To John Archibald Wheeler, the race to explain time was personal

http://nautil.us/issue/9/time/haunted-by-his-brother-he-revolutionized-physics

by dnetesn • 12 years ago

57 16 12 years ago

joaorico• 12 years ago

Freeman Dyson, another important physicist, wrote this a few years ago:

"I changed my mind about an important historical question: did the nuclear bombings of Hiroshima and Nagasaki bring World War Two to an end? Until this year I used to say, perhaps. Now, because of new facts, I say no. This question is important, because the myth of the nuclear bombs bringing the war to an end is widely believed. To demolish this myth may be a useful first step toward ridding the world of nuclear weapons."

Here's the rest of the text: http://www.edge.org/response-detail/11732

tzs• 12 years ago

Wheeler was worried about the war in Europe, though, and what could have been had the bombs been developed a year earlier. That would have put them early enough to consider delaying D-Day until after the bombs were available.

What, I wonder, would have turned out different if D-Day had started with nuking Berchtesgaden?

elpachuco• 12 years ago

>In 1984, Alley—along with Oleg Jakubowicz and William Wickes, both of whom had also been in the audience that day—finally got the experiment to run. It worked just as Wheeler had imagined: measurements made in the present can create the past.

I'm not a physicist so unfortunately I don't know whether this is true. However, of all the documentaries on Physics that I've seen they have never mentioned this.

Could anybody please chime in whether this is true.

Ono-Sendai2• 12 years ago

I can't tell you if it's true or not, but you can read more about it here: http://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experi...

ctchocula2• 12 years ago

Here's a Wheeler quote from the link: 'Actually, quantum phenomena are neither waves nor particles but are intrinsically undefined until the moment they are measured. In a sense, the British philosopher Bishop Berkeley was right when he asserted two centuries ago "to be is to be perceived."'

Could this be used as an argument that we are inside the simulation of the universe? I mean if I were to design a game, I might leave the state undefined until it's necessary for it to be known (by some consciousness perhaps).

tjradcliffe2• 12 years ago

It is very much a matter of interpretation. The notion that "measurements made in the present can create the past" is not generally accepted by physicists.

Furthermore, any idea that is put forward in terms of "X (a well-known concept) does not really exist" is almost certainly gibberish masquerading as hype. Of course time exists. If time is "really" an illusion it is still a perfectly real phenomena that is created by perfectly real causes and which results in perfectly real responses in knowing subjects who then mis-interpret their cause.

Illusions are real. When we see impute a bend in a stick that passes through the water's surface we make a mistake, but the bend is perfectly real: it is just that the bend is in the light that passes between the stick and our eye, not in the stick itself. Anyone who says the bend is not real has to explain how is it that we see it. We cannot see what is not real, although we may on very rare occasions misinterpret what we are seeing (we know the occasions are very rare because if they weren't we would have no ordinary, non-illusitory experiences to contrast illusions with.)

jsun• 12 years ago

Unsourced wikipedia articles seem to suggest that Wheeler believed the exact opposite on retrocausality. Since both pieces seem to cite no academic sources I will leave it to you to determine which is true.

http://en.wikipedia.org/wiki/Wheeler%27s_delayed_choice_expe...

AnimalMuppet• 12 years ago

I think this is a misunderstanding of what's going on in quantum mechanics. A photon is emitted as a probability wave function. (Note well: This is not the same as "sometimes a photon acts like a wave!) It travels as a probability wave function. It gets to the detector as a probability wave function. The form of the detector determines how the wave function collapses into an observable state. That's when the "choice" gets made. There is no projection of the choice onto the past; there is only the projection of the choice onto the probability wave function in the present. So this "retrocausality" stuff is actually a fundamental mis-understanding of quantum mechanics.

What the article got right: Relativity and quantum mechanics are talking about two fundamentally different things when they use the word "time". Reconciling those two ideas is going to be critical to a fundamental understanding of the universe.

graycat2• 12 years ago

Let's try to fool a wave function: Let's take one, run it through a beam splitter, take the two halves, and put one in a lab that is standing still and has a normal clock and the other half in a moving lab, a space ship, with a slow clock.

But, we want the wave function to be of a particle that also has a clock. So, let the wave function be that of an atom of a radioactive element with a short half life.

We get in our space ship and travel at, say, 90% of the speed of light, let the particle go, let it encounter the beam splitter, let one half of the wave function continue to bounce around inside our space ship with a slow clock and let the other half exit the space ship through a window, enter the lab that is standing still, and bounce around there.

Then the question is, what is the half life of the particle? Is the expected time, in the clock of the normal lab, to decay different for decays in the space ship than that for decays in the lab that is not moving?

I'm sure this is all wet in some sense, but I'm trying yet another students efforts to trip up quantum mechanics with special relativity!

tjradcliffe2• 12 years ago

The setup you describe doesn't bring QM and SR into conflict because the two split halves of the wavefunction never do anything interestingly "quantum". The radioactive decay process acts as a measurement of "which branch of the beam-splitter did the particle go down?" and so the experiment can be analyzed as if the particle went down one branch of the other in its entirety and was never split.

To get at the quantum effects you'd have to consider a case where the two parts of the wavefunction are eventually brought back into interference with each other, but any sort of decay process is going to make them mutually incoherent, so you won't see any interference fringes. You might get something interesting by having a moving mirror in an interferometer, which would redshift the reflected wave in that arm by a bit, and that would result in moving interference fringes due to the resulting frequency difference (you'd need to gate the interference pattern on the position/velocity of the mirror to see this.)

But even in that case, nothing very interesting would happen. In any experiment where you can detect which arm a particle goes down it will behave as if it went down that arm, even if your choice is delayed. In any interference experiment it will be impossible to tell which arm it went down. Nature is really good at doing the book-keeping to maintain the quantum veil.

physonaught• 12 years ago

That is true, but, (if you haven't already), I would recommend looking into the delayed choice quantum eraser experiment. I think it is probably more true to the essences of what the article was trying to communicate.

I don't think I can do the idea justice, but the basic idea is such: if you 'tag' the photons in the double-slit experiment by which path they went through, then you necessarily break superposition and won't see an interference pattern.

However, if you 'throw' away this information completely, the interference pattern re-emerges. The delayed-choice part of it is just a way to through away the 'tag' information after the photon has moved through the slits. It seems to suggest that you can uncollapse a wavefunction if you just erase any knowledge you have of the measurement you made. (I've probably butchered it, read here for more[1])

Although, honestly, you don't need retrocausality, and personally I think it has much more interesting applications to information theory (how does one destroy information? How does the wavefunction know about our records? What even is information, man? etc)

[1] http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

georgemcbay• 12 years ago

Related, worth watching (IMO):

http://youtu.be/dEaecUuEqfc

evanb• 12 years ago

The issue with this circumvention is that it assumes wave function collapse, which is equally mysterious. If you do away with collapse, you are left with a many-worlds type interpretation, in which the wave function simply evolves unitarily forever. It is in this sense that the current state "dictates" the previous states---the histories consistent with one measurement or another separated based on the kind of measurement made. If you do the interference experiment, the histories consistent with outcomes of that experiment are ones in which the photon takes no definite path. If you do the path-determining experiment, you no longer get interference patterns but also the consistent histories are ones in which the photon takes a definite path---either one slit or the other.

Since you get to decide which kind of experiment to do after the photon passes the slitted screen, your decision seems to force the histories to become either the interference kind or the definite-path kind after you might expect that to have already been determined. That's the sense in which the future seems to control the past.

I agree that this was not explained clearly in the article. It's a very slippery subject.

To John Archibald Wheeler, the race to explain time was personal

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