# relativity Definition and Topics - 349 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. ### B No way to catch up with galaxies currently receding at >c?

Consider a far-away galaxy that is considered to be currently receding from Earth at 2x the speed of light. (With this 2x c recession velocity, we are speaking of the Vnow, the imputed relative velocity of the galaxy compared to earth now, not the velocity of the galaxy relative to Earth at the...
2. ### B Time dilation for traveling to a distant star

If there is a spaceship traveling at 0.999c, the time to reach a star 100 lyr away would be approx 100 yr (assuming no accel and decel). But on the spaceship, It would be 100 yr * sqrt(1-0.999^2) = 4.5yr. Why do we take 100 yr as the time seen on Earth and not the time on the spaceship?
3. ### I Does charging my phone increase its gravitational force?

If the statement above is correct, I do not understand this concept. I guess by charging my phone I am not producing matter. Does it mean in this case, energy converts to mass (not matter)? Can someone please explain this?
4. ### B A twins paradox thought experiment

I have been looking through some of the threads about the twins paradox in relativity. It’s clear there’s a lot of confusion on this, and I am yet one more person very confused on this. So I was thinking about a hypothetical experiment, and I will lay out my hypothesis of what might...
5. ### B Relative speed of two oppositely directed light beams

Can someone give a meaningful explanation that the relative speed of two oppositely directed light beams is why only one light speeds? I understand that based on the Einstein relativity theory, the relative speed of two beams is C, because nothing can be quicker than light speed. However it is...
6. ### The rising railway coach

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...
7. ### B Apparent contradiction in force between 2 point charges across frames

So, i was trying to calculate, the net force between 2 point charges in their rest frame, and in a frame where they are moving. So, assume, there are 2 point charges each of charge +q. They are r distance apart from each other and moving parallel to each other with a speed v relative to a lab...
8. ### Constant acceleration in a rocket

I thought I'd start by writing the problem in a tensor formalism. I have identified with ##S## the earth and ##S'## the rocket. Since the acceleration provided is in the rocket's frame of reference, I can write the following four-vector. $$a'^\mu=(0, a, 0, 0)$$ Since we are interested in the...
9. ### Velocity addition formula in X and Y axes (Relativity)

The problem: Visualising the problem (My question is with regards to this): Why is the above set-up correct? In the above diagram, S would be moving at velocity -v relative to S', instead of v. Is this because the question says "speed v", and so we can set the direction as such? Why would the...
10. ### I Relativistic Energy

In Robert M. Wald's General Relativity he writes on page ##61##: The energy of a particle as measured by an observer - present at the site of the particle - whose 4-velocity is ##v^a## is defined by $$E=-p_a v^a$$ Thus, in special relativity, energy is recognized to be the "time component" of...
11. ### Rocket moving away from the Earth

I first began to identify the various events in the problem. I call the rocket ##S'## and the earth ##S##. Sending signal Tail signal reflection Head signal reflection Tail signal return Head signal return For the earth I know that: ##t_4 = T## ##t_5 = T + \Delta T## Since the two events...
12. ### Annihilation: calculation of photon energies

I set up this problem this way: ##p_a^{\mu}=(E, \sqrt{E^2-m^2}, 0, 0)## ##p_b^{\mu}=(m, 0, 0, 0)## ##p_c^{\mu}=(2E_\gamma, 2E_\gamma, 0, 0)## I have chosen to consider the two photons as a single particle of energy equal to ##2E_\gamma##. At this point I applied conservation of the...
13. ### Minimum energy of a photon to produce ##\pi^+##

I have a doubt about the first request: I suppose to find the minimum energy of ##\gamma## in the situation where ##p## is stationary, there is no reason to say that the proton is stationary if I were to calculate it in the CM, right?. So I have to consider che LAB-frame to find ##E_\gamma##...
14. ### 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...
15. ### I Relativistic velocity, perpendicular acceleration and momentum

A stationary observer sees a particle moving north at velocity v very close to the speed of light. Then the observer accelerates eastward to velocity v. What is its new total velocity of the particle toward the north-west relative to the observer? I ask because while the particles total...
16. ### B Does mass reduce an infinitesimal amount during combustion?

I have studied in high school that all chemical reactions obey conservation of mass, as the atoms are merely re-arranged, but when I read through special relativity, I was reading that you can show an infinitesimal change in mass (based on E=mc2) in combustion that's not noticeable that's being...

