# Tagged Questions

The union of special (SR) and general (GR) relativity. Use this tag if both SR and GR apply.

4k views

### Inertial Frames of Reference - Inertial vs. Accelerated Frames

According to Robert Resnick's book "Introduction to Special Relativity", a line states the following as the definition of an inertial frame of reference: "We define an inertial system as a frame of ...
830 views

### Reducing General Relativity to Special Relativity in limiting case

I understand that general relativity is applicable to gravitational fields and special relativity is applicable to case when there is no gravity. But is there a derivation on how to reduce General ...
226 views

### Does a light cone look the same from all reference frames?

If there were a light cone centered at some point $P$, and you were to look at that light cone from different reference frames, would it change its shape? I know that points inside and outside the ...
110 views

### Is simultaneity testable?

I was reading about Andromeda paradox, and I started wondering. How can we know that the situation in the Andromeda paradox is real ? How can you know that simultaneity is real ? How can you say that ...
119 views

### Does Contraction at High Speeds Happen in Any Dimension Besides Length?

I'm not sure if this is a duplicate. Whenever physics buffs talk about Einstein's relativity (I forget which kind) at high speeds, they always talk about "length contraction", or shortening of the ...
696 views

### “as measured in a local Lorentz frame”?

I've seen the phrase "as measured in a local Lorentz frame" tagged on the end of so many sentences. What does it mean precisely? To give an explanation with an example, consider the context of ...
78 views

### Length Contraction in Relativity

Let's suppose we have a one dimensional rod made of elementary point particles, in contact with each other placed along the x-axis.If the rod is moving along x-axis then we know (because of relativity)...
114 views

### Would a particle the size of a neutron, if it had enough mass, collapse into a blackhole?

For example, a neutron is a particle that occupies a certain volume. If you pack enough mass into that volume, it would collapse into a black hole (I assume there is not enough mass now). At least if ...
232 views

### Can all of physics be described by simple math? [closed]

Recently I was browsing through A Dynamical Theory of electromagnetic field by Maxwell and wondered because the paper did not seem to include any vector calculus or any vectors. I thought of the ...
126 views

### Consider relativity. If a stick accelerates from a stationary state, how will it look like during the acceleration?

I mean how it will be shortened in the sight of an observer in a stationary coordinate system. It seems I was not clear enough. If the stick becomes shorter, then while this is happening, the ...
405 views

### Is there a default notation for 4-vectors while handwriting?

In printed paper 3-vectors can be denoted bold italic while 4-vectors can be denote just bold. While handwriting 3-vectors are denoted by arrows above letters. Is there a similar way to denote 4-...
71 views

### A rope between Earth and Moon - Length and Speed of Light [duplicate]

An earlier question was asked on what would happen if a rope was attached between Moon and Earth, but the question was more about the impacts in terms of geology. We keep the same experiment setup : ...
634 views

### Proper time for an accelerating object

As far as I have read so far, proper time is the time measured on the clock of an inertial frame moving uniformly with respect to another inertial frame. The concept and the mathematical expression ...
791 views

### How do I adjust the kinematic equations to avoid reaching speeds faster than light?

I like some 'science' in my 'science fiction', so I started crunching out the kinematic equations for some of the scenarios my characters are getting involved in, and ran smack dab into an issue. (...
201 views

### Using Einstein's Relativity: Who is younger?

Suppose we have a person A and a person B. Person B travels very close to speed of light and never returns. He's constant in speed. Then, we can say two things: B is younger than A. A is younger ...
76 views

### Why can an object not reach the speed of light by falling in a gravitational field with constant acceleration?

I recently learned in school that the gravitational potential energy of an object is equal to $mgh$. I also learned that the kinetic energy of an object is $\frac12 mv^2$. If an object falls in a ...
214 views

### Does deceleration (or negative acceleration) feel like acceleration?

This may seem like a dumb question, but I'm having a hard time trying to intuit the answer. The question is in relation to Einstein's famous thought experiment which says that it is impossible for a ...
125 views

### Which is more fundamental,''constancy of speed of light'' or ''space-time''? [closed]

My friend asked me,''which is more basic or fundamental? constancy of speed of light which makes space-time behave dynamically OR dynamical behavior of space-time that makes light to travel with ...
202 views

### Has anyone yet been able to prove relativity wrong? [closed]

I was in a physics group, then a student (or a professor, I don't know) posted this- Mohammad Shafiq Khan > Physics > The space-time concept including the formula $E=mc^2$ are proved baseless in ...
858 views

