# What is Relativity: Definition and 993 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. ### I Is the electromagnetic 4-vector indeed a 4-vector?

About the electromagnetic 4-vector ##A^\mu=(\Phi/c,\mathbf{A})##. If it is indeed a four-vector, then its squared length ##A_\mu A^\mu=\Phi^2/c^2-\mathbf{A}^2## should be a Lorentz invariant. What is the physical significance of ##\Phi^2/c^2-\mathbf{A}^2##? Thanks for any help.
2. ### I Can someone falsify this analogy contradicting time dilation?

A spaceship travels at the speed of light from the sun to the earth and back, infinite acceleration and deceleration. Sunlight takes about 8 minutes to reach earth. On the trip to earth, the sun seems to be frozen. On the trip back, sun activity appears to resume and is seen at 2x speed. The...
3. ### B How are 'fringe shifts' in the Michelson-Morley experiment calculated?

Ok for the interferometer let the arm lengths be ##L##, speed of light relative to the aether be ##c## and the speed of the setup relative to the aether be ##v##. By calculation you can show that the distance between the two 'heads' of the beams of light is ##\frac{v^2L}{c^2}##, and using the...
4. ### A Correspondence between areal radius differences and proper distances

Other ways of wording this finding about the extended SC (Schwarzschild) spacetime: - in the local frame of a free faller, radial distance is given r coordinate difference - in either Fermi-normal coordinates, or Riemann-Normal coordinates built from a free faller at an event, coordinate...
5. ### B Help understanding the Relativity of Simultaneity

Hello everyone, So I think maybe what confused me with the symmetry of time dilation was not understanding relativity of simultaneity. After looking into it if you have two clocks P and Q that synchronize when they meet and if ##\gamma## is 2 then from P's reference frame P equaling 10 and Q...
6. ### I Is it only experience that's relative?

Reading Einstein Relativity. The trains experiences lightening strikes differently to the embankment. So the two events are experienced differently from the two reference points because they have different distances to the events. Light from the events taking different times to reach each...
7. ### B Question about the relative motion of an eye relative to light source

Hello everyone, I've been thinking about 2nd postulate of relativity, it seems that and the Michaelson-Morley experiment seems to imply that there is no ether, but I was thinking about a special situation that doesn't seem to go against that postulate. I think my question is basic so hopefully...
8. ### B Understanding the Twin Paradox: Simplifying with a Clear Experiment

I have seen many attempts to rationalise the 'Twins Paradox', but none of the seem satisfactory. They usually use acceleration or asymmetric differences in inertial frame, or other aspects of special relativity that tend to obfuscate the problem/explanation. So proposing an experiment that...

Consider a distant observer traveling at .867 c ( 𝛾=2 ) relative to the solar system along the line that is collinear with the sun's axis of rotation. As the clockwork solar system spins beneath him, the distant observer peers through his powerful telescope at Big Ben in London. After taking...
10. ### I Clock Synchronisation -- Do clock speeds go out of synch?

When reading about the one way speed of light, I think it's fair to say that most potential methods to obtain this fail because it is said to be impossible to have two non co-located clocks in synch. My question is really about terminology. Normally, two things can be said to be in synch if...
11. ### Misc. DIY Experiment of Special Relativistic Effects?

I'd like to perform an experiment that will build intuition for Special Relativity in the real world. While I do believe that it occurs in the real world, I'd like to be able to prove it for myself, and I feel that such an experiment would help others on this forum as well. Is there an...
12. ### I Terrell rotation is confusing!

Imagine in the distance you see a glowing spherical object hovering in the air. Suppose light from point B on the surface of the sphere facing away from you can arrive at the camera simultaneously with light from A: Since the object is moving at relativistic speeds, by the time light begins to...
13. ### I Doubts about the relativistic description of electrical interactions

I would like help with an issue that I have not yet fully mastered. Consider a particle resting on a plane, it is subjected to a gravitational force, which can be interpreted as the result of a deformation of space-time. It remains at rest due to the upward binding reaction provided by the...
14. ### B Can someone please explain Relativity Theory to me in as simple terms as possible?

Yes... Can someone please explain the relativity theory to me in as much simple terms as possible. It's really way in over my head. Thanks.
15. ### I What causes the magnetic force in a uniform magnetic field?

As is well known, when electric current flows in a straight wire, a charge in motion in the vicinity of the wire, parallel to it, is affected by a force called the magnetic force, which is attributable to the relativistic contraction effects that the wire undergoes in the frame of reference of...
16. ### I Existential problem on Electromagnetism and the combination of Relativity and Quantum Mechanics

In Feynman's famous Physics book, in a discussion of the generality of Maxwell's equations in the static case, in which he addresses the problem of whether they are an approximation of a deeper mechanism that follows other equations or not, he says: I was wondering first of all if this was a...
17. ### I I think the special theory of relativity self-contradicts

Consider two rulers that have the same rest length. Denote it by L0. Now let the two rulers be in relative motion. First consider things from the rest frame of the unprimed ruler illustrated below. A'_____B' ............A_______________B. State 1 ............A'_____B'...
18. ### I Relativity of simultaneity

When two remote events are observed/measured to be simultaneous in one inertial frame, the same events will not be simultaneous when observed from a second frame in uniform motion relative to the first. Why is this distinction in the kinematics of light not considered an operational distinction...
19. ### I Asymmetry in Length contraction?

