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. Kara386

    Special relativity question with lots of frames

    Homework Statement At exactly 00:00:00 hours, a group of convicts escape from a planet in a space-ship that travels at speed ##v=\frac{4}{5}c##. After 11 min, a patrol spaceship goes after them with ##v_P=\frac{24}{25}c##. Ignore all acceleration periods. (i) The convicts immediately notice the...
  2. S

    Calculating Velocity and Time Dilation in Special Relativity

    Homework Statement A spaceship travels from Earth to the vicinity of the star that is measured by astronomers on Earth to be six light-years away. The spaceship and its occupants have a total rest mass of 32 000 kg. Assume that the spaceship travels at constant velocity. The time taken as...
  3. F

    I Is motion through space or spacetime?

    I was chatting to someone recently about the motion of objects and whether they propagate through spacetime or not. They were/are convinced that motion through spacetime is simply not possible arguing something along the lines of the following: "Objects move through space. If you depict an...
  4. Stephanus

    B How does relativity impact our everyday lives beyond GPS and nuclear energy?

    Dear PF Forum, I'd like to have some opinions here. A friend of mine says, "From Newton , we got car, airplane, rocket, etc. From Einstein, we got sci fi movies". Okayy, forget him :smile: Now, what I want to know is, what relativity has direct impact in our everyday life? There are two things...
  5. M

    Derive p^2/2m from relativistic equations

    Given the relativistic equation for energy E2 = (pc)2 + (mc2)2 I want to find the non-relativistic approximation for kinetic energy in non-relativistic terms, Knr = p2/2m I start off with subtracting the rest energy E0=mc2 from the above equation. So K = E - E0 and assume that c is very...
  6. Approximation

    I Implications of Relative Simultaneity: Carl & Shirley's Observations

    Let's say you have a planet rigged to explode only if two bowling balls hit respective sensors at the same time. An unfortunate observer, call him Carl, releases two bowling balls at the same time from his reference frame. Meanwhile, Shirley passes by at nearly the speed of light. From Shirley's...
  7. F

    I Motivation for the usage of 4-vectors in special relativity

    I recently had someone ask me why we use 4-vectors in special relativity and what is the motivation for introducing them in the first place. This is the response I gave: From Einstein's postulates( i.e. 1. the principle of relativity - the laws of physics are identical (invariant) in all...
  8. P

    Centre of mass velocity special relativity

    Why is it that the ZMF velocity is given by the total momentum of the system divided by the total energy? Many thanks :)
  9. F

    I How can photons interact if they travel at the speed of ligh

    I was listening to a podcast about the solar neutrino problem, and they discussed how we have deduced that neutrinos are not massless due to the fact that they interact with other particles (even if this interaction occurs rarely). I paraphrase: "a particle traveling at the speed of light is...
  10. Zahidur

    Does light always travel at light speed?

    I've been told contradicting ideas about this. I've been told that light doesn't travel at a constant speed everywhere (i.e. light slowing down in speed after entering a more dense medium). However, I've also read that light speed is constant everywhere (i.e. if you could travel close to the...
  11. R

    Help Solve Physics Problem: Lamp Moving at v=c/4

    Hello, I need help with this problem. In 2D we have a lamp that emits light in 360 degrees around itself while stationary. Than it starts moving at velocity of c/4 what is the degree of the cone in which the light now emits to ? I hope you understand, like the angle of the field where the light...
  12. C

    B How can we tell that "fictitious forces" are not real?

    Earlier today I was reading through this entire thread: https://www.physicsforums.com/threads/implications-of-the-statement-acceleration-is-not-relative.670653/ And I remained confused about one thing in particular. The original poster made a statement (bolded below) on page 1 that seemed...
  13. little neutrino

    Relativity and Lorentz Transformations

    Homework Statement Spaceship A of length 30m travels at 0.6c past spaceship B. Clocks in frame S' of spaceship A and S of spaceship B are synchronised within their respective frames of reference and are set to zero, so that t' = t = 0 at the instant the front of spaceship A passes the rear of...
  14. L

    I Exploring General Relativity: A Beginner's Guide

    What is general relativity all about. I don't know anything about it. I am an undergrad.
  15. little neutrino

    The Classic Pole in Barn Relativity Question

    Homework Statement A pole-vaulter holds a 5.0 m pole. A barn has doors at both ends, 3.0 m apart. The pole-vaulter on the outside of the barn begins running toward one of the open doors, holding the pole level in the direction he is running. When passing through the barn, the pole just fits...
  16. little neutrino

