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Q & A: Observer Effect?

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Most recent answer: 09/11/2013
the theory of observing changing the observed
- Anonymous (age 55)
seward, mpls
In quantum mechanics we learn that the behavior of the very smallest objects (like electrons, for example) is very unlike the behavior of everyday things like baseballs. When we throw a baseball at a wall, we can predict where it will be during its flight, where it will hit the wall, how it will bounce, and what it will do afterward.

When we fire an electron at a plate with two closely spaced slits in it, and detect the electron on a screen behind these slits, the behavior of the electron is the same as that of a wave in that it can actually go though both holes at once. This may seem odd, but its true. If we repeat this experiment lots of times with lots of electrons, we see that some positions on the screen will have been hit by many electrons and some will have been hit by none. The observed "interference pattern" for these electrons is evidence of their dual wave-particle nature, and is well described by thinking of each electron as a superposition of two "states", one that goes through one slit, one that goes through the other.

To add to this already mysterious behavior, this interference will only happen if both possible paths that the electron can take are not distinguishable. In other words, if we could somehow tell which slit the electron went through each time, we would no longer get the interference. The act of making a measurement of the electrons path fundamentally changes the outcome of the experiment.


(published on 10/22/2007)

Follow-Up #1: quantum

I understand that Quantum Physics is complicated and I'm just trying to wrap my head around this answer. So a couple of questions. 1. Is this done in a complete vacuum? 2. If not how do you know you aren't striking another electron and the two electrons together are passing through each slit?
- TwoNames (age 32)
Kamiah, ID, USA
I'm not sure it's so much "complicated" as very unfamiliar.


1. It's nearly a complete vacuum. Collisions with gas make the paths distinguishable and destroy the interference pattern.

2. The pattern only appears for good vacuums. It remains good even when the electron beam is so weak that there are almost never two electrons present at the same time.

Mike W.

(published on 11/25/2009)

Follow-Up #2: observation and light

By observation, does that mean using light? Because, I could understand how "observation" would muddle things up if locating the electron at a point in time required possibly interfering with its path. ...I'm just wondering if the literal Copenhagen interpretation can be side stepped.
- Devon (age 23)
Our usual stories about observation involve light, but it isn't essential.  Anything that leaves some sort of external record which differs depending on which path was followed will do.
Mike W.

(published on 12/01/2009)

Follow-Up #3: measurement and consciousness?

Are electrons conscious i.e. do they know that they are being observed? Secondly, do they change their behaviour when someone tries to watch it?
- Indrajit Kuri
New Delhi, India
It would be extremely surprising if anything as simple as an electron could have any sort of consciousness. However, when someone tries to watch an electron, they usually do something to the electron to make its behavior more evident. That something (e.g. shining a light on it) changes the electron's behavior.

Mike W.

(published on 08/20/2010)

Follow-Up #4: free-will?

As a follow up to the previous answer, I'd like to share something I found interesting. If one defines free will as something like "non-deterministic", one can prove from three simple axioms that if you wish to claim we (experimenters) have "free will", then we must conclude electrons have "free will" as well. It is an interesting way to demonstrate the "measurement problem" in quantum mechanics, for in the traditional Copenhagen approach the evolution of a system is completely deterministic except for a measurement. If it turns out all of physics can be explained with the appropriate choice of lagrangian, can we really have the freewill to choose a random measurement?
- Kevin M (age 29)
Urbana, IL, USA
Thanks for the interesting link. Just to expand on a point mentioned in passing in that article, there is a strong distinction between the indeterminacy described by the theorem and the traditional concept of free will. Readers should be forewarned that what follows somewhat spills over the edge of physics into philosophy.

There are serious reasons (including the violation of the Bell Inequalities) to conclude that the sort of events described by quantum mechanics are "free" in the sense that no prior fact about the universe can tell us which outcome we will observe. That doesn't mean that the necessary determining facts are hard to find; it means they didn't exist.

On the other hand, when we think of "free will" we have the sense that there was some prior "will" which determined what we chose to do. However, the existence of any such will would violate the theorems as much as any other determining variable.

Thus since the peculiar randomness of quantum events undermines the deterministic picture of the world it could be said to indicate a sort of "freedom", but not anything resembling traditional "free will."

Mike W.

(published on 08/24/2010)

Follow-Up #5: confusion between the uncertainty principle and the observer effect

There's a lot of confusion between the uncertainty principle and the observer effect, leading to the new age, nonsensical claim that we can willfully create the world around us by altering our thoughts. So, to be clear (because there's a lot of conflicting info out there), when we talk about "observing" an electron and thereby changing its state, we're talking about using equipment to measure it, not simply observing with the naked eye, right?
- Ian (age 29)
Right, we have no indication at all that interaction with conscious beings (e.g. us) does something different than interaction with any other large object in which some record is left of the results. Of course, the only events we are aware of are those of which we are aware, but we can leave that worry for the philosophers. At any rate, the structure of quantum mechanics, in particular its violation of the Bell Inequalities, would run into big trouble if the random outcomes of quantum events were influenced by any local variable, including human will.

So you're right on all your key points. Nevertheless, there is a relation between the "observer effect" and the uncertainty principle. Mathematics requires that any wave, including purely classical ones,  have a "spread" relation: ΔkΔx >= 1/2. That says that the spread (Δ) in the wavevector (k, sort of the inverse of the wavelength) times the spread in position (x) is greater than or equal to 1/2. The classical wave simply must have spreads in both these attributes, just as you can easily picture for water waves. We don't call this "uncertainty" or make a philosophical fuss about it because, as you can see by eye, the spreads in position and wavevector are real, persistent things.
What's weird about quantum waves, though, is that when they're "observed" or "measured" we don't see the full spread that was there in the wave. If you set up apparatus to measure x, you see an output that has a very narrow range of x, even if the input is a big spread of x. Likewise if you measure k, the output has a narrow range of k.  It's as if the wavefunction "collapsed" in a way guided by the type of measurement made. As to which particular little range of, say, x it collapses to, there's just a probability rule. The detailed result is purely random, not guided by any prior content of the universe. That's what converts the quantum spread into quantum uncertainty.

So people have good reason to link these effects and to be very puzzled by the whole business. As is common in cases of confusion, some people use the occasion to claim to be the center of the universe and to have magical powers. Other people buy it.

Mike W., Shalin, Samson

(published on 12/13/2012)

Follow-Up #6: Quantum for the non-scientist

I have a friend (not a physicist) who reads unscientific articles on things like schroedinger's cat and superposition and the 2-slit experiment phenomena, and as a result comes to believe that quantum mechanics undermines the logical rules of our universe because you can say things like "the particle is in two places at once" and applies it to more of reality than it should be. I feel like I have a pretty good understanding that his problem lies somewhere in a bad understanding of the uncertainty principle and/or observer effect (similar to the I can make things happen that I want because of the observer effect issue discussed in the last comment, but not quite the same). I'm having a hard time articulating though, I was wondering if you could help?
- William (age 26)
Columbus, OH, USA

Hi William-

We'd love to help on that, but it would be much more effective if your friend could follow up with some specifics in his or her own words. We've written so much on the topic, generally searchable via the phrase "Bell Inequality", that we have little to add. I think it's fair to say that both quantum mechanics and relativity shake up a lot of basic ideas about the universe. That doesn't mean that they leave nothing of ordinary logic. just that what's left is modified- especially in the case of quantum mechanics.

Mike W.

(published on 09/11/2013)

Follow-up on this answer.