# I Question About The Role of Observation in Quantum Mechanics

#### ray3400

Summary
Does observing a particle cause it to exhibit a certain quality? What is the cause and effect relationship involved with observation?
In the double-slit experiment when a detector was placed before the two slits, a 2 strip pattern was produced after the two slits. When there was no detector placed before the two slits, a different pattern was produced after the two slits. Why does the presence of a detector before the two slits cause a different pattern to be produced after the two slits?

Last edited:
Related Quantum Physics News on Phys.org

#### StevieTNZ

Summary: Does observing a particle cause it to exhibit a certain quality? What is the cause and effect relationship involved with observation?

What constitutes an "observation" in quantum mechanics?
No one has resolved that issue. It could mean modifying quantum mechanics to allow for gravity to cause collapse of the wave function, for example.

A good book you can try reading is 'Sneaking a Look at God's Cards' https://www.amazon.com/dp/069113037X/?tag=pfamazon01-20

#### PeterDonis

Mentor
I don't understand how observation can "cause" anything.
"Observation" is an interaction. Interactions cause things. Certainly your observations of, for example, your door cause things to happen in your brain.

The reason observing your door can't make it open is that your door is a massive object and you observe it with piddly little photons that can't appreciably affect its state. But that's a particular fact about that particular observation; it's not a general property of all observations. In QM, many observations do not have that property.

#### ray3400

"Observation" is an interaction. Interactions cause things. Certainly your observations of, for example, your door cause things to happen in your brain.

The reason observing your door can't make it open is that your door is a massive object and you observe it with piddly little photons that can't appreciably affect its state. But that's a particular fact about that particular observation; it's not a general property of all observations. In QM, many observations do not have that property.
Is there any effort to mitigate the effects sensors have on the quantum particles being observed?

In the case of the double slit experiment with electrons, when the electrons are sent down the slits without detection they create an interference pattern and act like a wave. When the electrons are being detected they change to create a pattern of two strips and act like particles. What about the detector in this situation caused the elections to act like particles instead of waves?

Last edited:

#### ray3400

No one has resolved that issue. It could mean modifying quantum mechanics to allow for gravity to cause collapse of the wave function, for example.

A good book you can try reading is 'Sneaking a Look at God's Cards' https://www.amazon.com/dp/069113037X/?tag=pfamazon01-20
Thanks, I will look into it. This is certainly a huge jump from classical physics.

#### microsansfil

hi,

Concerning young's slits, in any case, the electron leaves an impact as a corpuscle.

As the electron/photon/.../"corpuscle" arrives on the Ꜫ plate, the following phenomenon occurs: their impacts are randomly distributed, and it is only when a large number of them have arrived on Ꜫ that the distribution of impacts seems to have a continuous aspect. The impact density at each point of Ꜫ corresponds to the interference fringes.

/Patrick

#### ray3400

hi,

Concerning young's slits, in any case, the electron leaves an impact as a corpuscle.

As the electron/photon/.../"corpuscle" arrives on the Ꜫ plate, the following phenomenon occurs: their impacts are randomly distributed, and it is only when a large number of them have arrived on Ꜫ that the distribution of impacts seems to have a continuous aspect. The impact density at each point of Ꜫ corresponds to the interference fringes.

View attachment 244801

/Patrick

Ok, but isn't it the case that when the electrons are being detected they act like particles, and instead of making a pattern like the one you posted, they make a pattern of two strips?

What aspect of the detection device in this experiment caused the electrons to act differently?

#### PeroK

Homework Helper
Gold Member
2018 Award
Is there any effort to mitigate the effects sensors have on the quantum particles being observed?

In the case of the double slit experiment with electrons, when the electrons are sent down the slits without detection they create an interference pattern and act like a wave. When the electrons are being detected they change to create a pattern of two strips and act like particles. What about the detector in this situation caused the elections to act like particles instead of waves?
If you have an hour to spare, it might be worth watching the Feynman lecture on the QM view of nature.

Also, there is a lot of stuff out there about QM that is designed to confuse rather than explain. For example: "that electrons sometimes behave like particles and sometimes like waves". This stuff makes it more difficult to get a grasp of what QM is really about. It might be an idea to watch the Feynman lecture with an open mind: forget what you think you already know about QM.

