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. A

    I Casimir effect and vacuum energy and a bit of relativity....

    Hi all, Silly question perhaps: I had understood that the energy density of the vacuum is constant throughout spacetime. But, with the Casimir effect, for example, the geometry of (real) matter (i.e., parallel plates), changes the vacuum energy density in between the plates - is this correct...
  2. B

    Insights Orbital Precession in the Schwarzschild and Kerr Metrics - Comments

    Bill_K submitted a new PF Insights post Orbital Precession in the Schwarzschild and Kerr Metrics Continue reading the Original PF Insights Post.
  3. Jlister

    Absolute Relativity (who needs virtual photons anyhow?)

    I've seen, in the description of a black hole's photon sphere, a mention of a "light-like ds." I was happy to see it because I've been thinking about a photon's physical environment; at the speed of light, it would experience no radial dimension & no time dimension. Might it make more sense to...
  4. A

    B Relativity Problem: Questions on Time, Movement & Reality

    hello . please pardon my ignorance when reading my simple question . -so if a magical spaceship flies from Earth at 80% the speed of light . time should go slower for the person inside the spaceship due to time dilation . so time goes slower for him . the thing is if we change the way we look...
  5. S

    B Help Understanding special relativity

    I'm trying to understand something about relativity that doesn't seem to add up. I will extend Einstein's carriage example to incorporate 4 clocks. At the beginning all these clocks are running at the same time when they are together. I put two of the clocks far away from each other (100 km...
  6. B

    Insights Precession in Special and General Relativity - Comments

    Bill_K submitted a new PF Insights post https://www.physicsforums.com/insights/precession-special-general-relativity/ https://www.physicsforums.com/insights/precession-special-general-relativity/
  7. B

    Insights Tetrad Fields and Spacetime - Comments

    Bill_K submitted a new PF Insights post Tetrad Fields and Spacetime Continue reading the Original PF Insights Post.
  8. Coco Hwang

    What do t and t0 represent in the relativity train problem equation?

    1. Two long trains pass each other head on with a relative speed of 0.97c. Bob, the driver of the first train notices that it takes 5.8 secs to pass the entire second train. How long do people on the second train say it takes for Bob to go by their train? 2. t = t0/(1-v2/c2)1/23. I understand...
  9. K

    A CPT symmetry and antimatter gravity in general relativity

    CPT symmetry and antimatter gravity in general relativity M. Villata Published 28 March 2011 • Europhysics Letters Association EPL (Europhysics Letters), Volume 94, Number 2 Abstract The gravitational behavior of antimatter is still unknown. While we may be confident that antimatter is...
  10. P

    I Can one determine the velocity of a photon in the fourth dimension using limits?

    Can one shed light on the velocity of the photon through the fourth dimension x4 using limits? To begin with, please study the mathematics from Brian Greene’s book An Elegant Universe. The upshot is that the faster an object moves through space, the slower it moves through the fourth...
  11. J

    Relativity General Relativity by Hobson, Lasenby and Efstathiou

    Hello, Is the book General Relativity by Hobson, Lasenby and Efstathiou good in the sense that is intuitive on the mathematics AND physics that it presents? Please vote and if you could also comment your opinion it would be much appreciated. Thanks!
  12. M

    A Need help with satellite navigation? Any textbook recommendations?

    Hey guys, looking to get some advice on satellite navigation. Can anyone recommend a nice textbook covering Newton, Kepler, Lagrange etc and their contributions to orbital motion. Also any textbooks on MATLAB or Maple examples of orbits - relativistic or Newtonian it doesn't matter. Not put off...
  13. Stephanus

    B Hubble flow and velocity addition

    Dear PF Forum, I've been wondering about this thing. The universe's radius is 46 gly. But a galaxy in 13.8 gly from us cannot be seen because it travels faster than the speed of light. So is there a galaxy in, say, 30 billions light years away from us? But before that, I'd like to know about...
  14. J

    I Understanding Relativistically Spinning Disk/Ring: Lorentz Boosts

    I'm trying to understand the relativistically spinning disk within the framework of SR (if that is even possible). I thought to first simplify the problem by considering a spinning ring/annulus, but I don't know if my analysis is correct. I imagined a spinning ring of radius R, spinning at an...
  15. Sebastiaan

    I Effect of Special Relativity on Spacecraft Thrust & Isp

    Let's say I have a Space craft traveling at 1% of speed of light and at rest speed it has an thrust of 600 kN with Specific impulse at 1.000.000s. We know we can calculate the relatavistic mass of the vessel with Einstein Special Relativity : γ = 1 / (1 - v2 / c2 )0.5 = 1.00005. Now the...
  16. P

    Ampere's Law: Explained and the Role of Magnetic and Electric Fields

    I know the ampere's law but I want an explanation on why it exist in that way, consider a wire where flows current, i know that when an observer is in relative motion to the current there is a magnetic field but why in this geometric configuration and not in another? What about the electric...
  17. A

    I What is the velocity of the photon through the fourth dimension x4?

