Is there a "direction of travel" at the quantum level ?

[geordief]

I think I have read that there are  circumstances where ,at the quantum level the direction of time  does not apply.

Is this is true ,is it  just for limited circumstances or is it across the board?

 

Do perhaps  quantum systems evolve in time generally  but some do not?

[Genady]

16 minutes ago, geordief said:

I think I have read that there are  circumstances where ,at the quantum level the direction of time  does not apply.

Is this is true ,is it  just for limited circumstances or is it across the board?

 

Do perhaps  quantum systems evolve in time generally  but some do not?

As QFT obeys SR, there is direction of time in the QFT as much as it is in the SR.

[geordief]

36 minutes ago, Genady said:

As QFT obeys SR, there is direction of time in the QFT as much as it is in the SR.

Is it just that there are some interactions where reversing the time signature in the maths  doesn't change the outcome?(I am fairly sure I have heard this more than a few times)

Edited December 18, 2023 by geordief

[Genady]

5 minutes ago, geordief said:

Is it just that there are some interactions where reversing the time signature in the maths  doesn't change the outcome?(I am fairly sure I have heard this more than a few times)

You mean, like in Newtonian mechanics?

[geordief]

8 minutes ago, Genady said:

You mean, like in Newtonian mechanics?

I don't "mean" anything.I am asking a question.

You seem to be suggesting that the question is trivial and might as well be considered in macro systems.

I have no way of knowing if that is right or wrong as I don't have the expertise.

[Genady]

20 minutes ago, geordief said:

I don't "mean" anything.I am asking a question.

You seem to be suggesting that the question is trivial and might as well be considered in macro systems.

I have no way of knowing if that is right or wrong as I don't have the expertise.

I just try to clarify the question because I don't know what they were referring to in the thing you've read. 

Let's consider an example, a particle in a Hamiltonian eigenstate \(\psi(x)\) with energy \(E\). It evolves in time as \(e^{-iEt}\psi(x)\). The probability density for it to be in position \(x\) is \(\bar {\psi}(x) \psi(x)\). This outcome does not depend on time and thus doesn't change if the sign of time is flipped.

Edited December 18, 2023 by Genady

[studiot]

1 hour ago, geordief said:

I think I have read that there are  circumstances where ,at the quantum level the direction of time  does not apply.

Is this is true ,is it  just for limited circumstances or is it across the board?

 

Do perhaps  quantum systems evolve in time generally  but some do not?

Yes of course there is, but direction of travel has quantum implications only sometimes.

 

By this I mean that there is a quantum solution for a particle freely travelling in space in any direction the solution is the same unless of course the travel medium  is not homogeneous and istropic.

Normally we do not employ this solution because the quantum levels are so closely spaced that it is effectively a continuum.

The direction of travel becomes important in the band theory of metals and semiconductors.

 

As regards your question in time, Genady is correct that some solutions are time independent, such as the particle in the box or the potential well models of an electron in an atom or a crystal.

But I don't see this has any relevence to travelling.

 

Does this help  ?

[geordief]

33 minutes ago, studiot said:

Yes of course there is, but direction of travel has quantum implications only sometimes.

 

By this I mean that there is a quantum solution for a particle freely travelling in space in any direction the solution is the same unless of course the travel medium  is not homogeneous and istropic.

Normally we do not employ this solution because the quantum levels are so closely spaced that it is effectively a continuum.

The direction of travel becomes important in the band theory of metals and semiconductors.

 

As regards your question in time, Genady is correct that some solutions are time independent, such as the particle in the box or the potential well models of an electron in an atom or a crystal

Especially

33 minutes ago, studiot said:

35 minutes ago, studiot said:

But I don't see this has any relevence to travelling

Thanks.(I hope nobody took the use of the word literally ,as in time travel.I just used it as a turn of phrase)

56 minutes ago, Genady said:

I just try to clarify the question because I don't know what they were referring to in the thing you've read. 

Let's consider an example, a particle in a Hamiltonian eigenstate ψ(x) with energy E . It evolves in time as e−iEtψ(x) . The probability density for it to be in position x is ψ¯(x)ψ(x) . This outcome does not depend on time and thus doesn't change if the sign of time is flipped.