Instead of a twin paradox, let’s just consider an inbound starship approaching Earth at relativistic speed. The traveler is on a flyby mission, he will never change speed or direction. We will disagree with the traveler on how much time will have elapsed when he passes Earth. The discrepancy can...
20. ### I What was the four-momentum meant to include?

Hello, was the four-momentum of relativity, Pν, supposed to include all mass and energy contributions from every field i.e. electromagnetic, strong, gravitational... Or is it just the momentum of what was known in Einstein's time?
21. ### B What is m in Kinetic Energy? Relativistic mass or Rest mass?

note: m = relativistic mass ##m_o## = rest mass v = velocity of the objectQuestion 1: If a particle is moving at relativistic speeds what would it's kinetic energy be? I think it's ##K.E. = \frac{1}{2} m_o v^2## and my friend thinks it's ##K.E. = \frac{1}{2} \frac{m_o...
22. ### Calculation of Thomas Precession in Rindler's relativity book

I am trying to work through rindler's relativity book. However, I got stuck at what I think should be a simple approximation. I calculated the angles ##\theta## and ##\theta''## using the formula for velocity transformation. If I simply expand ##\gamma(v)## and ##\gamma(v')## using taylor...
23. ### Speed of Message Relative to a Space Station: Relativistic Addition

u is the speed of the message relative to the station v is the speed of the spaceship relative to the station u’ is the speed of the message relative to the spaceship u=(-0.5c+0.7c)/(1+((-0.5c)(0.7c))/c^2 ) =0.2c/0.65=0.308c This just seems way too high, and I'm not sure if I'm doing it...
24. ### I How does an observer moving at relativistic speeds perceive a pulsar?

Let us suppose our velocity towards a distant neutron star roughly equated to a dilation factor of 10. On board our vessel we are equipped with a NIST-F2 atomic clock . Since the clock travels with us, it also experiences the same dilation; thus, in our frame, the clock keeps time the same as...
25. ### B Photons on a perpendicular bisector path from a black hole's core

Not directly from the core, but a trajectory that goes to the event horizon, and gets corrected to a perfect, perpendicular bisector path by the gravity of the core, when it reaches the event horizon. Would they escape the event horizon, since they have to always move at c? On this trajectory...
26. ### I Event Horizon movie, with some interesting concepts (papers cited)

Please hear me out; I've referenced some good papers below, and I think you'll find the movie clip intriguing. Is there anything solid to this movie? The faster than light travel. The dimensional gateway. The folding of spacetime, passing through the hole, and then the return of spacetime to...
27. ### I The speed of forces, and applying relativity to force carriers

Are there any limits to how fast forces can exert influence... or are they all instantaneous? How fast are the forces of a collision? I think frame dragging, with gravity, shows that at least gravity is not instantaneous, because if it is instantaneous, then how would it produce drag - it...
28. ### I Higgs Field & GR: Are They Related or Not?

Since these two pillars of science have mass as a common core, are they related, essentially the same or not?
29. ### I Relevant/irrelevant clocks for experimental tests of relativity

Recently, in this forum, highly respected members referred to clocks like pendulum and hourglass as if they are relevant for relativity. Are they really? Besides the lack of accuracy, they depend on acceleration/gravity, so they would not work at all in inertial frames and they could not...
30. ### IB Extended Essay research project on relativity

Hi guys, I am a first year IB student who has to pick his EE topic. The Extended Essay is an independent, in-depth study of a chosen topic within one of the IB subject areas. I chose to do mine on physics. I have to write 3500 - 4000 words for my EE. I've chosen a set up for an investigation /...
31. ### B Can Relativistic Mass Create a Black Hole?

Hello dear colleagues from the physics forum, Following question came to my mind, can you tell me which statement (s) is/are wrong?. Since relativistic mass is weighed with the Lorentz factor for an external observer, it is expected an increase with speed. Such mass (m1) would tend to...
32. ### A Relativistically invariant 2-body phase space integral

I encounter a function that I don‘t know in the calculation of Relativistically invariant 2-body phase space integral: in this equation, ##s##is the square of total energy of the system in the center-of-mass frame(I think) I don't know what the function ##\lambda^{\frac{1}{2}}## is. There are...
33. ### B Triplets Paradox: Is There a Solution?

