# What does this quantum experiment says about quantum world?

I am complete noob so please bear with me. I always read that in quantum world things exists as probability and only become one when they are observed...or wave collapses into particle.

But there was one recent experiment in which scientists observed the wave and it still showed interference pattern. So observing simply didn't collapse it.

Side note: it makes no sense that quantum world should be able to distinguish that it's being observed by human or whether sensor alone.

• It depends on different interpretations of quantum mechanics, but please first provide a link to the "recent experiment". Commented May 11, 2014 at 5:21
• It would help if you identified which recent experiment you are talking about. Quantum mechanics is quite a bit more complicated than observe → collapse, and there are several recent experiments which might be described that way. Commented May 11, 2014 at 5:21
• let me search it ...don't think i saved it. Commented May 11, 2014 at 5:47
• Here is the arxiv arxiv.org/abs/1208.0034 Commented May 11, 2014 at 15:35
• Possible duplicate: physics.stackexchange.com/q/35972/2451 Commented May 11, 2014 at 17:49

This experiment involves weak measurement. The standard dichotomy you've learned, "observe the state, and the wave collapses", doesn't cover everything you can do with quantum mechanics; you can also measure some statistical information about the state, and only partially collapse the wave. These are called "weak measurements", and what Heisenberg's uncertainty principle says for these is that the product of the noise in the measurement and the amount of disturbance of the quantum state is at least $\hbar/2$, where $\hbar$ is the reduced Planck's constant.

What is going on here is described fairly well in the abstract of a 2002 paper of Ozawa:

The Heisenberg uncertainty principle states that the product of the noise in a position measurement and the momentum disturbance caused by that measurement should be no less than the limit set by the reduced Planck’s constant, $\hbar/2$, as demonstrated by Heisenberg’s thought experiment using a gamma-ray microscope. Here I show that this common assumption is false: a universally valid trade-off relation between the noise and the disturbance has an additional correlation term, which is redundant when the intervention brought by the measurement is independent of the measured object, but which allows the noise-disturbance product much below the reduced Planck’s constant when the intervention is dependent.

Stating this in easier-to-understand terms, the proof of Heisenberg's uncertainty principle assumes that the disturbance in the measured system is independent of the state the system is in. If you set up an experiment where the disturbance depends on the state of the measured system (it isn't immediately clear how you can do this), then for some states the amount of disturbance can be lower than what Heisenberg's uncertainty principle says. Ozawa also gives a modified formula for the uncertainty principle which takes this into account.

The experiment described by the article is a demonstration of this. The experimental disturbance is shown to be lower than what Heisenberg's uncertainty formula yields, but is still larger than that given by Ozawa's modified formula. This experiment undoubtedly wasn't easy to do, but it agrees with the standard predictions of quantum mechanics.

I'm afraid this isn't going to be a very useful answer, but the phenomenon you're referring to is weak measurement. My answer isn't going to be useful because this is a highly technical area and impossible to describe to a non-physicist without the answer expanding into a book.

There is no consensus on what exactly happens during wavefunction collapse, but everyone agrees that the collapse occurs when the measuring system becomes entangled with the system being measured. The idea of weak measurement is to make the interaction between the measurement system and the system being measured very weak, so the degree of entanglement remains very small. This avoids the wavefunction collapsing on the timescale of the experiment.

Re your last paragraph, consciousness isn't an essential part of the collapse process. Search this site for something like collapse consciousness is:question for many, many questions on the topic or What is an observer in quantum mechanics? is a reasonable starting point.

• So could the act of measuring causes the collapse not because the "universe can't have you know both position and momentum" but because it interferes with things measured. Are the experiments true where sensors were left on but data wasn't saved and wave didn't collapse. But when data was saved wave collapsed. Commented May 11, 2014 at 17:01
• There isn't a simple answer to that because it depends what interpretation you prefer. Personally I like decoherence + many worlds, and in that case superpositions always collapse whether humans are involved or not. Commented May 11, 2014 at 18:06