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. dylanreynolds1

    I Thought Experiment Contradiction: Find the Answer

    Hello all, I have a question that's been bothering me the last few days and wasn't sure where to turn. Recall the original Special Relativity thought experiment: A spaceship travels at constant velocity v, moving in the positive x direction. An observer on the spaceship emits a photon directly...
  2. ibkev

    Relativity Morin's "Special Relativity: For the Enthusiastic Beginner"

    I just came across this recently released title by David Morin, "Special Relativity: For the Enthusiastic Beginner". If its anything like Morin's past books, it'll be excellent for self studiers. https://www.amazon.com/dp/1542323517/?tag=pfamazon01-20
  3. L

    A How these notions relate to the usual SR approach?

    In the context of General Relativity spacetime is a four-dimensional Lorentzian manifold M with metric tensor g, its Levi-Civita connection \nabla and a time orientation vector field T \in \Gamma(TM). In this context I've seem the following three definitions: A coordinate system is a chart...
  4. MetinErsin

    B Relativity Scenario Help: A Seeing B's Line Shorter or Longer?

    Let's assume there are two observers. Observer B is at rest and observer A is in a spaceship which has transparent walls. A draws line of 1 meter. When A arrives near B, B draws a line of 1 meter. And sees that A's 1 meter is shorter than own 1 meter. before forget i should say that meter is...
  5. J

    B Math Help for Advanced Simulation of Twin Paradox

    In my infamous simulation of the twin paradox, i assume a near instantaneous acceleration, with the results being almost identical with the "real" thing sub some extremely small values which you could not see with the naked eye anyway. But i might want to take the simulation a step further, to...
  6. I

    I Velocity Addition & Relativity of Simultaneity

    I have some issues understanding the following thought experiment: Suppose you are standing still, and two balls are moving towards you from opposite direction. From your own reference frame, Ball A is ##10^5## m away from you, moving towards you from the left with speed ##0.8c##, and Ball B is...
  7. N

    I Relativistic charged particle in a constant uniform electric field

    I'm doing some special relativity exercises. I have to find $$x(t), v(t)$$ of a charged particle left at rest in $t=0$ in an external constant uniform electric field $$\vec{E}=E_{0} \hat{i}$$, then with that velocity I should find the Liénard–Wiechert radiated power. I will show you what I did...
  8. Dimani4

    B Special Relativity: What Time is it?

    This question based on the site located here: http://galileo.phys.virginia.edu/classes/252/srelwhat.html The question is: If the object (spaceship) moving close to the velocity of light and I'm as a static object. The time goes slower in the object moving with constant velocity (inertial frame)...
  9. A

    I Consequences of Rotating Objects C on Coil A Frequency

    As per the above figure coil A is situated inside a magnetic field caused by small accelerating objects B which in turn are causing the larmor frequency in coil Object C or train of objects C have quadruple movement and are rotating around the coil A giving out gravitational waves. Objects C...
  10. V

    Understanding the Center of Mass Energy in Fixed Target vs. Collider Experiments

    Homework Statement While not explicitly a homework question, I am having some trouble with deriving expressions for the center of mass energy in a fixed target experiment versus a collider experiment. The question is effectively, "Derive an expression for the center of mass energy in a fixed...
  11. S

    B Mass-Energy Equivalence: Does E=mc2 Apply in Systems at Rest?

    Let's assume that a system has zero total momentum. The following relationship between mass and energy should apply: E=mc^2. If a system is overall at rest, does that mean that any internal changes to that system, assuming they leave the system with non-negative mass, will not be able to...
  12. N

    Partial Differential Equation in Special Relativity

    Homework Statement (a) Light waves satisfy the wave equation ##u_{tt}-c^2u_{xx}## where ##c## is the speed of light. Consider change of coordinates $$x'=x-Vt$$ $$t'=t$$ where V is a constant. Use the chain rule to show that ##u_x=u_{x'}## and ##u_{tt}=-Vu_{x'}+u_{t'}## Find ##u_{xx},u_{tt},##...
  13. ElPimiento

    Puzzled by an equation for relativistic time difference....