### Can we live forever by taking advantage of the time dilation effect?

I have been reading about Time dilation and twin paradox in Relativity. So,I thought that if It can be the difference of 10 yrs between the Girl in space and on Earth, then why can't anyone not age ...
6k views

### Why can't we accelerate objects past the speed of light? [duplicate]

Is there any intuitive reasoning behind why there would be this universal speed limit? It just seems so arbitrary. I know that there must be things that are unknown, but what reasoning is there behind ...
229 views

### Relativistic mass as seen by different observers

Imagine there are two observers $A$ and $B$ and a particle $P$. $A$ and $B$ are at about the same point, $P$ is some distance away. From $A$'s point of view, $P$ has velocity $V$ and $B$ has ...
350 views

Case 1: two people wake up in spaceships accelerating at 1g. They can measure or observe anything inside the room but not outside. They couldn't determine if they were on a spaceship or on earth. ...
604 views

### Where does the minus sign appear from in the metric tensor?

Trying to understand Schutz's AFCIGR, where does the minus sign appear from in the metric tensor? I understand that this expresses the invariance of the spacetime interval. Schutz says (I think) ...
167 views

### Is gravity Lorentz-invariant?

If I go whizzing past the Sun in my spaceship at 0.8c, I will see that the Sun looks like it has been squished. That is, it will be Lorentz-contracted along the direction of my motion (by a factor of ...
95 views

### Would Magnetic field be Zero if measured while moving along with charges?

According to Einstein's relativistic interpretation magnetic field caused by charges moving inside a wire is caused only by the relative speed between charges and the observer. So, if electrons are ...
215 views

### What would a nuclear explosion look like at relativistic speeds where time dilation and length contraction become significant?

Suppose a rocket carrying a nuclear weapon is moving past our planet at very near the speed of light while a stationary observer watches from here on Earth. As the rocket passes directly overhead, an ...
201 views

### If all energy is removed from an object, what will it stop moving relative to?

If you somehow removed all energy from an object, what would it stop moving relative to? According to relativity, an object's position is not relative to the universe, just reference points, and, as ...
265 views

### Do photons feel gravity of approaching objects only?

I have read that photons while travelling near massive objects such as the sun experience gravitational pull which is why we see some stars at different positions than they are when seen towards the ...
143 views

### Lorentz transformation - need a little clarification

So the question states: A's and B's systems are related by the standard Lorentz transform. B simultaneously fires off two photons from transmitters distance D apart along the x' axis, and in the ...
177 views

### Could light travel more slowly than the “universal speed limit”? Could this imply quantization of spacetime?

One description of relativistic effects that I've heard/read goes something like this: Everything moves through spacetime at a constant speed. An object's direction of travel through spacetime can ...
180 views

### When a high speed neutrino just misses an old neutron star, why isn't it trapped?

Suppose a neutrino is seen travelling so fast that its Lorentz gamma factor is 100,000. It races past an old, no longer active neutron star, narrowly missing it. As far as the neutrino is concerned, ...
67 views

### Need clarity about relativity/magnetism explanation

Please bear with me - this is not a duplicate question.... From what limited knowledge I have gathered, it seems that when two wires in parallel are carrying equal current, the electrons moving in ...
59 views

### Merging black holes makes them less dense, so

According to What is exactly the density of a black hole and how can it be calculated? (more specifically, John's answer here made me think: if you merge a whole load of chunks of an element heavier ...
146 views

### Newton's gravity vs Einstein's gravity

Hello so if i were to simplify the difference between gravity as seen from Newton and gravity as seen from Einstein, could i say that Einstein's gravity is just adding the fact that gravity does not ...
78 views

### Is there a gravitational radiation reaction force?

As you may know, the linearized relativity theory (or gravitoelectromagnetism), obeys equations analogous to Maxwell's equations in electrodynamics. These equations in ED result in the troubling ...
112 views

### Does the accelerated rate of expansion of the Universe have any effect on the speed of light in vacuum?

So I was just wondering about this as I finished reading about Michelson-Morley's experiment which disproved the ether theory. My question is since the Universe is constantly expanding and that too ...
161 views

### Since the Earth is moving through space, do we experience time dilation?

What I mean is, the Earth orbits around the sun at around 30km/s, while I understand that this isn't a very high velocity does time dilation still occur to say a person who is completely stationary in ...