Quantum physics experiment shows Heisenberg was right about uncertainty, in a certain sense

Quantum physics experiment shows Heisenberg was right about uncertainty, in a certain sense
Quantum particles are not really just particles… they are also waves. Credit: Shutterstock/agsandrew

The word uncertainty is used a lot in quantum mechanics. One school of thought is that this means there's something out there in the world that we are uncertain about. But most physicists believe nature itself is uncertain.

Intrinsic uncertainty was central to the way German physicist Werner Heisenberg, one of the originators of modern mechanics, presented the theory.

He put forward the Uncertainty Principle that showed we can never know all the properties of a particle at the same time.

For example, measuring the particle's position would allow us to know its position. But this measurement would necessarily disturb its , by an amount inversely proportional to the accuracy of the position measurement.

Was Heisenberg wrong?

Heisenberg used the Uncertainty Principle to explain how measurement would destroy that classic feature of , the two-slit interference pattern (more on this below).

But back in the 1990s, some eminent quantum physicists claimed to have proved it is possible to determine which of the two slits a particle goes through, without significantly disturbing its velocity.

Does that mean Heisenberg's explanation must be wrong? In work just published in Science Advances, my experimental colleagues and I have shown that it would be unwise to jump to that conclusion.

We show a velocity disturbance—of the size expected from the Uncertainty Principle—always exists, in a certain sense.

But before getting into the details we need to explain briefly about the two-slit experiment.

The two-slit experiment

In this type of experiment there is a barrier with two holes or slits. We also have a with a position uncertainty large enough to cover both slits if it is fired at the barrier.

Since we can't know which slit the particle goes through, it acts as if it goes through both slits. The signature of this is the so-called "interference pattern": ripples in the distribution of where the particle is likely to be found at a screen in the far field beyond the slits, meaning a long way (often several metres) past the slits.

But what if we put a measuring device near the barrier to find out which slit the particle goes through? Will we still see the interference pattern?

We know the answer is no, and Heisenberg's explanation was that if the position measurement is accurate enough to tell which slit the particle goes through, it will give a random disturbance to its velocity just large enough to affect where it ends up in the far field, and thus wash out the ripples of interference.

Quantum physics experiment shows Heisenberg was right about uncertainty, in a certain sense
Quantum particles are not really just particles… they are also waves. Credit: Shutterstock/agsandrew

What the eminent quantum physicists realised is that finding out which slit the particle goes through doesn't require a position measurement as such. Any measurement that gives different results depending on which slit the particle goes through will do.

And they came up with a device whose effect on the particle is not that of a random velocity kick as it goes through. Hence, they argued, it is not Heisenberg's Uncertainty Principle that explains the loss of interference, but some other mechanism.

As Heisenberg predicted

We don't have to get into what they claimed was the mechanism for destroying interference, because our experiment has shown there is an effect on the velocity of the particle, of just the size Heisenberg predicted.

We saw what others have missed because this velocity disturbance doesn't happen as the particle goes through the measurement device. Rather it is delayed until the particle is well past the slits, on the way towards the far field.

How is this 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 both wave and particle nature are manifest—the wave guides the motion of the particle according to the interpretation introduced by theoretical physicist David Bohm, a generation after Heisenberg.

Let's experiment

In our latest experiment, scientists in China followed a technique suggested by me in 2007 to reconstruct the hypothesised motion of the quantum particles, from many different possible starting points across both slits, and for both results of the measurement.

They compared the velocities over time when there was no measurement device present to those when there was, and so determined the change in the velocities as a result of the measurement.

The experiment showed that the effect of the measurement on the velocity of the particles continued long after the had cleared the measurement device itself, as far as 5 metres away from it.

By that point, in the far field, the cumulative change in velocity was just large enough, on average, to wash out the ripples in the .

So, in the end, Heisenberg's Uncertainty Principle emerges triumphant.

The take-home message? Don't make far-reaching claims about what principle can or cannot explain a phenomenon until you have considered all theoretical formulations of the principle.

Yes, that's a bit of an abstract message, but it's advice that could apply in fields far from physics.


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Are you certain, Mr. Heisenberg? New measurements deepen understanding of quantum uncertainty

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Jun 17, 2019
In the classical view, being somewhere is not the property of the particle itself but merely its relation to other particles in empty space, so it can be mathematically perfectly somewhere. In QM, where the particle is depends on what is known about the particle because for any observer (such as other nearby particles) the particle is not real any more than what information can be obtained of it.

The amount of information that you have about the particle defines how precisely it exists in space, because the particle does not exists as an independent thing - it exists only as far as the local universe "knows" that it exists, and the local universe has a finite amount of energy to encode that information.

That's why things in quantum mechanics are always a bit fuzzy, but it gets clearer when you include more stuff (information) over a larger distance and time scale, because this information adds to the knowledge of where that particle must be.

Jun 17, 2019
Hmmm...so there is uncertainty about the uncertainty?

Jun 17, 2019
Quick overview: spot the plural:

motion of the quantum particles
from many different possible starting points

They compared the velocities
change in the velocities

velocity of the particles
long after the particles
__________
Heisenberg was speaking to the measurement of a single particle and its position and momentum attributes. The experiment the author writes about is orthogonal to supporting or falsifying Heisenberg's claim, and the modern interpretation has presented an even more epistomologically stringent contraint: we not only cannot measure (more or less exactly) both velocity and position of a (given) particle, a simultaneous position and velocity does not actually   e x i s t .

Jun 17, 2019
Alfred Lande proved that uncertainty is due to the (evolving) limitations of human measurability.

Jun 17, 2019
The original article is at: https://theconver...e-118456 .

Jun 18, 2019
If we believe nature is uncertain, How can we explain the solar system, space craft, wave of gravity, LHC, air plane and automotive drive?

Jun 18, 2019
the distinction of nature is in reference to it not being a matter of just our ability to measure. It's not a byproduct of tools and our evolving technology and ability. The uncertainty is built into and comes from nature itself.

So the argument is, no matter how advanced we get and how amazing our tools become, we will never be able to remove uncertainty from the quantum realm.

Jun 19, 2019
Here is the nut:
The experiment the author writes about is orthogonal to supporting or falsifying Heisenberg's claim
.

Indeed, it is an opportunistic remark (which quite possibly may go back to the claim of being inspiring the experiment - I did not check) of using weak measurements which will converge on a classic, non-relativistic (Bohmian approximation) measurement.

It has little to do with fundamental quantum mechanics. It has even less to do with modern relativistic quantum field theory, hence the irrelevant claim of "duality" characteristics instead of field characteristics. The experiment it criticize was concerned with the nature of observation and how in real observations noise interferes with attempted measurements involving entanglement.

Jun 19, 2019
If we believe nature is uncertain, How can we explain the solar system, space craft, wave of gravity, LHC, air plane and automotive drive?


Because quantum physics converges to classical physics in the limit of large systems, see Bohr discovering that a century ago. This property is a quantum mechanics 101 area, check it out.

Though that does not get rid of observational uncertainty since classical systems more easily diverge exponentially or fold back on themselves in so called phase space. In which case they may give rise to chaos - fundamental unpredictability from lack of resolution in observation despite having theoretically fundamentally "certain" trajectories. That is chaos 101, you may want to check that out too.

Jul 11, 2019
In engineering, the precision of measurement is decided by the ability of measurement tool. Nature is certain beside the scope of calibration which is the commonsense of technicians. But I think it may not be true in subatomic scale because we still can not measure time and position precisely in that scale. That is to say: nature may or may not be certain in subatomic scale.

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