“You can observe a lot by just watching.” -Yogi Berra

Sure, the quantum Universe is a little bit spooky. Things that we’re used to being “determined” here in the macroscopic world, like where a particle will end up if you throw it, aren’t so simple if we head on down to subatomic scales.

Image credit: user Ufonaut99 from network54's GSJ Physics Forum, original via http://universe-review.ca/.

Image credit: user Ufonaut99 from network54′s GSJ Physics Forum, original via http://universe-review.ca/.

While you might have often heard that things are only determined by observation, does that have anything to do with you, the observer? Or is that just an anthropomorphized way of talking about what particles do?

Image credit: Till Jahnke, University Frankfurt.

Image credit: Till Jahnke, University Frankfurt.

Let’s take a look at Young’s experiment and Bell’s theorem and find out what a quantum observation really is!


  1. #1 rob
    July 19, 2014

    so what happen if we dont use active detection/measurement and just wait as particle react itself without any action from our side Q world is full of interaction with all particle surrounded our particle virtual etc etc so that is only matter of time as our particle send signal about hes actual state and location or maybe not ??

  2. #2 Sinisa Lazarek
    July 20, 2014

    @ Rob

    any interaction of any quantum system with any other is an “observation”… i.e. a sun ray hitting your window, at a point of interaction, each one is an “observation”. Not by you.. but that doesn’t matter. The ideal (mathematical) scenario is an interaction between 2 particles in absolute vacuum. But reality is not such. Trillions upon trillions of “observations” are done every second between atoms, photons etc. just so nature would work.

    But in general, yes…. the goal of experimentalists is to find a way to measure with there being as little disturbance to a quantum system as possible. And thus you reach heisenberg.. the smaller you probe, the higher the energy, and the disturbance…

  3. #3 Naren S.
    July 20, 2014

    With the Young Slit experiment with electrons being fired one at a time… can the pause between the firing of electrons be arbitrarily large and still cause the wave pattern on the screen?

  4. #4 Sinisa Lazarek
    July 21, 2014

    @ Naren


  5. #5 eric
    July 21, 2014

    @1: as Sinisa says, in RL you’re right, it typically is only a matter of time until an “observer” particle comes along. But you should look up the delayed choice quantum eraser concept. If you destroy all information gained from the interacting “observer” particle – even long after the interaction has occurred – the state of the system in the past goes back to indeterminant/wave-like. QM…schooling humans on the absurdity of reality since 1900. :)

  6. #6 John
    July 21, 2014

    I’ve always wondered if there’s some common sense that is being glossed over. You can’t simply look at a quantum particle because it is smaller than light. Are we using some sort of gamma or EM flashlight to look at them? If so, isn’t it the flashlight that causes the change and not the observation? Also, pebbles are subject to gravity. I don’t believe electrons are. Not at the same level anyway.

  7. #7 Michael Kelsey
    SLAC National Accelerator Laboratory
    July 21, 2014

    @John #6: Your first point is a re-invention (or a restatement without citation) of Heisenberg’s original gedankeneksperiment introducing his uncertainty principle.

    By “observation,” as Ethan pretty thoroughly explained in his article, we (nowadays) mean any sort of interaction, not just “eye-balling.” For example, an electron which strikes a pixel of a CCD chip and causes an electrical signal counts as an observation.

    Finally, so far as we know, electrons are certainly subject to gravity, just like protons, neutrons, atoms and molecules. Actually _measuring_ how electrons fall under gravity is absurdly difficult, just because it’s super hard to shield all of the confounding electrostatics. You might enjoy reading about measurements of ultracold neutrons in gravity (https://en.wikipedia.org/wiki/Ultracold_neutrons#Observation_of_the_gravitational_interactions_of_the_neutron), which are pretty cool (no pun intended…okay, pun intended :-).

  8. #8 Sean T
    July 22, 2014


    I’ll take Michael’s response to you a bit further. We have no reason to believe that electrons are not subject to gravity. However, the strength of the gravitational interaction depends on the mass of the two bodies that are interacting. In the case of the pebble, it has a mass of perhaps 1 gram or so. The electron has a mass of just under 10^-27 grams. Thus, the gravitational interaction between a body such as the earth and an electron is about 10^27 times greater for the pebble than it is for the electron. We typically measure the gravitational interaction between the earth and other bodies by using a scale to measure the weight of the smaller body. In the case of the electron, we have no hope of actually measuring this weight using a gravitational scale.

  9. #9 Michael Kelsey
    SLAC National Accelerator Laboratory
    July 22, 2014

    @Sean T #8 and John #6: We don’t actually measure the interaction force, or weight, for any atomic-scale particles. What we observe, though, is that if we launch such particles upward at very low speeds (few meters per second), they travel upward, stop, then fall back downward. See, for example, descriptions of “atomic fountain clocks”.

    We can measure their _acceleration_ in this case, and we see that neutrons, atoms, and molecules all fall at 9.8 m/s^2, the same acceleration as dropping a pebble. So the effect of gravity is the same, independent of the mass of the object, as predicted by both Newton and Einstein.

  10. #10 John
    July 22, 2014

    @Michael #7 and Sean T #8: Thank you so much for the well educated responses. I usually don’t have anyone to talk to about this stuff so I really appreciate it. So it sounds to me like Heisenberg is stating that the photon we send in is in fact causing the change so it doesn’t really sound that strange to me, we are just too big to look at things this small. It’s unfortunate but logical. Sorry about the lack of a citation. I’m kind of an armchair-scientist cheering the real scientists on. Following this train of thought, is there anything to rule out the possibility that electrons could be made of trillions of smaller particles that we would have no way of ever observing? Not without some massive breakthrough. I know String Theory touches upon this but I think it could be possible without String Theory as well.

