Quantum particles are not really just particles … they are also waves.
The word uncertainty is widely used in quantum mechanics. A school of thought is that it means that there is something in the world of which we are uncertain. But most physicists think that nature itself is uncertain.
Intrinsic uncertainty was at the heart of how the German physicist Werner Heisenberg, one of the initiators of modern quantum mechanics, presented this theory.
He put forward the principle of uncertainty which showed that we can never know all the properties of a particle at the same time.
For example, measuring the position of the particle would allow us to know its position. But this measure would necessarily disturb its speed, of a quantity inversely proportional to the precision of the position measurement.
Was Heisenberg wrong?
Heisenberg used the uncertainty principle to explain how the measure would destroy this classic feature of quantum mechanics, the two-slot interference model (more on this below).
But in the 1990s, prominent quantum physicists claimed to have proven that it was possible to determine which of the two slits a particle traversed without significantly disturbing its velocity.
Does this mean that Heisenberg's explanation must be false? In the work just appeared in Progress of science, my colleagues from experience and I have shown that it would not be wise to jump to this conclusion.
We show that a perturbation of speed – the expected size of the uncertainty principle – still exists, in a certain sense.
But before going into the details, we must briefly explain the two-slot experimental experiment.
The experience with two slits
In this type of experiment, there is a barrier with two holes or slots. We also have a quantum particle with a position uncertainty large enough to cover both slits if pulled at the barrier.
As we can not know which slit the particle is going through, it acts as if it passes through both slits. The signature of this is what is called the "interference pattern": ripples in the distribution of the places where the particle is likely to be on a screen in the far field beyond the slits, which means a long way (often several meters) after the cracks. .
But what if we put a measuring device near the barrier to find out which slot the particle is going through? Will we still see the pattern of interference?
We know the answer is no, and Heisenberg's explanation was that, if the position measurement is accurate enough to indicate the slit in which the particle passes, it will disturb its velocity randomly, just enough so that It is in the far field. and thus wash the undulations of the interference.
The eminent quantum physicists realized that the determination of the slit through which the particle passed did not require a positional measurement as such. Any measurement giving different results depending on the crack traversed by the particle will do the trick.
And they have developed a device whose effect on the particle is not that of a random kick as it passes. Therefore, they argued, it is not the Heisenberg uncertainty principle that explains the loss of interference, but another mechanism.
As Heisenberg had predicted
We do not have to go into what they claimed to be the mechanism of interference destruction, as our experience has shown that there is an effect on particle velocity, of the predicted size by Heisenberg.
We have seen what others have forgotten because this perturbation of speed does not occur when the particle passes through the measuring device. Instead, it is delayed until the particle has passed the slots towards the far field.
How is it possible? Well, because quantum particles are not really just particles. They are also waves.
In fact, the theory behind our experiment was one in which the nature of the waves and the particles were obvious – the wave guided the movement of the particle according to the interpretation introduced by the theoretical physicist David Bohm, a generation after Heisenberg.
In our last experiment, scientists in China followed a technique suggested by me in 2007 to reconstruct the hypothetical quantum particle motion from many possible starting points in both slits and for both measurement results.
They compared speeds in time when there was no measuring device present to those when there were any, and so determined the change in speeds resulting from the measurement.
Experience has shown that the effect of measurement on particle velocity persisted long after the particles had cleaned the measuring device itself, at a distance of about five meters (16 feet).
At this point, in the far field, the cumulative change in velocity was just large enough, on average, to erase the ripples of the interference pattern.
Thus, in the end, the uncertainty principle of Heisenberg emerges triumphant.
The takeaway message? Do not make big assertions about principles that may or may not explain a phenomenon until you have examined all the theoretical formulations of the principle.
Yes, it's a bit of an abstract message, but it's a tip that could apply in distant areas of physics.
Howard Wiseman is Director of the Center for Quantum Dynamics at Griffith University, Australia.
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