# Questions tagged [equivalence-principle]

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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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### 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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### 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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### Quick check: relativity, rockets, clocks and the equivalence principle

One common thought experiment that introduces relativity on gravitational fields is the "clocks on an accelerating rocket": Paraphrasing Mr. Feynman: Suppose a rocket, with two clocks, one on each ...
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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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### Different versions of Einstein's equivalence principle

As I understand it, there are two versions of Einstein's equivalence principle. The first states that "Locally, a frame in free-fall in a gravitational field is equivalent to an inertial frame in ...
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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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### Acceleration in Space

Lets assume a body with a certain mass is in a region of space that has zero $g$ (or pretty close to zero $g$). Would acceleration in a straight line produce $g$-forces? Would that mass acquire ...
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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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### 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 ...
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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 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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### 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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### How does the Equivalence Principle imply that derivatives of the metric vanish in a freely falling frame?

Why do the first derivatives of $g_{\mu\nu}$ vanish in a freely falling coordinate system? I would like to start from the Equivalence Principle that for any point in spacetime there exists a locally ...
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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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### Path of light rays in uniform velocity elevator

I have been told that light rays seen from inside an accelerated elevator will seen as following a curved path, parabola. Like the image below. But what happens when a pulse of light entered into a ...
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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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### 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 ...
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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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### 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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### 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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### 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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### 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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### Equivalence Principle and bending of light

Basically, the equivalence principle (EP) states that if someone is in a rocket in empty space with acceleration $g$ equal to that at the surface of the earth, any experiment he does cannot ...
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### Einstein's Principle of Equivalence - Infinite Acceleration/Energy?

Hello and thank you for your time. I've been wondering about Einstein's Equivalence Principle core idea. That is, from experiment alone, one cannot differentiate between a gravitational field and a ...
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### Some confusions about principle of equivalence

By principle of equivalence, an accelerated system and in general a non-inertial system of reference is equivalent to a certain gravitational field. If there is gravitational field, spacetime is then ...
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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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### 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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### Dark Matter and Equivalence Principle

Dark Matter effects show up in galaxies. Why isn't it considered as a violation of equivalence principle instead of assuming all these extra non-interacting particles as gravitational mass and ...
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### Showing $m_I = m_g$ follows from the equivalence principle

The inertial mass of an object is defined as its resistance to acceleration by $\vec{F}_{net} = m_I\vec{a}$. The gravitational mass of an object is defined as the scaling of the gravitational force an ...
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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 ...
190 views

### Will a stationary charge in a uniform gravitational field radiate?

Imagine a charged particle Q standing still in a uniform gravitational field g (i.e. on a table on the surface of a planet). Assume observers K1 and K2 when K1 is in free-fall and K2 is co-stationary ...
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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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### 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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### Why do we feel resistance to gravity rather than the acceleration of gravity?

What's the reason it behaves differently from all other forces? What I mean is, if you're in orbit you're accelerating toward the earth at almost 9.8m/s^2, but you feel nothing. If you are riding a ...
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### 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{...
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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 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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### On Einstein's equivalence principles

There are two foundative Equivalence Principles in General relativity: Weak Equivalence Principle (WEP): the dynamics of a test particle in a gravitational field is independent of its mass and ...
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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, ...