    Homework Statement Suppose that A', B', and C' are at rest in frame S', which moves with respect to S at speed v in the +x direction. Let B' be located exactly midway between A' and C'. At t' = 0, a light flash occurs at B' and expands outward as a spherical wave. (A', B', and C' are all on...
  14. T

    Special relativity application

    Homework Statement The rockets of the Goths and the Huns are each 1000 m long in their respective rest frame. The rockets pass each other, virtually touching, at relative speed of 0.8 c. The Huns have a laser cannon at the rear of their rocket that shoots a deadly laser beam at right angles to...
  15. sander69

    Test of Special Relativity Confirmed - ScienceMag.org

    On the following link it is mentioned that this experiment tests special relativity. Can someone please explain how it does that? http://www.sciencemag.org/news/2016/12/deep-probe-antimatter-puts-einstein-s-special-relativity-test
  16. Andy_K

    What are the Best Books for Learning Special and General Relativity?

    Dear All, I am trying to find some good books that provide a comprehensive one-stop education for special and general relativity, with concise coverage of key fundamentals of the maths involved. It is intended for self study, and I do not have strong fundamentals on advanced physics / maths...
  17. T

    I Change of special relativity formulas?

    We know that while deriving the special theory of relativity formulas like Lorentz Transformation, Length contraction, Time dilation etc, we assume that there is an observer at each point of the space in a certain frame and all these observers contain synchronized clocks and scales. But what if...
  18. A

    I The space-time interval definition in special relativity

    Hello, please note that the following is only about special relativity, not general. Of course, if there are any things to point out that fall in general relativity, feel free to do so, but I don't know GR, so I won't understand arguments based in GR. I also am not great with a geometry-based...
  19. C

    Special Relativity: Time Dilation Prob

    It has been 2.5years since I last did any special relativity so am rather rusty on it, I have a simple time dilation problem and its making my head hurt which way around it should be. Any help much appreciated! 1. Homework Statement Bob leaves Sarah on Earth and travels in a spaceship at 0.8c...
  20. B

    I Special relativity and sameness

    It seems if the clocks show the same time, the rod is at rest in our frame. If clocks are out of sync, rod is in motion. The closer velocity is to that of light, the more clocks out of sync, Another question - are all these rods the SAME rod? For example you ask me - what is velocity of the rod...
  21. MiLara

    I Special Relativity's effect on density

    Special relativity states that according to an observer at rest, a measuring stick on a moving platform will appear shorter. Would this observer still see the measuring stick as comprising of the same amount of atoms as the observer who is at rest with respect to the measuring stick? If this is...
  22. N

    Special Relativity (time/length)

    Homework Statement Spacecraft moving at 0.6c, length 180m with someone sitting at the back. He throws a cage with a mouse towards the front also at a speed of 0.6c. A stationary observer watches. Homework Equations What is length of spacecraft from observers POV? Length according to guy...
  23. F

    I D'Alembert equation and Galilean transformation

    The D'Alembert equation for the mechanical waves was written in 1750. It is not invariant under a Galilean transformation. Why nobody was shocked about this at the time? Why we had to wait more than a hundred years (Maxwell's equations) to discover that Galilean transformations are wrong...
  24. L

    I Electromagnetic Force in Special Relativity

    Hi! I came out with a problem last night I wasn't able to solve: Let's assume we have a condensator with a uniform electric field E confined in its inside, lying on the z axes. Let's also assume we have a piece of a ferromagnetic object aligned with the condensator at time t = 0, on the y-axes...
  25. B

    B Why Special Relativity have overcome Ether Theory?

