Nonlinear Schrodinger equation and linearity of Q.M.

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Discussion Overview

The discussion revolves around the existence of nonlinear Schrödinger equations, specifically the Gross-Pitaevskii equation, in the context of quantum mechanics (Q.M.), which is traditionally viewed as a linear theory. Participants explore the implications of this nonlinearity and its relationship to quantum field theory.

Discussion Character

  • Debate/contested, Conceptual clarification, Technical explanation

Main Points Raised

  • Some participants note that while Q.M. is fundamentally a linear theory, nonlinear Schrödinger equations can arise as approximations from quantum field theory.
  • One participant expresses concern that if linearity is violated, it could lead to non-reversible processes, potential cloning of quantum states, and faster-than-light signaling.
  • Another participant asserts that the Schrödinger equation remains exact within quantum field theory, suggesting that any nonlinearity is an approximation for practical calculations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of nonlinearity in quantum mechanics, with some expressing concerns about its consequences while others maintain that linearity holds in exact calculations.

Contextual Notes

The discussion highlights the dependence on definitions of linearity and the context in which nonlinear equations are applied, as well as the unresolved implications of nonlinearity in quantum mechanics.

Buddha_the_Scientist
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Hello all,
you may already know that Q.M. is a linear theory however there is something called nonlinear Sch. eq. for example Gross-Pitaevskii equation. How can such a thing exist considering that Q.M. is a strictly linear theory.
Cheers.
 
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It is an approximation obtainable from quantum field theory. This is nothing special; many PDEs arise in this way. For example, the Navier-Stokes equations are also approximations obtainable from QFT.
 
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Thank you Prof. Neumaier.
So it is an approximation for ease of calculations. Because I feel like if linearity is violated the theory shouldn't be reversible which might have serious consequences, also it might be possible to clone quantum states and faster than light signalling must be possible. But you are saying if one needs an exact calculation everything would be eventually linear, right?
 
Buddha_the_Scientist said:
if one needs an exact calculation everything would be eventually linear, right?
Yes, the Schroedinger equation still holds exactly in quantum field theory.
 

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