# What is Equivalence principle: Definition and 155 Discussions

In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.

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1. ### I How Do Interparticle Interactions Affect Buoyant Force in Gases?

This is for an article I'm writing with other members of my lab. There's a section on the equivalence principle applied to a balloon attached to an accelerated reference frame. I'd like to talk about any possible differences in buoyant behavior for different gas models (ideal, van der Waals...
2. ### I Is an accelerating frame the same as an inertial frame at a point?

If we have an observer that is accelerating in one direction (perhaps a rocket ship accelerating towards the sun), would its reference frame be identical to an observer at the same point that is not accelerating, but has the same instantaneous velocity? In other words, is an accelerating...

4. ### I Explaining how the Higgs fields gives particles mass

Physicists say the Higgs Field is like syrup and slows particles down from the speed of light. Wouldn’t it be easier and more correct to say there are no particles, just fields, and the strength of the coupling of the electron, photon, quark etc. fields with the Higgs field determines their...
5. ### I Wilbert Zweistein's Objection to Equivalence Principle

I'd like to share the following competition problem: My first reaction was that the thermometer will be able to measure heating due to the Unruh effect whilst his lab is accelerating.
6. ### B The Mini Rocket Experiment: A Test of Einstein's Equivalence Principle?

Alright, this is my idea of an experiment that could distinguish between the examples of the EP. Now, obviously I'm wrong (I don't say this because of your acceptance, i say this because I've tried with other ideas but they were wrong). The thing here is that i don't see where I'm wrong, and i...
7. ### I What if Einstein equivalence principle is proven wrong one day?

What would be the consequences of such thing? How it will affect physics theories and the world?
8. ### B Equivalence Principle and Rindler Horizons

I just thought of this. I'm not an expert, so cut me some slack if I get something wrong. Given: 1) Acceleration is supposed to produce a Rindler horizon. 2) If I understand it correctly, under Relativity and/or the Equivalence Principle, standing on (for example) the Earth is the same as...
9. ### I Equivalence principle question

The equivalence principle states that a person stood on Earth would experience “gravity” the same as if he was in an elevator in space traveling at 1g. I get this. but when Einstein was first exploring this, I read he came to the realisation that a person free falling on Earth (if in a vacuum)...
10. ### I Wavelength of Light Ray Affected by Gravity

I was reading Einstein's 1911 paper named "On the Influence of Gravitation on the Propagation of Light" when stated the formula for frequencies measured by observers at different fixed positions (heights) on Earth surface. One observer is at the origin of some coordinate system and measures a...
11. ### B Simple reasoning that the equivalence principle suggests curvature

I am a high-school teacher and a PhD. student. I am looking for ways to introduce my students to GR. In my treatment, I speak about the equivalence principle and later about curvature in general and consequently that of spacetime. I am missing a connection of these two parts that would be...
12. ### B Question regarding the equivalence principle

Hello, So I read that a person in a rocket accelerating at 9.8m/s^2 would feel the same pull downwards as a person standing on the Earth's surface. However, I can think of a few instances where you could tell the difference: - If you measure g (9.8N/kg) on Earth, you will notice that the...
13. ### A Gravitational Potential Energy & the Equivalence Principle

First, in section 20.4, after listing all the things gravitational potential energy does not do, they say the equivalence principle forbids it being localized. I thought I understood the equivalence principle, but maybe I don’t. Any comments explaining that would be appreciated. Second, they...
14. ### B Special Relativity & Einstein's Equivalence Principle

I was recommended a paper: <https://digital.library.adelaide.edu.au/dspace/bitstream/2440/101285/3/hdl_101285.pdf>. And in the opening sentence read: "The Einstein equivalence principle (EEP) is at the heart of special relativity." To me this didn't make sense. Firstly because Einstein...
15. ### I Evidence of Strong Equivalence Principle Violations?

This paper appears to be a major break though in observational evidence of a strong equivalence principle violation, something predicted in MOND and contrary to general relativity. The analysis is model dependent, but it seems to rule out the most plausible conventional GR based alternatives...
16. ### B Understanding Strong Equivalence Principle Errors & Misunderstandings

Please help me understand all the errors and misunderstandings, contained in the following paragraphs: 1. When a gravitational wave passes through earth, it 'squashes' the form of earth, hence gravity geometry is changed, hence gravity at any given point on Earth change at that moment, be it a...
17. ### I Equivalence principle and the Uniqueness theorem

We work with Maxwell's equations in the frequency domain. Let's consider a bounded open domain ## V ## with boundary ## \partial V ##. 1. The equivalence theorem tells me that if the field sources in ## V ## are assigned and if the fields in the points of ## \partial V ## are assigned, then I...
18. ### I Equivalence Principle & Geometry: Ben Crowell's General Relativity

In the first sentence of Chapter 2 in Ben Crowell's "General Relativity" he states: "The geometrical treatment of space, time, and gravity only requires as its basis the equivalence of inertial and gravitational mass". This is stated as if it's an obvious fact, but I don't understand why. Why...
19. ### I Equivalence principle: an electric charge and a coil

The near-range magnetic field ##\vec{B}## of a point charge ##q## at distance ##\vec{r}##, moving at a non-relativistic velocity ##\vec{v}##, is given by $$\vec{B}=\frac{q}{4\pi\epsilon_0c^2}\frac{\vec{v}\times\hat{r}}{r^2}.$$ Faraday's law of induction for the induced EMF ##V_c## in a coil...
20. ### I Falling EM system contradicts the equivalence principle?

