What is Special relativity: Definition and 1000 Discussions

In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates:
The laws of physics are invariant (that is, identical) in all inertial frames of reference (that is, frames of reference with no acceleration).
The speed of light in vacuum is the same for all observers, regardless of the motion of the light source or observer.

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  1. 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...
  2. 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...
  3. 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]
  4. 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...
  5. Sagittarius A-Star

    I Force on Scale in Rocket Scenario: Calculation & Discussion

    [Moderator's note: Spin-off posts from previous thread have been included in this new thread. Also, the OP's re-post of the scenario for discussion has been moved to this top post for clarity.] Yes. Physically, scales measure a force (and indirectly the energy) in their frame. Consider the...
  6. 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...
  7. 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 …
  8. 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...
  9. Ashshahril

    How to Visualize a 4 vector (e.g. a velocity)?

    Velocity is a 4-vector which has 3 space dimensions and 1 time dimension. It's space parts will be directed at the 3 space directions and time parts will be directed at the time dimension (But it is inverse. So, will it point at the inverse direction?). How can someone Visualize it? How they...
  10. Ashshahril

    Momenergy (momentum-energy)

    Why momenergy has magnitude equal to the mass? > The mom-energy of a particle is a 4-vector: Its magnitude is proportional to its mass, it points in the direction of the particle's spacetime displacement, and it is reckoned using the proper time for that displacement. How are these properties...
  11. H

    I How do forces change with speed?

    Hey dear physics community :) I ask myself what exactly happens to forces between materia when the materia hits near lightspeed. I know, that for an objective bystander watching let's say elon in his rocket with 99,99999% the speed of light, that the time goes slower, the mass of the rocket...
  12. 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...
  13. F

    De-excitation of a moving atom with photon emission

    The information I have are the following: ##p^\mu=(E, p, 0, 0)## ##p'^\mu=(E', p'\cos\beta, -p'\sin\beta,0)## ##k^\mu=\tilde{E}(1, \cos\alpha, \sin\alpha, 0)## Where: ##E=\sqrt{M^2+p^2}## ##E'=\sqrt{m^2+p'^2}## Using the conservation of the four-momentum ##p^\mu=p'^\mu+k^\mu##...
  14. J

    B Sine-Gordon model for better understanding of special relativity?

    Sine-Gordon model is looking trivial 1D model: just $$\phi_{tt} = \phi_{xx}-\sin(\phi)$$ which has physical realization as lattice of coupled pendulums, e.g. nice video: Despite looking so trivial, it e.g.: has analogues of massive particles ("kinks") corresponding to complete rotation -...
  15. Abhishek11235

    I Calculating Time Elapsed in Rocket Collision

    Consider an observer on Earth (Neglect any effect of gravity). Call him A. Let 2 rockets be moving in opposite direction along x-axis (x-axis coincides with the x-axis of A) with uniform velocities. Call them B and C. At t=0, in A's frame, the rockets are separated by length ##l## . Let ##V_a##...
  16. F

    Magnetic field generated by a current in a wire - special relativity

    First I wrote in ##S'##, by using Gauss theorem $$ \int_{\Sigma} \underline E' \cdot \hat n d\Sigma = \frac Q {\varepsilon_0} \rightarrow E'(r)2\pi rH=\frac{\lambda'H}{\varepsilon_0} $$ $$ \underline E'(\underline r)=\frac{\lambda'}{2\pi\varepsilon_0r}\hat r $$ Its components are...
  17. U

    A question on proper time in special relativity

    Homework Statement:: This isn't a homework but more of a conceptual question. Relevant Equations:: Proper time, ##\tau## Simply put, the proper time between two events as observed in an unprimed frame is calculated along the timelike worldline between the two events. This implies that the...
  18. U

    B Derive Special Relativity: Alternative Paths

    The only way I know of to derive special relativity is to start with the two postulates, derive the Lorentz transformations, and rewrite the laws of physics consistent with those transformations. Are there alternative ways to derive special relativity? Thank you.
  19. J

    Exploring a New Theory of Gravity: Filling in the Gaps of Newtonian Physics

    My name is Dilip (James) I am fascinated by physics and have written three books on the subject, which indicates my level of interest.. My latest book “The Electromagnetic Universe: A New Physics” is available on Amazon and describes in detail: a new theory on the propagation of light, a new...
  20. Frabjous

    Relativity Is Synge's Special Relativity Still Relevant Today?

    I stumbled across a cheap copy of Synge’s Special Relativity. I know that it was an important book, but there is an undercurrent in the comments that it is now dated. Is it still worth spending time with?
  21. Ashshahril

    Why do particles in a falling coach get closer together?