Last edited:

#### PeterDonis

Mentor
Is there any effort to mitigate the effects sensors have on the quantum particles being observed?
Past a certain point, you can't. In QM you cannot make interactions as small as you like; there is a minimum interaction.

#### PeterDonis

Mentor
When the electrons are being detected
You mean, when they are being detected at the slits, i.e., when the result when you send each electron through the apparatus is not just "electron made a dot at this point on the screen", but either "electron went through slit A and made a dot at this point on the detector screen" or "electron went through slit B and made a dot at this point on the detector screen". In the latter case there is no interference. But it is not true that electrons "act like particles" only in the latter case. Each electron makes a dot on the screen whether there is a way to detect which slit it went through or not. Making a dot on the screen is acting like a particle.

#### ray3400

You mean, when they are being detected at the slits, i.e., when the result when you send each electron through the apparatus is not just "electron made a dot at this point on the screen", but either "electron went through slit A and made a dot at this point on the detector screen" or "electron went through slit B and made a dot at this point on the detector screen". In the latter case there is no interference. But it is not true that electrons "act like particles" only in the latter case. Each electron makes a dot on the screen whether there is a way to detect which slit it went through or not. Making a dot on the screen is acting like a particle.

In the experiment when the electrons are being observed using a detector they produce a different output pattern than when they are not being observed with a detector. What about the detector observing them causes the output pattern to change?

#### PeroK

Homework Helper
Gold Member
2018 Award
In the experiment when the electrons are being observed using a detector they produce a different output pattern than when they are not being observed with a detector. What about the detector observing them causes the output pattern to change?
You should watch the Feynman lecture! There are four cases:

1a) Only first slit open

1b) Only second slit open.

2) Two slits with a detector.

3) Two slits without a detector.

The first three cases result in the same pattern, in the sense that 2) is the sum of 1a) and 1b). This leads to the conclusion that all the detector is doing is determining a slit through which the particle passes. It's not doing anything else.

The key point is that 3) is NOT the sum of 1a) and 1b).

Watch the Feynman lecture!

#### ray3400

You should watch the Feynman lecture! There are four cases:

1a) Only first slit open

1b) Only second slit open.

2) Two slits with a detector.

3) Two slits without a detector.

The first three cases result in the same pattern, in the sense that 2) is the sum of 1a) and 1b). This leads to the conclusion that all the detector is doing is determining a slit through which the particle passes. It's not doing anything else.

The key point is that 3) is NOT the sum of 1a) and 1b).

Watch the Feynman lecture!
Why does the observer collapse the wave function simply by observing? By what mechanism does this happen? How can looking at something cause it to physically change?

Last edited:

#### PeroK

Homework Helper
Gold Member
2018 Award
Ok, so its not the case that by simply observing the particles in motion (and not changing any other factor of the experiment) we are changing the output? Lots of people make it seem like observation in and of itself changes the output. If that's the case, I was wondering by what mechanism does observation cause a different output?
Every "observation" is an "output". With a detector you have two outputs: a definite slit and a final point on the screen. Without a detector there is only one output: the point on the screen.

It's at this point, to explain why 3) is not the simple sum of 1a) an 1b), that you need QM. That's is really the issue. Statements like "observations cause changes in output" are just words. They don't really mean anything.

Watch the Feynman lecture!

#### ray3400

Every "observation" is an "output". With a detector you have two outputs: a definite slit and a final point on the screen. Without a detector there is only one output: the point on the screen.

It's at this point, to explain why 3) is not the simple sum of 1a) an 1b), that you need QM. That's is really the issue. Statements like "observations cause changes in output" are just words. They don't really mean anything.

Watch the Feynman lecture!
Why does the observer collapse the wave function simply by observing? By what mechanism does this happen?

The bottom line is - the sensors caused a 2 strip pattern output as opposed to an interference pattern output. How can looking at something cause it to change? Is there some sort of energy or matter being emitted from the sensor that interacts with the electrons to cause them to change behavior?

If there is no transfer of matter or energy from the sensor to the electrons to form a cause and effect relationship, this is the equivalent of saying "because I looked at my door, the door changed positions".

#### PeroK

Homework Helper
Gold Member
2018 Award
Why does the observer collapse the wave function simply by observing? By what mechanism does this happen?

