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Think of it as the Undo Button

Via GeekPress, quantum weirdness just keeps on getting weirder:

In the latest issue of Nature News, Postdoctoral Fellow Nadav Katz explains how his team [took] a "weak" measurement of a quantum particle, which triggered a partial collapse. Katz then "undid the damage we'd done," altering certain properties of the particle and performing the same weak measurement again. The particle was returned to its original quantum state just as if no measurement had ever been taken.

Because theorists had believed since 1926 that a measurement of a quantum particle inevitably forced a collapse, it was said that in a way, measurements created reality as we understand it. Katz, however, says being able to reverse the collapse "tells us that we really can't assume that measurements create reality because it is possible to erase the effects of a measurement and start again."

Because quantum stuff always sounds so goofy anyway, it's hard to get a handle on just how significant this discovery may be. What we think of as "reality" * is the realization of trillions upon trillions of quantum events. Quantum particles exist in this extended, smeared out, many-places-and-states-at-the-same-time wave-form hyper-reality until they get observed or measured and then it turns out that -- Hey! It wasn't really in lots of different states, after all. It was there and it did that. Reality as we know it is the sum of all those there's and that's produced by all those collapsing waveforms.

We don't actually know much about how or why this is the case. The idea that observation or measurement can be interacting with physical reality to produce results is so patently bizarre that there's a tendency either to:

1. Conveniently ignore that that's what's going on

or

2. Turn it into some kind of spooky mystical thing

The first option is the path of cowards. The universe is weird. Let's deal with it. The second option is a dead end. As soon as we declare the strangeness to be magical, we're finished having a rational conversation about it (which we might not have been having anyway, but at least we were trying.)

So here's the thing. Let's analogize what's happening when a particle goes from an uncollapsed state to a collapsed state. Think of your iTunes when you're doing a random shuffle. A song sitting there on the disk is one of the many possible states of the Song I Am Currently Listening To. When a particular song is picked, the waveform of the entire music library gets collapsed down to just that one song. (It's just an analogy, okay? Stick with me.)

So the iPod plays me a Muddy Waters tune and then starts throwing some Blue Man Group my way. The transition is just a little too jarring, so I take the controls, find some Van Morrison, and (for now) put BMG back into the uncollapsed state. Everybody with me so far? Good.

Here's the problem with that analogy. Tunes playing on an iPod lack a characteristic that we normally associate with quantum waves in the process of collapse. Quantum collapse takes place along something we call the arrow of time -- or may in fact the the thing that defines it. Observation or measurement of quantum states helps push time along. Once we seal the deal as to what a particular outcome was, it's finished. Or at least it's supposed to be. But now Katz is showing us something else.

In other words, what Katz has done -- if I grasp the thing correctly, and I'm sure someone will tell me at great length why I don't -- is not to shut down Blue Man Group and play some other song. He is setting things up so that Blue Man Group never played.

It's not exactly time travel, nor is it even precisely time reversal, but those two concepts come as close as anything I can think of to what this experiment implies. This may be more weirdness of the universe that we're just going to have to get used to, or it may have implications about some very powerful technologies that we will someday have access to. It's hard to say right now.

But I'll tell you one thing. If we really are living in a computer simulation, Nadav Katz has stumbled across an intriguing snippet of source code.


* What a concept.

[Bumped on account of the Instalanche.]

Comments

Sounds like the enabling technology for a Heisenberg Compensator!

Does this destroy the ability of quantum computing to detect when a message has been compromised? The idea was that if someone had read the message, entanglement would be destroyed. If it can be reestablished...Does reestablishment require that the message be unread in some way?

The "spookiness" of quantum mechanics most likely arises from imprecision of the mathematics we use to model it. After all, the math we use to describe space involves the use of constructs such as points, absolutely perfectly straight lines, perfect planes, perfect circles etc. None of these constructs have actual real world instances.

The differences between the real and abstract matter little on the macrocopic scale but I think they pile up when we look at very tiny things. The existence of point means nothing when describing a football field but when your describing something near the size of Planck's limit a theoretical ability to infinitely subdivide space could introduce enough error to produce strange results.


Acutally, this isn't all THAT exciting. Once you accept the idea of quantum states and collapses, the idea that the reverse could occur isn't all that strange.

The next trick is going to be swapping a particle to and from quantum state an arbitrary number of times. Then you put it in a quantum state in one location, and collapse it in a _different_ location. And then you sell the patents to Googlesoft and retire because you've invented a teleporter!

Larry,
I am not a physicist, however it seems to hinge around a "weak" measurement. Most likely that measurement would not be detailed enough to get the data from a quantum encrypted message.