    As widely separated particles within a large enclosed space are differently affected by the nonuniform gravitational field of Earth, to use the Newtonian way of speaking, two particles released side by side are both attracted toward the center of Earth, so they move closer together as measured...
  22. C

    Calculation Involving Projection Tensor in Minkowski Spacetime

    In Minkowski spacetime, calculate ##P^{\gamma}_{\alpha}U^{\beta}\partial_{\beta}U^{\alpha}##. I had calculated previously that ##P^{\gamma}_{\alpha}=\delta^{\gamma}_{\alpha}+U_{\alpha}U^{\gamma}## When I subsitute it back into the expression...
  23. AdvaitDhingra

    Best (or at least good) GR and SR textbook for a 15-year-old highschool student

    Hi, I'm a 15-year-old high school student and I was wondering what textbook you guys recommend for Special- and General Relativity. I'm familiar with the concept of the Metric Tensor and Christoffel Symbols, but I wanted a good textbook where I can really learn derive it all and gain a deeper...
  24. Kaguro

    Deriving length contraction using spacetime

    Deriving time dilation was easy: Imagine two events in frame O' at the same location. ##ds^2 = -c^2 dt'^2## The same viewed in O frame is: ##ds^2 = dx^2+dy^2 + dz^2 - c^2 dt^2## ##\Rightarrow dx^2+dy^2 + dz^2 - c^2 dt^2 = -c^2 dt'^2## ##\Rightarrow (\frac{dx}{dt})^2+(\frac{dy}{dt})^2+...
  25. The Baron

    I A problem with how the speed of light connects to special relativity

    [Mentors' note: This question was orginally assumed a train traveling at the speed of light. Further discussion shows that this assumption is not required for the question; any relativistic velocity is enought to raise the question. The post has been edited, changes in boldface, to reflect...
  26. greg_rack

    Special relativity (length contraction, velocity composition) problem

    First, I calculated the velocity of the object with regards to earth, which is: $$v'=\frac{V-v}{1-\frac{vV}{c^2}}=0.34c$$ Now, the problem is solved if I consider the length ##l=45m##(so by calculating the ##\gamma## factor with ##v'##) to be the proper one... but since it's measured by the...
  27. Z

    I LIGO & Special Relativity: Is Spacetime Distortion Real?

    Hello Is the distortion of spacetime by gravitational waves directly related to the contraction of objects in motion predicted by special relativity ? In other words Besides the existence of the gravitational waves , did the LIGO experiment definitely confirm the contraction of the bodies in...
  28. U

    Trying to understand electric and magnetic fields as 4-vectors

    I was trying to show that the field transformation equations do hold when considering electric and magnetic fields as 4-vectors. To start off, I obtained the temporal and spatial components of ##E^{\alpha}## and ##B^{\alpha}##. The expressions are obtained from the following equations...
  29. MacWylie

    B Why Aren’t ALL SR Effects Cumulative?

    Why aren’t all the SR effects cumulative like Time Dilation? Why should the Space dimensional effect become null when v—>0 while the Time dimension does not revert back to the 2 frames being in sync if Space and Time are treated on an equal footing. Clearly, Space and Time are not treated on an...
  30. Q1111

    I Postulates of Special Relativity: Speed of Light in Inertial Frames

    Would the second postulate (The speed of light in free space has the same value c in all inertial reference frames.)be also true if it was in some medium instead of in free space? I know the value won't be c anymore but I want to know whether the speed of light in that medium would be the same...
  31. cianfa72

    I Reference frame vs coordinate chart

    Hello, here on PF I've seen many threads about the concepts of 'reference frame' and 'coordinate system'. In the context of SR my 'envision' about the concept of 'frame of reference' is basically the 'rods & clocks latticework' as introduced in the book Spacetime physics (Taylor, Wheeler)...
  32. Z

    B Does special relativity entail matter annihilation?

    hello Does special relativity entail matter annihilation ?
  33. L

    I Special Relativity: 3 Objects, Momentum & Time Dilation

    Let us have an observer at rest at (0,0) in a coordinate plane. Another at (1,0) moving away from the first observer at 5 m/s in the x direction as seen by observer 1 and another object at (1,1) moving away from observer 2 at 0.5C in the y direction as seen by observer 2. Should observer 1 see...
  34. M

    Why Is Time Dilation Considered a Real Phenomenon?

    Good day. I have read many books, forums, and articles, and watched many youtube videos, all in an effort to understand special relativity and time dilation. Or rather, more precisely, trying to find a valid explanation that does not result in contradictions and paradoxes, and if it does, one...
  35. S

    B Do photons travel instantaneously?

