# Quantum Decoherence - how fast does it propagate?

1. Dec 6, 2008

### Dmitry67

Here, in Saint Petersburg, Russia I open a box with a Schroedinger cat
I get immediately decoherenced so I see only one cat - dead or alive

How soon that decoherence 'hits' you in the US?
Photons can not travel directly to another part of the globe, they get absorbed/reemitted, losing the information on the way.

I believe this is a valid question (no philosophy, just calculations) but I can not answer it.

2. Dec 6, 2008

### cesiumfrog

"losing the information" seems like the wrong choice of words in the context of decoherence, but sure, decoherence should "propagate" at a medium-dependent speed (for example, you assumed at the start that the box could slow it dramatically). Just don't suggest that your lab could transmit the message faster than decoherence propagates (and since such indirect routes are the stuff of decoherence, I'm thinking that the answer to your question will be very close to c).

3. Dec 6, 2008

### Hurkyl

Staff Emeritus
I can't really make sense of the question. While I can imagine that:
1. You were never in a coherent superposition of "You see a live cat" and "You see a dead cat"
2. The joint (relative) state of you and the cat was* in a superposition of "The cat is alive and you see a live cat" and "The cat is dead and you see a live cat", but that pretty much instantly decohered as you interacted with the environment.
3. The (relative) state of the U.S. contains no information about you or the cat, so I have no idea what coherence you are wondering about decohering.

(This, of course, assumes that QM works on planetary scales, and reality doesn't contradict decoherence-based interpretations)

4. Dec 6, 2008

### Dmitry67

Lets assume that the box is ideal.

I understand that distant observer will be decoherenced quickly if he observes the experiment using video.

But what if not? In order to be de-coherenced, you need to absorb at least few photons from the Saint-Petersburg carrying the information of what is inside the box.

I doubt you will be receiving any photons from there. Then you need to wait for the photons to be absorbed by the walls of the laboartory (so they are de-coherenced with a box). Then, the walls emit photons outside, so the whole street is de-coherenced etc. I think it might takes seconds, minutes or even hours!

5. Dec 6, 2008

### Dmitry67

Hurky,

1,2. Yes, I am decoherenced with the cat almost instantly (about 10^-23 sec) based on the number of photons. I am aware of that.

3. I dont understand you point
In order for the decoherence to propagate objects must interact with each other.

To give you another example, lets say I open a box at 1am,
and you are on the Moon. So, at 1:00:01am on Moon light could not reach Moon, hence, the decoherence could not accur yet for you.

Not lets say you are on the invisible side of the Moon :)

6. Dec 7, 2008

### shaun_o_kane

Sorry Dmityr67 - I think you cannot avoid philosophy in this case - the "waveform" is not directly observable - you need to pick an interpretation for an "explanation". I don't think you should use the word decoherence as this has a specific meaning and it is not clear that's what you mean.

7. Dec 7, 2008

### WaveJumper

Edit: You are right. If the universe splits into different states/universes at each interaction, the whole universe must be entangled so that information about the decohered state could travel instantaneously. Hence if he's on Moon and his wife runs a red light, he would decohere(while he's still on the Moon) in a state where his wife is either dead or alive. He wouldn't know that state until he lands back on Earth, but for him to remain in the branch where his wife remained alive, decoherence have to be instantaneous regardless of distance.

Last edited: Dec 7, 2008
8. Dec 7, 2008

### shaun_o_kane

Is Wavejumper arguing against the Many-Worlds interpretation. It sounds like it...;-)

9. Dec 7, 2008

### WaveJumper

I was always under the impression that it was the deoherence interpretation that implied strongly the presence of multiple universes. How am i arguing against it when i am using it to form a thesis that the universe must be entangled so that a decohered state on Earth would reach instantaneously the Moon where a human will have to decohere in the same state?

10. Dec 7, 2008

### f95toli

Section V (the chapters on relativistic quantum processes) of Breuer's "The Theory of Open Quantum Systems" should cover this in some depth.

I haven't read section V (don't have any background in relativistiv QM since I almost exclusively work on solid state systems) but the rest of the book is very good.

You can get some idea about the contents by using the "look inside" feature on Amazon.

11. Dec 7, 2008

### shaun_o_kane

Decoherence is a somewhat ambiguous term - MWI talks about decoherence, but decoherence can exist on its own as a separate interpretation/hypothesis.

By the way, in which frame of reference does the waveform collapse instataneously? (This is a problem for all "real" waveform interpretations).

I don't expect to convince you of anything (I've argued this topic too many times) but I'd like to acknowledge their are other possible interpretations.

12. Dec 7, 2008

### Dmitry67

2 WaveJumper:

I believe there is a difference between the Many-Worlds interpretation and the current interpretation with the Decoherence (I would refer it is D)

In D the universe does not split: it is always linked together, even these links may be sometimes too weak to be detected. Also, if event X is too far away from the observer Y, then Y is not split at all - at least until it enters the time-like cone from the event X.

If right now someone opens a box in the Andromeda galaxy then you are not decoherenced with it for at list million years because there is no way how you could know about it (this is a different between this interpretation and many-worlds, where the whole universe is split immediately)

But my point is that observers are decoherenced much much slower then the light cone associated with the events. I read somewhere (forgot the URL) that to be decoherenced you must absorb about 5-6 photons. So you you are on Pluto and I open the box on the Earth I doubt you get any information about it even after you enter the same light cone.

13. Dec 7, 2008

### WaveJumper

There would be a difference only if you are not interested(if you disregard) what happens to the other superpositional states that did not decohere in the state you observed.

"Far too away" is not a concept that has any meaning with regard to quantum entanglement. There is simply no "far too away". If the universe does not split, you have to explain what happens to the other observed superpositional states. Do you have a better explanation?

I believe that could only happen if you could completely isolate Andromeda galaxy from interacting with the other galaxies, so that entanglement could not take place. But if you could do that, Andromeda galaxy would practically disappear.

From wiki:

"Since the system is entangled with its environment in some unknown way, description of the system by itself cannot be done without also referring to the environment (i.e. without also describing the state of the environment)."

http://en.wikipedia.org/wiki/Quantum_decoherence

Entanglement propagates instantaneously, as the experiments show and one photon is enough for a system to decohere in a particular state.

From the viewpoint of entanglement, the environment(the whole universe) is infinitely small, if you want to picture it - quantum entanglement "thinks" the universe is less than 1mm in diameter.

Last edited: Dec 7, 2008
14. Dec 7, 2008

### WaveJumper

Is there such a concept as "a frame of reference" in QM? What would that be supposed to mean?

15. Dec 7, 2008

### WaveJumper

Here is good description of Decoherence by Michio Kaku, professor of theoretical physics(one of the co-founders of String Theory, currently working on the TOE):

"" Today, there is still no universal consensus to the questions raised by Einstein back in the 1920s concerning the quantum theory’s outrageous paradoxes. But one idea is gradually catching on, and this is “decoherence.”