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. P

    I A paradox connected to General Relativity

    I am posting this paradox as a brain teaser If we drilled a tunnel through the earth, to the other side, and measured gravity in the tunnel, gravity would be zero in the center of mass. This is Newtonian gravity and is connected to the vector addition of the gravitational force; cancels in...
  2. A

    Special relativity - angles between fields

    Homework Statement In an electromagnetic filed, the elctric field ##\vec{E}## forms an angle ##\theta## with the magnetic field ##\vec{B}##, and ##\theta## is invariant for all inertial observers. Finding the value of ##\theta##. Homework Equations Tranformations of fields perpendicular to the...
  3. James Chase Geary

    Special Relativity problem: Lorentz transformation and Reference Frames

    Homework Statement S and S' are in standard configuration with v=αc (0<α<1). If a rod at rest in S' makes an angle of 45o with Ox in S and 30o with O'x in S', then find α. Homework Equations We are in the world of Lorentz transformations so we have t'=(t-vx/c2)/(1-v2/c2)1/2...
  4. A

    B Mass-Energy Equivalence: Matter as Potential Energy?

    I think mass as a form of potential energy and am always told that this is wrong. According to wiki: "In physics, potential energy is the energy possessed by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors." Why do this...
  5. A

    Special relativity - Gauge invariance

    Homework Statement In an inertial reference frame ##S## is given the four-potential: $$A^\mu=(e^{-kz}, e^{-ky},0,0)$$ with ##k## a real constant. ##A^\mu## fullfills the Lorentz gauge? And the Coulomb gauge? Which is the four-potential ##A'^\mu## in a reference frame ##S'## which is moving...
  6. A

    Special relativity - Transformation of angles

    Homework Statement Let's have a three-particle decay of equal mass ##m##; in the CM frame the three particles have equal energy ##E## and they form angles of ##\frac{2\pi}{3}## between each other. Which is the angle between two of the three particles in the rest frame of the other one. (The...
  7. Philip Dhingra

    I Tangible example of relativity of simultaneity

    I always liked the idea of starting relativity discussions with the idea that your father could travel at a substantial fraction of the speed of light and return back at roughly the same age as you. This makes time dilation seem interesting, and drives curiosity in students. But I'm struggling...
  8. K

    B How can the speed of light be constant for all observers?

    A rocket is in constant velocity. The velocity of the rocket is 150Mm/s (or 0.5 of the speed light, or 150 million meters per second) relative to us (we as observer). We observe two lights, one moving in parallell with the rocket, another is moving in the opposite direction. Below I have made...
  9. A

    Special relativity - kinematics

    Homework Statement A photon hits a proton at rest in the laboratory frame and there is the process: $$\gamma + p \rightarrow n+\pi^+$$ The mass of the pion is ##m_\pi## and assuming that the masses of the proton and the neutron are the same (##m##): Finding the threshold energy of the foton...
  10. stevendaryl

    Insights Rindler Motion in Special Relativity: Hyperbolic Trajectories - Comments

    Greg Bernhardt submitted a new PF Insights post Rindler Motion in Special Relativity: Hyperbolic Trajectories Continue reading the Original PF Insights Post.
  11. F

    Kinetic Energy Needed for Proton Acceleration

    Homework Statement a) Calculate the kinetic energy required to accelerate a single proton from a rest position to 0.9999c. The mass of a proton is 1.67*10^-27 kg. b) Find the ratio of the kinetic energy to the energy of a proton at rest. c) Explain why no particle accelerator will ever be able...
  12. T

    I Why is the conservation of electric charge expressed in the continuity equation?

