Observing particles

[Decay33]

If the wave state of a particle is collapsed when observed, if we stop observing it does it change back?

[fiveworlds]

But do you have knowledge of the equipment required to observe the particle??

[swansont]

If the wave state of a particle is collapsed when observed, if we stop observing it does it change back?

Once you have observed it in an eigenstate, it remains there unless there is an interaction, or you change into a different basis, e.g. light through a polarizer collapses into a polarization state, but you can regain superposition by using a polarizer rotated through some angle.

[fiveworlds]

Was it not the observation of an electron's wave-function in an atom that was the problem though? When studying electron position within atoms. Light would be shined on the electron in an attempt to observe the wave-function of the electron but this would alter the electron's wave-function.

[swansont]

Was it not the observation of an electron's wave-function in an atom that was the problem though? When studying electron position within atoms. Light would be shined on the electron in an attempt to observe the wave-function of the electron but this would alter the electron's wave-function.

 

There is no position eigenstate in the solution to the Schrödinger equation for the H atom. You are probably thinking about behavior of deBroglie waves vs wave functions. They aren't the same thing.

[Decay33]

When I thought of the question I was thinking of the double slit experiment. If we throw 1000s of particles thought it and measure them, they go through as particles, if we use the same particles and stop measuring them, do they go through as waves?

[conway]

decay33

 

so.....if a particle is collapsed, and as swansont says it stays so except for outside forces, would then these "used" electrons behave as particles if "not" observed in a double slit experiment? I think this a wonderful question. What about a machine that changes states of particles from wave to particle and back again, something like swansont suggested? How then would this device affect the experiments outcome. But wouldn't "using" the "particle/wave" device be akin to observing the electron. Thus always causing it to collapse every time the experiment was ran, no matter the configuration?

[swansont]

When I thought of the question I was thinking of the double slit experiment. If we throw 1000s of particles thought it and measure them, they go through as particles, if we use the same particles and stop measuring them, do they go through as waves?

That's not an example of wave function collapse (and collapse pretty much implies a discussion of wave functions). Quantum particles always act as waves when you aren't observing them.

[conway]

Swansont

 

I am confused, I thought you implied particles stay collapsed, unless acted on.

[Sensei]

That's not an example of wave function collapse (and collapse pretty much implies a discussion of wave functions). Quantum particles always act as waves when you aren't observing them.

 

You're confusing people with this statement.

Observation is not literal observation made by human eye.

 

Observation = measurement.

Measurement can be done by nature by itself. Without any human, animal or other living form intervention.

 

If we put digital/analog camera in front of our experiment and use it to record, it'll "steal" photons from environment, changing state.

I remember how I wanted to make photo of double slit experiment, taking photo by photo, I saw on wall all these interference dots, pressing "take photo" button in camera, and then they were gone in 9/10 of photos...

I could turn camera on, and go away of room, and the same.

 

I had to take 10-20 photos to get this:

In majority of cases of taking photo, it didn't look like this...

[swansont]

Swansont

 

I am confused, I thought you implied particles stay collapsed, unless acted on.

 

The fact that everything has a deBroglie wavelength, i.e. everything acts like a wave, is separate from the concept of a wave function. An electron moving at 100 m/s has a wavelength of about 7 microns. It will diffract, interfere — whatever wave interaction you want to look at. That says nothing about the states of the particle typically described by the wave function. It's not in a superposition of states, so there can be no wavefunction collapse. The wave function would describe e.g. its spin orientation. It's possible you could have two energy (or momentum) states, described by different speeds of motion, and thus different deBroglie wavelengths. That's something that could collapse to a single state. But the electrons will always have a wavelength up until you detect them, because you detect them at a localized spot.

[conway]

So....particles always have wavelength, but wave function(describes particle) exists when particle is "at its lowest intensity", and "can" give rise to different/more wavelengths?

Edited July 1, 2015 by conway

[swansont]

So....particles always have wavelength, but wave function(describes particle) exists when particle is "at its lowest intensity", and "can" give rise to different/more wavelengths?

 

The wave function always describes the state of the particle. But it is not the same as the particle's deBroglie wavelength. They are separate things.

 

wave nature ≠ wave function

[conway]

So while they are not equal they exist at the same time?

[swansont]

Yes. lambda = h/p is always true, and the wave function always describes the state of the particle, whatever that happens to be.

[MigL]

A quantum particle will show particle properties when you try to detect particles, i.e. a single photon through slits makes a single point at the detector.

And those same quantum particles will demonstrate wave properties when you try to detect waves, i.e multiple photons through slits will make points in the shape of a diffraction pattern at the detector.

 

The wave function on the other hand, is a mathematical construct, which simultaneously describes all possible states of the quantum particle. Any interaction, such as hitting the detector, will 'collapse' the wave function to a single state. It is a 'description', but has no actual physicality, akin to the event horizon of a black hole, or saying "I have zero apples" ( no actual apples are there ).

[conway]

MigL

 

Not that it can be proven but.......there is not "nothing" there. Something is. The apples went in. They may no longer be apples, but something is there. Additionally I think a separation should be noted. While it is akin to apples divided by 0. It is also different. That is mass's larger than any apples , and spaces smaller than any 0.

[swansont]

The description of the apple isn't a physical thing.