Who understood general relativity before Einstein? - by Brent Meeker - Quora Question Review

This document contains a review of the answer by Brent Meeker on the question in Quora: "Who understood general relativity before Einstein?"
To order to read all the answers select: https://www.quora.com/Who-understood-general-relativity-before-Einstein

• The text in italics is copied from the article.
  
• Immediate followed by some comments
  

Contents

• 1. Answer Review

Reflection

• Reflection 1 - Question Review part 1
• Reflection 1a - Question Review part 2 - General Relativity

1. Answer Review

It’s possible that David Hilbert arrived at Einstein’s equations before or a the same time as Einstein. Einstein made no secret of his work. He had been invited by Hilbert to lecture at Hilbert’s Göttingen University and the stayed at Hilbert’s home. If anything Hilbert learned about the problem from Einstein. Hilbert, being the better mathematician, may have derived Einstein’s equations first but he got side tracked by trying to unify electromagnetism with gravity and so Einstein who had independently solved the problem sent his solution to Hilbert in November 1915 and published first. Hilbert, who published his solution in March 1916 acknowledged Einstein’s priority.

Interesting information.

Does that mean Hilbert understood general relativity before Einstein?

That is the question. The problem is, what is the definition of the word: understood and what means the concept: general relativity.

No. Even Einstein didn’t understand general relativity at first.

How do you know that? What was Einstein missing?

He said that he thought it likely that no one would ever find any exact solutions to the tensor equations. But by Dec 1916 he received a letter from Schwarzschild, was fighting in the trenches in WW1, with the solution for a spherically symmetric field in empty space. Einstein applied his theory to cosmology and inserted the cosmological constant in order that the universe be in equilibrium as was thought at the time. But it was pointed out to him that the equilibrium was unstable and his model would either expand or collapse. Later Einstein and Rosen wrote a paper in which they “proved” that gravitational waves can’t exist. An error was pointed out and PhsRev declined to publish it. Einstein analyzed Schwarzschild’s solution and concluded that black holes couldn’t exist. There is still controversy as to whether black holes have a singularity at the center as the mathematical solutions do. Just recently it was proven that a Kerr-Newman (rotating) black hole is stable against perturbation. So there’s at least a hundred years between writing down the equations and understanding general relativity.

3.

In order to answer the question: "Who understood general relativity before Einstein?" you must first define two concepts i.e. the word "understand" and the concept "General Relativity". To get a feeling what the issues are, start with the following two questions:

1. Who understands how an airplane flies? What is involved.
2. Who understands Newton Mechanics?

IMO to understand how an airplane flies is only possible by performing experiments, first with a simple one and slowly making your experiments more complicated.
You can first explain how an inflatable balloon moves when you release it and why. Then the same with a kite, a glider and finally an airplane, using information from the previous examples and highlighting the differences.
What is not allowed is to use any existing law, because each law requires additional experiments in order to explain, which may be not relevant for the case discussed.

In order to understand Newton's Mechanics more or less the same strategy has to be followed with the difference that first you must define what is Newton Mechanics.
To be more practical the following sequence of experiments is helpfull:

1. A swing. To use a swing requires a person, which pushes at regular intervals, against the child to get the swing higher and higher.
   The purpose is to introduce the concept energy.
2. An atlete throwing a hammer. The most important thing is to describe what happens and why when the athlete releases the hammer.
3. How can you explain the movements the chair-o-plane swing ride carousel. See https://en.wikipedia.org/wiki/Swing_ride In a carousel the chairs are physical connected to the central rotating axis.
4. How is it possible that there are planets which rotate around the Sun.
   This system resembles the previous case, but here the planets are disconnected from the central object and move, more or less, free in space.
5. How is it possible that stars in a cluster rotate around each other.

The important issue is, that general speaking no mathematics is required, to understand physical processes.
The most difficult is the the concept: force of gravity. To explain simple, this force acts instantaneous between both or all objects involved. In reality, the force originates continuous from each object involved but the action is delayed, depending about the distance between both objects.

A whole different situation arises if you want to predict the future i.e. the future positions of for example the planets. Starting point is that you have to measure (and or calculate) at regular intervals the position of all the planets. In fact only one reference frame is used. Using Newton's Law it is than possible to calculate the masses of all the objects involved. Again using Newton's Law it is than possible to calculate the positions of all the planets at a date in the future.

In Reflection 1 - Question Review part 1 the question: "Who understood General Relativity before Einstein?" is not discussed. In stead the question the question "Who understands Newton Mechanics?" is discussed with the aid of 5 experiments. I don't know to what extend these same experiments can be used to understand GR.

A whole different situation arises if you want to predict the future i.e. the future positions of for example the planets using GR (compared with Newton Mechanics) The major issue is that the mathematics using GR is much more complex.

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