    I’ve seen a few places, here is 1 such link: http://physics.weber.edu/schroeder/mrr/MRRtalk.html I have a problem with how they represent the charges in the frame where the free charge is stationary and the charges are moving in the wire. This seems wrong for 2 reasons: 1. They show additional...
  13. DLeuPel

    I How can General Relativity explain the Moon drifting apart from Earth

    According to various sources, the Moon is separating from Earth 4 cm every year. I’ve searched for the explanation and I’ve found the following: The friction the seas and oceans from the Earth make with it’s soil causes the Earth’s rotation to slow down. This causes the Moon to speed up...
  14. P

    A Lense-Thirring effect - General Relativity

    Let us assume a "toy-metric" of the form $$ g=-c^2 \mathrm{d}t^2+\mathrm{d}x^2+\mathrm{d}y^2+\mathrm{d}z^2-\frac{4GJ}{c^3 r^3} (c \mathrm{d}t) \left( \frac{x\mathrm{d}y-y\mathrm{d}x}{r} \right)$$ where ##J## is the angular-momentum vector of the source. Consider the curve $$ \gamma(\tau)=(x^\mu...
  15. Adgorn

    Solution of basic special relativity problem

    Homework Statement This is more of a solution check than an actual problem, but I didn't see any other fitting thread to post this on. I'm starting to learn special relativity and have received 2 starting questions on the subjects, I solved them to the best of my abilities but have no actual...
  16. A

    Change in time due to accelerated light-clock

    Homework Statement A light-clock (a photon traveling between two mirrors) has proper length l and moves longitudinally through an inertial frame with proper acceleration ##\alpha## (ignore any variation of a along the rod). By looking at the time it takes the photon to make one to-and-fro...
  17. Space_Girl

    B Black Holes Must Have Singularities Says Einstein Relativity

    https://www.forbes.com/sites/startswithabang/2018/02/14/black-holes-must-have-singularities-says-einsteins-relativity/amp/
  18. Gene Naden

    I Sum Maxwell Lagrangian 1st Term: Use Minus Signs?

    So the first term of the Lagrangian is proportional to ##{F_{\mu \nu}}{F^{\mu \nu}}##. I wrote out the matrices for ##{F_{\mu \nu}}## and ##{F^{\mu \nu}}## and multiplied at the terms together and added them up, but some of the terms didn't cancel like they should have. Should I have used minus...
  19. pat65

    Relativity Gravity in a Nutshell is too chatty, would like something else

    Hello All, I recently purchased a book on relativity called "Gravity in a Nutshell" by Zee. While it is interesting I find his conversational style to be too chatty. I prefer a little more of a "get to the point" conversational style. I'm going to be modeling the evolution of the...
  20. A

    Mathematical prerequisites for General Relativity

    I am a working professional trying to get back to some physics that I used to enjoy a couple of decades ago. I still do remember the basic calculus (integrals, partial derivatives, basic ODE) and am interested in studying General Relativity. I have a decent understanding and memory of Special...
  21. A

    Special relativity - Maximum mass

    Homework Statement Finding the maximum mass ##M_x## which can be made from a collision of identical particles with mass ##m##, in the laboratory frame, in which one particle is at rest and the other one has energy ##E##. The reaction is the following: ##a+b \rightarrow a+b+x##.The Attempt at a...
  22. A

    Special relativity - constant acceleration

    Homework Statement A rocket is flying away from the Earth with a costant acceleration ##a## in a frame in which the rocket is at rest. Finding the equation of motion (the relation between the distance from the Earth and the terrestrial time). Showing that there is a maximum time ##T## after the...
  23. Destroxia

    Special Relativity - Length Contraction

    1. Homework Statement Why does the non-relative simultaneous frame have an additional term of ##v \Delta t## along with the contracted length ## L ## for ## \Delta x ##? 2. Homework Equations ## L + v \Delta t = \Delta x ## ---- (1) ## \Delta x = \gamma \Delta x' = \gamma L_{\star} ## ----...
  24. A

    Special relativity - inverse Compton scattering

    Homework Statement In the inverse Compton scattering there is a particle, with energy ##E## in the laboratory frame and mass at rest ##m##, which collide head on with a photon with energy ##E_\gamma##. Finding the maximum energy the photon can have after being scattered. The Attempt at a...
  25. M

    What are the speeds of P and M while riding Ents and playing catch with rings?