One person (A) stays on Earth, while another (B) goes on a long journey and returns later. We can calculate the time interval for A and the proper time interval for B. Let's say we get Δt=100y and Δτ'=50y. We then consider a second traveler (C) going on a different journey, departing and...
34. ### I Adler's New Book on Relativity: Misprint on Page 16?

I bought a copy of Adler's new book on relativity. Is there a misprint on page 16 regarding the Lorentz metric = diag (-1,-1,-1,-1) or am I missing something? The text itself after the equation suggests it is the same metric as on the earlier page so that index placement, two lower indices vs...
35. ### B Perception of Velocity in Special Relativity

If an object is going close to the speed of light, will outside frames of references perceive the moving object as going slow or fast?
36. ### General relativity - Using Ricc and Weyl tensor to find the area

I have the following question to solve:Use the metric:$$ds^2 = -dt^2 +dx^2 +2a^2(t)dxdy + dy^2 +dz^2$$ Test bodies are arranged in a circle on the metric at rest at ##t=0##. The circle define as $$x^2 +y^2 \leq R^2$$ The bodies start to move on geodesic when we have $$a(0)=0$$ a. we have to...
37. ### Einstein relativity between 2 coordinates systems

I calculated the speed using the information provided through the above equation and finding V' = 1.2 m/s. However, the first solution must be -1,2 m/s. I don't know how to reach it, any suggestion?
38. ### A Special & General Relativity: CTC Metrics in Cylindrical Coordinates

I am studying metrics that exhibit CTCs. I was looking at a few different metrics... Tipler's solution Godel metric Kerr metric For starters to compare them, I am trying to convert said metrics into cylindrical coordinates. Thanks in advance for any help😃
39. ### Relativity Recommended general relativity textbook

Hi, I am looking to study general relativity at my own steam (currently finishing 1st year physics at Warwick) during the summer. What textbook(s) would you recommend? I've heard good things about A. Zee's 'Einstein Gravity in a Nutshell'- is that worth it, and would it be suitable for someone...
40. ### B The synchronization of clocks and the relativity of motion

In his thought experiment on the relativity of simultaneity, Einstein synchronizes two clocks A and B (at either end of a rod) to the clock C in a stationary frame, while A and B (the rod) is moving relative to C. The equations Einstein finishes with demonstrate how observers moving with A and...
41. ### Special Relativity - Which reference frame experiences which time?

This was a practice question, so it had the answer with it, which is 31 minutes. However, I'm confused as to why Lisa experiences T0. It isn't exactly an event happening in Lisa's rocket, but rather her just moving through space. From her perspective, it should look like Earth is moving at the...
42. ### ChatGPT Results: Testing Special Relativity

Hi guys, I have heard a lot of claims about ChatGPT and how it is going to put a lot of people out of work. I have a friend who is a teacher of humanities and he is fearful of losing his job in the long term. People seem to be in awe at the thing and think it understand what is saying. So I...
43. ### I Determinism & General Relativity: A Physics Primer

I know nothing about physics, to be clear. My friend was saying due to general relativity, the faster you move through space, the slower you move through time. Objects with a heavy mass (like a blackhole) can distort the fabric of space time and being near its gravitational pull means that you...
44. ### Opening a portal to the center of the Sun

Whilst perusing a D&D forum, I stopped to answer a question someone put out which was "what would happen if we used the gate spell to open a portal to the middle of the sun?" I replied (this was a while ago) and whilst I'm reviewing it, I am troubled by the fact that this uses such high forces...
45. ### B Spin 2 and general relativity

I have some questions regarding the expected exchange particles for gravitation. From my understanding the following was valid: We can linearize the equations of GTR for weak fields "Quantum mechanics" (Schrödinger, Dirac equations) are linear Those linear equations allow eigenstates and...
46. ### I Explore Spacetimes, Metrics & Symmetries in Relativity Theory

I was discussing this paper with a couple of physicists colleagues of mine (https://arxiv.org/abs/2011.12970) In the paper, the authors describe "spacetimes without symmetries". When I mentioned that, one of my friends said that no spacetime predicted or included in the theory of relativity...
47. ### I Understanding Einstein's second postulate of special relativity

Einstein's two postulates of special relativity reads, according to Wikipedia: The postulates are most often formulated similarly to this. But in my opnion, the second postulate shouldn't be formulated as above, because then one misses the point. This is particular true for the second...
48. ### B Relativity of Measures: A vs B Frame & Light Source S

A frame “A” of three perpendicular axis (x, y, z) and an identical frame “B”, with an observer at rest in each, are moving toward each other at 1/2c on their common x axis. Centered between A and B on x is a light source S. At the time t1 when A, B and S coincide on x, S emits a light flash...

50. ### B A thought experiment of the relativity of light

A light source in uniform motion emits a flash of light. A spherically symmetric wavefront propagates from a central point, the source, or the “origin” of emission. The wave front remains at c relative to the origin as measured by all observers. How fast do these origins move? Am I missing...