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What Is the Heisenberg Uncertainty Principle?

By Christian Petersen
Updated: May 21, 2024

The Heisenberg uncertainty principle is a principle of nuclear physics, first described by theoretical physicist Werner Heisenberg. It states that one cannot accurately and precisely measure the momentum and position of a given sub-atomic particle simultaneously. The principle also states that the accuracy of the two measurements is inversely related — the accuracy of one measurement is correspondingly reduced as measurement of the other approaches the limit of its accuracy. Heisenberg clarified the principle, by stating that it had nothing to do with experimental techniques or measurement apparatus. Even under theoretically ideal and perfect conditions, it would remain valid.

In Heisenberg's paper on uncertainty with respect to sub-atomic particles, the Heisenberg uncertainty principle states that "The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa." This statement seems simple but had important implications for the very new sciences of quantum mechanics and quantum physics. It revolutionized the way scientists understood physics, the universe, the nature of matter, and reality. Prior to the development of this idea, physics was based on the supposition that, theoretically, there was an exact and precise value for every aspect of every particle in the universe, even if the means for measuring those properties did not exist.

The Heisenberg uncertainty principle says that not only is this not the case but that it can never be the case and that this fact is a result of the fundamental structure of matter and the way in which the particles that make it up behave. Rather than exact values for the various properties of sub-atomic particles, quantum mechanics instead deals with the probabilities of such values and of how the particles will behave. It is also related to the ability of light to act as both a wave and a particle and the finite speed at which it travels.

As part of his work in developing the principle, Heisenberg worked out what are called uncertainty relations. As the basis for this work, he used a hypothetical single electron moving through a vacuum. Observations of the electron are described in terms of its momentum, which is defined as its velocity — speed and direction — multiplied times its mass, its charge, and the time involved in the observation. He used a thought experiment, using an imaginary gamma ray microscope, to show that his principle indicates that it is impossible to know the exact value of all the variables of such a particle's properties.

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Discussion Comments
By nony — On Aug 25, 2011

@Charred - I’ll take the point you raise a step further, hopefully without getting too steeped in philosophy.

I think that uncertainty raises another question. How do the decisions we make affect our outcomes?

Perhaps the uncertainty exists because in the universe, any number of choices can result in different outcomes, and the uncertainty principle captures this fact at an atomic level; hence, position and momentum are given relative probabilities, not certainties.

Okay, that’s as deep as I’m going to get.

By Charred — On Aug 24, 2011

@David09 - I am no by means an expert on quantum mechanics, but it’s a weird and wild world from what I’ve heard and read.

I’ve heard that some subatomic particles actually change position in response to external events. For example, the simple act of observing a particle causes it to act funny.

Don’t ask me how, but scientists say that a particle acts one way when it’s on its own, but the moment you “look” at it, it acts differently. Perhaps this is a corollary of the uncertainty principle, which deals with probabilities of the exact location of matter and not certainty.

By David09 — On Aug 24, 2011

@NathanG - I think that there are other problems with the notion of a transporter besides what you state.

From what I’ve heard, it would take massive amounts of energy to reduce people to their atomic constituents, making the whole thing kind of impractical.

However, I don’t know if I agree with your take on the uncertainty principle. It just says that the exact position of the particle and its momentum cannot be measured at the same time, not that the position cannot be measured.

I suppose you could argue that you would need both pieces of information though if you were taking people apart, atom by atom.

By NathanG — On Aug 23, 2011

So let’s take this Heisenberg uncertainty principle, and ask what it implies for the so called physics of some science fiction shows.

Take Star Trek for example. They have this thing called a transporter, which takes people apart and puts them back together. People are reduced to their essential atoms in the process.

If it’s not possible to know the exact position of subatomic particles (you could only know probabilities, as the article seems to suggest) then would it not destroy the whole notion of a transporter?

I say it would. It would completely upend the scientific basis of the machine, assuming there was much science to begin with. People cannot simply be disassembled and reassembled like jigsaw puzzles, because their atoms could wind being in different locations.

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