quantum mechanics - Is there a difference between observing a particle and hitting it with another particle?

First, let me state that I'm a lot less experienced with physics than most people here. Quantum mechanics was as far as I got and that was about 9 years ago, with no use in the meantime.

A lot of people seem to think that the act of observing the universe we are changing the universe. This is the standard woo hand waving produced by people who don't understand the physics -- thank you Fritjof Capra.

In reading about quantum physics out there on the Internet, I find this idea is propagated far too often. Often enough that I question whether my understanding is accurate.

My questions:

1. 
   

   Does observing a particle always mean hitting it with something to collapse the wave function?

   
   


2. 
   

   Is there any other type of observation?

   
   
   


3. 
   

   What do physicists mean when they say "observe"?

   
   

Related, but doesn't get to the question of what observation is: What is the difference between a measurement and any other interaction in quantum mechanics?

Answer

Assuming that the incoming "first" particle is prepared in a pure state, interaction with another particle does seem necessary. Such an interaction might simply be the spontaneous emission of a photon or other particle by the original incoming particle, however.

Most importantly, such an interaction is not itself sufficient. For a measurement event to occur (wave function collapse in the Von Neumann formalism) we must also "physically lose track" of the some of the information of the interacting particle after the interaction has taken place, so that we must replace the entangled state description of the second particle after the interaction with a probabilistic mixture of such states, forcing a description of the first particle after the interaction in terms of a real valued probability density matrix rather than as the complex valued pure state amplitude we started with. This change of description automatically includes an increase in entropy, which also occurs physically.

Unless the second, interacting, particle either escapes the apparatus or interacts with a third particle which so escapes, i.e. "interacts with the environment", no measurement has yet occurred, the entire interaction is in principle reversible, and the complex amplitude description remains appropriate. Measurement requires "loss" (via decoherence) of the entangling information by further entangling with the environment and dissipation.

The escaping third particle is often an emitted photon or phonon. See the reference in the linked answer What is the difference between a measurement and any other interaction in quantum mechanics?, particularly the 1939 article by London and Bauer (but avoid their metaphysics) for details. More recently, see this book on quantum measurement theory, particularly page 102 referring to the view of Zeh.

You may have noticed that some ambiguity remains in this descriptipn. This has been analyzed in great detail and resolved by Zurek, but it gets a little tricky. See e.g. http://arxiv.org/abs/1001.3419 and references therein.