?

    I have for a long time been pondering the concept of 'Absolute velocity'. Or, 'Proper motion'. The velocity of an object, with respect to the center of mass of the universe, and the cosmic microwave background radiation. Ways, it seems to make more sense, than merely relative velocities, with no...
  44. J

    I Does General Relativity explain inertia?

    As far as I understand it general relativity does not explain the origin of the inertial mass ##m_i## in Newton's law of motion ##\vec{F}=m_i\ d\vec{v}/dt## but rather it simply applies the concept to curved spacetime. For example if we have a particle with inertial mass ##m_i## and charge...
  45. R

    B General Relativity, Twins & Schopenhauer: Analyzing Timing Diff.

    Consider a mass ##M## that generates a curvature of the space-time and an observer ##O##, fixed and positioned at such a great distance from ##M## that the time ##t## of its clock is not affected by ##M##. Suppose that the observer ##O##, in his polar coordinates reference system centred at...
  46. jk22

    A Nonlocality Theorems & Relativity Principles

    Do nonlocality theorems imply that the local inertial frame principle of general relativity is wrong (since it is local) ?
  47. greg_rack

    B Space is Relative: A 17 Yr Old's Exploration

    Hello everybody, my question may sound stupid, especially speaking of such a mind-blowing and important theory... but here I am! I'm 17 and I'm reading a fabulous book by Stephen Hawking, "A Brief History of Time", and it introduced me to relativity theories... I literally started looking the...
  48. D

    B Why the stay-at-home twin is not considered to be accelerating?

    I am not a physicist. I need your kind help in removing my following doubt about twin paradox. What I have been able to understand about twin paradox is this- 1. Special relativity deals with non-accelerating (inertial) motion. 2. The traveling twin (A) moves at a high speed in relation to the...
  49. Bob Walance

    B General Relativity & External Forces: 2 Cases Examined

    It seems that there are two distinct gravitational cases to consider: a) Object with no external contact with any type of stuff (e.g., a person in free-fall in a vacuum) b) Object WITH external contact (e.g., a person standing on the ground) I've enjoyed reading and listening to various...
  50. A

    I Solve Constant Acceleration Problem in Special Relativity

    Basically I just want to work out a constant acceleration problem in relativity, of the same kind of introductory physics. Vo= 0.9999c Vf = 0 D= 50 Au Accel, Earth frame? Accel, Ship frame? Time of transit, Earth frame? Time of transit, ship frame? Motion is 1-D. All origins line up at the...
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