Einstein & Relativity: Metric, Tensor Fields, Scalar Product

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Discussion Overview

The discussion revolves around the role of the metric in the context of relativity, particularly its foundational status among tensor fields, and the historical mathematical framework used by Einstein compared to contemporary approaches. It explores theoretical implications, historical context, and mathematical structures relevant to general relativity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Historical

Main Points Raised

  • One participant questions whether all concepts related to tensor fields and scalar products can be derived from the invariance of the metric under coordinate changes.
  • Another participant notes that Einstein utilized Riemann geometry, which was established before his work.
  • A different viewpoint suggests that while the metric allows for the definition of a scalar product and geometric constructions, it is not more fundamental than other tensor fields.
  • It is proposed that connections are a broader concept not reliant on the metric, although the metric can be used to derive a specific connection known as the metric connection.
  • Historical context is provided, indicating that Einstein's mathematical challenges were influenced by the work of Italian geometers, and that he was initially unaware of the Bianchi identities, which affected his formulation of field equations.
  • Further clarification is made that metric compatibility alone does not uniquely define a connection; additional conditions, such as being torsion-free, are necessary.

Areas of Agreement / Disagreement

Participants express differing views on the foundational status of the metric compared to other tensor fields, and there is no consensus on whether the metric's invariance can lead to a complete derivation of related concepts. The historical context of Einstein's mathematical influences is acknowledged, but interpretations of its significance vary.

Contextual Notes

There are limitations regarding the assumptions made about the foundational role of the metric and the definitions of connections. The discussion also highlights the dependence on historical mathematical knowledge that may not be universally recognized or understood in contemporary contexts.

kent davidge
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I was reading a document from the time of Einstein where the metric is referred to as "the fundamental tensor". That made me wonder if it's possible to derive all concepts of tensor field, scalar product, connectivity etc. starting from the requirement that the metric is invariant under change of coordinate patches?

Also, did the mathematics of the time of Einstein and other physicists/mathematicians differ from the current mathematics for Relativity that we encounter currently in Textbooks/etc?
 
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For TOR Einstein applied Riemann geometry that had been established at that time.
 
The metric is not more fundamental than any other tensor field, but it does allow us to define a scalar product and thus to do geometry (e.g. construct geodesics). Connections are a more general concept not dependent on defining a metric, but you can use the metric to construct a unique connection, the metric connection.

The math Einstein used was due mainly due to Italian geometers like Ricci, Levi-Cevita, and Bianchi. Einstein had some trouble finding his field equations because he did not know of the Bianchi identities, which were not widely known outside this Italian school of differential geometers.

https://en.wikipedia.org/wiki/Contracted_Bianchi_identities

Some discussion of the history here.
 
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Daverz said:
but you can use the metric to construct a unique connection, the metric connection.
Just to add that metric compatibility is not sufficient to uniquely define a connection. You also need to impose that the connection is torsion free.
 
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