    My question is very simple. As far as I know, once upon a time there was the Lorentz Ether Theory. This theory admitted existence of so-called Ether as a certain preferred frame, which was undetectable because of physical contraction of moving material bodies in direction of their motion. In...
  26. R

    Special Relativity: Rotated Michelson Interferometer

    Homework Statement The aether-wind theory of the Michelson-Morley experiment was discussed in class for the special case where the arms of the interferometer (each of equal length L) are parallel and perpendicular to the wind. In this problem we consider the general case for an angular setting...
  27. B

    I Question on §3 of Einstein's 1905 paper

    Hi guys, This is my first time posting on PF! I have a question on §3 of Einstein's paper "On the electrodynamics of moving bodies."My problem is with the following mathematical statements: Hence, if x' is chosen to be infinitesimally small, or I have just finished high school, and...
  28. O

    Special relativity simultaneity

    Homework Statement [/B] Homework Equations The rear clock ahead example gives vL/c^2: The Attempt at a Solution I think the solution is the same, because even if there is a time dilation due to u (downward velocity) both clocks would slow down at the same rate and so the time difference...
  29. Conservation

    I Relativity of Simultaneity and frames

    Hello everyone, sorry if this is a noob question; I'm just starting out with special relativity. I was wondering whether relativity of simultaneity is a direct consequence of our ability to "know" being dependent on sight (light reaching a point). If, for example, we could only judge an event...
  30. Y

    Relativistic elastic collision

    Homework Statement Homework Equations et Em and pm be the energy and momentum of the mass m after the collision. Let p and p' be the momentum of mass M before and after the collision. From conservation of 4 momentum: \begin{bmatrix}E+m \\ p\end{bmatrix}=\begin{bmatrix}E_m+E' \\...
  31. J

    Relativity Books about Special Relativity for preparation for QFT

    Hello, I want to learn QFT but I feel that my understanding of Special Relativity is not good enough. Could you please recommend to me any good relativity books to fill my gaps? My gaps are mostly conceptual. Thanks in advance!
  32. J

    A How is the invariant speed of light enocded in SL(2,C)?

    In quantum field theory, we use the universal cover of the Lorentz group SL(2,C) instead of SO(3,1). (The reason for this is, of course, that representations of SO(3,1) aren't able to describe spin 1/2 particles.) How is the invariant speed of light enocded in SL(2,C)? This curious fact of...
  33. E

    I Collision in special relativity

    Let us assume that we have inelastic collision of masses ##m_1=1## and ##m_2=k## This means ##m_2=k m_1##. (##k>>1##) ##v_1=v##, ##v_2=0##, Velocity after collision is ##v'##. Units are such that ##c=1##. Let us assume that ##v_1## is close to one. At inelastic collision we respect conservation...
  34. Inderjeet

    B Length Contraction equation derivation

    I recently saw the derivation of length contraction in Special Relativity . At the end , it said x' = (x - vt) γ(gamma) x = (x' + vt') γ(gamma) Where γ(gamma) is Lorentz transformation . It is = 1/√(1- v²/c²) Then derivation continued , with expansion of x' = (x + vt)γ As t = 0 in this case...
  35. G

    B Special relativity and sequence of events

    Sorry for my ignorance... still trying to get to grips... If a lady in the middle of a moving train sends out beams to the front and back of the train. They reflect off mirrors back to her and arrive simultaneously because she can't do an experiment to give away that she is moving forward...
  36. H

    I Mass in Special Relativity: Misconception or Reality?

    Hi PhysicsForum, I'm currently reading Spacetime Physics by Taylor and Wheeler and I can't wrap my head around the concept of mass in SR. In the textbook, mass is described as the magnitude of the momenergy 4-vector and is invariant no matter which inertial reference frame you choose. So does...
  37. redtree

    I Special relativity and inertial frames

    What in the mathematics of the derivation of special relativity limits the model to inertial frames? How is an inertial frame defined in the context of the derivation?
  38. J

    A The de Sitter group and minmal length?

