# Questions tagged [equivalence-principle]

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### What mathematical constraints does the equivalence principle impose on GR

I'm trying to determine exactly how the equivalence principle affects GR mathematically (rather than conceptually). I found this StackExchange post which more or less says that the equivalence ...
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### Screening tensor-scalar mechanics with equivalence principle

Modified gravity with scalar degrees of freedom generally requires some screening mechanism to pass solar system tests, and that generally breaks down the equivalence principle. Question: is there ...
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### Einstein's equivalence principle applied to a constant accelerated system comprising two mirrors

Let us consider a system comprising two mirrors with an observer placed in between. The distance from the observer to each of the mirrors is $d$. At $t=0$ the observer emits a light signal in both ...
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### Does a vertical light beam violate Einstein's equivelance principle?

Imagine the classic Elevator thought experiement. Einstein said there's no physical experiement that shows the difference between gravity and upwards acceleration. What if we casted a light beam ...
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### What is the relationship between curved vs flat and Lorentzian vs Newtonian?

Poisson's equation $\Delta \Phi = 4 \pi G \rho$ is not Poincare invariant, hence the need for a new theory of gravity after Special Relativity (SR). The motivation for gravity as curvature in ...
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### Help Understanding this Free Body Diagram (Eotvos Experiment)

I am reading through Hartle's "Introduction to Einstein's General Relativity" and it discusses the Eotvos Experiment in Chapter 6. The free-body diagram (shown below) has me a little puzzled. This ...
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### Do gravitational mass equalling inertial mass when object is at rest just a coincidence?

Rest mass can be thought of as the measure of energy that object posses. This mass that appears in Newton law of motion measures inertia and hence called inertial mass. Gravitational mass is like ...
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### Weightless at the center of mass of two merging black holes?

I have read this question: What is the general relativity explanation for why objects at the center of the Earth are weightless? When $r = 0$ the Christoffel symbol $\Gamma_{tt}^r$ is zero and ...
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### Is there an absurdity in Einstein's equivalence principle? [closed]

A friend of mine, Amir Assarzadeh, upon noticing my thought experiments [1&2], asked me an intriguing question to which he also made an answer defending Einstein's equivalence principle (EEP). ...
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### A possible consequence of LIGO measurement of Gravitational Waves [duplicate]

LIGO measured Gravitational Waves. Presuming here that the Gravitational Waves can be considered as a disturbance or change in a Gravitational Field, as described here on this Quora question: What ...
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### Is quantum gravity consistent with the principles of equivalence and relativity?

I saw an article about physicist testing quantum mechanics in accelerated frames, hoping for insights into quantum gravity. And then I suddenly wondered, is quantum gravity inconsistent with general ...
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### Free fall and gravitational waves

If you take the Earth-Moon system, from what I understand, the Moon for instance is in free fall towards the Earth-Moon center of mass. However Einstein's equivalence principle says that a body in ...
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### Gravitation as force and torsion [duplicate]

To what extend is valid that the equivalence principle and that gravity is locally removable when torsion fields are present? That is, is the equivalence principle (or any version of it) valid when ...
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### Making tidal forces “disappear” via coordinate transformations

I'm watching this introductory lecture on general relativity by Leonard Susskind (see 31:50 onwards for the relevant part) [Video title: General Relativity Lecture 1, channel: Stanford]. To give ...
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### G-force: difference between gravity and acceleration of an object

I'm trying to simulate the perception of weight by a human in a system but I'm struggling to treat gravitational acceleration and other accelerations in the same way. As I understand, when I stand on ...
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### If Einstein was wrong: gravitational mass different from inertial mass [closed]

Assume for a second (well, a few) that Einstein was ever so slightly wrong. That Inertial mass is different from graviational mass. This raises a number of questions, most probably answered but I am ...
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### Is there an absurdity in GR prediction about time dilation inside a thick spherical shell?

In his book, Relativity: The Special and General Theory, Einstein claimed that the clocks in a gravitational field, as well as those located on a rotating disc, run slower solely due to the ...
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### Does acceleration create gravity since inertial mass is found to be the same as gravitational mass?

Does acceleration create gravity since inertial mass is found to be the same as gravitational mass? If not what would account for acceleration behaving like gravity and vise versa?
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### Incorporating Gravity in special relativity and in Newtonian limit [closed]

Consider the following metric: $$ds^{2} = -c^{2}dt^{2} + dx^{2}+dy^{2} + dz^{2}. \tag{1}$$ This is the Minkowski Metric which describes a spacetime without Gravitational interaction. Furthermore, ...
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### Einstein's equivalence principle (EEP) and indefinite acceleration

My first question was going to be: Does EEP hold at all time scales? I believe this was successfully answered at Einstein's Principle of Equivalence - Infinite Acceleration/Energy? The answer ...
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### Why do we know that light must follow a geodesic?

THE CONTEXT: Some context to my question: Einstein once posed the thought experiment of a man falling inside a closed box. Just before the box was dropped, a photon was fired horizontally moving from ...
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### According to GR, gravitational acceleration is illusion. by analogy is the acceleration caused by lorentz force is also an illusion?