How can looking at something cause it to physically change? Did the sensors they used to observe the electrons before they went into the slits interact with the electrons in a way to force them to act like particles and form a 2 strip pattern as opposed to an interference pattern which occurred when there is no sensor observing?
An observation gives you information about something. To say it changed implies it was definitely doing something and then it changed to definitely doing something else. The first thing you should learn about QM is that a particle does not have a definite position unless you measure where it is.

Furthermore, particles at the QM scale do not have classical trajectories.

Fundamentally, you have not learned the basics of QM. You're now just asking the same questions, based on the same preconceptions and misconceptions. At some point, be it IT, chemisty or QM, you do actually have to learn the basics.

If you use a phrase like "electrons being forced to act like particles", then you need a fresh start.

#### ray3400

An observation gives you information about something. To say it changed implies it was definitely doing something and then it changed to definitely doing something else. The first thing you should learn about QM is that a particle does not have a definite position unless you measure where it is.

Furthermore, particles at the QM scale do not have classical trajectories.

Fundamentally, you have not learned the basics of QM. You're now just asking the same questions, based on the same preconceptions and misconceptions. At some point, be it IT, chemisty or QM, you do actually have to learn the basics.

If you use a phrase like "electrons being forced to act like particles", then you need a fresh start.
Referring to the double slit experiment:
I do the same experiment twice, and keep everything exactly the same except in one case the electrons are being observed, and in the second case the electrons are not being observed. In the first case I get a two strip pattern output on the screen, and the second case I get an interference pattern output on the screen.

Why does the experiment produce different results when the electrons are observed as opposed to when they are not observed?

Is it correct to conclude that observing the electrons caused the output to be different on the screen?
If so, by what mechanism does observing the electrons cause the output to be different on the screen?

Last edited:

#### vanhees71

Gold Member
Ok, but isn't it the case that when the electrons are being detected they act like particles, and instead of making a pattern like the one you posted, they make a pattern of two strips?

What aspect of the detection device in this experiment caused the electrons to act differently?
The confusion comes only from the fact that some people, obviously and unfortunately many of which write popular-science articles/books, that the world is not behaving as they think from everyday experience with macroscopic systems, which appear to behave as described by classical physics. This is, because for macroscopic objects we need to describe only quite rough macroscopic observables and neglect (average over) the many microscopic degrees of freedom they consist of.

The intuitive worldview from this experience breaks usually down when dealing with small systems like electrons. You cannot describe them as classical particles nor as classical fields. Rather, from the most fundamental point of view we have today about them, they are certain states of a quantized Dirac field.

Under "non-relativistic" circumstances, i.e., when the electron runs with speeds much less than the speed of light, you can describe it by a wave function $\psi(t,\vec{x})$ (where I neglect the spin degree of freedom, which is not important for the explanation of the double-slit experiment). This wave function obeys the Schrödinger equation, which describes the time evolution of the wave function.

Though there are many people, who cannot accept this view, and I'm sure there'll be tons of following answers to your question denying it, yet the only consistent interpretation of this wave function (it's called the minimal statistical interpretation, since there's a plethora of other interpretations all trying to somehow deny the very fact of generic indeterminism in our description of nature, which is due to some philosophical prejudices but not founded on any observations): The function $P(t,\vec{x})=|\psi(t,\vec{x})|^2$ describes the probability distribution to find the electron at a place $\vec{x}$ when looked there at time $t$.

The specific form of the wave function is determined by the "preparation of the electron" at some initial time $t=0$. Then its time evolution is given by the Schrödinger equation. So given the preparation you can determine the probability distribution of its location at any time. You can also calculate the probability distributions for any other observable, like momenta, by certain mathematical manipulations (in this case going from the position representation to the momentum representation it tells you to take the Fourier transform of the wave function).

This interpretation implies now that observables of a particle are usually not determined but have only certain probabilities do be found when you measure them, given by the wave function.

Now take the double-slit experiment. Now say you want to figure out through which slit each electron comes you shoot at the double-slit, but it should not be determined beforehand, that each electron goes through one of the specific slits (this possibility is not very surprising, yielding a priori the same thing as you expect from classical physics).

