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This isn't really an invention - it's a
proposal for an experiment. Skip to the
sixth paragraph if you already know about
the two-slit experiment. The classic two-
slit experiment is a manifestation of the
weirdness of quantum mechanics. You
start with a light source shining on a
detector
(for instance, a piece of
photographic film or a sensitive camera).
Between the light and the detector, you
place a barrier which has two thin, vertical
slits in it to let light through.
If light behaved like a particle, you'd just
see two bright strips on the detector, as
the light shone through the two slits.
However, because photons have wavelike
properties, the light coming through the
left slit interferes with that coming
through the right slit. It's a bit like two
sets of ripples on a pond interfering with
eachother: in some places, the light waves
coming through the two slits are 'in phase'
and add up; in other places, they are 'out
of phase' and cancel. The result is that,
on the detector, you get a series of many
light and dark stripes where the light adds
or cancels.
So far so good. Now you turn down the
intensity of the light, right down, to the
point where it's only giving out one
photon every second. Now, of course,
each photon has to go through either the
left or the right slit, and there is no other
photon for it to interfere with - they're
going through alone - so there can't be an
interference pattern. But the spooky thing
is that there is *still* an interference
pattern. It's as if each photon, going
through alone, splits into two and goes
through both slits, interfering with itself.
OK. So now you add something. It's
possible to build a detector which can 'see'
a photon going past it. So, you put one of
these detectors next to each slit, to see
which slit each photon goes through.
When you run the experiment (still with
only one photon at a time), you find that
each photon goes through *either* the left
slit *or* the right slit - never through
both. And now, spookily, there's no
interference pattern.
In other words, when you *don't* follow
each photon, it splits in two and goes
through both slits at once and interferes
with itself. When you *do* follow each
photon, you see it going through one slit
or the other, and there's no interference.
This experiment has been done, and all of
this really happens.
OK, so here's the experiment I want to do.
We build our two "photon watchers" to
detect the passing photons, but we make
them differently. Each detector records, in
itself, when a photon goes past, and it
stores this information internally. At the
end of the experiment, we can either
download this information and get a
record of which way each photon went,
*or* we can press a button which erases
the information, so we don't know which
way each photon went.
If we download the information, then we
have 'watched' each photon to see where it
went, and we should therefore abolish the
interference pattern. But if we press the
'erase' button, we haven't 'watched' each
photon, and therefore we should get an
interference pattern. But, we don't decide
whether to download or erase until *after*
we've looked to see whether there is, in
fact, an interference pattern.
This system should, therefore, be able to
predict whether we're going to download
or erase the data. If we see an
interference pattern, we know that we are
going to press the 'erase' button. If we
see no interference pattern, we know that
we are going to download the data about
which way the photons went (which, of
course, means that we've observed them
and therefore abolished the interference
pattern).
But then, suppose we automated the
system so that, if we get an interference
pattern, we automatically download the
data; and if we see no interference
pattern, we erase the data.
(?) Bacon
http://www.constitu...r/baconsemiosis.htm
[pertinax, Nov 28 2007]
Can't resist... must...self promote...
Schr_f6dinger_27s_20Dyson_20Shell
For [Maxwell], not for me, honest. [theleopard, Nov 29 2007]
A REAL causality experiment being done
http://cosmiclog.ms...7/07/17/274531.aspx
QM is spookier than most folks imagine, heh. [Vernon, Nov 30 2007]
Lotsa stuff about virtual particles
http://www.nemitz.net/vernon/GHOSTLY.pdf
Much more detail about something mentioned in an annotation here. [Vernon, Nov 30 2007]
Another experiment, already done
http://www.analogsf.../0412/altview.shtml
This one boggles, too. [Vernon, Nov 30 2007]
The Paper describing the preceding experiment
http://www.irims.or...mentarity%20All.PDF
Includes diagrams toward the end of the document. [Vernon, Nov 30 2007]
Wikipedia describing the same experiment
http://en.wikipedia...i/Afshar_experiment
The explanation is less technical and the diagrams are in color. [Vernon, Nov 30 2007]
What I was thinking of...
http://arxiv.org/PS.../0610/0610241v1.pdf
commonly referred to as a delayed choice quantum eraser, first proposed by Wheeler. [4whom, Dec 01 2007]
the wiki on delayed choice quantum erasers
http://en.wikipedia...ice_quantum_eraser.