Gravity (considred to be a force) causes an acceleration, and the samething goes for lorentz force its causes charged objects to accelerate, So my question is, is lorentz force acceleration, an ...
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### 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 ...
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### Doubt on Newtonian weak field metric, accelerated frames and metric tensor transformation

Suppose we do not have yet General Relativity conclusions (like, Schwarzschild Gemetry and Weak Field Approximation) , but rather, just Minkowski space-time, newtonian gravity, principle of ...
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### If gravity is a fictitious force, could there be a real force counterpart for it atleast locally? [closed]

I wanted to ask the same question in different ways. 1.Consider the below example whereby we have a man M in a box B in outer space. B is acted upon by a force, F. B moves in direction of F. As the ...
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### How much of general relativity does the equivalence principle actually imply, why is it different?

From EP we have that gravity is not a force but a pseudo force, i.e an inertial force due to a gravitational field an accelerating a body independantly of its mass, in other words, the trajectory of ...
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### Time dilation, mass-energy equivalence, and implications for the passage of time [closed]

I'm an amateur physics enthusiast, and there is a question that's been in the back of my mind for some time that I haven't been able to answer on my own. First, I want to define a couple of terms and ...
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### 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 ...
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### Equivalence principle doubt

There is something about Einstein Equivalence Principle that I don't quite get. This is my reasoning: Equivalence principle $\rightarrow$ locally, acceleration is equivalent to a gravitational field ...
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### How does Newtonian Gravity explain the difference between the experience of 1G and zero gravity? Or can it?

Humans can feel the difference between being on earth's ground, and being in space (zero gravity) This can be explained by general relativity, where in (1), force is applied by the ground upwards, ...
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### Do hot gasses actually weigh more than cold gasses in SR?

I was reading: https://plato.stanford.edu/entries/equivME/#PhysMassEnerEqui Where they begin to discuss a weighing a relativistic gas against a cold gas. The idea is that the relativistic gas ...
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### Does the equivalence principle imply that light must move slower when moving away from a massive object?

Thought experiment: Elevator going up at an extreme acceleration, pulse of light bouncing up, and down between mirrors on the floor, and the ceiling. Won't it take light longer to travel from the ...
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### Why does things travel in a straight line in inertial frames?

Why does physical entities travel in straight paths in a flat space-time and in geodesic in curved spacetime? Is it due to Inertia? If it is, then why does waves also follow the same pattern?
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### Entanglement of Black Holes with the outside universe

Black holes are thermal objects which means the density matrix of the black hole is in a thermal state. This means that the black hole has to be maximally entangled with the rest of the universe to ...
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### Can we apply the equivalence principle in electromagnetics to multiple regions simultaneously?

Note: I posted the same question on the electrical engineering stack exchange with no luck so far, possibly because it's quite theoretical, so I'm posting it here again. I'm not sure if it's ...
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### Acceleration in general relativity

Let's say that, from my point of view, another observer is accelerating. Now, from his point of view, he is standing still: all he feels is an overall fictitious force of gravity, which is just a ...
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### Can we feel gravity while free falling?

I watched some video saying that there is still gravity in space station and almost same strong as it on Earth, the reason they feel no gravity is because the centrifugal force of the orbit is same as ...
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### On local inertial frame and acceleration in general relativity

[Q1] In the general relativity, a local inertial frame is mentioned. The local inertial frame is a notion, which is related to (or represents) the equivalent principle. Here, I understand that, in ...
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### 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 ...
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### Equivalence Principle: Why is sitting in space the same as failling in a uniform gravitational field [duplicate]

The equivalence principle states that no experiment can determine whether one is being accelerated or if there is gravity present. This is insightful and intuitive. However, I was sort of surprised. ...
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### Why gravitational mass and inertial mass being same was a problem before Einstein solved it?

Why is it so that Einstein is credited with telling us the reason for the equivalence of inertial and gravitational mass of an object? Did Newton ever make a distinction between the two masses? What ...
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### Does semi-classical gravity obey the equivalence principle?

Question I was recently wondering about semi-classical gravity : $$G_{\mu \nu} = \frac{8 \pi G}{c^4} \langle \hat T_{\mu \nu} \rangle_\psi$$ Does this obey the equivalence principle? My ...
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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 ...
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### Falling bodies in an accelerated elevator

If a sealed elevator were subjected to a massive acceleration in interstellar space and two weights were dropped from ceiling to floor, as far apart as possible on opposite sides of the elevator, ...
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### Is a body in passing gravitational wave still in free fall?

I have a simple apparatus: a box and inside it a test particle (small bullet). If the device is in inertial motion or affected only but (static) gravitational field of another massive body (in free ...
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### Invariance of forces

In Newtonian mechanics, the inertial forces depend on the reference frame and the other forces are invariant. Are they still invariant when special or general relativistic effect are involved?
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### What is proper name for non-inertial forces in GR?

General relativity works in all reference frames, so inertial forces are real in it. And due to the equivalence principle, gravity should be also considered inertial. So what is a good term for the ...
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### How do we know who is moving? [duplicate]

If I am in a free float environment and there is another object at rest relative to me, and then I start to accelerate towards this object, do I know I am the one who is moving, since I am the one who ...