Equivalence Principle

The Equivalence Principle is a fundamental principle of physics which states that gravity operates as if the observer was in an upwardly accelerating frame of reference. Despite that it takes more force to move a more massive object through space (Newton's Second Law), when in free-fall bodies of different masses 'fall' at the same rate without exhibition of different inertial resistances, just as if the bodies were inert and the Earth were accelerating upwards into them. The Equivalence Principle is also used to explain why observers in free-fall feel weightless, rather than feeling their own weight pulling them down (ie. while in a Zero G plane). Experiments with light and atomic clocks at various scales have further confirmed the nature of the Equivalence Principle.

=Description=

Thought Experiments Behind Einstein's Theory
A Space.com article Relativity: The Thought Experiments Behind Einstein's Theory (Archive) by astrophysicist Paul Sutter (bio) explains that under a plain interpretation of how Newtonian Gravity pulls on objects, a body with twice the mass of another should fall faster. Newtonian Gravity requires a separation of inertial and gravitational mass and their equivalence for bodies to fall equally. It is suggested that this is an ad-hoc mechanism to explain physical phenomena.

4th Cosmology School at Cracow, Poland
See this slide from the Introduction to Cosmology by the 4th Cosmology School at Cracow, Poland (Archive):



Coincidence in General Relativity
The Equivalence Principle is also said by some to be a coincidence in General Relativity. See the following quotes.

Ryan Martin
On p.112 of Introductory Physics: Building Models to Describe Our World (Archive) by astrophysicist Ryan Martin (bio), Et al., it says:

American Institute of Aeronautics and Astronautics
Another quote from a publication of the AIAA on p.99:

Anatoly Alekseyevich Logunov
Theoretical Physicist Anatoly Alekseyevich Logunov (bio) remarks:

https://pdfroom.com/books/the-theory-of-gravity/X623zYb6g4Z

GRT = General Relativity Theory, as defined earlier in the paper:

Classroom Aid - Equivalence Principle
The following video briefly describes the Equivalence Principle (Runtime: 5m):

=Examples=

Water Balloon and Accelerometer
The effect of the surface's upwards acceleration can be seen in a hanging and falling water balloon (left) and a mechanical accelerometer (right).

http://gravityprobe.org/GravityProbe%20Links/Galileo-Undone-Mar-10-2020.pdf (Archive)



Monkey and the Hunter
https://web2.ph.utexas.edu/~gleeson/RelativityNotesChapter8.pdf (Archive)

8.3.1 The Monkey and the Hunter



''Figure 8.3: Monkey and the Hunter A popular lecture demonstration is to fire a projectile at a hanging toy monkey. The monkey is released at the instant that the gun is fired.''

Science at the Amusement Park
https://web.archive.org/web/20200704180931/http://physics.gu.se/LISEBERG/callab.html

Carousel with Swings



Roland Eötvös
In Equivalence of Gravitational and Inertial Mass (Archive) by Benjamin Crowell, Ph.D. (bio), we read about Roland Eötvös' investigation into the Equivalence Principle:



''Figure a: The two pendulum bobs are constructed with equal gravitational masses. If their inertial masses are also equal, then each pendulum should take exactly the same amount of time per swing.''



Figure b: If the cylinders have slightly unequal ratios of inertial to gravitational mass, their trajectories will be a little different.



''Figure c: A simplified drawing of an Eötvös-style experiment. If the two masses, made out of two different substances, have slightly different ratios of inertial to gravitational mass, then the apparatus will twist slightly as the earth spins.''



''Figure d: A more realistic drawing of Braginskii and Panov's experiment. The whole thing was encased in a tall vacuum tube, which was placed in a sealed basement whose temperature was controlled to within 0.02°C. The total mass of the platinum and aluminum test masses, plus the tungsten wire and the balance arms, was only 4.4 g. To detect tiny motions, a laser beam was bounced off of a mirror attached to the wire. There was so little friction that the balance would have taken on the order of several years to calm down completely after being put in place; to stop these vibrations, static electrical forces were applied through the two circular plates to provide very gentle twists on the ellipsoidal mass between them.''

=General Relativity and Accelerating Upwards=

The Equivalence Principle is a fundamental tenet of General Relativity, which describes that the surface of the Earth is accelerating upwards through curved space-time to cause the Equivalence Principle effects as experienced on Earth. The following citations describe why space was decided to be curved in the Round Earth Theory.

Gravity: A Very Short Introduction
From Gravity: A Very Short Introduction (Archive) by Cosmologist Timothy Clifton (bio), we read:

Why Is Spacetime Curved?
In a section titled Why Is Spacetime Curved? of the book Time Travel in Einstein’s Universe by John Richard Gott III (bio), professor of astrophysical sciences at Princeton University, we read:

Tony Goldsmith
Tony Goldsmith, author of a mass-media book Space-time for Absolute Beginners and his Absolute Beginner book series, explains the Equivalence Principle as follows (Archive):

Why Gravity is NOT a Force
This popular science video by Veritasium with over 10 million views explains at the 9:57 mark how in General Relativity you accelerate upwards without changing your spatial coordinates with the General Relativity equations:

Inner Life of Numbers
In a book on how math relates to the universe One to Nine: The Inner Life of Numbers by mathematician Andrew Hodges (bio), he describes that the earth's surface is accelerating upwards against your feet in the geometry of curved space-time (Archive):

Earth’s Surface Accelerates Up (and Out)
A physics student, Berry, came across this subject and made us a brief paper about what he learned in his upper level physics classes, showing the math on how in the globe model of gravity the surface of the earth is actually accelerating upwards (Archive).

Experiments
A great many experiments have taken place in the effort to find violation of the Equivalence Principle. The following are of interest:


 * Gravitational Time Dilation - Time dilates in accordance with the uniform prediction of the Equivalence Principal to various heights
 * Time Dilation by Latitude - The Equivalence Principle is used to explain why time does not dilate at different latitudes due to the different latitudinal speeds of Earth
 * Vertical Michelson-Morley Experiment - Light velocity changes when photons are directed upwards or downwards
 * Pound-Rebka Experiment - The frequency of light red-shifts or blue-shifts when moving upwards or downwards in accordance with Doppler Shift

Variations in Gravity
It is often claimed that gravity varies by location on a larger scale. An assessment finds that the experiments either do not show variations, show unexpected results, or are contradicted by other experiments. See Variations in Gravity