The following is an improved version of my previous post https://www.physicsforums.com/threads/falling-electric-dipole-contradicts-the-equivalence-principle.964594/ Consider the following system comprising a particle on the left with charge ##+q## that is a large distance ##d## away from two...
21. ### I Gravitational time dilation using an accelerating light-clock

I've been trying to understand gravitational time dilation by considering a light-clock of length ##l## undergoing an equivalent acceleration ##a## from rest along the direction of the bouncing light pulse. I find that the time ##t## that the light pulse takes to travel to the forward receding...
22. ### The Equivalence Principle -- Is this a way to distinguish between a gravitational field and an accelerated rocket?

If we are in a cabine in a gravitational field and inside, we have a racket and a ball. We put strings in each side of the racket and we connect the racket to the ceiling of the cabine. This strings only allows us to keep the weight of the racket. Then, we drop a ball to the racket. We do this...
23. ### I Equivalence Principle: Locally Flat Spacetimes Explained

Hi all, I have ran into some confusion about the equivalence principle; perhaps I should state what I understand and then proceed to ask questions. It is my understanding that the equivalence principle states that spacetimes are locally Minkowski, and so the rules of SR apply in that locality...
24. ### I Gravitational wave interactions and the equivalence principle

According to wikipedia, the strong equivalence principle states “the gravitational motion of a small test body depends only on its initial position in space time and velocity, and not on its constitution, and the outcome of any local experiment (gravitational or not) in a freely falling...
25. ### I The Equivalence Principle as a Starting point of GR

Hello World, I have understood the following: in SR, time intervals and space intervals (distances, lengths) are relative and inertial reference frame dependent. Space and time is not absolute anymore. However, acceleration is still absolute: different inertial frames see the same acceleration...
26. ### I Falling electric dipole contradicts the equivalence principle?

Consider an electric dipole consisting of charges ##q## and ##-q##, both of mass ##m##, separated by a distance ##d##. If the dipole is given an acceleration ##a## perpendicular to its moment the total electric force on it, due to each charge acting on the other, is given approximately by...
27. ### I Einstein's Equivalence Principle: Freely Falling Local Frames

Einstein's equivalence principle states that: The sets of inertial frames in the real world that correspond to (portions of) the ideal set of inertial frames discussed in special relativity consist of freely falling local frames. In other words,can we say that since all the local frames are in...
28. ### I Equivalence Principle: Controversy & Misinterpretations

Is there a controversy over the EP? What I mean is: is it considered to be false beyond any doubts or on the contrary it is absolutely true and doubts about its validity are only misinterpretations? I know that the question it's not so simple because there are many EPs, strong, weak, etc. but I...
29. ### I Local meaning of the equivalence principle

[Moderator's note: Spun off from another thread due to topic change.] Can I say that "being at rest in a uniform gravitational field is locally equivalent to accelerating in flat spacetime, free falling in a uniform gravitational field is locally equivalent to nonacceleration in flat spacetime ?"
30. ### A Why doesn't an electron sitting on the Earth radiate?

I have some puzzles when I combine the Equivalence principle and Unruh's effect. Equivalence principle says that acceleration is equivalent to a uniformly gravitational field, or locally equivalent to gravitational field. Then the electron sitting on Earth should have Unruh's effect, i.e...
31. ### I Time dilation vs the Equivalence principle

Some time ago there was a similar thread https://www.physicsforums.com/threads/clock-hypothesis-gravity-time-dilation-and-equivalence-principle.929838/ but what I want to discuss is similar but not the same and I would like to specify my question in such way, that it hopefully won't go sideways...
32. ### I Can anyone help me understand the Equivalence Principle

Equivalence principles explains why lite bend towards massive objects, Einstein uses a moving lift to illustrate this, the light will seem to be bending if the lift is moving, but for a stationary lift, it will not because the position it strikes is stationary. So I think it is not correct...
33. ### I Inertial & non-inertial frames & the principle of equivalence

One particular form of the equivalence principle states that The laws of physics for freely falling particles in a gravitational field are locally indistinguishable from those in a uniformly accelerating frame in Minkowski spacetime My question is, does one arrive at this conclusion from a...
34. ### I Questions about the general principle of relativity