    Homework Statement P and M are each riding Ents while playing catch with two rings. P throws his gold ring with a speed relative himself that is twice the speed that M throws his silver ring relative to himself. P and M start off going with the same speed and direction, 30m apart with M in...
  26. A

    Special relativity - scattering angle

    Homework Statement Finding the maximum scattering angle of a particle whose mass in ##m_1## which hits with relativistic velocity ##v## a particle at rest with mass ##m_2<m_1##. The Attempt at a Solution I've written the 4-momenta (p before the collision, k after the collision and the z-axis...
  27. S

    Help me find my error in a relativistic kinetic energy calculation

    Hi everyone. Given: an asteroid with the mass of 50,000,000 kg, which is moving with the velocity of oh-my-god particle -- 99.99999999999999999999951% of c. Due to relativistic effects, its total kinetic energy will be 1.44 E+36 Joules (Lorentz factor = 3.2 E+11). A hypothetical particle...
  28. A

    Special relativity - transformation of electromagnetic fields

    Homework Statement In a reference frame ##S## there is a particle with mass ##m## and charge ##q## which is moving with velocity ##\vec{u}## in an electric field ##\vec{E}## and in a magnetic field ##\vec{B}##. Knowing the relativisitc laws of motion for a particle in an EM field, find the...
  29. ZapperZ

    B MinutePhysics Special Relativity Series

    MinutePhysics is attempting to produce a series of video lessons on Special Relativity, using an approach, according to the video, that will be different and "simpler" than the traditional method that SR has been taught in schools. Since we often get questions on here about this topic...
  30. F

    Abstract Special Relativity, particle at rest and one moving

    Homework Statement In frame S particle 1 is at rest and particle 2 is moving to the right with velocity u. Now consider a frame S 0 which, relative to S, is moving to the right with velocity v. Determine the value of v such that the two particles appear in S' to be approaching each other with...
  31. P J Strydom

    I Can I be taught general and special Relativity on this forum

    If I am wasting anyone's time, please advise so I can leave and find another source to assist me in my quest. I have been thinking about the theory of General and Special Relativity for most of my life. When I was 16 years old, I saw a documentary about Einstein and how he used his thoughts as...
  32. J

    Special Relativity Problem from Rindler

    Homework Statement In the inertial frame ##S'## the standard lattice clocks all emit a 'flash' at noon. Prove that in ##S##, this flash occurs on a plane orthogonal to the ##x##-axis and traveling in the positive ##x##-direction at speed ##\frac{c^2}{v}##. Homework Equations Lorentz...
  33. A

    How Does Relativity Affect the Measured Distance Between Cities at High Speeds?

    Homework Statement A man is flying with a speed of 0.7c and at a height 1 km. During his flight, he measures the distance between Toronto and Montreal to be 450 km. What is the real distance between them. I don't think the answers are based on real life, they are just examples of places...
  34. B

    Angled Length Contraction (Relativity)

    Homework Statement Find the length of a meter stick if in the laboratory frame it is angled at 60 degrees relative to the direction of motion, and it is traveling at .8 c. I have calculated gamma to be 1.667 Homework Equations Lorentz transformations The Attempt at a Solution a' = l' cos(x)...
  35. S

    I Relativistic Particles: Reaching Absolute Temperature?

    Hello fellow physicists.Recently I imagined a particle far away from everything (infinitely away) such that there is no potential energy affecting the particle. Ep = 0 Ek = mc2δ : δ = (1-v2/c2)-1/2 L = EP- Ek = -mc2δ ∂L/∂v = vE0/c2 *δ^3 where E0 = mc2 d( ∂L/∂x)/dt = 0 ∴ vE0/c2 *δ^3 = 0 mvδ3...
  36. Hypercube

    B I cannot grasp simultaneity in Special Relativity

    Hi everyone, I'm struggling to grasp simultaneity in Special Relativity. Since I am struggling conceptually, I have posted my question here. But if you think this problem I made up modified should go to homework, I apologise (and feel free to move it). Suppose we have two light sources...
  37. Chris84567

    I Measuring Speed of Light Relative to Earth

    The question: If the max speed I can go in the universe is the speed of light measured from? Background: If there are three rocks in a vacuum along a line (the x axis), one is stationary (rock 1), one is moving 5 m/s in the x direction (rock 2) and one is move -5 m/s in the x...
  38. M

