# Schrodinger Equation and wavefunction collapse

#### feynmann

If we solve the Schrodinger Equation for hydrogen atom, we get discrete energy levels that agree with experiment. But no where we need the wave function collapse. So my question is where the wave function come from and why do we need it?

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#### Jackson Tan

The collapse of the wavefunction is associated with measurement. If you perform a measurement on the hydrogen atom, you force the wavefunction to collapse into one of the eigenvectors of the measured observables.

Before measurement, the atom is in a superposition of all possible states. You get a superposition of all the discrete energy levels. But it is only upon observation that you get a specific spectral line (or photon). And repeated measurements (or observing many atoms at once) gives the whole atomic spectrum as we know it.

#### feynmann

The collapse of the wavefunction is associated with measurement. If you perform a measurement on the hydrogen atom, you force the wavefunction to collapse into one of the eigenvectors of the measured observables.

Before measurement, the atom is in a superposition of all possible states. You get a superposition of all the discrete energy levels. But it is only upon observation that you get a specific spectral line (or photon). And repeated measurements (or observing many atoms at once) gives the whole atomic spectrum as we know it.
Do you really believe that before measurement, the atom is in a superposition of all possible states. How about cat, before measurement, is the cat in a superposition of life and death?

#### Jackson Tan

Do you really believe that before measurement, the atom is in a superposition of all possible states. How about cat, before measurement, is the cat in a superposition of life and death?
I leave it up to your philosophy to answer that ! It is a postulate of QM that a measurement changes a superposition to a definite eigenstate. Prior to that, it is a superposition of states. What this means in reality, well, is beyond experiments.

#### alxm

Do you really believe that before measurement, the atom is in a superposition of all possible states. How about cat, before measurement, is the cat in a superposition of life and death?
What makes you think you can treat that which is measured and that which is measuring as separate systems?

#### dragonwhip

I leave it up to your philosophy to answer that ! It is a postulate of QM that a measurement changes a superposition to a definite eigenstate. Prior to that, it is a superposition of states. What this means in reality, well, is beyond experiments.
Why is the Copenhagen interpretation being taken for granted here?

#### sokrates

See Quantum Decoherence.

Schrodinger's cat 'problem' is not considered to be a problem at all.

The box is in contact with the environment, there's decoherence and the cat dies or lives.

#### Ilja

See Quantum Decoherence.

Schrodinger's cat 'problem' is not considered to be a problem at all.

The box is in contact with the environment, there's decoherence and the cat dies or lives.
If Schroedinger's cat is a problem in some interpretation, decoherence cannot solve it. With decoherence, a macroscopic superposition becomes even more macroscopic, without remaining to be a superposition.

Decoherence may be a useful tool for some applications, not more.

#### sokrates

I don't think there is any 'equivalence rule' between the interpretations that states 'if there's a problem in some interpretation, it exists in all interpretations'.

This is just not true. There are certain, tangible problems in the Copenhagen interpretation and Schrödinger's Cat is one of them. And decoherence 'supposedly' (and succesfully) solves the so-called 'measurement problem'.

But it would be absolutely unfair to bring all the interpretations down to some base level and say they are equivalent.

If they are just saying the same thing, why on earth physicists are still discussing QM even after 80 years later?

#### f95toli

Gold Member
why on earth physicists are still discussing QM even after 80 years later?
They don't, nowadays it is mostly a topic that comesup on Internet forums
Most working physicists simply do not care that much about different interpretations of QM, mianly because there is no way to tell which one is correct.
There is a small number of people (some of them well known) that work on the "foundations" of QM but most people who use QM in their daily woprk (which means most physicists) adhere to the "shut up and calculate" school.

#### sokrates

Good point :)

you convinced me.

But the proportion may not be that small, after all how many physicists work on, say, electronic transport properties of proteins??

The venerable Murray Gell-Mann is working on this field! That's enough! :)

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