Dirac's equation and anti-matter

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Discussion Overview

The discussion centers on the derivation and implications of Dirac's equation, specifically regarding the existence of antimatter. Participants explore the theoretical foundations of the equation, its relation to symmetry considerations, and the conceptual framework surrounding antimatter in both historical and modern contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asks how Dirac deduced the existence of antimatter from his equation and whether the equation was derived or an empirical law.
  • Another participant explains that Dirac aimed to create a relativistic version of the Schrödinger equation, introducing spinors and noting that the Dirac equation can be derived from symmetry considerations without needing Einstein's field equations.
  • It is mentioned that the equation ##E^2 = (mc^2)^2 + (pc)^2## has two roots, allowing for both positive and negative energy solutions, which leads to the concept of antimatter.
  • A participant questions whether antimatter arises from the 4-component spinor, suggesting that two components represent matter and the other two represent antimatter.
  • Another participant counters that while Dirac spinors can represent antimatter, the existence of antimatter is not limited to this representation, as it can also be found in scalar fields.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between Dirac's equation, spinors, and the existence of antimatter. There is no consensus on the necessity of the 4-component spinor for the existence of antimatter, as some argue it is not exclusive to Dirac's formulation.

Contextual Notes

The discussion highlights the complexity of deriving antimatter from Dirac's equation and the various interpretations of the role of spinors and scalar fields in this context. Some assumptions and definitions remain implicit, and the mathematical steps leading to these conclusions are not fully resolved.

Salamon
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Can someone explain how Dirac was able to deduce that anti-matter exists? How did this follow naturally from Dirac's equation? Did Dirac have to derive his equation or was it just an empirical law of nature like Newton's gravity or Einstein's Field equations?
 
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Dirac's intention was to find a relativistic generalization of the Schrödinger equation which is first order in time derivative. In order to do that he had to introduce new 4-component objects called spinors. At his time this was pioneering guess-work. Today I would say that the Dirac equation follows (almost) uniquely from symmetry considerations, i.e. from the requirement of a Lorentz-covariant wave equation for spin 1/2 fields. Einstein's field equations (GR) a not required, SR is sufficient.

Antimatter followed from the equation

##E^2 = (mc^2)^2 + (pc)^2##

which has two roots, i.e. allows for both positive and negative energy solutions. In addition some handwaving arguments like the Dirac sea, absence of an electron with negative energy equals presence of a positron with positive energy etc. is required. Today the framework of QFT is much more satisfactory to deal with the Dirac equation and antimatter, however one cannot fully avoid the Dirac sea which appears in normal ordering (regularization).
 
tom.stoer said:
Antimatter followed from the equation

##E^2 = (mc^2)^2 + (pc)^2##

Doesn't antimatter follow from the 4-component spinor? 2 components are for matter, the other 2 for antimatter.
 
That's a specific representation for Dirac-spinors. But antimatter exists for scalar fields as well, therefore its existence does not require antimatter.
 

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