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. B

    I Judge Original Time & Length Special Relativity

    How to Judge original time and original length of special relativity
  2. rudransh verma

    Classical Understanding Frames of Reference in Force and Motion for 10+2 Students

    I was starting out some problems on force and motion at 10+2 level. I was told you don’t need to know about frame of reference. But I want to. So which books are there to understand frames? Is it a part of relativity? If yes then which books? Thank you guys.
  3. J

    I Angular momentum of an atom within a rigid body in motion

    Considering an atom within a rigid body, does the angular momentum of an electron within the atom vary when the body is put in motion? My intuition is that, whether considered in a classical sense or quantum sense, the speed of a given electron in its motion within an atom will be constant and...
  4. PeterDonis

    I Kinematic Decomposition for "Rod and Hole" Relativity Paradox

    In a recent thread, I said that if there was interest, I would post in a separate thread the calculations for the kinematic decomposition of the congruence of worldlines describing the rod in the "rod and hole" relativity paradox discussed in that thread. Since there was interest, I am posting...
  5. R

    I am confused by the concept of space-time in special relativity

    I am confused by the concept of space-time in special relativity, I have 3 spatial dimensions and 1 time dimensions. In Newtonian physics don't I also have 3 spatial dimensions and 1 time dimension? Then why call it space time in special relativity and not in Newtonian physics?
  6. s__1000

    Question on Einstein's opening passage of relativity

    Einstein says ' we cannot whether it is true that only one line goes through two points' ' We can only say Euclidean Geometry deals with things called straight lines to each of which is ascribed the property of being uniquely determined by two points situated on it, - What does he mean? and...
  7. alan123hk

    B The Paradox of Relativity Length Contraction

    A rigid rod with length ## l_0## slides on a smooth and flat tabletop along the length at speed of ## ~\frac {\sqrt{3}}{2}c~ ##, there is a hole of width ##~l_0~##on the table. The observer who is stationary relative to the desktop thinks that the length of the rigid rod ##~ l=l_o...
  8. Alpha2021

    A General Relativity: Section 5.1 Homogeneity & Isotropy Analysis

    From the section[5.1] of 'Homogeneity and Isotropy' from General Relativity by Robert M. Wald (pages 91-92, edition 1984) whatever I have understood is that - ##\Sigma_t## is a spacelike hypersurface for some fixed time ##t##. The hypersurface is homogeneous. The metric of whole space is ##g##...
  9. Yink

    An introductory question about special relativity

    The way I was doing is that I list events 1. lightning hits x=0 this is (x_1=0,t_1) 2. lightning hits x=12 (x_2=12,t_2) 3. left lightning reaches "me" (x_3=9,t_3) 4. right lightning reaches "me" (x_4=9,t_4=t_3) t_4=t_3 since "I" see the lightning at the same time Then the...
  10. K

    B Explaining a Stronger Relativity Paradox than the Twin Paradox

    The twin paradox can be explained by changing reference frames. But I’m really curious how this paradox can be explained. In the situation below there are three observers: A: Standing at a moving train platform moving at a speed of c/2 relative to “the ground”. B: Standing at a moving train...
  11. alan123hk

    B Solving the twin paradox with special relativity

    I saw a book that uses special relativity to solve the twin paradox, the inference process is roughly as follows. Suppose a spacecraft sets off from the Earth to travel to a distance black hole and then return to the earth. We divide this process into three stages, that is, the process of...
  12. alan123hk

    B Special Relativity & Lorentz Transformation Q: Clock C2 Reading?

    I believe this does not belong to the homework category. I hope I won't be mistaken. I am reading a book to self-study special relativity, the following is an example mentioned in the book. When clock C' and clock C1 meet at times t'=t1=0, both clocks read zero. The Observer in reference frame...
  13. M

    Relativity and Simultaneity: A Cosmic Perspective

    Posed question is that a comment plunges into Jupiter. Same time as my physics class starts on Earth. My friend is traveling towards Jupiter at a significant percentage of c. Would the person in spaceship say that the comment collision occurred before, after or as class started? My answer: same...
  14. only1god

    I Is movement really relative or absolute?

    I was discussing in another page this topic with someone. He said it's relative in everything, meaning that energy and force doesn't count. This is obviously wrong because the fact that today we can't (maybe tomorrow we can) know which thing is moving towards the other and which not, doesn't...
  15. FMJalink

    I Proof of Special Relativity w/ Michelson–Morley Experiment

    Dear readers, Maybe someone can enlighten me on the understanding of the proof given by the Michelson–Morley experiment on the special relativity. Just as introduction to detail the setting: There are 2 coordinate systems A and B. A stands still and B moves with the velocity v along one of...
  16. BadgerBadger92

    I Time Dilation vs. Doppler Effect: Similarities & Differences

    Does time dilation in Special Relativity relate to the Doppler effect? If you move near the speed of light you experience time differently and the sound is stretched. Are these similar phenomenon?
  17. P

    A Why are General relativity texts so much more formal?

