Quantum Physics - Questions about Photon Test

Okay, I don’t even know where to begin, but let me babble out a question:

My curiosity was spurred most recently by the latest Discover magazine.

Quantum theory was discussed, but this time with an incredible leap that there are many, many other parallel universes that are similar to this, but reflect an infinite number of variations. In other words, there any number of me out there, some who decided to come to work, so who didn’t…and so on and so on. We exist simulataneously.

I am vaguely familiar with Quantum theory, but what I’ve outlined above was quite a leap from what I’ve heard before.

A popular test that was discussed was the projection of a single photon through a single slit in a screen that had two slits. When the photon left it’s mark on the receiving material, their were signs that two photons went through the screen, one through each slit. This test started the wave/particle concept…in other words, the photon had the properties of both, and left the marks on the receiving material as evidence…contrasting the notion that the very same photon was in two places simultaneously.

The leap I heard was that what was captured was a phantom photon…or the very same photon was actually in more than one place at the same time…proof that there are other universes.

If the photon was in another place, how could we measure that? How do we make the leap to conclude that matter is in many places simultaneously? So, the photon test allowed us to realize that matter exists in different places simultaneously ergo multiple universes??

Sorry this isn’t a simple question - I just don’t see how the single photon test could show the effects of two photons (or the same photon in two places) if the other photon/place were in another universe.

Help?

The “Many worlds” interpretation of QM is just one of many.

In the particular experiment you describe, it goes like this:
[ul][li]Shoot a stream of particles (electrons, photons, whatever) at a sheet with two slits in it, and detect the particles as they hit a screen beyond them.[/li][li]Note that the particles interfere with each other just like a wave would.[/li][li]Reduce the flow rate of particles until just one at a time is shot through the slits.[/li][li]Note that over time, the same interference pattern is detected! This means that even if you send one paricle through the device, it somehow interferes with itself.[/li][/ul]
Now, how do we interpret this? There are several ways. One way is that the particle isn’t really a simple particle, but is a bundle of probabilities.For example, we could not say that “The electron is at location X”, and we could not even say “There is a 50% chance the electron is at location X, and a 50% chance it is at location Y.” Instead, we have to say that the particle is smeared out, and isn’t in either place until we measure it.
This means we could say that the particle passes through both slits. If you treat the particle as a bundle of probabilities, this is not so hard to swallow, you just say that the probability wave passed through both slits.
However, if you have a hard time with that you might say something like the particle causes a split into two universes, in universe A the particle goes through one slot, and in universe B the particle goes through the other slot.

Weird, huh? The main thing to keep in mind is that the interpretations of QM can get into the metaphysical realm, so we may never know some of these things for sure.

I highly recommend books by John Gribben such as “In Search of Schroedinger’s Cat” for a great intro to quantum mechanics.

doug,

Thanks for the breakdwon.

So is there a consensus about whether the particle is a wave or exists in two places at once? Has any progress been made, or do we resign ourselves to the fact that it’s both?

What if you had one particle and three slits?

You have to resign yourself to the fact that no one knows. Quantum mechanics is outside the realm of your intuition, and probably always will be. As Richard Feynman said:

Von Neumann proved that if a quantum system is interacting with a measuring device, then the superposition of these systems remains a smoothly evolving wave(function). And that only when the superposition encounters a system (a conscious brain) which is not subject to the time symmetrical laws of QM does it collapse.

This collapsing of the wavefunction is one of biggest sticking points in QM, and it is also why so many theories have been proposed to get around this need for consciousness. Many worlds is one of these theories and it is mathematically correct, but so are all the others - so you buy your tickets and take your chances.

Rest assured, this is not a simple question. There are a lot of subtle issues here. The first thing is to understand exactly what the double-slit experiment shows. douglips has made a reasonable attempt in the space, but, frankly, I doubt anybody can capture the significance in a forum like this. That said, one can do it given a bit more space, at least provided your name is Richard Feynman. Offhand, I don’t know exactly what Ring is quoting as (1967 a), but Feynman laid out the famous explanation of it in two places: The Character of Physical Law and volume one of the Lecture on Physics. I’m pretty sure the relevant chapter of the latter is reprinted in his Six Easy Pieces and that’s probably the easiest place to start. It should be accessable to anybody (and what you read in Discover surely derives from it at no more than a few removes). To paraphrase the man himself, understand that and you’re face-to-face with why nobody really understands quantum mechanics.

The John Gribben recommendation is not bad, but secondhand compared to the above.

