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I am a bit confused about the Heisenberg's Uncertainty Principle - just read about it in How to Teach Physics to Your Dog, by Chad Orzel. He states that the reason electrons can't be measured is because the photons used to measure electrons, collide with them causing changes in momentum and velocity.

Couldn't this simply be fixed by creating a vacuum and using a different method of observation? Maybe something that doesn't exist today?


A full disclaimer: I never took a physics course higher than a basic intro to physics.

All the answers below are still referring to light (photons) as the method for measuring the momentum/location of the particle. My question is: is it not possible to create a vacuum without any possible light source from entering; using this vacuum we can device a more advanced method which does not affect the momentum or position of the electrons.


marked as duplicate by Michael Brown, Waffle's Crazy Peanut, Emilio Pisanty, Manishearth Mar 20 '13 at 17:31

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    $\begingroup$ In my understanding, a measurement necessarily involves an interaction, and thinking simplistically, electrons interact with other electrons by exchanging photons. In theory you could have a device 'tuned" to some other interaction, then perhaps you don't have to detect photons. By measure I presume you wish to measure position? The question is not very clear. $\endgroup$ – Antillar Maximus Mar 20 '13 at 13:29
  • $\begingroup$ @AntillarMaximus, yes, by "some new technology", I am referring to an interaction that doesn't involve photons at all but something else altogether. $\endgroup$ – TheOne Mar 20 '13 at 13:53
  • $\begingroup$ The only two forces that interact with electrons are electromagnetic (photons) and weak(W and Z bosons). It's hard to imagine a "measuring device" tuned to electron position that uses the weak force. $\endgroup$ – DWin Dec 12 '18 at 20:20

What is missing in your question, (and maybe not emphasized properly in the book), is the domain of application of the statement :"measure".

Here are individual electrons in a bubble chamber interacting with a magnetic field and turning into helical paths.

electron positron

It shows an electron and positron pair generated by a photon interacting with a nucleus in the bubble chamber liquid.

We can measure the position and the momentum of the electron and positron, and after accumulating a number of such interactions we will know the probability that a photon of this energy has to create an electron positron pair.

The difference with your statement lies in the domain of application of the word "measure".

If we want to go into dimensions much smaller than the micron measurement accuracies of a bubble chamber, then the uncertainty principle becomes important.

$\Delta(x) \Delta(p)>\hbar/2$ with $\hbar=1.054571726(47)×10^{−34}Js$ is satisfied macroscopically since $\hbar$ is practically $0$.

It is in the very small dimensions, less than picometers, of the elementary particle interactions where the Heisenberg uncertainty principle effect is significant and unavoidable.

In those dimension it has little meaning to visualize the electron as a small billiard ball. It is an elementary quantum mechanical entity whose domain is described by solutions of quantum mechanical equations. These solutions give the probability of finding the "particle electron" in a specific (x,y,z,t) and depending on the experiment and the boundary conditions, this probability displays a wave nature or a particle nature . In any case it obeys HUP because the HUP arises from the basic assumptions of the quantum mechanical formalism.

The "good enough technology" at those small dimensions has to follow the Quantum Mechanical solutions, so no, the better the position measurement,the worse the momentum knowledge is inevitable.

Now you also state from the book

He states that the reason electrons can't be measured is because the photons used to measure electrons, collide with them causing changes in momentum and velocity.

If there existed no HUP there would be no problem in measuring the position of an electron the way one would measure trajectories of billiard balls in classical mechanics.

It is the quantum mechanical nature which is probabilistic that changes the rules and only a probable position can be predicted or postdicted. This indirectly has to do with the HUP since the HUP is at the center of the mathematical formulation of Quantum mechanics, arising from the operator algebra and the commutator relationships of these operators.


The uncertainty principle that arises from Quantum Mechanics, tells us that uncertainty is built in (embedded) into the "nature of reality. Put it another way, the uncertainty principle describes a fundamental property of quantum systems and it isn't a statement about the limitation of technology. when you talk about measurements and observation, you mustn't think of this as in the classical sense. In QM, a measurement is performed by probing the system with a photon, etc. and doing this the photon interacts with the system and disturbs it. So no matter how advanced technology becomes, there will always be an uncertainty in the system you measure. Take a look at http://plato.stanford.edu/entries/qt-uncertainty/


That effect you're referring to is the observer effect, wherein the act of observing something changes the thing we observe (and when used on people, interestingly, it's called the Hawthorne Effect -- but anyway).

But the uncertainty principle is a real thing that is apart from the observer's effect. No matter how good your technology is, the uncertainty principle makes it impossible to know both the position and momentum of your particle.

This is partly because your particle exists in a "cloud of probability." That is, there is actually no one specific point in which the particle (such an electron) exists. Rather, it exists in a distribution around the nucleus (for example). For every collection of points in space, there is a probability that is associated with the electron being present there.

The uncertainty principle makes it so that the more you know about the position of the electron, the less you know about the momentum, and vice versa. The act of knowing the position causes its momentum to be very uncertain. At the same time, the act of knowing the momentum makes its position very uncertain. It has nothing to do with observing it, though -- whether we observe it or not, it still acts in that way.

I know, it's weird. You can take a look at the wiki page: http://en.wikipedia.org/wiki/Uncertainty_principle

Edit --

You might also want to look at wave-particle duality. It's a very poorly understood thing AFAIK, especially with these quantum particles. But an electron has both a wavelength, like a wave, and momentum, like a particle.

The crazy thing is, if you treat the electron as a particle, it acts like a wave. And if you treat it as a wave, it acts like a particle!

  • $\begingroup$ Was any reason for this effect provided? $\endgroup$ – TheOne Mar 20 '13 at 13:04
  • $\begingroup$ Do you mean the observer effect? Yes -- you already said the reason yourself. Basically, the act of measuring something changes the way it behaves. $\endgroup$ – markovchain Mar 20 '13 at 13:09
  • $\begingroup$ No, in regards to your statement, "But the uncertainty principle is a real thing that is apart from the observer's effect. No matter how good your technology is, the uncertainty principle makes it impossible to know both the position and momentum of your particle." What's the reasoning behind this? What causes a particle to exist in a "cloud of probability"? $\endgroup$ – TheOne Mar 20 '13 at 13:56
  • $\begingroup$ Maybe because of the wave-particle duality... $\endgroup$ – TheOne Mar 20 '13 at 13:57
  • $\begingroup$ You will have to read up on the cloud of probability wherein an electron exists in. It's not a mini-solar system with the electron orbiting the nucleus, but rather a probability density enveloping the nucleus. This is way more complex when taken in depth and involves a lot of maths, unfortunately. $\endgroup$ – markovchain Mar 20 '13 at 14:14

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