includes some errors [4whom, Dec 01 2007]
Airy Optical Beams
http://www.physorg.com/news115556629.html
Photon, meet hole (CCD version of Whaka-mole) [reensure, Dec 03 2007]
Many-worlds interpretation
http://en.wikipedia.org/wiki/Many_worlds
This cab driver believes the "wavefunction collapse" is a bit irrelevant. [ed, Dec 03 2007]
Soliton waves
http://en.wikipedia.org/wiki/Soliton
Persitant propagating wavefronts. [8th of 7, Dec 03 2007]
Ten Commandments
http://en.wikipedia...ki/Ten_Commandments
No other gods/do not worship idols. [8th of 7, Dec 04 2007]
Schrodinger's Comic
http://xkcd.com/45/
The observer decides the outcome. [Vernon, Dec 06 2007]
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Annotation:
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The mere capability of recording data will erase the superposition. Sorry. If you don't believe me, read "Entanglement" by Amir Aczel for a good source on this peculiar facet of quantum mechanics. |
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And, why is it Schrodinger's? Young was the inventor of the experiment. At least attribute the new experiment to the founder of the download: Al Gore, of course. |
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Thank you, [MB], for providing my daily fix of awe and wonder. |
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It's interesting (to me) that your subtitle echoes Francis Bacon's metaphor of experimentation as a form of torture, with the aim of extracting confessions from nature. |
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[daseva], if I've understood you correctly, the interference pattern is prevented at the point when one or other of the built 'photon watchers' spots a photon, whether or not the information is forwarded to a human observer; so, you are predicting that this experiment will never yield an interference pattern. Is that right? |
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<somewhat out of my depth>
[MB], if that is right, does it mean that you were supposing that the role of 'observation' in quantum mechanics necessarily implied human (or other 'conscious' or 'intelligent') observation, whereas in fact (IIRC), 'to be observed' in quantum terms is simply 'to have a measurable effect on something' (which itself might or might not subsequently be observed by a human)?
</soomd> |
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<slightly trollish whimsy>
If all of the above is correct, we may return to the 'confession' theme by affirming that the extraordinary rendition of the photon into the non-human jurisdiction of the photon-watching devices (while we look the other way) yields no actionable intelligence beyond what the photon would confess under our immediate oversight. This proves nothing, but struck me as darkly humorous.
</stw> |
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I'm a little curious how you detect the passage of the photon without affecting it's passage. |
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I'm sure Heisenburg would have had something to say about that. - So how is it done? |
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//The mere capability of recording data
will erase the superposition.// I'm not
sure this is the case - at least it's
something that's heavily disputed. The
argument is the same as for
Schrodinger's cat (hence the title). In
that case, the quantum event is the
decay of the atom, and it determines
the life or death of the cat; but the cat
is meant to be in a superposition of
states until it is observed by opening
the box. More generally, there is the
question of exactly what is needed to
collapse a quantum state. Clearly, it is
not "any" type if interaction; yet
observation by a human is sufficient to
collapse it. There is no reason to
assume that a detector cannot be built
which becomes entangled with the
passing photons but then erases that
information without any external
manifestation. Such a detector would
not collapse the state; the basis of
quantum computing is that quantum-
scale interactions don't collapse things
until they are coupled to the macro
world. |
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//And, why is it Schrodinger's//
Because the philosophy is the same as
his cat. You could, for example, use a
series of cat-boxes to record the
passage of each photon, and then
decide whether to open the cat boxes
and look inside. |
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//I'm sure Heisenburg would have had something to say about that//
But how can you be sure? |
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//does it mean that you were supposing
that the role of 'observation' in quantum
mechanics necessarily implied human
(or other 'conscious' or 'intelligent')
observation,// That is one of the
fundamental paradoxes in QM -
observation collapses the state, at least
from the observer's perspective. The
'cat' experiment is designed to highlight
this paradox - can the cat not collapse
the state itself? What happens if the
whole experiment is done in a closed
room - does the state of the cat remain
ambiguous until the experimenter
leaves the room and reports the results
to someone else? |
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The experiment I designed is intended
to force this paradox to show itself -
clearly there has to be a contradiction at
some point, but it's not clear where the
paradox lies, in the path from quantum
event to human observation of a
diffraction pattern. |
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//I'm a little curious how you detect the
passage of the photon without affecting
it's passage// Me too, but I'm assured it is
possible, and has been done. Moreover, if
your detectors are not very efficient, then
you see a weak diffraction pattern -
midway between the 'observed' and
'unobserved' states. |
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Very clever, [MB] - bun for sheer creativity. I'm sure [daseva]'s first comment is right, but I'd like to try this just to make sure. After all, that's the point of experiment. |
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It's not self-evident that recording data
should collapse the state. In many
situations, you have a series of
quantum events interacting, and it's
only when (and if) you read out the final
outcome that the intermediate states
collapse. Likewise, it should be
possible to have a photon-passage-
detector which puts an internal device
into a state which can either be read out
(collapsing everything), or annhialated.
