Why no higher derivative in physics?

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    Derivative Physics
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Discussion Overview

The discussion centers around the question of why higher derivatives beyond the second are rarely encountered in physics, particularly in relation to equations governing physical phenomena. Participants explore theoretical, mathematical, and practical implications of higher derivatives in various contexts, including classical mechanics, wave equations, and fluid dynamics.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants note that while higher derivatives exist conceptually (e.g., jerk, jounce), they are not commonly used in physical equations.
  • Others argue that most physical phenomena can be described adequately with second derivatives, suggesting a practical limitation rather than a theoretical one.
  • A participant mentions that equations like the wave equation and those in general relativity predominantly involve second derivatives.
  • Some contributions highlight that higher derivatives can appear in specific contexts, such as in the theory of elastic bending of plates or in certain numerical methods.
  • There is a discussion about the locality of derivatives, with some participants asserting that higher derivatives describe behavior at points further away, while others challenge this notion, emphasizing that differentiability is a local property.
  • One participant points out that in practical applications, such as in servo drives, jerk is utilized to manage motion profiles, indicating that higher derivatives can have relevance in engineering contexts.
  • Another participant mentions that higher derivatives are sometimes necessary in simulations of complex systems, like thin-film flow, without significant issues arising from their use.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and application of higher derivatives in physics. While some acknowledge their existence and occasional use, others emphasize their rarity and question their relevance in most physical contexts. The discussion remains unresolved regarding the fundamental reasons for the limited presence of higher derivatives in physical equations.

Contextual Notes

Some participants highlight that the discussion may depend on specific definitions of derivatives and the contexts in which they are applied, suggesting that the scope of the question could be narrowed to particular areas of physics.

  • #31
Ksitov said:
Dear all,

I'm asking why there is no higher derivative than two in physics ? I never encountered a third (time or space) derivative in physics.

Have you some litterature about this?

Thank you.

Regards.

I would say the short answer is that the all phenomena can be represented as a perturbation upon a differential equation having constant coefficients. The solutions of such a differential equation map out to functions that are the exponential functions (i.e., of the natural logarithm base, e) of the input variable scaled by the values that are the root of the polynomial equation that corresponds to the original differential equation such that the order of the differential is the power of the polynomial term (this is standard material covered by a course in differential equations). Now, for a polynomial with real coefficients, the roots must either be real values or pairs of complex conjugate values, and those pairs when multiplied together yield a quadratic factor; hence whatever is modeled mathematically can be resolved down to a function in which is no more than quadratic in nature, and hence, all that is needed to describe it is the use of at most the 2nd differential.
 
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  • #32
The Blasius equation relates to boundary layer flow on along a semi-infinite plate

2f''' + f''f = 0
 

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