    Relativistic Time Dilation and Length Contraction in Overtaking Rockets

    Homework Statement Two rockets are each 1000m long in their rest frames. Rocket A, traveling at 0.800c relative to the earth, is overtaking Rocket B, which is moving in the same direction at 0.600c. (i) According to the crew on B, how long does A take to completely pass? I.e. how long is it...
  17. davidbenari

    I Understanding 4-Momentum in General Relativity

    Conserved quantities in GR deal with ##p_\mu## not ##p^\mu## and while in Minkowski spacetime its easy to see what each of the components mean (since the metric is so simple) in general relativity I think its not and its starting to confuse me. Why exactly is ##-p_0## the energy in general...
  18. Seanra

    I My lecturer says "Special relativity is absolutely wrong"

    Hi guys In an assignment I wrote for university I was penalised for claiming that FTL neutrinos would violate special relativity. Below is the relevant part of my assignment and the response from my lecturer. Could somebody please explain what he could mean by that because as far as I can...
  19. P

    B Understanding Gravity & Orbits in Light of Relativity

    I've been re-reading Hawking's Brief book (for the fourth time :-) and trying to understand a bit more. He discusses how General Relativity tell us that the planets don't orbit due to gravity, but due to the bending of space-time by mass/energy. That in fact, they are actually following a...
  20. N

    I Understanding 4-Momentum in Special Relativity

    Hello, I am studing elementary particle physics and want to ask something, just to check if I have understood properly. So, as I understand, this is true about four-momentum in special relativity: 1. The square of the sum of particles' four momenta is invariant under Lorentz transformations...
  21. F

    I Proving Orthogonality of Vector w/ Schnutz Special Relativity Tensors

    There's a question in Schnutz - A first course in special relativity Consider a Velocity Four Vector U , and the tensor P whose components are given by Pμν = ημν + UμUν . (a) Show that P is a projection operator that projects an arbitrary vector V into one orthogonal to U . That is, show that...
  22. A

    I Stanford: objects in spacetime all move at constant speed c?

    In this http://web.stanford.edu/~oas/SI/SRGR/notes/SRGRLect6_2007.pdf, it is stated: Likewise, objects in spacetime all move at constant speed c in spacetime but if you change its direction, say by moving at speed v in the x direction, then spatial speed will change and so will the speed along...
  23. A

    I Deriving Einstein's Relativity Postulates from Minkowski Spacetime

    Does the constancy of the speed of light for all observers naturally emerge from the Minkowski spacetime metric? Do Einstein's two postulates of relativity emerge from the Minkowski spacetime metric? Suppose we begin with Minkowski spacetime and the Minkoswki metric...
  24. Anjum S Khan

    Einstein's theory of relativity

    A falling apple made Newton to think about gravity. What made Einstien to think and develop his theory of relativity ? What devices/apparatus/equipment did he use for his theory of relativity ?
  25. Perturbative

    I General Relativity within the confines of a Hilbert Space

    Introduction If Quantum Mechanics is more fundamental than General Relativity as most Physicists believe, and Quantum Mechanics is described using Hilbert Spaces wouldn't finding a compatible version of General Relativity that operates within the confines of a Hilbert Space be of utmost...
  26. LarryC

    I What specific momentum should be used in the matter wave?

    According to de Broglie's hypothesis, the matter wavelength could be described by lamda=h/p. But which momentum, the classical p=mv, or the relativistic one p=mv(1-v^2/c^2)^(-1/2) should be used in this equation? Additionally, an even more confusing issue to me is regarding the other relation...
  27. Holden Kenne

    B Understanding Newton's Laws of Motion: Exploring Errors and Contradictions

    Hi, I'm a frehshman in High school writing a paper about Isaac Newton. One of my paragraphs is about the laws of motion. I came across a tidbit of information - there are instances in which Newton's laws are not correct. One example I found was that, at speeds approaching that of light, an...
  28. ibkev

    General relativity math self-study - what next?

    My goal is to develop an intuitive understanding of the math underlying general relativity and ultimately be able to take a book like Wald or Carroll and, as someone on these forums commented once, “be able to casually read it while sipping my morning coffee and listening to the news.” :) So...
  29. Israel Eydelson

    How do we simulate time flow in video games? EE/PHYS

    Hi Everyone, I am very ignorant and uneducated but I have a few questions about a difficult thought experiment. 'How does time flow in a video game/simulation or even in our imagination in relativistic terms?' Let's say a processor operating at 4.2 Ghz, 42 billion instructions per second at...
  30. ChrisVer

    I Solving General Relativity's Schwarchzild & FRW Metrics

    I had the following question how are the Schwarchzild metric that describes a spherically symmetric matter distribution (such as a star) be compatible with the FRW metric that describes the 'overall universe' that the star resides in/is part of its matter distribution? Then we say that FRW...
  31. D

    Schools A difficult choice - General Relativity or Dark Matter....