    Hi all, What I notice is that there's a significant difference in style between the GR texts and the other textbooks. In particular, GR texts very much try hard to read like a math textbooks, emphasizing theorems and abstract definitions, which I'm not sure are practically useful (though...
  18. L

    Relativity Special relativity in Lagrangian and Hamiltonian language

    Some introduction books on Lagrangian and Hamiltonian mechanics use classical mechanics as the theoretical framework, and when it come to special relativity it goes back to the basics and force language again. I would like to ask for some recommendations on good books that introduces Lagrangian...
  19. L

    I Understanding Special Relativity and Coordinates

    I'd like to get some help on checking my understanding of special relativity, specifically I'm trying to clarify the idea of coordinates. Any comment is really appreciated! The spacetime is an affine space ##M^4##, which is associated with a 4 dimensional real vector space ##\mathbb{R}^4##...
  20. BadgerBadger92

    I Could Tachyons Discredit Relativity?

    I hear these massless and hypothetical particles could travel faster than light, would that prove relativity wrong?
  21. Andreas S-H

    B Proving Special Relativity w/ Standard Equipment

    Hello everyone, I am preparing to write an exam project in college about special relativity, however i am missing the critical experiment to prove that it is true. I thought about using the life time of muons, but i don't have a scintillator to detect them, so unless a standard geiger counter...
  22. LCSphysicist

    Special relativity and simultaneity: Spacecraft and Target

    I made an exercise whose question is like that: A spacecraft moves with speed v at the axis x. There are two target at the axis x, in which the distance between them is L at the ground frame. Suppose that the spacecraft shoot the target simultaneously when it passes by the target. A|so...
  23. S

    Angle observed in rest frame related to relativity

    My answer is (A) since I think the motion of the spacecraft will alter the length of the spacecraft (length contraction) but not changing the orientation so the angle will stay the same But my teacher said my answer is wrong. What is my mistake? Thanks
  24. LCSphysicist

    Inside glass, can I apply relativity of speed for light?

    I was doing a exercise which considerst he time it takes for light travels a glass with thickness proper D and velocity v. The speed of light is c/n inside the glass. Now, my approach was to go to the glass frame, take the relative speed between the glass and the light using the trivial formula...
  25. BadgerBadger92

    Relativity Good Beginner's General Relativity Books?

    I want to learn General Relativity so I am looking for a good beginners book with low amounts of math and clear explanations. Any suggestions?
  26. LCSphysicist

    Rod colliding with the ground (Special relativity question)

    Ok, so basically: There is a frase with a particle moving horizontally with velocity v and a rod parallel to the ground, with rest length L, falling with speed u vertical constant. The rod bounces off on the ground. Describe what happens in the particle frame and find the angle the rod makes...
  27. BadgerBadger92

    B Special Relativity and Light -- How does speed of an object affect time?

    I am an amateur in physics and I have a question. How does speed of an object affect time?
  28. Z

    I Do Fictitious Forces Still Matter in General Relativity?

    hello it is well known that gravitationl force is actually a fictitious force generally speaking,are fictitious forces still necessary in general relativity ? the fictitious forces which we experience on a bus or on a car can also be understood as due to the spacetime distortion ?
  29. R

    Relativity I want to learn special relativity. More details below

    I want to learn special relativity.I have read a tiny bit of 2nd edition of Spacetime Physics: Introduction to Special Relativity and am liking it. Is it a good book? I also want problems to solve. I tried Special Relativity: For the Enthusiastic Beginner but found it to difficult. Does anyone...
  30. LCSphysicist

    Special relativity, a train and a light pulse

    i am having some hard time thinking about this problem: It is basically this: Imagine a bulb and a receptor distant L from each other (at the same axis x) inside a room, the roof of the room is at a height d from the bulb and receptor. Now you are at a train moving horizontally, parallel to...
  31. C

    B No way to catch up with galaxies currently receding at >c?

    Consider a far-away galaxy that is considered to be currently receding from Earth at 2x the speed of light. (With this 2x c recession velocity, we are speaking of the Vnow, the imputed relative velocity of the galaxy compared to Earth now, not the velocity of the galaxy relative to Earth at the...
  32. ergospherical

    Relativity "Must-read" papers in general relativity

    I found an interesting list of "must-read" papers in the field of general relativity compiled by Emanuele Berti: https://pages.jh.edu/eberti2/posts/must-read-paper-list/ Are there any notable exceptions, or other "classic" papers that - in your view - every relativist ought to have read?
  33. Harry Case

    I Understanding Jacobian Matrix Transformation in Special Relativity

    While learning about Special Relativity I learned that we use the Transformation matrix to alter the space .This matrix differs for Contravariant and Covariant vectors.Why does it happen?,Why one kind of matrix (Jacobian) for basis vectors and other kind(Inverse Jacobian) for gradient...
  34. A

    I Time Dilation on 3D Torus: Clock Speed & Why It Matters

    Assume that space is a three-dimensional torus ( a 3D donut) . If there is a clock traveling at a CONSTANT speed in a direction parallel to the torus (inside out of the hole) and one clock that is still. Which clock ticks faster and why? I know that the clock rotating will tick slower, but I...
  35. S

