# When will a quantum measurement occur?

I am confused about the nature of quantum measurements.

In short: if a particle is coupled with a field, when will the wave collapse. A field might be coupled to a particle for a long time, and then suddenly decides to releases its energy to the particle. What determines when this happens?

I ordered my thoughts in a number of points, but maybe its all the same question. Any help would be much appreciated!

Suppose there is a particle in a electromagnetic field. The particle has the transition available to absorb a single excitation, photon, of the EM wave.

1: is the likelihood of phenomenon such as scattering or absorption determined by a probability distribution? and hence is photon absorption a wavefunction collapse phenomenon. (i guess so right? it is the photoelectric effect)

2: Can the collapse of the wavefunction, occording to a probability distribution, only be a result of a measurement? ( I mean measurement in general, including interaction with surroundings )

3: The way i see a measurement, it is some sort of extraction of information from a system at a particular time. like a snapshot. (The collapse happens (almost) instantaneous, so measurements spread out over time, can be considered multiple measurements?)

4: So the particle is continuously coupled with the field. Absorption is a result of measurement, or collapse. When then occurs such a measurement?

5: if measurements would happen continuously when the field and particle are coupled, wont the wave collapse right away, and then be trapped in this state (quantum zeno effect)?

6: Or if the measurement does not happen continuously, when then? Is there another probability distribution that determines if a measurement is occurring? never heard of this.

• I mean no offence, but you have a lot of physics concepts linked together that, AFAIK, have no connection. If you read through similar questions regarding quantum measurements, you may find some of your questions already answered, or your assumptions in this post clarified. – user108787 Sep 6 '16 at 15:53
• Thanks for your reply, i quess i formulated my question ambiguous. I basically have only one question, but ordered my assumptions in questions. My question is, if a particle is coupled with a field, when will the wave collapse. Or an other way saying, when will a measurement occur? I edited my quenstion slightly – Julian Sep 6 '16 at 16:14

My question is, if a particle is coupled with a field, when will the wave collapse. Or an other way saying, when will a measurement occur?

Forgive me if you know this already (and I am no expert, so if someone contradicts me, they are probably right.):

There are two theories, Q.M. and its later development, Quantum Field Theory.

Q.M. does not stress the importance of fields, You can read an entire textbook on Q.M. without dealing with fields. Rather it concentrates on particles and on using the concept of a wavefunction collapse (which is a topic of much debate, to say the least). There are least 8 different basic idea about how quantum mechanics works. It's does say, when a particle is observed, it's wavefunction collapses. As I say, intrepting that simple statment is not easy and entire books have been written about it. But no fields involved, yet.

QFT takes the notion of fields as the basic underlying reality. It treats a particle as an excitation of the field, it's also known as a quanta of the field. If you take the field as fundamental, (and accept that there is a field for 1. every property of each particle, 2. for the particles themselves and 3. every force that we know about), then you don't need to deal so much with the wave particle duality that QM gets caught up in, and the wavefunction of QM is replaced by the field concept of QFT.

Another very important aspect of QFT is that it combines special relativity with quantum mechanice. The Schrodinger equation of QM is replaced by the Dirac equation, which was the forerunner of the field concept.

To try and answer your question, it still remains a subject of debate as to how a measurement "produces" a particle. QFT does not directly answer this question, so we still don't know for example, is a human observer needed to make the measurement, that creates the particle? QFT is a highly developed and sophisticated refinement of QM, but the basic difference between the classical and quantum world's still remains.

I hope you get a better answer than this, as there are other users of this site with vastly more experience and knowledge than I have.

• No problem, but I have to say I still don't follow what you mean by: when will a measurement occur? Could I suggest you expand on that important question, and post another question based purely on an expanded version of that, and the best of luck with it. Google decoherence for another view of the question. – user108787 Sep 6 '16 at 18:02
• I mean that QM states that if a measurement happens a collapses will occur, but not when the measurement happens. A field might be coupled to a particle for a long time, and then suddenly decides to releases its energy to the particle. What determines when this happens? – Julian Sep 6 '16 at 18:14
• I read some more about collapse in QFT here (link) where it basically sad that a field would collapse when a particle would be made / destroyed. Which sounds to me like circular reasoning, since a particle is the product of a collapse in the first place. Confusing.. – Julian Sep 6 '16 at 18:14

As I understand it, QM collapse is the conversion of a system from having a QM state or superposition of QM states (the square of the probability that it has a given position, speed, etc.) to an "actual" or physical or Newtonian position, speed, etc.

An example of collapse in nature would be the conversion of a photon of the Sun's energy to the chemical energy of the key step in photosynthesis in a plant's leaf.

An example of collapse in a physics experiment would be the measurement of whether a photon passes through a given slit in the two-slit interference experiment. One important point concerning this type of collapse is that the energy of a single photon is too small for us to see in a lighted room, so it must be amplified in some way to be visualized or recorded. Amplification of a photon is usually done by destroying the photon or increasing or decreasing its energy such as to generate electrons. The energy of such electrons is much easier to detect, since electrons carry a charge.

It has been said that human consciousness can collapse a QM field (a wave) into a particle having position, speed, etc., but a simpler explanation (Occam's razor) is that the act of measurement, in which a large energy is directed at a tiny QM-regime phenomenon, perturbs the phenomenon and thus forces the collapse, which is then observed by humans.

I hope others better educated than I can correct this explanation.