The Toronto experiment actually measured the spin of a single photon, not the position of a particle. In fact, it can even equal zero, a fact which has only been fully appreciated in the last ten years. According to QM, multiplying the error by the disturbance can be less than ħ/m. The surprising thing is that this guess is wrong. This is a very reasonably guess, and it is essentially the “measurement–disturbance relation” which Heisenberg guessed in 1927. You might guess that they should be related by This fact is another form of the HUP, relating the error in a measurement of position, e(q), and the associated disturbance in the velocity d(v). But this doesn’t work, because a perfect (zero error) measurement of position will necessarily disturb the velocity, and vice versa. You might be think that if you want to know both you could find them out by first measuring the velocity, then the position. Hence you cannot be certain of both the position and velocity of the particle – you cannot know everything. Here Δq is the uncertainty in the position of a particle (in metres), Δv is the uncertainty in its velocity (in metres per second), m is its mass in kg, and ħ is a very small constant ( Planck’s constant) approximately equal to 10 -34 = 0.00 … 001, where there should be 34 zeros here.īecause the two uncertainties multiplied together in equation (1) must be greater than some number, this means that it is not possible for both Δq and Δv to be zero. The simplest uncertainty relation, which can be derived quite easily using QM, can be expressed as follows: ![]() The most useful forms are quantitative relations between various quantities related to uncertainty. To appreciate the paper, you have to understand that the HUP is a general principle which has many different forms. So … if the HUP is a consequence of QM, but QM has not been violated, what exactly is the paper claiming? One principle, many relations. ![]() Moreover the experimental violation they report was predicted using QM, in a 2010 paper by Austin Lund and me. Indeed, the design of their apparatus relies on QM being correct. The experimentalists in Toronto, led by Aephraim Steinberg, do not claim to have disproven quantum mechanics. This theory applies to all forms of matter and energy (as far as we can tell) and is extremely well tested. It is actually a consequence of something more fundamental, namely quantum mechanics (QM). Not really a principleįirst, as I explained previously, Heisenberg’s “uncertainty principle” is not really a fundamental principle at all. How could the Heisenberg Uncertainty Principle (HUP), which has been fundamental to physics for more than 80 years, suddenly be disproven? The Toronto experiment actually used some of my own research, so I am well-placed to explain what is really going on. ![]() As I explained a few months ago, it has the profound implication that we cannot know everything.Įarlier this month, a paper from physicists in the University of Toronto was published in the prestigious journal Physical Review Letters ( public-access pre-publication version here) that at first sight seems to show that their experimental results violate Heisenberg’s principle. The Uncertainty Principle, introduced by Heisenberg in 1927, applies to observations of the properties of the quantum world, which is typically microscopic in scale.
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