Didn't the Russians discover this effect back in Stalin's time?

What this leads to, first and foremost is to the concept of "quantumdisestablismentarianism" and a new "longest word in the English language" to bedevil school kids for another couple of centuries. At least they won't have trouble with the definiton -- they see that every time a teacher imparts meaning in a class, said meaning relaxing to nothing after the class completes and for which the student will never again be called up to know.

This is huge as it could mean the end of Postmodernism.

http://www.jim-rose.com/2008/08/postmodernism-undone.html

"The universe is weird. Let's deal with it."

I always laugh when I see a comment like this. The universe can't be weird; it's the only reality we know. It's only people's thinking about the universe that's weird. If it is what it is, and has always been so, on what basis does anyone expect it to be different?

Andy --

Sure, we're talking about our perceptions of the universe when we attribute certain characteristics. If we call it "big" or "dynamic" or "uncaring" or "weird," we're just describing our emotional response to it -- nothing else.

I'm starting a new franchise. Get in early while its cheap.

Quantum Auto Body. "Like It Never Happened, Because It Didn't"

Using your analogy, I reset the IPod to not play Blue Man Group, and I leave it on a table. Another observer picks it up. How would he know that BMG had played? He wouldn't know. To him, BMG has never played.

When we review the video of the security camera above us, and we see that I reset the IPod, then we are sure that BMG played, even though our observer couldn't tell.

It isn't strange to reset an isolated system to a state that it was once in. It is not demonstrated that we can reset the universe to a state that it was once in.

Well, if he did what he appears to claim he did: "so much for quantum cryptography."

Here's a thought experiment. Let's say Katz et al perform the first measurement and someone is making a video at the time. Katz says to the camera, "It's result A."

Then the team resets so that the first measurement never happened. They measure again and Katz says this time, "It's result B."

If they look at the recording, will Katz's first statement no longer exist in recorded form? If it does exist, isn't this evidence that the first measurement did in fact happen? Perhaps the existence of a recording of Katz's first statement is evidence that Katz got the measurement wrong, or that he didn't actually measure and was lying. If this is so, doesn't this reflect poorly on Katz as a researcher?

If human consciousness is a quantum phenomenon -- and it's certainly weird enough -- there is a third possibility, and the correct one if you trust QM. The wavefunction does not collapse when you make a measurement. Rather the quantum system that is your consciousness goes into a superposition of states that matches the superposition of the system being measured. If you're measuring the spin on an electron, your consciousness goes into a superposition of "I see the spin is up" and "I see the spin is down."

This is the "many worlds" or "parallel universe" interpretation of QM. That's bad terminology, because while superpositions evolve independently, they can interact, and therefore remain in the same world or universe. The research you report is consistent with this. The significance is that the research weakens the "collapse" interpretation, and strengthens what is better termed the "quantum decoherence" interpretation.

I think it would be simpler, or at least less worrisome, to assume that you are not correctly grasping it. He didn't "make it never have been measured", he "uncollapsed the wave". He didn't reverse the time arrow, he just showed that the time arrow is not defined by whether one particular wave is collapsing or expanding. Or at least that's what I think. More caffein is definitely required.

My head hurts from thinking about this. I don't like questioning my notion of reality. I like it just the way it is, or isn't.

Phil: The wording of the press release is vague almost to the point of being meaningless. I took a stab at reading the original paper and this statement seems to explain why this is a new result: "In order for the uncollapsing procedure to work, we have to erase the information that was already extracted
classically. This distinguishes this measurement-induced
uncollapsing from a �quantum eraser�, in which only potentially extractable information is erased." I'm not sure what this means. Restoring the phase state of a qubit is a basic operation in quantum computing. I guess the processes result in the qubit having different entropies, since the paper states that this experiment did not result in any transfer of energy.

Shannon Love: This is a good point. I believe physicists working in the fields of quantum chromodynamics and string theory have techniques for dealing with discreet volumes at the Planck scale.

Jim Rose: Gasbags the lot of them. Postmodernism doesn't seem to be taken seriously by many people who aren't begging the question anymore. I'm not sure if you can say this finding is a refutation of Professor Aronowitz's argument as that was a non-sequitor anyway. Sources for effective arguments against this philosophy that I can think of off the top of my head are Steven Pinker's "The Blank Slate" and Richard Feynman's "The Meaning of It All" though there are many others. I guess this is a nitpick but it seems pretty unlikely to me that Ayatollah Khomeini was both an Islamic fundamentalist and a postmodernist. You might want to change that.

Director: lol

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