As an experimental physicist you now have to ask, how to realize this case, i.e., you have to think how you have to prepare each electron to describe this situation. Now the above described formalism tells you that if you prepare an electron to have a very well defined position initially, this implies that the wave function is narrowly peaked around this position. Now the momentum wave function is given by the Fourier transform of this wave function that's narrowly peaked in position space, which implies that it is a pretty broad distribution in momentum space. This is quantified by the famous position-momentum uncertainty relation: If the standard deviation (which quantifies the uncertainty, i.e., the width of the wave function around the expectation/average value) of the position-vector-$x$ component is $\Delta x$, then there's a an uncertainty momentum-vector-$x$ component $\Delta p_x$ which is so large that $\Delta x \Delta p_x \geq \hbar/2$. So if you initially localize the electron very well, you get a quite uncertain momentum, and the Schrödinger equation for a free electron accordingly tells you that the electrons position-wave function gets broader and broader with time.

In order to avoid the case that your electron's position wave function is still so narrow and the experimentalist aimed at on slit well enough (i.e., he's chosen the momentum, though broad, to be pointing into the corresponding direction) that with certainty it goes through one slit, you simply have to put your slit far away enough from the electron source, so that the wave function gets so broad that its peak covers well both slits. You can arrange this such that it goes with equal probabilities through one or the other of the two slits, if it goes through at all, i.e., many electrons will simply be absorbed by the material making up the double slit, but some will go through, and you cannot with certainty say through which of the two slit any individual electron will have come.

However, at the position of the slits, each electron only goes as a whole through one of the slits. This implies that those electrons that went through the slits at all, at the slit it's with certainty localized with an accuracy given by the width of this slit, and that implies that directly at the slits each single electron is uniquely at the one or the other slit.

Closely behind the slits the wave function thus now shows two narrow peaks corresponding to the pobabilities to go through one or the other slit. The two peaks are well separated, i.e., their width (given by the width of each slit) is much smaller than their distance (given by the distance of the centers of the slit).

Behind the slits the electron's wave function again goes on according to the Schrödinger equation, i.e., it broadens again, but not too far from the slit, i.e., after not too long times, the wave function will still show two well-distinct bumps. Although these bumps got a bit broader, they are still not too much overlapping, so that for each electron the probability distribution is such that with quite high certainty you can say through which slit they came.

This means that, if you want to know (with high certainty) through which of the two slits each electron came you simply have to put your electron dectector (e.g., a CCD cam like the one in your cell phone) close enough to the double slit. Sending through a lot of such prepared electrons in this setup you'll just see two well-disitinct bumps according to the probabilities given by the wave function at the position of the CCD cam. Each electron being in one of the bumps very likely came through the corresponding slit. Thus, here you obviously have which-way information with high certainty.

On the other hand, if the wave function develops further from the slits the two bumps get broader and broader and then overlap more or less. The longer you wait and the farther the electron goes away from the slits the less certain you can determine through which slit this electron came. I.e., if you put the CCD cam far enough away from the slits, there's no possibility anymore to know from the position the electron hits the cam, through which slit it has come.

Now the Schrödinger equation is mathematically a wave equation, and this implies that the partial waves of electrons coming through the one or the other slit simply add like $\psi=\psi_1+\psi_2$. Now the probability is given by $|\psi|^2=|\psi_1|^2+ |\psi_2|^2 + \psi_1^* \psi_2 +\psi_1 \psi_2^*$. The last to terms are called the "interference terms". In the situation where the CCD cam is placed far away from the two slits $\psi_1$ and $\psi_2$ have a large overlap and thus the interference term is significant, and depending on the specific position at the CCD cam the contributions of the partial wave may add (constructive interference) or cancel each other out (destructive interference) or something in between, and that's precisely the interference pattern you expect from wave phenomena, i.e., in this situation where you don't know which way the electron took the probability distribution shows a significant interference pattern.

Of course you can argue with the interference piece also in the case discussed above, when the CCD cam is close enough to the slits to still resolve which-way information with high certainty: Then the partial waves do not significantly overlap, corresponding to the still well separated two bumps, and thus $\psi_1 \psi_2^*+\psi_1^* \psi_2 \simeq 0$ everywhere at the CDD screen. Thus you get just well distinct bumps on the CCD screen but no (significant) interference.

Note that still, each electron makes one single dot on the CCD screen, i.e., a single electron cannot appear as some smeared distribution at the screen. In this sense it has always particle features when detected, and the above given probabilistic "interpretation of the wave function" is at least non-contradictory with these observational facts and also not contradicting any other fundamental laws of physics (like causality and all that). I know of no other interpretation that is consistent in this sense (except Bohmian mechanics, which however is consistent only in the here discussed non-relativistic limit, and also doesn't lead to other phenomenological implications of the quantum-theoretical formalism than the minimal interpretation).

