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Questions tagged [equivalence-principle]

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69
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7answers
4k views

If gravity is a pseudoforce in general relativity, then why is a graviton necessary?

As far as I’m aware, gravity in general relativity arises from the curvature of spacetime and is equivalent to an accelerated reference frame. Objects accelerating in a gravitational field are in fact ...
53
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10answers
34k views

Why does a free-falling body experience no force despite accelerating?

Note: For the purposes of my question, when I refer to free fall assume it takes place in a vacuum. From my (admittedly weak) understanding of the equivalence principle, falling in a gravitational ...
42
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3answers
17k views

If I drop a ball in an accelerating rocket, will it bounce? If so, how?

Einstein's equivalence principle says that you cannot distinguish between an accelerating frame or a gravitational field. However, in an gravitational field, if I drop a tennis ball, it will bounce, ...
41
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4answers
3k 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 ...
33
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5answers
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Is floating in space similar to falling under gravity?

In the case there is no air and your eye are closed, then does falling from the sky under gravity have the same feeling as floating in space? Can our body feel that we are accelerating without the ...
28
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6answers
6k views

Why do objects follow geodesics in spacetime?

Trying to teach myself general relativity. I sort of understand the derivation of the geodesic equation $$\frac{d^{2}x^{\alpha}}{d\tau^{2}}+\Gamma_{\gamma\beta}^{\alpha}\frac{dx^{\beta}}{d\tau}\frac{...
25
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5answers
6k views

Does a person inside a falling bus fall to the front of it?

So I was watching Final Destination 5 and something caught my attention. There's a part where a bridge collapses and everything falls apart, so there's this bus that has a person inside (unaware of ...
22
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4answers
6k 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 non-inertial frame accelerating in the sense opposite to the gravitational field - is ...
21
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8answers
16k 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?
19
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3answers
1k views

Do accelerated charges radiate or not? [duplicate]

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 ...
17
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4answers
576 views

What does the equivalence principle mean in quantum cases?

We know that electron trapped by nuclear, like the hydrogen system, is described by quantum state,and never fall to the nuclear. So is there any similar situation in the case of electron near the ...
15
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2answers
2k 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?
14
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3answers
9k views

How Is the Inertial Mass Different From the Gravitational Mass? [duplicate]

I recently read that the mass that we deal with in the equation $F=M_{i}a$ is called the inertial mass and the mass that we deal with in $F=M_{g}g$ is called the gravitational mass. I find a hard ...
13
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4answers
3k views

Gravitational shielding and equivalence principle

I read on wiki that gravitational shielding is considered to be a violation of the equivalence principle. Is that so, How? A conceptual description without lot of mathematics will be helpful. I am ...
11
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4answers
1k views

Is the lay explanation of the equivalence principle wrong?

The common explanation/trope for the equivalence principle always has something to do with you being inside an elevator or spaceship, and your supposed inability to differentiate, say, gravity from a ...
11
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4answers
7k 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 ...
11
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6answers
666 views

Charged particle as observed from an inertial and a non-inertial frame of reference

A charged particle fixed to a frame $S^\prime$ is accelerating w.r.t an inertial frame $S$. For an observer A in the $S$ frame, the charged particle is accelerating (being attached to frame $S^\prime$)...
10
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5answers
3k views

In circular motion, with a constant distance, why does the mass of the orbitting object have no effect on its revolution at all?

Using Newton's universal equation and some circular motion equation, the orbiting object's mass cancels out. But can someone please explain why this is without using pure algebra?
10
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2answers
639 views

How can a point-like particle “feel” gravity, if locally the curvature of spacetime is always flat?

I imagine a point-like particle can only experience the local properties of spacetime. But locally there is no curvature and no gravity, as it is often stated that Locally, as expressed in the ...
9
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8answers
1k 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 ...
9
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3answers
2k views

Why do we say “Spacetime Curvature is Gravity”?

Although "Spacetime Curvature is Gravity" is the first statement that is told to an infant by the "popular science", I believe that really really misrepresents what GR has to say. I am posting this ...
8
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4answers
5k 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 ...
8
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3answers
1k views

Can one assign an equivalence principle of some kind to the EM field?

Introduction: Consider the EM field. There was a time when the field was defined in a similar manner to that of the gravitational field. This changed when the view on gravitation evolved to this ...
8
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3answers
7k views

How did Einstein derive general relativity?

How did Einstein derive general relativity (GR)? Did he use: the equivalence principle? The principle of least action? Anything else? Note, I'm not looking for a full mathematical derivation of GR! ...
8
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1answer
395 views

Is quantizing acceleration equivalent to quantizing gravity?

It's been said that gravity is locally equivalent to acceleration. It is also been said, that we need to quantize gravity. But doesn't that imply that this would have to be equivalent to quantizing ...
8
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5answers
4k views

Is it correct to say that falling object are standing still?

As I was browsing youtube I came across the BBC video "Brian Cox visits the world's biggest vacuum chamber - Human Universe: Episode 4 Preview - BBC Two" He drops a bowling ball and a feather in a ...
8
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3answers
2k views

Active gravitational mass of the electron

In PSE here electrons are added to a sphere and gravitational modifications are expected. My question is: Is there any experiment that show that a negatively charged object is source of a stronger ...
8
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2answers
434 views

Silly doubt about tidal effects and Einstein Field Equations

Suppose I give you the following line element: $$ ds^{2} = -(1+2\phi)dt^{2}+(1-2\phi)(dx^{2}+dy^{2}+dz^{2}) \tag{1}$$ Without saying anything you might think that this is just another line element ...
8
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1answer
507 views

Is special relativity compatible with Einstein's Equivalence Principle (EEP)?

We know that, according to EEP, an emitted photon would travel a curvilinear path in a uniformly accelerated compartment as well as a compartment that has been located in a uniform gravitational field ...
8
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1answer
812 views

Why must inertial mass and gravitational mass be equal, not just proportional?

I'm certain I'm just under a grave misapprehension. I really just don't understand how one can conclude that inertial mass is equal to gravitational mass. It seems to me that we've simply scaled our "...
8
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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 ...
8
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2answers
781 views

General covariance from the equivalence principle

Einstein's equivalence principle (EEP) tells us that there is no way in principle to locally distinguish between inertial acceleration and the effects of a gravitational field by carrying out any non-...
7
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4answers
1k views

Some confusion regarding the equivalence principle

If I'm right then principle of equivalence says that there is no difference between the experiments performed in a room present in a gravitational field and an identical room accelerating upwards with ...
7
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5answers
743 views

Taking selfies while falling, would you be able to notice a horizon before hitting a singularity?

I am generally interested in the role of "pings"(0a) between participants (a.k.a. "signal roundtrips"(0b), as familiar for instance from Synge's "five point curvature detector") in the determination ...
7
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4answers
927 views

Why do gravitational mass and inertial mass appear to be indistinguishable? [duplicate]

I have learnt that heavier the object is (the more gravitational mass it has), the more resistance to the change of motion it is (the more inertial mass it has). I can accept this fact but I can't ...
7
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3answers
897 views

What is “a general covariant formulation of newtonian mechanics”?

I am a little confused: I read that there are general covariant formulations of Newtonian mechanics (e.g. here). I always thought: 1) A theory is covariant with respect to a group of transformations ...
7
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1answer
1k views

Difference between Fermi and Riemann normal coordinates

What is the difference between Fermi normal coordinates and Riemann normal coordinates? Which one of them is related to the vanishing of the Christoffel symbols?
7
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2answers
391 views

Equivalence principle and the meaning of the coordinate speed of light

Short version: Does the equivalence principle give us a means to tell if variations in the coordinate speed of light have absolute or only relative significance? Background In general relativity ...
7
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1answer
423 views

Electromagnetism and equivalence principle?

The Maxwell equations for the electromagnetic Potential $A_\mu$ are (using Lorenz gauge). $$\nabla_\mu \nabla^\mu A^\nu - R^{\nu \mu} A_\mu = j^\nu$$ However the equivalence principle states: No local ...
7
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4answers
5k views

Counteracting g-force

Is it possible to counter-act g-force for a jet-pilot, by him putting on a scuba-diving suit and filling the cockpit with water? On earth we are constantly pulled down, or accelerated with one g. In ...
7
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2answers
157 views

Einstein's initial clue that spacetime is curved [closed]

I did General Relatively years ago at Uni. I have revised a lot of the maths demo Dirac''s book. It is incredible the leap in thought to noting from the Bianchi identities that the curvature term's on ...
7
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4answers
5k views

Why acceleration is not relative in General Relativity?

I was thinking of it, If I say: "I'm moving at a velocity $v_1$ relative to a reference frame $M$ then the acceleration will be the derivative of $v_1$ relative to the reference frame $M$." In other ...
7
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2answers
768 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 ...
7
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1answer
167 views

Graviton detector thought experiment

I was recently thinking of a thought experiment: Assumptions Graviton detectors can exist The equivalence principle will hold in the final theory of quantum gravity We can accelerate the graviton ...
6
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8answers
3k 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 ...
6
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3answers
766 views

Confusion with the equivalence principle and inertial frames

This is an explanation of the equivalence principle. A. says accelerating frames can simulate gravity. Then, I am confused. Are the frames presented in A. inertial frames? Compared to B. I have the ...
6
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3answers
32k 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 ...
6
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4answers
613 views

Is there a distinguished reference system, after all?

The equivalence principle, being the main postulate upon which the general relativity theory rests, basically states that all reference systems are equivalent, because pseudo forces can (locally) be ...
6
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1answer
353 views

Geometric formulation of the equivalence principle

Let $(M,g)$ be a $4$-dimensional Lorentzian manifold. It is well know that given $(U,\psi=(x^1,\ldots,x^4))$ local chart around some $p\in M$, it is posible to find a change of coordinates given by $(...
6
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2answers
3k views

Where is Strong Equivalence Principle stronger than Weak Equivalence Principle?

Where is Strong Equivalence Principle stronger than Weak Equivalence Principle? In my note, the two equivalence principles are stated as follows Weak Equivalence Principle: Gravitational and ...