As douglips implies, there’s a crucial difference between understanding what quantum mechanics (QM) is and what it implies. Certainly, while all current experiments confirm QM, none force us to believe in multiple universes. The latter is a matter of how we interpret the fact that QM represents the observable universe. Again as douglips says, there are other interpretations. All predict (currently) identical experimental results. However, most interpretations introduce the idea that making a measurement alters the universe. The big metaphysical advantage of multiple universes is that a measurement by us doesn’t alter the totality of all universes - it merely switches us from one branch to another, rather like the way switching from one railroad track to another doesn’t change the total rail network. The huge disadvantage is the sheer redundency of stuff involved in explaining even the most trivial event.

For completeness, it should be mentioned that David Deutsch (Oxford) argues that the advent of practical quantum computation will force people to believe in multiple universes. The argument is simply that the experimental concepts become too complicated to think about in any other way. I perhaps should note that Deutsch is both a firm believer in the many-worlds interpretation of QM and a pioneer of quantum computation. Or perhaps not.

It’s one of the most famous science quotations of all time. Try any of the following – there must be a hundred of them.

http://www.google.com/search?q="Nobody+knows+how+it+can+be+like+that"+&btnG=Google+Search

Each measurement creates an entirely new universe.

Every measurement creates another us in the new universe.

Of course I use the term “new universe” as shorthand since both universes are created at the time of the measurement. Since practically everything we do amounts to a measurement, there are literally accountable universes per this interpretation.

That should read “uncountable”

OK - I follow. Two basics, if that’s possible in QM, that I’ve heard and accepted for a long time is that light acts as particle and wave, and to take a measurement means you’ve altered what you are measuring.

The leap is that, presto, we have many worlds. Deutsch grabbed my attention with that - and he is backed-up by outstanding credentials, and not to mention the bullet proof reputation the theory of QM has earned.

Now, I don’t mind someone just saying, “Phil, stop there, it is just so complex it isn’t worth worrying about.” If Deutsch’s proposal of many worlds is that complex, so be it.

I guess I thought, “Well, apparently there is some cross-over between these many worlds, and they aren’t seperate because in this world, we can cognitively determine that there are other worlds, and by some stroke, we’ve measured the effects of other worlds in this world.”

Maybe we shouldn’t declare them other worlds, universes, etc. Maybe they are infinitely progressively different levels/phases within this world.

Now I have a headache.

Keep in mind that it’s not merely taking a measurement that alters what you’re measuring, but having a conciousness interpret that measurement. Physicists have thought up all kinds of tricky schemes to separate the measurement from the concious interpretation, and in every case it’s the interpretation that does it, not the test instruments.

I think this is what is so troubling to the philosophers. Then someone thought up the many-worlds idea, which is philosophically more pleasing, except for that minor point about the universe splitting into nearly infinitely many branches at each instant of time.

My opinion is that this doesn’t help things at all.

QM leaves everyone bewitched, bothered and bewildered but it is possibly the most exiciting field of study in this century. Philster, I higly recommend two basic overviews of QM: Brian Green’s The Elegant Universe and the classic, The Dancing Wu Li Masters by Gary Zukav.
Zukav quotes physicist E. H. Walker, who speculates that photons may be conscious: “Consciousness may be associated with all quantum mechanical processes… since everything that occurs is ultimately the result of one or more quantum mechanical events, the universe is “inhabited” by an almost unlimited number of rather discrete conscious, usually nonthinking entities that are responsible for the detailed working of the universe.” In other words, those photons somehow knew where they were going when they went through the slits.
Mind blowing, no?

Quantum mechanics - the dreams that stuff is made of!

While you’re citing QM books, I should cite another that I just mentioned in another thread a few minutes ago.

Read “The Cosmic Code” by Heinz Pagels. It’s the best layman’s introduction to QM that I ever saw, and will easily and simply take you quite far into the math even if you flunked algebra. IIRC, he deals with the photon experiment in detail.

After reading The Cosmic Code, I felt ready to tackle The Feynman Lectures. Then I looked at em and chickened out.

Photon Pixies eh? - are they pink by any chance? :smiley:

Mindblowing, yes.

I am going to spend some time grappling the idea that consciousness (which is an abstract to me) is an issue within Quantum Mechanics.

From CurtC, “…it’s not merely taking a measurement that alters what you’re measuring, but having a conciousness interpret that measurement.”

I don’t think anyone has ever explained that to me. I recall some QM convos with past professors and they always stuck with me, but I sort of stumbled upon the points above.

I do have some reading to do (thanks for the referalls), but I find it curious that QM - one of the most respected theories I know of - is loaded with material for philosophical debate. I’ve kind of been ignorant of the deeper philosophical issues, but not all. Sorta disappointing that so many “scientists” stay away from the tougher issues wrapped up in QM.