    What is the velocity of the photon through the fourth dimension x4? Photons are real, physical entities. The fourth dimension is a real, physical entity. Therefore, photons must have a relationship with the fourth dimension. They must have some velocity relative to it. What is the velocity...
  18. robphy

    Insights Relativity on Rotated Graph Paper - Comments

    robphy submitted a new PF Insights post Relativity on Rotated Graph Paper Continue reading the Original PF Insights Post.
  19. MelissaBlaze

    How to Calculate Peak Speed and Time Dilation for a Relativistic Spacecraft?

    Hi, If I want my spacecraft to accelerate at a rate that produces 1 G for a set distance (half way to another solar system) then decelerate at the same rate for the second half of the trip, what formula would I use? I would like to calculate the peak speed of the ship and the time dilation...
  20. N

    B General Questions about Special Relativity

    There are a few fundamental questions I wanted to ask about related to special relativity: 1) Firstly, is there an intuitive explanation for length contraction and why lengths are relative? For example, the fact motion is relative is intuitive. E.g. someone sitting inside a train moving 60...
  21. MelissaBlaze

    Question about relativity and distance

    If someone left the Earth on a spacecraft traveling .99% the speed of light for a distance of one light year, and time slowed on the spacecraft relative to the earth, approximately how much time would the person on the spacecraft experience when arriving at the destination? (I'm leaving out...
  22. J

    Relativity Differential Forms and the Geometry of General Relativity

    Hello, I would like to know if anybody here has used the book "Differential Forms and the Geometry of General Relativity" by Tevian Dray and how they found it. Thanks!
  23. scientepid

    I How can you visualize the Higgs field, or can you?

    Just when I thought I had finally wrapped my brain around relativity, Quantum theory took off. Then the Higgs Boson was discovered. How does the Higgs field under-pin relativity, namely space-time? How is the Higgs field distributed? Does it have curvature like space time, or is omnipresent...
  24. V

    Relativity of motion and astronomy

    Greetings, One of the basic postulates of relativity is that the laws of physics hold equally well in all frames of references. This got me wondering about the geocentric model of solar system which necessarily gives rise to motions of peculiar kinds. For example, the epicycles, which are...
  25. TheAnt

    B Study Special Relativity: 6th Formers, Books & Websites

    Hello, I have often heard special relativity was not too difficult to study. I was wondering as a 6th former what knowledge is needed to do so and if it was even possible at my level. If yes does anyone suggest any good books or websites? I am searching for a more in depth mathematical...
  26. A

    B Einstein's relativity of simultaneity & quantum paradox.

    Einstein's relativity of simultaneity & quantum measurement paradox. Suppose a rocket traveling close to the velocity of light which emits a single photon from its midpoint at point A, illustrated below. The rocket is equipped with a single detector drawn in green at the front of the rocket...
  27. Deepak K Kapur

    Nobel Prize in Physics for Relativity?

    Why didn't Einstein get Nobel Prize for ' Relativity' Has anyone else been awarded Nobel Prize for 'Relativity'?
  28. tomasbede

    I Could I travel to andromeda in my lifetime?

    Hi, this is my first post on here. Let's say I departure from Earth in a spaceship and reach a speed very close to c, as I go faster time would run slower and I would feel as if I was going faster and faster with no speed limit, even though I'm not going faster than c. Because my time is...
  29. Daleri Mc Rileda

    B How is the Age of the Universe Determined?

    How can the age of the universe be determined when there is no absolute measure of time or distance in the universe. Every other place in the universe has another rate of the passing of time. Since time expands with the expansion of space, trying to use expanded time to measure time is circular...
  30. C

    A problem regarding Galilean relativity

    Homework Statement Two bodies are moving on the same line. When they move away from each other the distance between them changes for 16m in a time interval of 3 s (Δd1 = 16 m ; Δt1 = 3 s). When they move towards each other the distance between them changes for 3 m in a time interval of 3 s (Δd2...
  31. t_r_theta_phi

    I Metric for Infinite Rod in Gen. Relativity

    What is the metric for the spacetime around an infinitely thin, infinitely long, uniform rod? Could it be written in the form ds2 = A(r)dt2 + B(r)dr2 + C(r)dh2 + r2dθ2 where h is the coordinate along the rod and r is the radial coordinate, or would it be something more complicated?
  32. Daleri Mc Rileda

    B How Far Is It To The Center Of The Milky Way/Tootsie Pop?

