Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

In Quantum Mechanics the value of an observable results from the interaction between the "system" with the "Measuring gadget".

But when the experimenter[or the technologist concerned] is devising/constructing the gadget he has a view the "system" with its stand alone properties/attributes.It seems to be so.

But theory itself cannot ascribe the value of some observable to the system alone. How does one explain this?

share|cite|improve this question

If you have some system in a superposition of states, then when your measuring gadget interacts with it it becomes entangled with the original system and it too is in a superposition of states. If you walk over to look at your measuring gadget you too become entangled and enter a superposition of states, and so on up to and including the journal you publish the results in.

In real life we don't see macroscopic objects, cats or otherwise, in a superposition of states, and the reason is generally considered to be decoherence. The describes the interaction of a system with the rest of the universe and the loss of coherence that results. The rate at which a system decoheres increases with the complexity of the system, so while a superposition of states can be maintained for an atom or molecule, it is lost almost instantaneously for something the size and complexity of lab equipment, and your experiment returns a definite value for whatever you were looking at.

The largest system that has been observed in a superposition of states contains about ten trillion atoms.

share|cite|improve this answer
"Decoherence can be viewed as the loss of information from a system into the environment (often modeled as a heat bath),[2] since every system is loosely coupled with the energetic state of its surroundings."...from Wikipedia[]. When we talk of the "loss of information from a system" we seem to consider certain well defined stand alone properties of the system.This "leaked" information interacts with the evironment, the gadget being a part of it to produce the value of the measurement. – Anamitra Palit Aug 3 '12 at 0:48
Information leaks out of the system----this information "leaked" is indicative of properties of the system $independent{\;} of{\;} the{\;} environment$ . The leaked information gets modified by interaction with the environment,the gadget being a part of it.Is it possible to eliminate the effect of the environment in the process of measurement? – Anamitra Palit Aug 3 '12 at 0:56
In the last sentence of the previous comment I have tried to indicate at a mathematical/theoretical elimination of environmental effects to get to the stand alone properties of the system in a theoretical manner – Anamitra Palit Aug 3 '12 at 1:06
You might want to ask this as a separate question. It is certainly possible to calculate the properties of an isolated system, and in fact that's what we normally do. The problem is that eventually a measurement is looked at by a human and we are part of the environment. – John Rennie Aug 3 '12 at 5:41
The comments to the Original Posting "The Lorentz transformations in the Micro-world" may be related to the issue at hand. Link:… – Anamitra Palit Aug 3 '12 at 7:05

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.