    Hi all, I've just been made offers to two different institutions - one to study General Relativity and Early Universe Cosmology, and one to study particle physics phenomenology and dark matter at PhD level, and I'm having a hard time choosing! Relativity and Cosmology is Queen Mary University...
  32. F

    A What math do I need to know to understand General Relativity

    I'm a 16 year old whose summer goal is two understand general relativity, but I'm lost on what math to have to understand it, I understand topological spaces and a topological manifold. but then it becomes more complicated math, and I know I simply don't understand because of the mathematics.
  33. Chris11235

    General Relativity and Hawking Radiation at the Event Horizon

    The question is to resolve a logical conflict. GR says as we fall into a black hole, an outside observer will see that event come to a stand still as if the falling object is hovering at the horizon. This stand still extends to infinite time. Unfortunately, I've read and hear the term...
  34. A

    B Solving Special Relativity Problems: Interval vs Lorenz Equations

    Why is it that in SR we always seem to jump to the Lorenz equations when there is a simpler way. This is the concept of an interval. The interval is defined as the square root of ( T squared minus X squared) . In Special relativity the time and distance are different for different...
  35. Alltimegreat1

    B How much will astronauts age on a journey at the speed of light?

    Let's assume we have the technology to travel in space at exactly the speed of light and embark on a journey to a star located 50 light years away. When the ship arrives at its destination, people on Earth will have aged 50 years. How much will the astronauts on board have aged?
  36. E

    I Special theory of relativity argument

    Some layman people are against special relativity and stubbornly persist on their theories. One man put the model that the speed of photon is c + v, where v is speed of source. In such case Michelson interferometer is not a good anti-argument. What is, in your opinion about the most simple and...
  37. W

    I Relativity of Simultaneity: An Overview

    I recently started studying Special Relativity an my book discusses the following: Say I have synchronized two separated clocks in a reference frame S, if then an observer in another reference frame S' for whom the clocks are moving sees the clocks he would say those clocks are out of...
  38. V

    How fast must the spacecraft travel? Special Relativity

    Homework Statement [/B] 1. Homework Statement A spacecraft is prepared to visit α-Centauri, which is at the distance 4.37 light years from the Sun. Provisions for the crew are prepared for the duration of 16 years. How fast must the spacecraft travel for this provision to be enough? Answer...
  39. pixel

    Special Relativity Comic Strip: Today's Edition

  40. R

    I Experiment to test the basic assumption of SR

    Has there ever been an experiment to support the principle assumption of relativity? For example an experiment where an atomic clock is sent on a probe to , let's say, Pluto. We on Earth get transmissions of the time on that ship and, after taking account of the light travel time delay, we...
  41. R

    I Relativity: Coming to Terms 1: Starting Assumptions

    Coming to terms with the theory of relativity is a long and difficult process that requires shedding all the popular misconceptions and hype surrounding the subject matter. Science doesn't sell but throw in carefully worded claims of time travel, matter materialization, shape shifting and...
  42. henrco

    Relativity - relative velocity

    Hi, Could i please get advice on this problem and confirm that my approach and solution are correct? Thanks in advance. 1. Homework Statement An enemy space rocket traveling at speed 0.8c towards the planet Zog fires a missile in the direction parallel to its own path. Relative to the rocket...
  43. R

    Special Relativity problem -- An electron travels at 0.422c....

    Homework Statement An electron travels at 0.422c. Calculate the following. (a) the relativistic momentum kg · m/s (b) the relativistic kinetic energy J (c) the rest mass energy...
  44. T

    I A special relativity explanation for the Lorentz Force?

    I am still a bit puzzled by this video: Does this mean that force exerted on an electrically charged particle facilitated by moving through magnetic field B is due to special relativity, and not virtual photons acting as magnetic force carriers? Thanks
  45. Jefffff

    Special Relativity and Space-Time Diagrams

    Homework Statement [/B] S' is moving at 0.5c relative to S. Two events, stationary with respect to S, occur at a distance of 4 light years from the origin at time 3 years and 6.5 years. Estimate the time between the events as measured by an observer in S'. Check your solution with the time...
  46. R

    A How Does Arm Length Affect Doppler Shift Measurement?

    full screen (high res) http://images.tinypic.pl/i/00765/8g7j8biccn4d.jpg Em Wave = LIGHT Em Wave = RadioEquator Source------L---------SensorHow big L we need use to register Doppler ? I wan to measure 360/24h speed Big Question ?! single arm we have Doppler (RED ) [ more long arm --->...
  47. M

    I Relativity Theory: General & Special Contexts

    I want to know in which context should i take relativity with both general and special .
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