For instance, you could encode the
passage of a
photon in the spin-state of two nuclei -
up/down or down/up; then you either
read the spin states and extrac the
data, or let the two nuclei interact in
such a way that you can no longer tell
which is which, in which case the data
has been lost with no possibility of
recovery. |
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That's fascinating. I'm sceptical, but can't deny your logic. Bun, for making me think (not that that's a hard thing to do, but I enjoy the opportunity!). |
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If I believe you can prove that you have
not looked at your idea again [MB], I will
give you this croissant, so have I given
it or not? |
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As you'll see at the top of the page,
[xenzag], I have been given both a bun
and a non-bun. It would be foolish to
suppose that these result from two
different bakers - the only sane
interpretation is that they arise from a
single baker in a superposition of states. |
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[Pertinax] - sorry, I missed your
annotation. In regard to //
<somewhat out of my depth>...// the
whole question of what constitutes "an
observer" capable of collapsing the
states is a very spooky one. Certainly,
with no "observer", there really is a
superposition of states. At the other
extreme, with a human "observer" the
states collapse. Schrodinger's cat is an
oddity - it surely should qualify as an
'observer' of its own life, but from the
perspective of someone outside the
box, it doesn't. The idea of this
experiment was really to force quantum
mechanics into a corner; clearly, QM
should not be able to "predict" whether
we are going to download or erase the
data, but pinpointing the flaw in the
argument would be informative. |
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I suspect (in a very fuzzy way) that the
answer to all this lies in a sort of
"quantum relativity", whereby a
quantum state can collapse from the
perspective of one observer, yet remain
uncollapsed from the perspective of
another. But this is almost certainly
bollocks. |
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I think this was previously alluded to in a dyson shell implementation, I shall leave the author to link to it. |
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More people failing to be unconvinced by David Hume *yawn*... |
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//alluded to in a dyson shell
implementation// Can you expand a little,
[4whom]? |
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//We build our two "photon watchers" to detect the passing photons// - as [CustardGuts] says, this is why your experiment is doomed to fail. You can't detect the presence or absence of a photon without affecting the photon itself. It will *have* to give up energy to your detector. |
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This idea contains exctly the right amount of science. Enough to make me feel clever but not enough to make me realise I'm not. |
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//You can't detect the presence or absence of a photon without affecting the photon itself.// Yes, I too am unconvinced by this point. And by inferrence, the whole "whoooo - wave/particle duality" mysticism that appears to surround it. It could easily be that I'm missing something, but if so, I'd love to know what it is. |
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([zen] - what? - you think you *can* detect the photon without affecting it, or you agree you can't?) |
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The author has linked, MB. |
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//this is why your experiment is
doomed to fail.// My understanding is
that that's not the case, counterintuitve
though it may seem. For one thing, it is
possible to perform "interaction free
measurement" - people have produced
images despite the relevant photons not
having interacted at all with the imaged
object - but having had the potential to
interact. All very spooky. The detection
of the passing photon is a sort of
inverse of this. |
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In any event, suppose that the photon
*does* have to give up some energy -
it's still not a problem. Imagine we
have material which absorbs a photon
at wavelength A, and emits a photon at
a longer wavelength, B. In doing so, an
atom of the material is kicked into an
excited state. This is just fluorescence. |
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Now imagine that each of our slits has
a window of this fluorescent material.
