# B Recoherence possible?

1. Jan 29, 2017

### oquen

When the wave function has collapsed or one of the outcomes chosen. Under what conditions can recoherence still occur or the branches reverting back to superposition? Like when you remove the information of which way path, etc.?

2. Jan 29, 2017

### atyy

Recoherence cannot occur after a measurement has been made and the wave function has collapsed.

All recoherences occur only when there has been no true measurement, and there has been no true collapse.

3. Jan 29, 2017

### oquen

In a macroscopic object like trees.. I understand each of the billions of particles are entangled with the environment.. is there a true measurement in each entanglement or has there be no true collapse and recoherence can occur?

4. Jan 30, 2017

### oquen

"After wave function collapse, the time evolution of the wave function is again deterministic and governed by the Hamiltonian via the Schoredinger equation. So if you change the Hamiltonian, you will affect the evolution."

Aren't you describing recoherence above which you stated cannot occur. It's so confusing. Please elaborate.. Thanks a whole lot!

5. Jan 30, 2017

### Demystifier

Nobody knows. It's a matter of interpretation.

But even if recoherence is possible in principle (which it is, according to some interpretations), it is impossible in practice. A typical time needed for recoherence is much longer that the age of the Universe.

6. Jan 30, 2017

### oquen

I was not referring to the recoherences of the entire system at once. But just an atom or particle. Let's say one particle in the marble ball has formed entanglement with a photon in the environment. The particle is a system and photon the environment. It turns into mixed state as described by the statistical density matrix. And mixed state implies born rule. Meaning the particle in the system is in an eigenstate with particular value. But after this, can the same particle/photon revert back to one where the value is erased? This is what I meant by recoherence. Is this not a correct term. What is then I wonder. I appreciate all help.

7. Jan 30, 2017

### MathematicalPhysicist

Is it even meaningful to give the universe an "age"?

Doesn't it assume it's finite in nature?, and that there's something that predates it?

In that case, whatever you choose multiverse or universe you must eventually posit eternity for self consistency reasons.

8. Jan 30, 2017

### oquen

I think I am mixing things up. So let me break down my inquiries:

1. When the electron in an atom in the marble ball entangles with the photon in the environment. The electron and photon are in entanglement. What breaks the entanglement? I was asking if what broke the entanglement can reestablish the original entanglement.

2. Decoherence is "where a sufficiently large quantum object interacts with its environment. Because of the nature of the interaction the resulting state describing the system looks like classical probabilities -- the interference vanishes (although in principle the quantum system + environment remain in superposition)." I was not asking about Recoherence in connection to this. I think this is what you have in mind when I mentioned Recoherence?

9. Jan 30, 2017

### Staff: Mentor

Substitute "the time since the Big Bang" if the term "age" bothers you. Anything more than that is off topic for this thread.

10. Jan 30, 2017

### MathematicalPhysicist

I am not even sure there was a big bang;
But to prove or disprove this claim I'll need to go back in time.

So when did the big bang occur? give me a specific date 02/05/-14000000000 :-)

11. Jan 30, 2017

### oquen

Before this thread locked out and my question unanswered for a long time. Let me get to more details. It is mentioned how "Decoherence is an interaction between systems, usually the environment, that transforms a pure state into a mixed state in a particular basis. Most of the time its in the position basis. And that basis is stable meanting it does not change as the interaction evolves. Its got to with most interactions are of the radial type."

But remember I'm just trying to understand what goes in one entangled pair. Without measuring apparatus that breaks the entanglement.. the entangled pair is never broken. So for the above I'd like to take an example the position basis. For the entangled pair of one electron in the atom and the one photon in the environment (remember not the entire marble balls).. what breaks the entanglement that would put one pair (or one of the pairs entangled with thousands (is this the case?) in position basis? This is compared to entanglement where one pair alone won't be put in a position basis. Is this the answer? that it's because its not a pair but breaks the entanglement. Because as I understood it. When it is put in position basis. It has collapsed. When it did. I was asking in the original question if it can uncollapse (or the perhaps false meaning of recoherence). Hope someone can clear this up, before thread locked. Thanks!

12. Jan 30, 2017

### Staff: Mentor

Do you know what the term "big bang" properly refers to? It refers to the hot, dense, rapidly expanding state that is the earliest state of the universe of which we have reliable knowledge. So if you are not sure there was a big bang, you are not up to date on the current state of cosmology.

About 13.7 billion years ago according to a comoving observer.

Anything more on this topic is off limits for this thread.

13. Jan 30, 2017

### Staff: Mentor

If you expect the thread to be locked, you must realize that you are asking questions that are not answerable at a "B" level. You need to have more background in QM to even be able to have a meaningful discussion about this. So, given that your original question has been answered as much as it can be at the "B" level, thread locked.