    I Relativistic particle in non-uniform magnetic field (math)

    Hi everyone, I am currently working on a project in Physics analyzing the possibility of magnetic fields protecting against radiation particles such as fully ionized nuclei or electrons. I wrote a code in Matlab that simulates a 3-D magnetic field based on a combination of wires and wire loops...
  39. I

    Finding the relativistic speed of a spacecraft

    Homework Statement If a spacecraft is traveling to a star which is located at a distance of 1 lightyear and it would take the spacecraft 1 year to reach the star in its own frame, how fast would the spacecraft actually fly? Also, how long would the journey take for an observer on Earth...
  40. stickman76

    I Graphic Anim of Special Theory of Relativity: Exploring Interrelationships

    There is a relationship between each of the mathematical values in each of the paradoxes with regard to each observers. There is obviously also a relationship between every situation we study in the Special Theory of Relativity. The Lorentz Transformations obviously prove this. So, I began to...
  41. Gabriele99

    I Spacetime of Flatland: Exploring a 2D Universe

    I started this post on physics.stackexchange but it's too vague for that site, so here I am! :) I'm trying to really get the intuition of spacetime. This video explains how Minkoswki was the first to think that maybe our universe does not consist of a 3d space which evolves in time, but rather...
  42. Q

    B Does this connect Quantum Theory and Special Relativity?

    This is probably extremely wrong I just want to know how. If photons don't experience time doesn't that mean they are every where at once, and if that is true doesn't part of quantum physics say before a particle is observed its in all the states it can be in? So doesn't that apply in that sense...
  43. WilliamMGiraldo

    B Common Time: Explaining Einstein's Definition

    Hi. Thanks for letting me ask this question. It is a stupid one. I'm a newbie at relativity theory, and I'm reading On the electrodynamics of moving bodies, By Einstein, A. He associates time with space, and tells us that you can measure the "time" of an event if you are in the coordinates of...
  44. A

    B Special relativity equation question

    Good evening. I think I've got it as far as time dilation is concerned. Could someone please let me know if this calculation is correct? - From frame C( stationary camera) Bob is moving at 50,000KM/second parallel to Alice's path - Bob is also 50,000KM behind Alice on the parallel path - Bob's...
  45. J

    I General Relativity & The Sun: Does it Revolve Around Earth?

    The discussion here https://www.physicsforums.com/threads/general-relativity-the-sun-revolves-around-the-earth.245334/ concludes that in general relativity there are no privileged reference frames so that, for example, it would be valid to use the Earth as a reference frame and say that...
  46. A

    B Solving Special Relativity Equation: Rail Car Thought Experiment

    I know that c is the same in all reference frames. I am trying to understand the rail car thought experiment. I've got it down pretty well except for one part - the difference between an observer moving toward a light source versus an observer moving away from a light source at the same...
  47. J

    B Is Special Relativity Universally Accepted?

    Hi pf, When I was taught about SR, I was told it was one of the most successful theories in science and has been verified over and over again. However, I have been watching some videos online and wanted some guidance. I initially came across this: <link deleted> This guy claims to have a PhD...
  48. jedishrfu

    B Young Student Wins $250K Explaining Relativity

    This young student won $250K for explaining Relativity http://www.sciencealert.com/a-high-school-student-just-won-250-000-for-explaining-relativity-using-a-pick-up-truck-and-usain-bolt
  49. bhobba

    Only Three People Understand General Relativity

    From Verifying the Theory of Relativity by Chandrasekhar Sir J.J. Thomson, as President of the Royal Society at that time, concluded the meeting with the statement', I have to confess that no one has yet succeeded in stating in clear language what the theory of Einstein's really is'. And...
  50. K

    Relative velocities in special relativity

    Homework Statement Two spaceships fly toward a space station as shown in the figure. Relative to the station, spaceship A has speed 0.8c. Relative to the station, what speed is required of spaceship B such that its pilot sees A and the station approach B at the same speed? (a) 0.40c (b) 0.50c...
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