Reality doesn't exist until we measure it, Quantum Experiment confirms.

Is this one more experiment which is the victim of how news generally hypes stuff like this?

This is a section from the article:

Australian scientists have recreated a famous experiment and confirmed quantum physics's bizarre predictions about the nature of reality, by proving that reality doesn't actually exist until we measure it - at least, not on the very small scale.

That all sounds a little mind-meltingly complex, but the experiment poses a pretty To successfully recreate the experiment, the team trapped a bunch of helium atoms in a suspended state known as a Bose-Einstein condensate, and then ejected them all until there was only a single atom left.

This chosen atom was then dropped through a pair of laser beams, which made a grating pattern that acted as a crossroads that would scatter the path of the atom, much like a solid grating would scatter light.

They then randomly added a second grating that recombined the paths, but only after the atom had already passed the first grating.

When this second grating was added, it led to constructive or destructive interference, which is what you'd expect if the atom had travelled both paths, like a wave would. But when the second grating was added, no interference was observed, as if the atom chose only one path.

The fact that this second grating was only added after the atom passed through the first crossroads suggests that the atom hadn't yet determined its nature before being measured a second time.

So if you believe that the atom did take a particular path or paths at the first crossroad, this means that a future measurement was affecting the atom's path, explained Truscott. "The atoms did not travel from A to B. It was only when they were measured at the end of the journey that their wave-like or particle-like behaviour was brought into existence," he said.

Some of the things about this really confuse me, especially the claim that a future measurement affected the path the atom would end up taking. How literal are they being when they say this? (I don't think they are.)

Does this have any serious implications on QM or its interpretations?

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    $\begingroup$ I always like to quote this when things like that happen: quantamagazine.org/… $\endgroup$ – image Jun 1 '15 at 17:34
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    $\begingroup$ Generally speaking, whatever you read in a news article about science is almost certainly 100% wrong. This is especially true for content about quantum mysticism. $\endgroup$ – CuriousOne Jun 1 '15 at 17:58
  • $\begingroup$ QM is weird, we know that and every so often, on a slow news day, the media figure it out too. $\endgroup$ – user81619 Jun 1 '15 at 18:02
  • $\begingroup$ @Acid Jazz: QM isn't weird. It'a actually a much cleaner theory than classical mechanics. Think about it: it's fully linear! For the finite particle number case you can basically calculate everything, which is a feature that classical mechanics lacks completely. All of the "weirdness" that you ever keep hearing about stems from frustrated people who didn't get the message of how nature really works (we posted it 80 years ago) who are still trying to find "particles" where there are none. Let go of the idea that there is a single particle in the universe and QM is as straight as an arrow. $\endgroup$ – CuriousOne Jun 1 '15 at 18:16
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    $\begingroup$ "Think about it: it's fully linear! For the finite particle number case you can basically calculate everything, which is a feature that classical mechanics lacks completely." How do you mean? Why does classical mechanics lack completely the feature of calculating everything? $\endgroup$ – Ján Lalinský Jun 1 '15 at 21:48

Yes, it is hocus pocus from scientific reporters.

Going to the original press release from the experimenters:

Common sense says the object is either wave-like or particle-like, independent of how we measure it. But quantum physics predicts that whether you observe wave like behavior (interference) or particle behavior (no interference) depends only on how it is actually measured at the end of its journey. This is exactly what the ANU team found.

This is 100% consistent with quantum mechanics, and I would stick to that.

Quantum mechanics does not predict trajectories or energy/mass waves , it gives the probability for detecting a "particle" at (x,y,z,t) . The probability distribution may show interference patterns characteristic of wave equations. The "particle" is a quantum mechanical entity, and we keep using the term because at the limit of macroscopic manifestation, the track of an electron for example, it acts like a classical particle.

As probability distributions need a great number of measurements whereas a "particle" manifestation is one instance of that distribution the term particle is still used.

In this particular experiment to detect an interference pattern for the alpha particles a large number must have been accumulated, one by one, similar to the double slit experiment with single electrons at a time.

The way I read the description is like a complicated double slit experiment, "one slit no interference, two slits interference". They use light grids to run the gauntlet for the alpha particles . One could randomize one of the slits in the double slit experiment and get the same result . So no new implications .

I would like though to see the published paper , to evaluate the claim

"However, the random number determining whether the grating was added was only generated after the atom had passed through the crossroads."

In general with interference phenomena in QM changes in boundary conditions change the accumulated distributions.

  • $\begingroup$ Maybe the hocus pocus comes from the experimenters, to garner media attention. IMHO electrons are waves. It's quantum field theory, not quantum point-particle theory. And the Einstein-de Haas effect "demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". So they're "standing waves going round and round". Only when you detect an electron, you perform something akin to an optical Fourier transform and turn it into a dot, so it goes through one slit only. Ditto at the screen. No magick is required. $\endgroup$ – John Duffield Jun 1 '15 at 19:59
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    $\begingroup$ @JohnDuffield: Electrons aren't waves, either. Quantum fields are objects that are neither particles nor waves. It's not clear to me why people want to exclude a third option when it stares right at them. What's wrong with calling something that behaves differently than the previous two by another name? It's not like there are only dogs and cats in the world. Does anybody here feel a necessity to ask the question whether a horse is like a dog or a cat? Of course not. So why are so many folks doing it in physics? $\endgroup$ – CuriousOne Jun 1 '15 at 23:20
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    $\begingroup$ Anna: The only reason why quantum states in high energy physics experiments show particle behavior is because we are performing series of weak measurements on them, which is an observer dependent feature. Go into the rest system of a 60GeV muon and you will not be able to measure nice straight particle tracks on it at all. $\endgroup$ – CuriousOne Jun 1 '15 at 23:24
  • $\begingroup$ @CuriousOne we are in agreement, except we can only do lab measurements as that is where we live. $\endgroup$ – anna v Jun 2 '15 at 3:21
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    $\begingroup$ @JohnDuffield You are confusing the beautiful mathematics of QFT with "reality", at best you are saying "it is mathematics that creates reality". Suppose I analyze a potted plant with Fourier transports in order to create a three dimensional image of it. Does this mean that the plant is spread all over the universe according to the fourier variables? $\endgroup$ – anna v Jun 2 '15 at 3:24

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