    I do realize that electrons have some mass which adds to the mass of the atoms and molecules they are part of so in that respect they are affected by gravity. I just never heard of any interaction with electrons that wasn’t trumped by a more powerful force, so their movement is not really subject to gravity but the result of the stronger force. So there is still some gravity in the equation but its statistically insignificant. Just like how we are not subject to the centrifugal force or inertia of orbiting the sun because it is trumped by the Earths gravity. I’m not sure if anyone has ever tested if we weigh less at night and more during the day. I didn’t know about the atomic fountain clock, that’s awesome. Everyone has heard of atomic clocks, I just didn’t know about the fountain part. Thanks

  11. #11 ACORN1
    July 23, 2014

    Einstein proposed the glove analogy where if I send a left hand glove to one of the universe in a shoe box and a right hand glove to the other end of the universe in another shoe box. If I open the first shoe box and find a left hand glove, well, duh, the other glove is a right hand glove. No superposition required. What’s wrong with this explanation?

  12. #12 Michael Kelsey
    SLAC National Accelerator Laboratory
    July 23, 2014

    @John #10: Good questions, all, but too many to really address appropriately in comments on somebody else’s blog :-)

    Many of them are things you could attack by doing some Web searching (with care to avoid the woo!). The Wikipedia physics articles are generally pretty good, often being written by grad students in the area of interest.

    Heisenberg used the “microscope” analogy to derive and to provide some intuitive sense for the uncertainty principle. If you’re familiar with Fourier transforms, you can get exactly the same result without worrying about “shooting photons at electrons”, if you realize that momentum and position (or energy and time) are related to one another as Fourier conjugate variables. If you’re not familiar with FT’s, look them up on Wikipedia.

    The evidence for electrons being pointlike elementary particles(and protons *NOT* being so) comes from “deep inelastic scattering” (look it up). If we shoot electrons at each other, we can calculate exactly how they should scatter (angular deflection) as a function of how close to each other they get. If their charge were smeared out in a sphere, that would affect the angular distribution. What we have observed is that electrons scatter as though they were “perfect point charges”, with a spherical radius less than 10^-18 m. For protons, the same kind of experiments show that at low energies, they act like spherical charge distributions of ~10^-15 m (1 fm), but at very high energies, they have pointlike constituent charges, not smeared out.

    It isn’t that a stronger force “negates” the weaker (as your comments imply). If multiple forces are involved, we can certainly measure their different effects quantitatively: if I pull on a rope in one direction, and my six-year-old daughter pulls in the other, the rope is going to come to me. But it won’t come to me as fast as if I were the only one pulling. That difference is measurement, just as it is for gravity vs. electrostatics, or centrifugal vs. gravitational force (your weight at the equator is very slightly lower than it would be if the Earth were not spinning, and you can work out the difference).

  13. #13 eric
    July 24, 2014


    I do realize that electrons have some mass which adds to the mass of the atoms and molecules they are part of so in that respect they are affected by gravity.

    This is more of an interesting aside than it is a correction or answer, but the electron rest mass is (approximately) 511 keV. Using Einstein’s famous equation, you can convert that into a unit of mass (kg, for instance). However, most scientists in the relevant fields don’t bother. They typically use the elecron volt as the ‘working unit’ for all particle masses, because its extremely convenient to do so. Cuts down on the unnecessary math. This is why, for example, the recent discovery of the Higgs Boson reported its mass in GeV.

    I just never heard of any interaction with electrons that wasn’t trumped by a more powerful force

    !!! All of chemistry is interactions with electrons that aren’t trumped by a more powerful force! :) ” Interactions with electrons that aren’t trumped by a more powerful force” is what’s holding your body together, as well as the table, chair, computer etc… around you.

    Bonding interactions and other interactions between electrons typically have energies in the eV to keV range. So long as there isn’t a background ‘soup’ of particles and EM radiation at higher enegies than that, interactions between electron systems such as atoms will occur. It took the universe about 400,000 years to reach that point, but since then, interactions between electrons have only been ‘trumped by more powerful forces’ in environments such as hot stars and particle accelerators.

  14. #14 James T. Dwyer
    July 27, 2014

    Doesn’t the weirdness go away if we resist attributing causation to the act of observation, and instead simply consider that particles propagate as disperse waves in spacetime but interact as localized particles – and that the physical interactions that are necessary to produce entanglement also introduce the (probabilistic) alignment of quantum particle states?

  15. #15 Caleb Howard
    August 20, 2014

    so… what is the theory within the field of quantum computation – which allows the superposition of multiple “q-bits” to factor products of large primes efficiently (for example)? The computation is unobserved, but the observed result is somehow constrained by a filter of some sort to only allow the desired result… It seems (to my naive mind) to work as if there is massive parallelism involved.

    I want to be rock solid in my understanding – but the view I keep slipping into of Everett’s many-worlds interpretation of superposition and the collapse of the wavefunction is of a literal existence of all possible states (within a higher-dimensional context). With the *conscious* apprehension determining which state amongst the infinitude of possibilities we become associated (entangled?) within.

    Please – Lead me to clarity – and deliver me from woo, o-brainy ones!

    …and thanks!