    The de Sitter group is often used as an extension of the Poincaré group, because its a simple group and preserves, in addition to a velocity c, a length L. A natural candidate for this length scale is the Planck length. Thus it seems to make sense to think about the invariant Planck length as...
  39. P

    Directional Acceleration at Relativistic Speeds

    Homework Statement A particle flies along in the positive +x direction. It has a constant force F applied 30º clockwise to the x-axis. It is moving at .6 c. What is the angle of acceleration? Homework Equations a = F/(mγ3) The Attempt at a Solution [/B] I'm pretty sure I know how to do...
  40. Axidecimal

    Explain how postulates are synthesized into Special Relativity?

    Homework Statement [/B] List two postulates of special relativity and then in your own words explain how these two statements are synthesized in Einstein's theory. Homework Equations 1. The laws of physics are the same in all inertial reference frames. 2. Light propagates through a vacuum...
  41. V

    A How parity exchanges right handed and left handed spinors

    Reading through David Tong lecture notes on QFT.On pages 94, he shows the action of parity on spinors. See below link: [1]: http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdfIn (4.75) he confirms that parity exchanges right handed and left handed spinors. Or for an arbitrary representation of...
  42. bananabandana

    Link between 'time' component of 4-momentum and energy

    Homework Statement $$ E = -\vec{v_{obs}} \cdot \vec{p} $$ Where ## \vec{p} ## is the four momentum, and ## \vec{v_{obs}}## the velocity of the observer. Homework EquationsThe Attempt at a Solution [/B] This was a stated result in a GR course. I look through my SR notes and find that I...
  43. Jeff B L

    B Special case of the twin paradox in special relativity

    There is something about the twin paradox in special relativity that has always bothered me. One twin sets out on a journey at a large fraction of the speed of light, turns around and returns. The fact that the returning twin is the one who is younger is explained by the fact that they are the...
  44. K

    Special Relativity: Find speed of 3rd object.

    Homework Statement Rest length of Ship B is twice that of Ship A. Ship A travels at 0.5c relative to a "fixed" observer. Ship B travels at such a speed that the same "fixed" observer measures Length B = Length A. How fast is Ship B traveling? 2. Homework Equations [/B] All given in statement...
  45. TheSodesa

    Escape Velocity of a Neutron Star: Relativistic Calculation

    Homework Statement Calculate the escape velocity on the surface of the neutron star in the previous problem (##m = \frac{2}{3} \cdot 2,1 \cdot M_{\odot}##; ##R = 15km##). Hint: Basic physics. Note, however, that the escape velocity is not going to be small when compared to the speed of light...
  46. MarSuper

    B Why is there a limit to the speed of light in special relativity?

    I have a question about Special Relativity. If a person is in a rocket ship traveling at 99.999999 percent the speed of light and they are standing at the back of the ship. Will they be able to walk to the front of the ship or will inertial resistance prevent them from doing so?
  47. lonewolf219

    Help with tensor formulation of special relativity

    Homework Statement Hi, I can't seem to understand the following formula in my professor's lecture notes: F_αβ = g_αγ*g_βδ*F^(γδ) Homework Equations Where g_αβ is the diagonal matrix in 4 dimensions with g_00 = 1 and g_11 = g_22 = g_33 = -1 and F^(γδ) is the electromagnetic tensor with c=1...
  48. T

    Special relativity momentum and energy conservation

    Homework Statement Two identical particles of mass m travel towards each other at speed v; they combine and form a single new particle. By employing conservation of momentum and conservation of energy, what is the mass of this new particle in Homework Equations Relativistic momentum and total...
  49. J

    B Measuring c from Earth: Is the Speed of Light Affected?

    I understand that c is the "ultimate speed" and that it is a result of the fundamental constant, the permeability of free space. But if this is a constant only to a "stationary" frame of reference, how can we accurately measure it from Earth? Since Earth is moving relative to a supposed "fixed"...
  50. Toby_phys

    Paradox found in my first relativity sheet

    We have got some SR work to do, however we have only had 1 introductory lecture. I have a problem with one of the problems. Question: A rod of 1m parrallel to the x-axis, travels at un-relativistic speeds at 45 degrees. It passes through a slit - also parallel to the x-axis - that is 1.1 meters...
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