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In QM, would you ever have to deal with forces on i.e particles?

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- Thread starter pivoxa15
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In summary, the concept of force is not directly dealt with in quantum mechanics, as all forces are derived from a potential. The potential is crucial in the Schrodinger Equation and affects the solution to problems. However, the concept of force is obscured in the formalism of QM, and some physicists believe that it needs further development. In stationary states, the time derivative of the square of the wave function is zero, and thinking in terms of probability is important in understanding QM.

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In QM, would you ever have to deal with forces on i.e particles?

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pivoxa15 said:In QM, would you ever have to deal with forces on i.e particles?

You really ought to make sure you understand classical mechanics before doing QM. In Lagrangian/Hamiltonian mechanics, there are no "forces" either. So this really is not a necessary concept in solving the dynamics of a system, classical or quantum.

Zz.

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ZapperZ said:You really ought to make sure you understand classical mechanics before doing QM. In Lagrangian/Hamiltonian mechanics, there are no "forces" either. So this really is not a necessary concept in solving the dynamics of a system, classical or quantum.

Zz.

However, at my uni a second course in classical mechanics is not a prereq for upper QM courses. But you think it should be? If so why?

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In mine too, however, they are taught simultaneously. Also many staff members criticize the decision to teach QM before a better coverage of Analytical Mechanics.pivoxa15 said:However, at my uni a second course in classical mechanics is not a prereq for upper QM courses. But you think it should be? If so why?

The question itself is interesting - I suppose one can define a force operator as the time derivative of the momentum operator, find expectation values etc. In a rather poor year and a half since I took QM1 I have never encountered it.

I've heard once that Pauli's exclusion principle actually creates pressure which can be interpreted as actual force acting on electrons. I wonder how similar it is to classical forces (can you derive it from some field? can can it be considered an elementary force - I guess not but I didn't read very deep into it)

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pivoxa15 said:In QM, would you ever have to deal with forces on i.e particles?

In quantum mechanics we typically don't talk about forces. It's not that the concept of force doesn't exist, but this concept is obscured behind the formalism of QM. Potential, however, is defined in terms of force (recall that the potential is some function whose gradient gives the force on a test mass), and potential is an extremely important concept in QM. It appears in the Schrodinger Equation, and greatly affects the solution to any particular problem. So in that sense, force is a very important concept in QM.

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arunma said:In quantum mechanics we typically don't talk about forces. It's not that the concept of force doesn't exist, but this concept is obscured behind the formalism of QM. Potential, however, is defined in terms of force (recall that the potential is some function whose gradient gives the force on a test mass), and potential is an extremely important concept in QM. It appears in the Schrodinger Equation, and greatly affects the solution to any particular problem. So in that sense, force is a very important concept in QM.

Good point, Potential and force are related by a derivative. But in QM we usually only deal with the potential in our calculations. It is also easier to measure than force? I only know how force is measured classically, that is a=F/m by using a spring balance.

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rlduncan said:In QM physicists use the SE and potential to calculate probabilities. A potential implies a force or acceleration which means a charged particle will radiate.

Er.. come again?

We use the central potential to solve the Schrodinger Equation for a Hydrogen atom. These are

Want another one? Try the Kronig-Penney potential in solid state physics. Solve for that to get your Bloch wavefunction. Where is the "radiation"?

Zz.

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ZapperZ said:Er.. come again?

We use the central potential to solve the Schrodinger Equation for a Hydrogen atom. These arestationarysolutions. Where is there any "radiation"?

Zz.

I agree these are the stationay solutions which are in agreement with hydrogen's spectral lines. I disagree that stationary states exist in a point charge field, which is the case for the hydrogen atom. For in this field, stationary states are mechanically impossible. If the electron is stationary , it will be accelerated and fall to the nucleus. If it is in a dynamic equilibrium with the proton it will radiate and fall to the nucleus.

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Is it the case that stationary here means 0 net force rather than 0 net velocity?

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NeoDevin said:

So in QM I should think about everything in terms of probability? In this case the stationary state is when the rate of change of the probability of the particle with respect to time is 0. I can see now why they saw QM is unintuitive. For one thing, probability is quite unintuitive as most people get probability type problems/puzzles wrong.

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rlduncan said:I agree these are the stationay solutions which are in agreement with hydrogen's spectral lines. I disagree that stationary states exist in a point charge field, which is the case for the hydrogen atom. For in this field, stationary states are mechanically impossible. If the electron is stationary , it will be accelerated and fall to the nucleus. If it is in a dynamic equilibrium with the proton it will radiate and fall to the nucleus.

i am confused on two levels here:

1) the situation you have described has already been disproved - it is called the Bohr model.

2) by stationary states ZapperZ is referring to solutions who's probability density has no time dependence (e.g. standing waves). This is an important result in quantum chemistry.

For example, general solutions to the time-dependent SE in the position representation are stationary states of the form (where the potential carries no time dependence):

[tex]\Psi(x,t) = e^{\frac{-iEt}{\hbar}} \psi(x)[/tex]

the probability amplitude is then:

[tex]|\Psi(x,t)|^2 = |\psi(x)|^2[/tex]

and the time dependence is gone.

a nice way to think about [tex]\Psi(x,t)[/tex] is that the standing wave solution [tex]\psi(x)[/tex] is rotating through the complex<->real planes with a phase factor proportional to the energy (apply Euler's trig formula to exponential propogator). It is the projection of this rotation into real position space that makes the wave appear to have time dependence. In more advanced topics, this leads to the plausibility of performing analytic continuation to go from path integral state transitions to dynamics! (e.g. setting [tex]\beta = \frac{it}{\hbar}[/tex])

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it is clear that in the formalism of QM you talk in terms of potentials.but note that there are the Ehrenfest's equation which give you the connection with classical mechanics.if you can experimetally measure an observable then there's an operator corresponding to that whose expection value is measuarble!

and as for talking of derivatives like -grad V(x) then you should have a look at what cohen-tannoudji has to say in his book(vol 1, chap 3)!

and as for talking of derivatives like -grad V(x) then you should have a look at what cohen-tannoudji has to say in his book(vol 1, chap 3)!

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rlduncan said:I agree these are the stationay solutions which are in agreement with hydrogen's spectral lines. I disagree that stationary states exist in a point charge field, which is the case for the hydrogen atom. For in this field, stationary states are mechanically impossible. If the electron is stationary , it will be accelerated and fall to the nucleus. If it is in a dynamic equilibrium with the proton it will radiate and fall to the nucleus.

quetzalcoatl9 has described your misunderstanding of the word "stationary". Try looking up calculus of variation and figure out what the term 'stationary state' means.

You still have not shown where, in the solution to the H atom or the Kronig-Penny model, that there is a "radiation". All you have given are hand-waving argument that is based on a complete misunderstanding. Please do this immediately in your next post. Show exactly in the QM solution that incorporate the potential term where is this "radiation".

Zz.

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