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

Jan 17, 2012

Heisenberg's Uncertainty principle is arguably one of the most famous foundations of quantum physics. It says that not all properties of a quantum particle can be measured with unlimited accuracy. Until now, this has often been justified by the notion that every measurement necessarily has to disturb the quantum particle, which distorts the results of any further measurements. This, however, turns out to be an oversimplification. In neutron experiments carried out by professor Yuji Hasegawa and his team at Vienna University of Technology, different sources of quantum uncertainty can now be distinguished, validating theoretical results by collaborators from Japan. The influence of the measurement on the quantum system is not always the reason for uncertainty. Heisenberg's arguments for the uncertainty principle have to be revisited – the uncertainty principle itself however remains valid. The results have now been published in the journal Nature Physics.

It is well established that some physical quantities cannot be measured at the same time. The question is, how this fact should be interpreted. "Heisenberg's famous thought experiment about using light light (?-rays) to measure the position of an electron is still quoted today", says Jacqueline Erhart from the Institute for Atomic and Subatomic Physics at the Vienna University of Technology. To measure the position of a particle with high precision, light with a very short wavelength (and therefore high energy) has to be used. This results in momentum being transferred to the particle – the particle is kicked by the light. Therefore, Heisenberg argued, it is impossible to measure both position and momentum accurately. The same is true for other pairs of physical quantities. Heisenberg believed that in these cases, an error in one measurement leads to an inevitable disturbance of the other measurement. The product of error and disturbance, Heisenberg claimed, cannot be smaller than an a certain threshold.

However, the effect of the measurement on the and the resulting disturbance of the second measurement is not the core of the problem. "Such disturbances are also present in classical physics – they are not necessarily linked to ", Stephan Sponar (Vienna UT) explains. The uncertainty is rooted in the quantum nature of the particle. Quantum particles cannot be described like a point-like object with a well-defined velocity. Instead, quantum particles behave as a wave – and for a wave, position and momentum cannot be defined accurately at the same time. One could say that the particle itself does not even "know" where exactly it is and how fast it travels – regardless of the particle being measured or not.

"In order to describe the fundamental uncertainty and the additional disturbance due to the measuring process, both particle and measurement device have to be treated in the framework of quantum theory", says Georg Sulyok (Vienna UT). This was done by the Japanese physicist professor Masanao Ozawa in 2003, leading to a generalized . His equations contain different "kinds of uncertainty": On the one hand the uncertainty which comes from the measurement, as it disturbs the particle (this is the uncertainty described in Heisenberg's thought experiment of the position-momentum-measurement), on the other hand the equations contain the fundamental quantum uncertainty, which is present in any quantum system, regardless of the measurement.

A sophisticated experimental design now made it possible to study these contribution to uncertainty at the Vienna University of Technology. Instead of a particle's position and momentum, the spin of neutrons was measured. The spin in x-direction and the spin in y-direction cannot be measured simultaneously, they fulfill the uncertainty relation, in much the same way as position and momentum. With magnetic fields, the neutron spins were rotated into the right direction, then the spins were measured in two consecutive experiments. Carrying out a large number of with small, well-defined changes in the measurement apparatus, the physicists could study the interplay between different sources of uncertainty.

"The smaller the error in one measurement, the larger the disturbance of the other – this rule still holds. But the product of error and disturbance can be made arbitrarily small – even smaller than Heisenberg's original formulation of the uncertainty principle would allow", says professor Yuji Hasegawa.

But even if two measurements hardly influence each other: quantum physics remains "uncertain". "The uncertainty principle is of course still true", the researchers confirm. "But the uncertainty does not always come from the disturbing influence of the measurement, but from the quantum nature of the particle itself."

Explore further: The importance of three-way atom interactions in maintaining coherence

Provided by Vienna University of Technology

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User comments : 10

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de_la_meu
not rated yet Jan 17, 2012
Fourier transform adpets rejoice, another myth debunked.
Isaacsname
5 / 5 (1) Jan 17, 2012
And empirical measurements will forever remain approximations with a lessening degree of uncertainty ?
Callippo
1 / 5 (3) Jan 17, 2012
Even at the water surface you can never have pure transverse or longitudinal waves - there always exists mixture of both. Therefore the perspective mediated with pure transverse or longitudinal waves (corresponding the relativistic or quantum mechanical perspectives) will be always mixed. For example, in recent time physicists succeeded in measurement of quantum wave functions or exact path of photons during double slit experiments, which could imply, the limit implied with uncertainty principle could be beaten. But these measurements were taken as a running average of many consecutive "weak" measurements - it means the uncertainty related to single measurement didn't change, "we" just averaged it. With using of the multiple measurements or more general scope of experiment we could limit the uncertainty related to mean value - but the uncertainty related to knowledge of individual measurements decreases the more at the price.
Seeker2
1 / 5 (2) Jan 18, 2012
"But the uncertainty does not always come from the disturbing influence of the measurement, but from the quantum nature of the particle itself."
Ergo, quantum fluctuations. Got it.
Vendicar_Decarian
4.1 / 5 (14) Jan 18, 2012
"But the product of error and disturbance can be made arbitrarily small even smaller than Heisenberg's original formulation of the uncertainty principle would allow."

The only important part if the story.
Cynical1
1 / 5 (2) Jan 18, 2012
if everything was perfectly predictable - none of anything we "observe" would have ever existed...
daywalk3r
3.4 / 5 (14) Jan 18, 2012
It says that not all properties of a quantum particle can be measured with unlimited accuracy.
Nothing can be measured with unlimited accuracy. Unless you define a custom framework with predefined "limits", which essentialy introduces error tolerance/rounding. Then you can get "unlimited accuracy", but only within that speciffic framework. Wait a minute..

Quantum particles cannot be described like a point-like object with a well-defined velocity.
No particles can/should - only virtual/imaginary ones. That's why we use approximations.

And empirical measurements will forever remain approximations with a lessening degree of uncertainty?
Exactly. With the uncertainty converging to (but never reaching) zero. And when introduced to a framework, one can evaluate limits.

Another thing that should be mentioned is, that neither velocity nor spin can be evaluated by a SINGLE measurement. It takes a minimum of TWO measurements for each..

1000 char limit ate the rest :(
Noumenon
4.1 / 5 (56) Jan 18, 2012
"But the product of error and disturbance can be made arbitrarily small even smaller than Heisenberg's original formulation of the uncertainty principle would allow."

The only important part if the story. VD


Actually, this is the only important part of the story,...

"the uncertainty does not always come from the disturbing influence of the measurement, but from the quantum nature of the particle itself."

Heisenberg gave an intuitive description when describing the measurement as unavoidably disturbing the system. But QM is anything but intuitive, therefore this was merely an analogy. deltaX*deltaP>=h (etc,..) still stands for conjugate variables.
Noumenon
4.1 / 5 (55) Jan 18, 2012
@TheGhostOfOtto1923 [http://www.physor...ard.html]

"[regarding inequality tests devised by A. Leggett and performed by A. Zeilinger] These experiments tells us rather emphatically that we can never perceive reality 'as it really is'. We can only reveal aspects of an empirical reality that depend on the nature of the instruments we use and the questions we ask. Quantum Physics, it seems, has completed it's transformation into experimental philosophy. . . . Quantum theory pushed us to the edge of an epistemological precipice " - J. Baggott (2011)

:)
MSghauri
not rated yet Jan 30, 2012
well according to a research there are 10^10^16 multiple universes, as multiverse theory is a part of Quantum mechanics... why are they giving an amount to multiverses arnt they infinite? according to heisenberg's principle???

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