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I was wondering, what is the status of Newton's Third Law of Mechanics in modern physics? I'm told that it does not always hold. In that case, when and how does it break down? Thanks.

Molu

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I was wondering, what is the status of Newton's Third Law of Mechanics in modern physics? I'm told that it does not always hold. In that case, when and how does it break down? Thanks.

Molu

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Well, Newton's Third Law is a concept that is both defined and valid in classical physics. So this law always holds.loom91 said:

I was wondering, what is the status of Newton's Third Law of Mechanics in modern physics? I'm told that it does not always hold. In that case, when and how does it break down? Thanks.

Molu

However, when going to special relativity the third Newton Law is also valid. Beware that we are talking about F = dp/dt here and NOT F=ma. After the calculation of the dp/dt term, you shall see that force is no longer parallel to acceleration in special relativity. The reason for this is the presence of the Lorentz factor (the gamma) in special relativity. This factor contains a v²-part in the denominator that is also dependent of time.

For more info click on nr. 11

regards

marlon

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Does it also hold in classical fields? What about quantum fields?

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Just to be clear, i did not say the F = dp/dt is wrong in the case of QM. What i say is that this law is NOT DEFINED for QM-theory.

marlon

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But that's the second law, what about the Third? And also, does the third law work for classical fields (Maxwell/Einstein)? Thanks.Manchot said:

Molu

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No that is very much untrue for several reasons. First of all, you are working with observables here and nut just p, a or F vectors. Secondly, haven't you read my remark on the violation of the HUP ? Doesn't that count ?Manchot said:

marlon

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Over all I think Newton' s third law is valid in qunatum mechanics (whether we don't know it fully or not).

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Since Newton's laws deal with dynamics we need to work with wavefunctions satisfying the time-dependant Schroedinger equation, so for example the wavefunction is a time-evolving superposition of constant momentum eigenstates? How do you find dp/dt of this?

I don't want to rule it out because of the uncertainty principle alone, since we can work with momentum and have no need for position- and the energy/time relation is not so clear cut in my head since time is not an observable like energy.

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