QM is a breeding ground for great philosophical and scientific debate. Usually when you get both together, the understanding of our world grows exponentially. Just seems like QM is not being tapped for the ultimate dual resource in thinking that it seems to be.

“The universe is not only stranger than we know, it is
stranger than we can know.”

Or From Futurama “And it’s a quantum finish! We’ll go to
the electron microscope for results. … And it’s number
four by a Planck length!”
“No fair! By observing it you’ve changed the results!”

I've had many discussions about QM and QP. The person who has the most insight and has been able to help me the most, thinks (and in fact is unil he is tested and one probability front collapses) that he is a werewolf. In many ways QP is like Zen koans. Don't attempt to understand how a certain things is possible (light being simultaneously a wave and a particle for instance) . Convince yourself that it is possible. Once you've truly done that, and destroyed your previous understanding of reality in the process, can you begin to understand QP.

Philster, I’m just going to make a comment about combining philosophy and physics. One reason you may be disappointed by many scientists is the attitude (which I think I agree with to some degree) that in fact science and philosophy are completely separate. The idea is that science is only concerned with making predictions that can be tested; whereas philosophy is concerned with things that can’t be tested (except perhaps with regards to logical consistency). That perspective says that the ‘many worlds’ interpretation has nothing to do with science because it doesn’t help us to predict anything, and can’t be tested. It’s a kind of interesting image - trillions of realities branching out each nanosecond as quantum probabilities are resolved – but it doesn’t say anything about the particular reality we find ourselves in (or why this one instead of any other). If nothing in this reality interacts with anything in any other reality, as far as science is concerned, the other ones don’t exist.

Many physicists would agree that the ‘many worlds’ image is neat, and just as valuable in its own way as say, the images in “Dune”, but it’s not science, even if inspired by science. So don’t be too disappointed if physicists aren’t as excited about it as you might expect.
With regards to electrons/photons and the wave/particle things, I think DocCathode has it right. Just because you have this simple model in your head of a mathematically idealized billard ball doesn’t mean that real electrons have to follow that model. They’re electrons, not particles or waves, and they act like electrons do. Sometimes that behaviour is pretty close to our simple model of ‘particles’ , and sometimes that behaviour is close to our simple model of ‘waves’, but they’re always just acting like electrons.

About the consciousness affecting the outcome, if you want to do some web reading, I’ve found a good article at http://www.pipeline.com/~rhodesr/bottom/reality/chap2.html , which I’ve kept bookmarked for the past year or so. It gets down to the point where you put a detector at the slits to observe whether it went through one or the other, and this causes it to be a particle go through one or the other and not be a wave. Then there are variants:

  • If you turn off the detectors but leave them in place the electron acts like a wave.

  • If you leave the detectors turned on but don’t record the data, the electron acts like a wave.

  • Record the data from the detectors, but erase it before analyzing it. The electon will have acted like a wave.

Then there’s a more complicated scenario where the data is recorded in a way where there are different methods to analyze it, and a visiting politician at some point later decides which method to use to analyze the data. In this case “So it seems that a random choice (represented by the politician who has no personal interest in the experiment)
made hours, days, months, or even years after the experiment is ‘complete,’ will change the result of that completed experiment. And, by changing the result, we mean that this random, delayed choice will affect the actual location of the electron hits as recorded by the electron detector at the back wall days, months, or even years in the past.”

This strangeness is what causes some to look for explanations in other ways. They’re desperate enough that the many worlds view seems attractive.

[hijack]

What I’ve never understood is why Bohm’s interpretation doesn’t get more attention. I don’t know much about it, largely because it gets ignored so much and I don’t have enough interest to go out and learn about it, but IIRC it makes the same predictions and spares you the philosophical conundrums as long as you’re content with accepting this odd unobservable “pilot wave,” or some such. Anyone know more about it?

[/hijack]

I wish to understand more about what, in these cases, is menat by “measurement”. Probably what I’m after is to understand what sort of interaction with the electron is necessary to “measure” it, and what sort of process is involved in “recording” the detection, both at the slits and on the board where the pattern does/doesn’t form.

I guess that amounts to technical descriptions of the equipment and processes used.

Where can I find this? Or is there a doper generous enough to attempt an explanation here?

Quite frankly, CurtC, I have a hard time swallowing the whole consciousness interferes with the experiment thing. If this is true, then I’m really pissed off at my QM professors for leaving this out of the material - this would be the most funky thing about QM, and they didn’t tell me. Further, I don’t recall reading this in any of the layperson materials I’ve read about this. Still haven’t read Schroedinger’s Kittens, though.

So, does anyone have a more direct source for this info? Anyone know more about this Ross Rhodes guy CurtC links to? I just have a hard time taking this at face value and want to dig into this further.