You'll still get interference: it'll be
interference between the longer-
wavelength emitted photons rather than
the original incoming ones. So now,
each "window" has absorbed a tiny
amount of energy from the passing
photon. We can then choose whether or
not to detect this absorbed energy and,
by inference, the passage of the
photon. |
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//we have material which absorbs a photon at wavelength A, and emits a photon at a longer wavelength// - but how do you know it's emitted a photon? The answer is, you don't - that's the point. |
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[hippo] //eh - what? etc // I agree that it can't. However, I do remember specifically the bit of the experiment where someone starts 'measuring' which slit the individual photons go through, and the subsequent collapse into particulate behaviour (over time). And have always wondered about this. |
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What I've never understood is how this measurement gets to take place - perhaps it's by means of this fluorescence thing [MB] is talking about - but that doesn't quite add up to the same thing - not sure exactly why - it just doesn't feel as though it does. Maybe Heisenberg and all that. |
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It's not that 'observation' is some magical act that collapses wave functions, or kills/unkills cats, it's that at such tiny scales, the only way of observing what's going on is akin to a blind man observing where other road users are by colliding with them at full speed, in a tank. The observation (read fatal traffic accident) will tend to significantly effects the results. It's not mysterious, it's just that small delicate things get so small and delicate that we just can't tell they are there without mashing them all up. |
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So fluorescence as a test, might be a bit like having a bus so packed full of people that, when Godzilla throws a hapless human in at one end, another pops out the other side, screaming and running directly for shelter, in a particulate manner. |
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//We can then choose whether or not to detect this absorbed energy and, by inference, the passage of the photon.// I suppose my question here is - is it the windows that causes the change (I imagine it will be) or is it the act of looking at the windows? If two scientists at opposite ends of the room look at two different parts of the experiment; Scientist A is looking at the projected interference patterns/line pairs, and Scientist B is looking at the fluorescent windows. Scientist B is distracted for a moment by an abandoned sandwich - does this mean that the twin lines on the wall suddenly revert to interference patterns while Scientist B is temporarily indisposed? I think not. |
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//but how do you know it's emitted a
photon// because the emitted
wavelength is longer than the absorbed
wavelength, which means that the
material has absorbed some energy,
which must in theory be detectable. |
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Failing that, there are "wavelength
doublers" which absorb one photon and
emit two of longer wavelength. There's
nothing to stop you detecting one of
these two photons whilst its twin goes
on to diffract (or not). |
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It's not true to say that *any* interaction
collapses the waveform. That's why
quantum computing works - you can
couple quantum events without
collapsing them. The question is this:
in between quantum-scale interactions
which don't collapse the waveform, and
macro-scale interactions which do, at
what point does collapse happen? |
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I will try and find the link to a paper that tries to resolve this. It is quite an elegant "solution". If I have not done so by tommorrow 19h00 GMT, give me a reminder, I will have more time on the weekend. |
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Thanks, [4whom]. [zen tom] can you
remind me to remind [4whom]? |
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MB, Tom as a calendar? Try Tim. He is less difficult to spot. |
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Good point. Can we get Tim to remind
Zen Tom to remind me to remind you? |
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// treat light as an EM field which can
be absorbed in quanta// Yes, that's
precisely the point (or a point, anyway).
Light has both field properties and
quantized (i.e., particulate) properties,
as indeed do electrons and elk. |
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However, that's not the real spookiness.
The real spookiness comes back to this
question of what consititutes
"observation" sufficient to collapse the
behaviour of the system. Simple
quantum interactions *don't* (hence the
possibility of quantum computing);
measurement and human observation
*does* (hence the two-slit result). But
any observer and any measuring device
is just a whole pile of quantum
interactions anyway. So where is the
threshold between a non-collapsing
quantum interaction and a collapsing
one? (I am using the verb 'collapse'
transitively here.) |
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"// treat light as an EM field which can be absorbed in quanta//" This is the exact start of the precipitation of quantum theory, a description proposed by Einstein to explain the photo-electric effect. It is damn near verbatim. If it was intentional, I appologise, but me thinks not. |
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// other than the probability of extracting
a quantum from the field// Exactly. But
the bottom line is that the system behaves
differently - in a tangible physical sense -
when it's being watched. |
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I hate coming to someone's defense but I will say this: |
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Is it impossible for a broken glass, that has fallen from a table, to reconstitute itself? Nay! And this is correct. But only because the *probabilty* that it will not, completely outweighs the *probability* that it would.