    With length contraction being ever increased while approaching the mass of a black hole singularity, how does one measure the distance to the virtual infinite contracting frames of reference of time and space /distance of the singularity? Related question : How can one measure the size of a...
  33. Basicsofphysics

    A A Unified Theory of Relativity and Particle Physics

    Is there a peer reviewed paper that proposes a way to connect the relativity of time dilation, length contraction and the relative effects of gravitation per acceleration with respect to mass with the standard model of particle physics? An established theory which also takes into account both...
  34. Meddusa

    I Action in Relativity: Non-Relativistic vs Relativistic Particles

    Hi, I found that action in relativity is written in two ways: 1) S = ∫ ds 2) S= -mc ∫ ds is (1) for non-relativistic particles and (2) for the relativistic particle ? why there is a minus sign in front of it? I read that writing (2) is up to a guess (-mc term), is it very interesting...
  35. J

    Relativity On Sean Carroll's General Relativity textbook

    I have heard/read that Carroll's Spacetime and Geometry: An Introduction to General Relativity is one of the best books on General Relativity but its price is insanely high! Does anybody know any place which sells it at a lower price than, say, Amazon? If not, what are other books that cover...
  36. P

    Special Relativity- Photon/Mirror Collision

    Homework Statement A photon of frequency ν is reflected without change of frequency from a mirror, with an angle of incidence θ. Calculate the momentum transferred to the mirror. Homework Equations E= hν Conservation of four-momentum The Attempt at a Solution If the mirror is in the x-y...
  37. U

    A simple Proper Time Calculation

    Homework Statement Suppose a spaceship starts from rest from a space station floating in deep space and accelerates at a rate of |a| relative to the space station for 1.0 Ms. It then decelerates for the same amount of time at the same constant rate |a| to arrive at rest at another space station...
  38. mertcan

    A Stress energy tensor general relativity

    Hi, I would like say that in this link ( ) and starting from 56.28 Suskind tries to find the energy tensor equation using \phi, afterwards he finds a equation similar to wave equation in terms of \phi. My question is: For what does \phi stand ? I could not capture the meaning of \phi. Could...
  39. Tony Weston

    A Exploring Bell's Theorem with Relativity (No Superluminal Comm.)

    Hi... New to this forum. Be kind! I did not study physics at university, and consider myself an armchair physicist. I am a computer programmer by trade. I first came across Bells inequalities a few years ago, while working with a fello programmer who did have a PHD in physics. Its pretty...
  40. B

    Programs Should I Transfer Math PhD Programs for Better Research Opportunities?

    Math grad student here. I'm currently finishing up my first year in a PhD program, and I based on my experiences so far I can definitely say graduate school is something I want to continue. I spent my first year taking courses in preparation for prelims rather than doing research, but I did well...
  41. D

    I Special relativity and magnetism

    I've read in various places that magnetism can be explained in terms of the effects of special relativity. However, all of the explanations of this only mentioned the case of current flowing in a wire. Can special relativity explain the magnetism of free flowing electrons and other moving...
  42. A

    I Galilean Relativity: Can Experiments Tell Motion Relative to Other Frames?

    In Galilean Relativity, laws of mechanics are invariant across frames. In all the frames they are the same. So, in Dynamics and Relativity by W.D.McComb, it is written that this implies you cannot perform any experiment in an inertial frame that can tell whether an inertial frame is moving or...
  43. A

    A Metric with Harmonic Coefficient and General Relativity

    Goodmorning everyone, is there any implies to use in general relativity a metric whose coefficients are harmonic functions? For example in (1+1)-dimensions, is there any implies for using a metric ds2=E(du2+dv2) with E a harmonic function? In (1+1)-dimensions is well-know that the Einstein...
  44. A

    I Understanding General Relativity

    I am reading general and special relativity from a book and I am stuck with these lines please can someone provide its detailed explanation "Einstein demanded that the special principle of relativity should be valid also for Maxwell’s electromagnetic theory. This was obtained by replacing the...
  45. G

    I Parametrizing a path through spacetime

    It is customary, when discussing a particle's motion through spacetime, to talk about its path x^{\mu}(\lambda) , where x^{\mu} are the the spacetime coordinates of the particle in some frame, and \lambda is some parameter. I have a doubt regarding this parameter. Everywhere I've looked...
  46. E

    I Meaning of the equivalence principle in General relativity

    It is known that equivalence principle is good for understaning of general relativity (GR). This means comparison of the elevator and falling in homogenic gravitational field. But, I here somewhere something like "in the higher lever of GR, let us forget on principle of equivalence". How it is...
  47. Sachin_Vaidya

    Exploring Why Time Perception Changes: Is Relativity at Play?

    Why time perception changes continuously...? Could relativity be playing some role?
  48. nomadreid

    I Reconciling Simultaneity & Relativity: Navigating the Spacetime Maze

    Oops, forgot to finish the title, but don't see how I can edit it. Sorry. How does one reconcile the idea that, according to relativity, one cannot talk of simultaneity, yet one can talk of a slice of spacetime to be space where all the points are at the same time? That is my main question...
  49. PabloAMC

    A Problems between Quantum Mechanics and General Relativity

    I have read several times that general relativity has some problems with quantum mechanics and they are not compatible. However, special relativity can be introduced in quantum mechanics mainly by Dirac equations (so I am pretty sure that the problem of passing from a frame where the parameter...
  50. S

    Relativity What Are Good Beginner Books on General Relativity with Mathematics?

    HI, can anyone suggest a good book on general relativity with fairly good mathematics for beginner for self study?
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