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4
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6answers
2k views

Surely space-time Curvature does not explain gravity, it just describe its effects?

In special relativity co-moving objects see the other's 4-velocity as being only temporal. When they move relative to each other they see the other's 4-velocity has rotated so that it points less in ...
10
votes
4answers
1k views

Is there a fundamental reason why gravitational mass is the same as inertial mass?

The principle of equivalence - that, locally, you can't distinguish between a uniform gravitational field and a noninertial frame accelerating in the sense opposite to the gravitational field - is ...
13
votes
3answers
1k views

Why do objects follow geodesics in spacetime?

Trying to teach myself general relativity. I sort of understand the derivation of the geodesic equation ...
14
votes
7answers
5k views

Would it help if you jump inside a free falling elevator?

Imagine you're trapped inside a free falling elevator. Would you decrease your impact impulse by jumping during the fall? When?
5
votes
2answers
395 views

How (or why) equivalence principle led to Einstein field equations?

If equivalence principle was origin of general relativity what was the process that this principle led Einstein to developed his theory of general relativity?
3
votes
5answers
723 views

How are the comoving coordinates NOT a prefered reference frame?

Physics me this: The equivalence principle has rigorous physical definitions that say, for one, that the laws of physics are the same in all inertial reference frames. This is to say that the ...
2
votes
9answers
3k views

How does this thought experiment not rule out black holes?

How does the following brief thought experiment fail to show that general relativity (GR) has a major problem in regards to black holes? The full thought experiment is in my blog post. The post ...
33
votes
4answers
1k views

Does quantum mechanics violate the equivalence principle?

I have a question about equivalence principle in quantum mechanics. Consider a Schroedinger equation under gravitional field $$\left[ - \frac{1}{2m_I} \nabla^2 + m_g \Phi_{\mathrm{grav}} \right]\psi ...
5
votes
4answers
2k views

Why is the equivalence principle so important to general relativity?

In its simplest form, equivalence principle states that the inertial mass and the gravitational mass should be the same. This is easy to understand. But why is it so important to the formulation of ...
3
votes
1answer
601 views

How does Newton's 2nd law correspond to GR in the weak field limit?

I can only perform the demonstration from the much simpler $E = mc^2$. Take as given the Einstein field equation: $G_{\mu\nu} = 8 \pi \, T_{\mu\nu}$ ... can it be proved that Newton's formulation ...
4
votes
1answer
239 views

Understanding the different kinds of mass in gravity

On this site, the Phys.SE question Is there a fundamental reason why gravitational mass is the same as inertial mass? has been asked. See also this Phys.SE question. The 'answer' provided on this ...
2
votes
3answers
257 views

Why can't we think of free fall as upside down rocket?

/\ / \ | | | m | | | ------ <--- floor (Rocket A) This rocket is accelerated (g) upwards then mass(m) falls on the floor. ...
-3
votes
4answers
4k views

What is the difference between gravitation and magnetism?

If you compress a large mass, on the order of a star or the Earth, into a very small space, you get a black hole. Even for very large masses, it is possible in principle for it to occupy a very small ...
5
votes
1answer
116 views

Explanation for “if all accelerated systems are equivalent, then Euclidean geometry cannot hold in all of them”

I'm doing an EPQ (mini college research paper) on gravity, and I found a site that explained things in simple terms. I am having trouble understanding how Einstein came to his revelation space-time ...
0
votes
1answer
446 views

Weightlessness for astronauts

Well, this question has been puzzling me for kinda long time, many people believe that orbiting astronauts feel weightless because they are "beyond the pull of Earth's gravity"...How far from the ...
8
votes
2answers
2k views

Does the equivalence between inertial and gravitational mass imply anything about the Higgs mechanism?

For example: the role it might play in a theory of quantum gravity (ie causing space-time curvature)? I realize that inertial mass can result from binding energy alone. Has the equivalence principle ...
1
vote
1answer
283 views

Accelerating elevator?

If your in an elevator accelerating downwards faster than gravity, would you be pushed up to the ceiling? Since gravity cannot match the acceleration of the elevator, you're wouldn't be able to move ...
14
votes
2answers
383 views

Do accelerated charges radiate or not?

This questions has been asked all over the net (here included) but I can't find a satisfactory answer or discussion. Some say it does not radiate if the acceleration is caused by a uniform gravity ...
8
votes
8answers
595 views

Gravity theories with the equivalence principle but different from GR

Einstein's general relativity assumes the equivalence of acceleration and gravitation. Is there a general class of gravity theories that have this property but disagree with general relativity? Will ...
8
votes
4answers
2k views

Water pressure in free fall

The increasing water pressure as you go deeper is generally explained in terms of the weight of the water column above the observation point pressing down. The question, then, is what would happen if ...
7
votes
2answers
461 views

Equivalence principle and radiation from falling particle

I am currently having a hard time solving a problem of GR from Lasenby's book. I can't make it more clear than by quoting the exercise: 7.2 A charged object held stationary in a laboratory on the ...
5
votes
3answers
295 views

Did people realize that gravity accelerated things before Einstein's elevator thought experiment?

I'm reading about the (very near) equivalence of gravitational mass and inertial mass in my undergrad GR course, and the text (Lambourne) describes this equivalence as the inspiration for Einstein's ...
5
votes
3answers
307 views

Is the equivalence principle strictly fulfilled by general relativity?

The equivalence principle states The outcome of any local experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime. Any real local ...
3
votes
1answer
116 views

How to explain centripetal force in terms or relativity

At the end of a video of dropping a ball and feathers in a vacuum, Brian Cox explains that the Ball and Feathers, as understood in terms of General Relativity, aren't falling. (apologies I can only ...
1
vote
0answers
133 views

Use of Principle of Equivalence

Let $x^\mu$ be the coordinates of a reference frame, $K$, where all bodies feel the same constant and uniform acceleration $\textbf{a}=\textbf{g}=-\nabla\varphi$; let $\xi^\mu$ be the coordinates of a ...
4
votes
1answer
362 views

Weak equivalence principle tests

on the wikipedia article about the equivalence principle there is a mention about testing the EP against parity-violating masses; "The equivalence principle is untested against opposite ...
3
votes
2answers
4k views

Orbital speed for a circular orbit?

I wanted to look up the formula for orbital speed for a circular orbit on Wikipedia, and I found 2 formulas: All bounded orbits where the gravity of a central body dominates are elliptical in ...
1
vote
4answers
205 views

Equivalence principle and acceleration vs a gravitational field

I picked this up on the net: Einstein came to realize the principle of equivalence, and it states that an accelerated system is completely physically equivalent to a system inside a gravitational ...
1
vote
1answer
1k views

How do Einstein’s equations support mass gain in particle accelerators? [duplicate]

Possible Duplicate: Why does the mass of an object increase when its speed approaches that of light? A charged particle that is accelerated through a particle accelerator like CERN ...
0
votes
2answers
3k views

Jumping in an elevator?

I've always been under the impression that jumping in an elevator wouldn't help at all, especially after reading a snippet of physics where Einstein said that free fall was identical to zero gravity. ...
0
votes
2answers
3k views

Projectile motion without air resistance [closed]

Does the mass of projectile affect its trajectory when it is projected in no air resistance environment and why?