Need for GR: Historical Indications

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Discussion Overview

The discussion revolves around the historical context and necessity of General Relativity (GR) in light of Special Relativity (SR) and existing gravitational theories. Participants explore theoretical and experimental motivations for GR, including Einstein's thought processes and the implications of SR on Newtonian gravity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Historical

Main Points Raised

  • Some participants suggest that Einstein's dissatisfaction with the nonlocal nature of Newton's law of gravitation was a significant theoretical motivation for GR.
  • Others argue that the inconsistency between SR and Newtonian gravity necessitated a new theory of gravity, prompting Einstein's exploration of GR.
  • It is noted that Einstein initially attempted to construct a simpler theory of gravity that ultimately contradicted observations, leading to the development of the equivalence principle.
  • Some participants highlight that the precession of Mercury's perihelion was recognized as a problem prior to GR, but question whether this alone justified the need for a radical new theory.
  • There is mention of the "experiment in Principe" as a pivotal moment that demonstrated GR's applicability to scientific problems, although the exact nature of this experiment is debated.
  • Some participants express differing interpretations of what constitutes a "need" for GR, with references to Mach's principle as a motivating factor for Einstein.
  • There are discussions about the implications of Mach's principle and its relevance to the development of the Einstein field equations, with some asserting that Einstein did not fully succeed in this regard.
  • Participants also engage in a discussion about the geographical context of the observations made during the experiment in Principe, seeking clarification on the location.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of GR, with some arguing that historical context shows no pressing need before the experiment in Principe, while others contend that the inconsistencies between SR and Newtonian gravity created an implicit need for a new theory. The discussion remains unresolved regarding the extent of Einstein's motivations and the implications of Mach's principle.

Contextual Notes

Limitations include varying interpretations of historical motivations for GR, the dependence on definitions of "need," and unresolved questions about the specifics of the experiment in Principe.

greypilgrim
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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 curious what would happen if he applied the equivalence principle exhaustively?

From the theoretical side, I assume Einstein was not happy with the fact that Newton's law of gravitation is nonlocal. Experimentally, I can only think of the perihelion precession of Mercury that was known before the theory and couldn't be explained otherwise. Einstein's other two classical tests, light deflection and gravitational redshift, hadn't been observed before.

But those two indications don't seem to be enough for me to convince someone that a new theory as radical as GR is needed. So was there other evidence?
 
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greypilgrim said:
From the theoretical side, I assume Einstein was not happy with the fact that Newton's law of gravitation is nonlocal.

It was more than just "not happy". As soon as SR was established, it was obvious that Newtonian gravity as it stands is inconsistent with it. So the natural question was, how do we make a theory of gravity that is consistent with SR. That was Einstein's primary impetus towards GR.

greypilgrim said:
those two indications don't seem to be enough for me to convince someone that a new theory as radical as GR is needed.

Initially it didn't convince Einstein either; he tried a simpler way of constructing a theory of gravity as an inverse square force with a light-speed time delay, similar to the way electrodynamics can be formulated in terms of retarded potentials. Unfortunately this didn't work: the simple theory he constructed this way grossly contradicted observation--for example, it didn't predict stable orbits for planets.

It was only after these abortive attempts that Einstein had what he called "the happiest thought of my life"--as he put it, "if a person falls freely, they will not feel their own weight". This led to the equivalence principle, which turned out to be the key idea that started him on the road to what we now know as GR.
 
Specifically, the things that fell out of SR that required Newtonian gravity to be revisited were:

1 - Gravitational fields are caused by mass, but SR revealed the matter-energy equivalence. Thus energy, in all of its forms, should be sources of gravity.

2 - Gravitation fields contain gravitational potential energy, but SR revealed that energy cannot travel faster than c. This conflicts with Newtonian action-at-a-distance.

3 - Gravitation fields contain gravitation potential energy, thus the presence of gravitation field is a source of field itself. This is responsible for the non-linear (& difficult to solve) nature of the Einstein field equations. This is why we cannot simply emulate electrostatics (inverse square law & fields traveling a c). To find the net field of a collection of charges, we can consider them one at a time and apply linear superposition. We can do this because charges create fields, but the fields themselves are not sources of field.
 
greypilgrim said:
But what about GR? Was there a "need" for this theory or was Einstein just curious what would happen if he applied the equivalence principle exhaustively?
Historically speaking there was no need.

Only after the experiment in Principe did it prove a solution to a scientific problem.
 
MeJennifer said:
Historically speaking there was no need.

Only after the experiment in Principe did it prove a solution to a scientific problem.
Perhaps you have a more conservative interpretation of "need".

As has been pointed out, as soon as SR was established, it was clear that Newtonian gravity was inconsistent with it.

In physics, inconsistencies between theories need resolution.
 
the_emi_guy said:
As has been pointed out, as soon as SR was established...
Perhaps you have a more conservative interpretation of "established".

And by the way PPN is inconsistent only in the most extreme gravitational experiments.
 
MeJennifer said:
Only after the experiment in Principe did it prove a solution to a scientific problem.
Hi @MeJennifer:

Can you provide some details about the "experiment in Principe"? Citing a specific reference would be very helpful.

Regards,
Buzz
 
MeJennifer said:
Historically speaking there was no need.

Only after the experiment in Principe did it prove a solution to a scientific problem.

The OP mentioned the anomalous(according to Newtonian gravity) precession of the perihelion of Mercury, which had been recognized as a problem since the 1850s.

Personally, I feel that Einstein's stated motivations of extending the principle of relativity to include something like Mach's principle, and of finding relativistic field equations for gravity is enough of a need.
 
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The Bill said:
... to include something like Mach's principle, ...
In that respect he failed miserably.
Take for instance Gödel's rotating universe.
 
  • #11
MeJennifer said:
In that respect he failed miserably.
Take for instance Gödel's rotating universe.

Whether he failed or succeeded in this is irrelevant to the fact that Mach's principle helped motivate him to develop the Einstein field equations.
 
  • #12
MeJennifer said:
Sure here are two interesting links:
Hi $MeJennifer:

Thank you much for the two links. I of course knew about the astronomical observations in 1919 to test whether GR was correct about the gravitational bending of light rays. I had forgotten where these observations took place, and that Principe was the name of the location.
The following quote is from Wikipedia:
This eclipse was photographed from the expedition of Sir Arthur Eddington to the island of Principe (off the west coast of Africa).​

I looked at the map of the island on Google maps, and I find it a bit confusing. Apparently the name of the island country is São Tomé and Príncipe, which consists of two main islands and some very small others. The large island is São Tomé, and the other much smaller one is Ilhéu das Rolas. I could find nothing on the maps specifically named Principe. Is there a place on the island specifically named Principe, and is that where the observations took place?

Regards,
Buzz
 
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  • #13
Google Maps (for me, at least) and Wikipedia both label the second-largest island as "Ilha do Principe" = "Island of Principe".

Tp-map.png
 
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