# Particle wave duality question: 2 different detectors observing different properties?

I hear a lot of people saying an observation of the double slit experiement collapses the wave function and doesn't allow you to view the particle in 2 places at the same time or as both wave and particle (or something like that).

Another often-heard phrase is: If you know the location, you don't know the momentum and vice versa. However, I wanted to know if having 2 different detectors observing the same thing would be of use: 1 detecting the location of particle in the wave and 1 detecting the momentum or wave itself as opposed to just having 1 observer watching one thing at 1 time.

• I strongly suggest you forget about "wave-particle duality" and study quantum mechanics from a modern perspective. Wave-particle duality in the naive form is wrong and in the correct form the name is misleading. Particles are particles, but the states in which they can be found obey quantum versions of probability distributions (aka wave functions whose modulus squared are probabilities) satisfying a wave equation. When you have only a few particles you see the particle properties, when you have lots of them the whole thing looks like a wave of something. – Bubble Oct 4 '14 at 15:06
• Take @Bubble's advice! The most important thing to keep in mind is that our classical world is not fundamental, so there is no reason why quantum mechanics should be intuitive; we really have to let go of our (naive) ideas about what the world is made up of. – Danu Oct 4 '14 at 18:00

Having multiple observers trying to do different observations to ascertain all properties of the particle at the same time will not make any difference. The fact of the matter is that the position and the momentum of a particle cannot be well-defined at the same instant in time.

This follows from the wave-particle duality that applies to all particles, and is not an artifact of our methods of measurements, but a fundamental fact that one cannot work around with any complicated experimental setup whatsoever. Some time ago, I answered a related question, where I go a little deeper into the reason why this is true.

• Thanks for ur answer :) I'm new to all this but I thought if It's possible to know either the momentum or the position but not both then is it because the speed and direction of the particle randomly changes all the time? – Michael Connor Oct 4 '14 at 11:50
• No, the speed is just not well-defined for a localized particle: The entire concept doesn't make sense in this context. This goes against your (classical) intuition, but it's something those who study nature have to come to terms with. – Danu Oct 4 '14 at 11:51
• Thanks again I shall try gain more knowledge on the subject I'm clearly out of my depth at this moment in time ha – Michael Connor Oct 4 '14 at 11:57

This phenomena is present in every measurement. If you measure the voltage in a wire your instrument has to have a high but not endless high resistance. The current flowing thru this instrument makes the measurement a little bit uncertain. The same happens if you measure the current.

A little more sophisticated is it if you want to measure photons. You can hit them only by other photons, perhaps with lower energy. But at the end you change the directions of the photons or the intensity or both or the spin or something else. This is much more dramatically for the uncertainity when measuring a voltage. This is a process of a lot of a lot of particles. It is a statistical process. Then closer you come to the measurement of single particles then bigger will be the uncertainty.

• What you describe is experimental uncertainty because of the way our apparatus is constructed. The Heisenberg uncertainty of quantum mechanics is emphatically not due to the measurement process, but follows from non-commutativity of the corresponding quantum operators alone, and is a fundamental uncertainty. – ACuriousMind Oct 4 '14 at 14:00