Is there such a thing as objective reality? Vlatko Vedral looks into an experiment to test how far quantum indeterminism persists in the everyday world.
From Vlatko Vedral:
I seem to be writing about "tests" quite a lot. One of my last posts talked about a possible lab test that could detect parallel universes and now I find myself writing about the work of a former student of mine, Koji Maruyama, and his colleagues to devise an experiment to test another fundamental and dramatic prediction of quantum mechanics--one that touches on our sense of how objective and "real" reality is. To over-dramatize it a bit, they are effectively asking: Does the color of your eyes depend on how we decide to look at you? Or, to take another macroscopic feature: Does the very existence of life on Earth depend on the context within which we ask this question?
Image: Noodlez222
There are many ways of describing the fundamental difference between quantum and classical physics. The main issue is that of the existence of superpositions in quantum physics, namely that an object can exist in many different states at the same time. For example, an electron can exist in two different spatial positions within an atom. But also, larger objects, such as complex molecules, can exhibit the same property. This is clearly something that classical Newtonian physics rules out. A classical object has a well-defined position as well as a well-defined velocity with which it moves.
The superposition phenomenon itself leads to two big surprises. One is the existence of entanglement--a phenomenon in which two quantum objects can become intertwined, so that changing the properties of one immediately affects its partner--which I have blogged about before. Entanglement has been demonstrated regularly in the lab. (Check out Grace Stemp-Morlock's article on attempts by FQXi researchers to create entanglement on the largest scales yet: "Quantum Upsizing".)
But there's another surprise that hasn't really been tested
to such an extent: How your system responds to being probed is dependent on the order in which you perform measurements, that is, on the _context_ of your probing. What this means is that a quantum system does not have a predetermined outcome to a given measurement, but that the outcomes emerge and are created directly as a result of the measurement process.
Speaking somewhat loosely, measuring the position of your object and then speed is not the same as measuring the speed and then position. This property, known as contextuality, is also a consequence of quantum mechanics, first formalized more than 40 years ago by two mathematicians, Simon Kochen and Ernst Specker. (You can read more about Kochen and Specker's theorem and how it has led physicists to ponder whether or not we really have free will, in this post by Zeeya Merali.)
So how do you test contextuality? Clearly, for classical objects, it makes no difference which order you measure the position and speed--you should always get the same result. This is because the position and velocity are objective properties of systems in classical physics existing independently of measurements. Classical systems in other words are completely non-contextual.
There haven't been many tests of contextuality. But what is interesting is that this property should be true for any system (if quantum mechanics is correct) independently of its size and complexity (at least as far as our theory suggests). Now, Maruyama and his colleagues have written a paper showing how to test contextuality by measuring quantum fluctuations in a device called a "Josephson junction"--which consists of a very thin insulator sandwiched between two superconducting layers.
What is novel here--and this is in sharp contrast with previous tests--is that Josephson Junctions are effectively macroscopic objects. They consist of something like one billion electrons, existing simultaneously in different states. The question is: if we make different measurements with these, will the outcomes still be dependent on how we make the measurement, that is, on the context? The paper describes how to do this in detail, with a technology fully available at present.
If junctions prove to be contextual--and I am betting all the money I don't have on this outcome--then we are facing an interesting question. If the macroscopic properties of one billion atoms turn out not to exist independently of measurements and the context, that is, they don't have an objective independent existence, then how far does this feature persist in the macroscopic world? Does it for example apply to living systems? Which brings me back to my initial questions: Does the color of your eyes depend on how we decide to look at you? Does the very existence of life on Earth depend on the context within which we ask this question?
Bizarre as it may sound, the answer could, at least in principle, be "yes" to both questions. Only time can tell what kind of reality our world ultimately embodies.
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Vlatko Vedral is a professor of quantum information science at the University of Leeds, UK, and a professor of physics at NUS, Singapore.