    I Angle-Preserving Linear Transformations in 2D Space for Relativity

    I'm watching this minutephysics video on Lorentz transformations (part starting from 2:13 and ending at 4:10). In my spacetime diagram, my worldline will be along the ##ct## axis and the worldline of an observer moving relative to me will be at some angle w.r.t. the ##y## axis. When we switch...
  36. Thytanium

    A Radial Excess Calc: GR, 4πGρ & R00

    Which is the mathematical procedure to obtain ##\delta r = \frac{GM}{3c^2}## from ##\nabla^2 V = R_{00} = 4\pi G\rho## where ##\nabla^2 V## is volume contraction of a spherical mass of density ##\rho## and ##R_{00}## is the 00 component of Ricci tensor ##R_{ij}##?
  37. LCSphysicist

    Special relativity and acceleration

    A particle is moving along the x-axis. It is uniformly accelerated in the sense that the acceleration measured in its instantaneous rest frame is always g, a constant. Find x and t as functions of the proper time τ assuming the particle passes through x0 at time t = 0 with zero velocity.I n...
  38. L

    I Relativity of simultaneity doubt

    (excuse me for my english, but I'm studying physics and I am not a native English speaker) One observer OE, is on the ground, we take him as the fixed frame of reference. The other OT is on the train that is moving relatively to the OE at a costant velocity ( they are both inertial frame of...
  39. V

    B Time Dilation: Traveling to a Distant Star in 4.5yr

    If there is a spaceship traveling at 0.999c, the time to reach a star 100 lyr away would be approx 100 yr (assuming no accel and decel). But on the spaceship, It would be 100 yr * sqrt(1-0.999^2) = 4.5yr. Why do we take 100 yr as the time seen on Earth and not the time on the spaceship?
  40. P

    Question on special relativity from "Basic Relativity"

    Summary:: Require confirmation regarding answers to a question posed by the book "Basic Relativity" by Richard A. Mould. Here is a problem which I encountered while going through Basic Relativity by Richard A. Mould- I'd like to receive a confirmation regarding the answers I've come up with to...
  41. P

    I Violation of Special Relativity Principle?

    From the michelson-morley experiment, if a clock were to measure the time period of light hitting the mirror and returning back, it would be 2L/c, where L is the distance between the laser nd the mirror. For a moving observer, the time period would have a factor of *gamma*, the boost factor...
  42. U

    Rotation of a photograph of an object due to relativity

    I came across an interesting question in the Hartle's textbook, "An Introduction to Eisntein's General Relativity". The question is as follows: Explain why a photograph of an object moving uniformly with a speed approaching the speed of light, parallel to the plane of the film appears not...
  43. LCSphysicist

    Special relativity - Trivial exercise with spacetime interval

    For a observer on Earth, a rocket takes Mike from Earth to Pluto with a speed of 0.82 c for 33.72 yr. Find the space-time interval for the two events such as Mike leaving the Earth and reaching Pluto considering Pluto is at rest relative to Earth for the observer on Earth. I confess that i am...
  44. G

    I Velocity in Relativity: Theoretical Analysis of Multi-Dimensional Universes

    There are several theoretical analyses of 2 dimensional or of multi-dimensional universes. Now, we live in a special universe that follows Newton's first law (A body at rest or a body in motion shall continue to be at rest or in motion till acted upon by an external force), and consequently...
  45. T

    I Does Special Relativity Affect Mass?

    Is mass relative? does the faster you go change your mass? Because according to the Energy calculations E = ymc^2 at speed and mc^2 and since the speed of light is constant, does that mean that mass changes? [Thread edited by a Mentor to remove personal speculation]
  46. M

    I Relativity of Simultaneity Questions

    Hi All, I've been doing some reading on the above but having some problems understanding certain parts of it (maybe it's wrong from Wikipedia!) To simplify it for me I will first pose a simple scenario where we are not factoring in speed yet, then go from there. Say we have two planets A and B...
  47. cianfa72

    I Principle of relativity for proper accelerating frame of reference

    Hi, I've a doubt about the application of the principle of relativity as follows. Assume as principle of relativity the following statement: It is impossible by any experiment performed inside a "closed" laboratory to say whether we are moving at constant velocity or staying at rest. Consider...
  48. Frabjous

    Relativity Second book on special relativity

    What are the best books for a second exposure to Special Relativity? I can find plenty of threads on introductory books …
  49. P

    Rear Clock Ahead Effect (Special Relativity)

    Time when the left beam hits the left wall in the ground frame ##vt_1-x = c t_1## ##t_1 = \frac{x}{c+v}## Time when the right beam hits the right wall in the ground frame ##ct_2 = vt_2 + y## ##t_2 = \frac{y}{c-v}##Setting the times equal to each other with the constraint x+y=L to find x and...
  50. ohwilleke

    I Scope of General Relativity: Assumptions & Implications

    Assumptions 1. General Relativity is the modern and most complete widely accepted theory of gravitation, formulated in a background independent, geometric way. 2. General Relativity is formulated in a manner consistent with Special Relativity and I could imagine that it might be possible to...
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