One of the founding principles in GR is the principle of general relativity, which loosely states that all coordinate frames (inertial and non-inertial) are equivalent in the sense that the laws of physics are invariant. My question is, does the justification for this come from Einstein's...
35. ### I How Does Energy Source Curvature? Exploring the EEP

TL;DR Why does the Einstein equivalence principle imply that all forms of (non-gravitational) energy source curvature? Now, as understand it, the Einstein equivalence principle (EEP) implies (or at least suggests) that gravity is the manifestation of spacetime curvature, the reason being that...
36. ### I Understanding the Einstein Equivalence Principle

The Einstein equivalence principle (EEP) states that “The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its position in spacetime.” I’m trying to make sure I’ve understood this correctly. I’m I correct to...
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### I Clock hypothesis, gravity time dilation and Equivalence Principle

1. The Clock Hypothesis states that the rate of a clock does not depend on its acceleration but only on its instantaneous velocity. This has been experimentally verified at very high accelerations. 2. A clock in a gravitational field experiences time dilation and runs slower that one not in a...
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### I Spinning disk, length contraction, & equivalence principle

Suppose we have a spinning disk with a very fast spin, an observer in the center, and an observer on the edge. Suppose that the observer on the edge measures the circumference of the spinning disk. (1) Now, the observer on the edge at a given instant will be moving at a faster speed than the...
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### I Really parabolic light ray in an accelerated elevator?

In an elevator "vertically" accelerated at g in outer space, the equivalence principle says a "horizontal" light ray in the elevator looks like a parabola. I completely understand that the light ray is curved but don't understand why the deflected light ray is an exactly parabola. Almost all...
40. ### B Einstein's Equivalence Principle: What's New?

The equivalence principle states that the laws of physics are the same in any inertial frame. Translating this into mathematics language, the equivalence principle states that a given equation should retain its form when one transform between the coordinates of two intertial frames, correct...
41. ### B Does a moving bar magnet create charges?

So I understand that a charged particle moving relative to me has a magnetic field, and does not have one if its stationary relative to me... So is it true that a magnet moving relative to me has a charge? Say I took a bar magnet and threw it away from me way out in space, would it acquire a...
42. ### I Einstein's Equivalence Principle.

So if someone is in an elevator moving up at g in outer space, vs someone in an elevator in a uniform gravitational field of g, there is no way telling whether one is in an elevator in outer space or resting on earth. But what is the big insight about this? Isn't this true classically as well...
43. ### B Bending of light in a gravitational field

I have a few conceptual issues following a standard thought experiment to argue why light bends in a gravitational field and I'm hoping I can clear them up here. Consider an observer in a lift in free-fall in a uniform gravitational field and an observer at rest in the uniform gravitational...
44. ### I How can the Equivalence Principle hold when we consider tidal forces?

Tidal Forces: "It arises because the gravitational force exerted by one body on another is not constant across it". which implicitly implies that the acceleration is not constant on that body. Equivalence Principle: "weightlessness sensation occurs when one free falls in gravity" - which...
45. ### A Gravitational Redshift in Newtonian Equivalence Principle?

So I was reading that the equivalence principle of Newton doesn't work because of a thought experiment. They said that an experimenter shoots bullets( 1 per second) from the bottom of the elevator to the top. This happens in outerspace where the elevator moves up with accleration g. And this...
46. ### I Detecting Earth in an Elevator: Thought Experiment

I have in mind thought experiment where physicist is in elevator falling towards the Earth. Question would be if he is not allowed to look outside, how would he detect the presence of the planet? Let's not take in consideration tidal forces and assume he is taking local measurements during small...
47. ### I About the Equivalence Principle

Hey all, I was trying to compare the similarity between time dilation due to gravity and the scenario of an accelerating rocket through the equivalence principle and there's something tha blocks my understanding about their similarity. I found a good distance-time graph of the accelerating...
48. ### The World’s Largest Vacuum Chamber Settles an Old Question

Recently I came across an excellent video that brings to life an age-old physics teaching theme... The world’s largest vacuum chamber is NASA’s Space Power Facility in Ohio, pictured below. They remove around 30 tons of air from the chamber in order to test equipment during in-space conditions...
49. ### I Need for GR: Historical Indications

Hi. There were theoretical (find transformations under which Maxwell's equations remain invariant) and experimental (speed of light is constant, Michelson-Morley) indications that made the development of SR inevitable. But what about GR? Was there a "need" for this theory or was Einstein just...
50. ### I Meaning of the equivalence principle in General relativity

It is known that equivalence principle is good for understaning of general relativity (GR). This means comparison of the elevator and falling in homogenic gravitational field. But, I here somewhere something like "in the higher lever of GR, let us forget on principle of equivalence". How it is...