Thanks.It doesn't  seem that extraordinary to my untrained ears. 

Would probability  equations in QM  normally have a time component?(it comes as a surprise to me that they would but I only aspire to a layman status )

[Genady]

33 minutes ago, geordief said:

Would probability  equations in QM  normally have a time component?

Here is an example: a superposition of two Hamiltonian eigenstates, \(\psi_1\) and \(\psi_2\), with the energies \(E_1\) and \(E_2\): \(\frac 1 {\sqrt 2}(e^{-iE_1t}\psi_1+e^{-iE_2t}\psi_2)\). The probability is squared modulus of this function, which includes a time component, \((E_1-E_2)t\).

[joigus]

4 hours ago, geordief said:

I think I have read that there are  circumstances where ,at the quantum level the direction of time  does not apply.

Can you provide a quote?

[TheVat]

https://www.quantamagazine.org/how-quantum-physicists-flipped-time-and-how-they-didnt-20230127/

 

Physicists have coaxed particles of light into undergoing opposite transformations simultaneously, like a human turning into a werewolf as the werewolf turns into a human. In carefully engineered circuits, the photons act as if time were flowing in a quantum combination of forward and backward.

“For the first time ever, we kind of have a time-traveling machine going in both directions,” said Sonja Franke-Arnold, a quantum physicist at the University of Glasgow in Scotland who was not involved in the research.

Regrettably for science fiction fans, the devices have nothing in common with a 1982 DeLorean. Throughout the experiments, which were conducted by two independent teams in China and Austria, laboratory clocks continued to tick steadily forward. Only the photons flitting through the circuitry experienced temporal shenanigans. And even for the photons, researchers debate whether the flipping of time’s arrow is real or simulated....

[studiot]

52 minutes ago, TheVat said:

https://www.quantamagazine.org/how-quantum-physicists-flipped-time-and-how-they-didnt-20230127/

 

Physicists have coaxed particles of light into undergoing opposite transformations simultaneously, like a human turning into a werewolf as the werewolf turns into a human. In carefully engineered circuits, the photons act as if time were flowing in a quantum combination of forward and backward.

“For the first time ever, we kind of have a time-traveling machine going in both directions,” said Sonja Franke-Arnold, a quantum physicist at the University of Glasgow in Scotland who was not involved in the research.

Regrettably for science fiction fans, the devices have nothing in common with a 1982 DeLorean. Throughout the experiments, which were conducted by two independent teams in China and Austria, laboratory clocks continued to tick steadily forward. Only the photons flitting through the circuitry experienced temporal shenanigans. And even for the photons, researchers debate whether the flipping of time’s arrow is real or simulated....

That article was all journalism and not enough substance (for me) to figure out what they were doing , let alone how they did it.

 

Please also remember that a standing wave can also be modelled as two  travelling waves going in opposite directions.

[geordief]

3 hours ago, joigus said:

Can you provide a quote?

Not really.I think I have heard this from time to time  but can't remember where I did.

Not recently ,anyway.

[joigus]

18 hours ago, geordief said:

Is it just that there are some interactions where reversing the time signature in the maths  doesn't change the outcome?(I am fairly sure I have heard this more than a few times)

This sound more like the dynamical law is invariant under time inversions, which is quite different from saying that the direction of time doesn't apply.

Not even that is true, since electroweak interactions violate CP (charge conjugation + parity conjugation). Parity conjugation being the corresponding generalisation to quantum mechanics of mirror reflection. As we have very good reason to believe the world is CPT-invariant (the combination of the 3 relevant inversions in QFT), it follows that T must be violated.

As Genady said,

19 hours ago, Genady said:

As QFT obeys SR, there is direction of time in the QFT as much as it is in the SR.

Physicists sometimes like to play with metaphors, and conceptual hell breaks loose.  

When the metaphor constitutes the argument, you can rest assured the argument cannot be trusted.

[MigL]

Most simple interaction at the quantum level is reversible in time.
This means that if you observe a video of the interaction, it will look normal playing forward as well as backwards.
( except for the neutrino case that Joigus mentions above, and some other violations of C symmetry )

It is only when you start dealing with large groups of interactions that many more degrees of freedom make time reversal unlikely.
( a concept better known as entropy )