What happens between interactions?

[geordief]

A particle (which in  QFT is a wave travelling through a field) can only be measured at the beginning and the end of its journey. 

In between ,if it is measured then the journey is different .(shorter)

 

If ,for a test  particle where the initial conditions are identical(can that be  guaranteed, I wonder?)  and the particle is then measured at different times  from its starting point do we get identical measurements?

I mean ,if we have an observer on the Moon and we ,on the Earth** fire a particle  with known attributes at them   do we know precisely what  their measurements will be ?

And ,suppose an observer half way between Earth and Moon  intercepts the same ( or an "identical") particle  will they be guaranteed  to make the same measurement as the measures on the Moon?

 

Or do the potential  measurements along the line of the particle's line of transit vary with distance from the source?

 

**if the source on the Earth and the receiver on the Moon can be imagined as being at rest wrt each (which they aren't of course but let's just suppose they are -or find other places that are,if that is a problem)

Edited October 5, 2022 by geordief

[swansont]

8 minutes ago, geordief said:

If ,for a test  particle where the initial conditions are identical(can that be  guaranteed, I wonder?)  and the particle is then measured at different times  from its starting point do we get identical measurements?

It depends on the measurement being made.

If I measure the charge on an electron, the result is not going to depend on earlier measurements. The energy of a photon won’t depend on the determination of its polarization. (with a properly designed experiment)

Other measurements will affect subsequent ones.

[geordief]

7 minutes ago, swansont said:

It depends on the measurement being made.

If I measure the charge on an electron, the result is not going to depend on earlier measurements. The energy of a photon won’t depend on the determination of its polarization. (with a properly designed experiment)

Other measurements will affect subsequent ones.

Thanks

So ,if we know the spin attribute of a photon as it leaves a measuring device on the Earth do we know what a similar device on the Moon will measure provided there are no interactions along its path?

And if there is an interaction along the path will that measurement be the same as it would have been on the Moon?

Edited October 5, 2022 by geordief

[Sensei]

There are lab frame of reference and center-of-mass frame of reference. Interaction between particles at relativistic velocity must be calculated in CoM FoR, and converted to lab frame to know how much energy particle should have to give expected results e.g. creation of charged pion particle after collisions of two protons, in CoM, protons must have >= 139.6 MeV kinetic energy, but because of conservation of momentum they need more. From two particles with kinetic energy (in CoM), there are created three particles with kinetic energy after collision. In lab frame e.g. CERN/LHC, one particle is at rest ("target" which is hit), and the other one, beam of them actually, is accelerated. After collision there is shower of new particles.

55 minutes ago, geordief said:

And ,suppose an observer half way between Earth and Moon  intercepts the same ( or an "identical") particle  will they be guaranteed  to make the same measurement as the measures on the Moon?

Calculate in CoM FoR of these particles to learn the outcome.

At low enough kinetic energy we skip relativistic part, and use lab FoR, instead of proper CoM FoR (as they would not differ much).

"the same", "an identical".. such humanistic PoV.. When you measure the sound emitted by a person/car approaching you, and moving away, you get different results, that is not identical.. even in classical physics..

 

Edited October 5, 2022 by Sensei

[swansont]

2 hours ago, geordief said:

Thanks

So ,if we know the spin attribute of a photon as it leaves a measuring device on the Earth do we know what a similar device on the Moon will measure provided there are no interactions along its path?

And if there is an interaction along the path will that measurement be the same as it would have been on the Moon?

If we send polarized light somewhere and there are no interactions along the way, the polarization will be the same on arrival. There are experiments which rely on this.

If there is an interaction that affects polarization, all bets are off. As before, it depends on the interaction.

[MigL]

Depends what kind of interactions you are considering.

Classical interactions have clearly defined and determined trajectories, and classical ( macroscopic ) particles can be tracked all along that trajectory.

Quantum particles have no clearly defined trajectory, and, in QED, the 'in-between' is a summation of the probability amplitudes of all possible trajectories. This then leads to infinite probability amplitudes, and must be remedied by renormalization.

[geordief]

55 minutes ago, MigL said:

Depends what kind of interactions you are considering.

Classical interactions have clearly defined and determined trajectories, and classical ( macroscopic ) particles can be tracked all along that trajectory.

Quantum particles have no clearly defined trajectory, and, in QED, the 'in-between' is a summation of the probability amplitudes of all possible trajectories. This then leads to infinite probability amplitudes, and must be remedied by renormalization.

I just had the quantum interactions (or any interaction involving a quantum particle if a quantum particle can interact with a macro system )

 I was wondering whether ,within the quantum system it was possible to define the tocks of time by the interactions that occur.

Is there any way ,within that system to distinguish  btw a "long" interval between  interactions and a small interval?

 

Can the interval ,whether long or short  be considered  in any way as a unit of "quantum  time" (in any useful way)?

[swansont]

38 minutes ago, geordief said:

I was wondering whether ,within the quantum system it was possible to define the tocks of time by the interactions that occur.

Atomic clocks involve quantum systems, so obviously the answer is yes. I think an issue here is you are asking very general questions, when specific ones need to be asked.

[geordief]

26 minutes ago, swansont said:

Atomic clocks involve quantum systems, so obviously the answer is yes. I think an issue here is you are asking very general questions, when specific ones need to be asked.

The atomic clock involves an interaction btw quantum objects and a macro system ,doesn't it?

 

Can the process of the passage of time  be defined and quantified without recourse to a macro system  and just by means of interactions  within the quantum system itself?

[swansont]

21 minutes ago, geordief said:

The atomic clock involves an interaction btw quantum objects and a macro system ,doesn't it?

Eventually it does, but the ticking is an interaction between photons and atoms.

 

 

[geordief]

1 hour ago, swansont said:

Eventually it does, but the ticking is an interaction between photons and atoms.

 

 

Are photons "more quantum" than atoms ?

If there were no atoms (which is supposed to have been the case at an earlier epoch)  would it then be impossible to  count any tocks?

[swansont]

11 hours ago, geordief said:

Are photons "more quantum" than atoms ?

Photons are quantum particles, so you've already excluded classical behavior that you could include if you just said "light"

Atoms can exhibit classical behavior and quantum behavior, depending on what you're looking at. So photons are more quantum than atoms. 

 

11 hours ago, geordief said:

If there were no atoms (which is supposed to have been the case at an earlier epoch)  would it then be impossible to  count any tocks?

I'm not sure (I'm not a cosmologist), but we do know how long it took for recombination to occur, so there have to be some features that are time dependent. 

[geordief]

1 hour ago, swansont said:

 

I'm not sure (I'm not a cosmologist), but we do know how long it took for recombination to occur, so there have to be some features that are time dependent. 

A bit like time  as we understand it  "going into and exiting its own black hole"?

Not sure if I am going off topic ,but I have heard the expression countless times that the  photon in a vacuum is not a valid frame of reference.

 

I can, I think see that the mathematical equations might not allow this (infinities involved?) but is there also a physical reality that this impossibility  indicates?

Edited October 6, 2022 by geordief

[swansont]

2 hours ago, geordief said:

A bit like time  as we understand it  "going into and exiting its own black hole"?

Not sure if I am going off topic ,but I have heard the expression countless times that the  photon in a vacuum is not a valid frame of reference.

 

I can, I think see that the mathematical equations might not allow this (infinities involved?) but is there also a physical reality that this impossibility  indicates?

Nothing I've discussed requires you to try and be in a photon's frame  of reference.