    Special Relativity tells us - the faster things travel their time is slower relative to a stationary observer. Do massless particles, like photons traveling at the speed of light, experience zero time and in their frame of reference travel "instantaneously".
  36. U

    Help with Tensors: Using Einstein Summation Convention

    Assuming Einstein summation convention, suppose $$R^2=\eta_{\mu\nu}x^{\mu}x^{\nu}$$ I was able to show that $$\partial_{\mu}R=\frac{\eta_{\mu\nu} x^{\nu}}{R}$$ by explicitly doing the covariant component of the four-gradient and using the kronecker tensor. However, how do I use the equation...
  37. D

    I Does string theory falsify the theory of special relativity?

    Hello, I am spending time learning more about the theory of special relativity and string theory. One of the things that I have read about string theory is that it includes other dimensions in relation to space (space has 9 dimensions in string theory, supposedly). However, from what I...
  38. Athenian

    [Special Relativity] - Finding the Proper Length at Rest in Frame S'

    Currently, the only part of the textbook question that is completely throwing me off is "an angle of ##l=\sqrt{2}c##". If I am not mistaken, how am I suppose to interpret that as an angle and calculate for the answers of (a) and (b) accordingly? As for my attempted solution process of this...
  39. R

    Doppler effect in Special relativity -- Solution verification

    I am assuming that B is a stationary observer here. For the first part of the trip, using the formula, rocket A is approaching B at velocity $$v_A$$ =0.6 c. The length that A travels is $$L_A = v_A t_1 $$ where $$t_1 = $$ 6 hrs. For the first part of the trip, B is receiving signals at a rate...
  40. agnimusayoti

    What should I do to understand special relativity?

    Homework Statement:: There are plenty of special relativity problems on Time dilation, length constraction, and Lorentz Transformation. But, I can't solve anyone, even the simplest problem. Relevant Equations:: Time dilation, length constraction, Lorentz Transformation What should I do to...
  41. R

    Intro special relativity problem regarding time dilation

    This example is worked out in the book, and at the beginning, they make the assumption that the muon is traveling at c, and then find the change in time from the Earth reference frame using delta t=100km/c. Then delta t is plugged into the time dilation equation on the left side and we solve for...
  42. potatoleg

    Calculate time passed on a ship clock seen by Earth observers

    In this case, γ = 1/√(1-v^2/c^2) = √(1-0.6^2) = 0.8 However, I'm not sure if time observed by Earth is proper time or moving time. The definition of proper time in my textbook states that it is 'the time measured in a frame of reference where the events occur at the same points in space. I'm...
  43. bq1892

    Describe the length of an electron's journey in its own frame of reference

    Lv = Lo / γ 1/γ =√(1-v^2/c^2) = √(1-0.8^2) = 0.6 Therefore Lv = Lo x 0.6 = 150 x 0.6 = 90m Therefore electron travels 90m in its own frame of reference (answer key solution) However, shouldn't the electron be assigned rest length, Lo, as its frame of reference is at rest with itself instead...
  44. G

    Possible to publish a paper that contradicts Einstein's special relativity?

    Summary:: Is it possible to publish a paper contradicting Einstein's special relativity in any high-ranked Journal? Which Journal is the best, i.e. open minded, for such a paper? I plan to publish a new theory of electromagnetics in a Journal. The findings are purely through strict...
  45. Azrion

    I Exploring the Concept of Time Traveling

    I am curious about time traveling. According to Einstein's special relativity, the easiest way to travel into the future is by approaching the speed of light or faster. Now assume this setup: Sunlight travels to Earth at c (speed of light). We have the technology to reach speeds faster than...
  46. T

    B On the four dimensions of special relativity

    Hello. Questions: How can special relativity describe four dimensions when we only see three spatial dimensions and we experience time?Why time is the fourth dimension and not another physical quantity? Is this only a generalisation that was needed to describe more physical phenomena and the...
  47. J

    I What is the four-momentum of a photon?

    I'm trying to understand the Energy-Momentum relativistic relationships for a light particles. It is commonly said that the Energy of a photon depends on the observer by the relationship ## E = - \mathbf{p} \cdot \mathbf{u}## where p is the 4-momentum of the source emitting light particles and u...
  48. T

    Time dilatation between straight and curved lines in Minkowsi space

    Summary:: Special relativity - 2 astronauts syncronize their clocks and moves in different paths at different velocities, which clocks is left behind? and why? Hi everyone, i have the following problem and I'm not understanding if my strategy to solve it is correct: Two astronauts synchronize...
  49. J

    I Movement of a guitar string at relativistic speeds

    Hey! I'm and undergrad in the third year of my applied physics program. I'm taking a course in Special Relativity, and due to Corona the exam has been replaced by a pretty free project where we delve deeply into a topic related to the course. I'm interested in music, so my professor suggested I...
  50. 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...
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