#### ray3400

The confusion comes only from the fact that some people, obviously and unfortunately many of which write popular-science articles/books, that the world is not behaving as they think from everyday experience with macroscopic systems, which appear to behave as described by classical physics. This is, because for macroscopic objects we need to describe only quite rough macroscopic observables and neglect (average over) the many microscopic degrees of freedom they consist of....
Thanks for the explanation. I think I have a much better idea of how the double-slit experiment works after reading. There is still one aspect of it I don't understand - Why does the experiment produce different results when the electrons are observed as opposed to when they are not observed? Or is this not a feature of the double slit experiment? If it is the case, I don't understand how the act of observation can cause different results.

#### vanhees71

Gold Member
What do you mean the experiment produces different results when the electrons are observed as opposed when they are not observed? If you don't observe the electrons, there's no experiment, because nothing is observed. Physics is about observations, and measurements are just observations giving precise numbers. If you don't measure the electron's position, all you know are the probabilities to find it at any given position, provided you know its wave function well enough due to some knowledge about it at an earlier time (and this also only if you have accurate knowledge about how the electron interacts with anything around it, i.e., if you can solve the Schrödinger equation given the Hamiltonian of the electron).

#### ray3400

What do you mean the experiment produces different results when the electrons are observed as opposed when they are not observed? If you don't observe the electrons, there's no experiment, because nothing is observed. Physics is about observations, and measurements are just observations giving precise numbers. If you don't measure the electron's position, all you know are the probabilities to find it at any given position, provided you know its wave function well enough due to some knowledge about it at an earlier time (and this also only if you have accurate knowledge about how the electron interacts with anything around it, i.e., if you can solve the Schrödinger equation given the Hamiltonian of the electron).
In the double-slit experiment when a detector was placed before the two slits, a 2 strip pattern was produced after the two slits. When there was no detector placed before the two slits, a different pattern was produced after the two slits. Why does the presence of a detector before the two slits cause a different pattern to be produced after the two slits?

#### ray3400

Here is another study that explores the same question:
https://www.sciencedaily.com/releases/1998/02/980227055013.htm

"REHOVOT, Israel, February 26, 1998--One of the most bizarre premises of quantum theory, which has long fascinated philosophers and physicists alike, states that by the very act of watching, the observer affects the observed reality. "

"the very act of watching, the observer affects the observed reality"

How does the "observer effect" cause the two different output patterns in the double slit experiment for electrons? There is a different output pattern when the electrons are being observed before the going through the slits and when they are not being observed before going through the slits.

Last edited:

#### vanhees71

Gold Member
In the double-slit experiment when a detector was placed before the two slits, a 2 strip pattern was produced after the two slits. When there was no detector placed before the two slits, a different pattern was produced after the two slits. Why does the presence of a detector before the two slits cause a different pattern to be produced after the two slits?
If you put a detector in front of one slit to somehow register, that an electron goes through, there's some interaction of this electron with this device, and this has to be taken into account when solving the Schrödinger equation. Usually such an interaction destroys the intereference pattern, because it destroys the coherence of the partial waves concerning the case that the electron goes through the slit, where you detect it, or through the other slit. To analyze this in detail you have to give a specific example for the detector and analyse its effect on the electron. That's why I provided the most simple thinkable example of the dependence of what's observed from the choice of which experiment you do, i.e., an experiment which shows interference patterns vs. one that provides which-way information.

#### A. Neumaier

Is there any effort to mitigate the effects sensors have on the quantum particles being observed?
Yes, see nondemolition measurements. But this doesn't apply for the double slit.
by the very act of watching, the observer affects the observed reality. "
The pattern changes since a detector at the slit has a serious impact on the microscopic stuff going through the slit:
How can looking at something cause it to physically change?
The change in an observed object due to an observation is large if the means of observing it is comparable (or larger) in size and impact to the observed object. If you magnify the situation sufficiently strongly, it is qualitatively like (though not really like) observing a sand castle by a big wave from the shore.

Last edited:

"Question About The Role of Observation in Quantum Mechanics"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving