# Questions tagged [equivalence-principle]

The tag has no usage guidance.

85 questions
Filter by
Sorted by
Tagged with
17k 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?
35k 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 ...
759 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 ...
2k 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 ...
7k 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 ...
6k views

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?
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 ...
1k views

### Curved space-time VS change of coordinates in Minkowski space

I'm looking for a rather intuitive explanation (or some references) of the difference between the metric of a curved space-time and the metric of non-inertial frames. Consider an inertial reference ...
1k 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 ...
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 ...
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?
463 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 ...
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 ...
3k views

### When objects fall along geodesic paths of curved space-time, why is there no force acting on them?

On cseligman.com, it is written that So, we see things falling with an acceleration which we call the acceleration of gravity,and thinking that we live in a straight line , uniformly moving or ...
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 ...
1k 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 ...
593 views

### Equivalence principle question

I understand the equivalence principle as "The physics in a freely-falling small laboratory is that of special relativity (SR)." But I'm not quite sure why this is equivalent to "One cannot tell ...
80 views

### EM wave generation from different frames of reference

Take 2 observers and a charged particle, Observer S is stationary to the particle and sees the electric field and no magnetic field as he is stationary to the particle. Observer A decides to ...
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 ...
428 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. ...
238 views

### Equivalence principle for test fields

My question is very simple. We all know that, for a test particle(classical) in a gravitational field, the motion is only determined by the geodesic lines(let's forget about the initial conditions for ...
699 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 $S^\prime$)...
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 ...
402 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 ...
407 views

### Auto-parallel Transport or Principle of Extremum Action?

In an affinely connected spacetime with a metric compatible connection, the equation of the curve in which the tangent vector at each point is the result of the parallel transport of every tangent ...
246 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 ...
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 ...
496 views

### Does GR imply a fundamental difference between gravitational and non-gravitational acceleration?

Does the equivalence principle imply that there is some fundamental difference between acceleration due to gravity and acceleration by other means (because there is no way to 'feel' free fall ...
772 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 ...
422 views

### Shouldn't Torsion be eliminated on the basis of Equivalence Principle?

An affinely connected spacetime with a metric compatible connection can, in principle, have a non-vanishing anti-symmetric part; where the definition of the connection is given by defining parallel ...
955 views

### Time dilation in a gravitational field and the equivalence principle

A clock near the surface of the earth will run slower than one on the top of the mountain. If the equivalence principal tells us that being at rest in a gravitational field is equivalent to being in ...
32k views

### I think I am misunderstanding Einstein's equivalence principle and his elevator

I'm having difficulties understanding why a gravitational acceleration can be guaranteed to be locally equivalent to an accelerating frame. Doesn't it matter on how the force is being applied? If the ...
2k views

### How exactly and WHY does matter affect space-time? [closed]

According to general relativity, inertial mass and gravitational mass are the same, and all accelerated reference frames (such as a uniformly rotating reference frame with its proper time dilation) ...
2k 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 ...
110 views

### Now when we can measure Gravitational Waves, how does the 'Principle of Equivalence' still hold true?

LIGO measured gravitational field waves. The whole thought experiment of Einstein, leading to ‘principle of equivalence’, assumes that there is no way to know inside the rocket that whether its ...
65 views

### How did Einstein generalise the Einstein elevator thought experiment to conclude with the equivalence principle?

Suppose a reference frame S1 is accelerated in the x direction with a constant acceleration g. A second frame S2 is at rest in a homogeneous gravitational field which imparts to all objects an ...
142 views

### Gravity, falling bodies, and the equivalence principle

Why is that bodies in a box accelerating uniformally in space that is sufficiently removed from gravitational fields fall identically to bodies in a box located in a homogeneous gravitational field (e....
279 views

### LIGO discovery: if the space “time” metric is changed, how is it measured? [duplicate]

Regarding the wonderful 2016 news about gravitational waves. Travel time in one arm of the LIGO is ~ 30μs. A gravitational wave affects the arm for some few hundred of these laps. Then for example ...
170 views

### Equivalence Principle holding in Special Relativity? (let alone QFT)

Motivation I am pretty confused of why people are hopeful to find a version of the equivalence principle ("the complete physical equivalence of a gravitational field and a corresponding acceleration ...
12k 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 ...
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 ...
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 ...
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 ...
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 ...
766 views

### Doesn't Einstein's General Relativity violate the spirit of Principle of Relativity?

As is well established, Special Relativity ensures that no observer can ever tell from the experiments he has been doing in his car whether the car is moving or not as long as the car constitutes an ...
405 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 ...