18. ### Compton Effect with the electron initially moving

Hello! I do not understand how to prove the exercise. I have searched all over but I have found no hints on how to get started. Can anybody help me?
19. ### Calculate a specific boost and rotation

Let's begin with the first point. a.I) Apply a generic boost in the y-z plane (take advantage of the arbitrariness in deciding the alignment of the y and z axes). \begin{equation*} B_{yz} = \begin{pmatrix} \gamma & 0 & -\gamma v_y & -\gamma v_z \\ 0 & 1 & 0 & 0 \\ -\gamma v_y & 0 &...
20. ### I What can relativity tell us about how the particles view us?

Some background, I am an undergraduate electrical engineering student with a knack for physics. I plan to attend graduate school for physics but for the meanwhile I've only taken an undergraduate course in QM mechanics, which used griffith's book, and a modern physics course, which covered some...
21. ### I Beginner question about tensor index manipulation

For instance, using the vector ##A^\alpha e_\alpha##: ##g_{\mu \nu} e^\mu \otimes e^\nu (A^\alpha e_\alpha) = g_{\mu \nu} (e^\mu, A^\alpha e_\alpha) e^\nu ## ##g_{\mu \nu} e^\mu \otimes e^\nu (A^\alpha e_\alpha) = A^\alpha g_{\mu \nu} \delta_\alpha^\mu e^\nu = A^\mu g_{\mu \nu} e^\nu = A_\nu...
22. ### B Some questions on the fundamentals of lightspeed and its limits

Hey there, I'm aware this is a bit of a stupid question, and I think that I understand the principle fundamentally, however, my intuition is still having a little trouble catching up, and I'm trying to figure out if it is because of an important detail that I have missed/misinterpreted. I think...
23. ### I SR: which clock was slower?

I don't know much about the math of SR, but this is what's bothering me: if a moving clock B ticks slower than the stationary one I have (A), then from the viewpoint of B, my clock (A) is ticking slower. So if we turn around and meet each other in the middle, which clock was slower than which...
24. ### Special Relativity Math Thing (Fresnel's Drag Coefficient)

I am wondering if there is a typo in my textbook. Please see the attachment. The textbook says "...keeping only the lowest term in x = v/c." I am wondering if it should be "x = v/(nc)," as I circled in blue on the left side. It is a binomial expansion of the denominator. Shouldn't x be v/(nc)...
25. ### B Why is "space" relative?

Hello everybody, my question may sound stupid, especially speaking of such a mind-blowing and important theory... but here I am! I'm 17 and I'm reading a fabulous book by Stephen Hawking, "A Brief History of Time", and it introduced me to relativity theories... I literally started looking the...
26. ### B Why the stay-at-home twin is not considered to be accelerating?

I am not a physicist. I need your kind help in removing my following doubt about twin paradox. What I have been able to understand about twin paradox is this- 1. Special relativity deals with non-accelerating (inertial) motion. 2. The travelling twin (A) moves at a high speed in relation to...
27. ### B Can the relative speed of an object meet/exceed the speed of light?

I was curious if the relative speed of an object can exceed the speed of light. Specifically, I am curious about the following thought experiment. I am not a physicists (and if I were asking the following would make me a poor one) and it has been 20 years since college physics. If a vessel is...
28. ### B Decay in c caused by Universe expansion

I'm currently writing a research paper about the speed of light. I have researched universe expansion, specifically, the quantised redshift spectral index fluctuations of distant galaxies and other structures over time, however, I need to suggest why universe expansion possibly causes a recorded...
29. ### B Does time dilation work in 1d space?

Imagine this question in 2 dimensions, time (t) and distance (x), that is (t,x). Alice (A) is at the origin, x=0. Bob (B) begins at x=c. Thus we have A(0,0) and B(0,c). Both Alice and Bob send a light signal towards the other but let's say the signal changes colour every second by the colours of...
30. ### I Lorentz arguments about the ether (original paper)

I bought the book "The principle of Relativity" by Einstein et al. and was really surprised by the (low) level of explanation by Lorentz regarding the compression of rods on the experiment carried out by Michelson & Morley. I reproduce part of it below: Well, he gave absolutely no arguments to...