We are not afforded this luxury in the quantum world. All probabilities exist equally (in the quantum environs). It is a consequence of the speed of light and its time dilation effects.
The critique of "What constitutes observation?" remains valid (IMHO). Unfortunately, previously covered in theleopard's previous iteration. |
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//Rubbish. You're not psychotic at the
moment are you ? // Not as far as I
know. |
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bigsleep, far be it for me to correct your view, and/or misinterpret another's post. But having said that, I think you underestimate our spotted friend. |
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The question of observation is an open one. I think theleopard had a good shot at it, (at least the best I have seen that exists outside the realms of possibilty). MB has taken another good crack at it (within the realms of possibilty).
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I challenge you to read *and* understand theleopard's last anno on his idea.
Post Script: Hey, at least I remind you of someone! |
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Fresh observational eyes are needed .
Temporarily forget particles , temporarily forget waves , think total anew .
A new 'wrong' approach might just uncover the insight needed . |
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Knowing how arm moves through its degrees of motion (behaviour) and the neuron muscle firing chemistry (structure action) are two different things . |
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<figuratively a taxi driver> |
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[bigsleep] (or another well-informed person), would you be willing to offer an answer to the question in my first anno which I originally addressed to [daseva], but which [daseva] decided (perhaps rightly) was beneath him? |
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'Ere, I 'ad that Professor Schroedinger in the back of the cab once... or did I? |
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No, sorry. You are correct. I stand that there will exist no interference pattern. The only way to have the interference is not to watch it. You can't watch it, and then decide not to. You have watched it and you know which slot it went through. Erasing the data before checking it out is not going to change the fact that information was collected. You gotta burn that data, spread the entropy, and now you data is an element of reality wether it wants to be or not. |
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Also, this idea is somewhat magical in the fact that if led to further conclusions, one might be able to just backup the data before erasing it. Now, you see an interference pattern, but you're damn sure they were single particles going through one or the other hole like bullets: you recorded it! |
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So, you're going to force measured particles to behave like waves. Now, that would be cool. This is essentially forcing real particles to behave as if they were superimposed waves. Get in line, little particles! you are all going to pretend to interfere and not go in a probabilistic straight line but move over here and there in a cool pattern!!! |
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I doubt their going to listen ;) |
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[MaxwellBuchanan], are you familiar with the "virtual particles" that fill the vacuum between ordinary particles? Ordinary and virtual particles are always interacting with each other, and much of QM can be explained as a consequence of that. For example, an ordinary particle might temporarily transfer some of its momentum to a virtual particle, and get it back less than a nanosecond later. This can explain the two-slit experiment completely. The ordinary particle (ANY type) goes through one slit, and the virtual particle goes through the other slit. The interference pattern is the natural result of the momentum getting back into the ordinary particle afterward, since the MOMENTUM went through the two slits simultaneously, and not either particle. |
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If you perform some interaction with the ordinary particle, to detect which slit it went through, you ALWAYS change its total momentum, and that's why the interference pattern disappears. |
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Now I do understand that you would like to do some kind of interaction that does not mess up the momentum of the particle in question. This could actually be done, if you could control virtual particles. The trouble is, you are not allowed to DETECT virtual particles, so even if you did use them to detect the which slit the ordinary particle went through, you couldn't get the information/result from the virtual particles that did the interacting! |
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I've added some links. There is an experiment in which the photon's path is determined AFTER the interference pattern has been created. |
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Does this count as interaction-free observation? Start with the two slit experiment. There will be places in the interference pattern that get no photons due to the interference. Let's call one such place Fred. Now cover one slit with a photon detector. Fire a photon. Perhaps it hits the detector. Obviously in this case there is interaction. On the other hand, it may not hit the detector. In this case it may go through the other slit and hit Fred. If so, there was no interaction with the detector and yet the observation (that the photon didn't hit the detector) destroyed the interference pattern so the photon could land on Fred, which would otherwise be impossible. Just the potential for interaction changed something, even when the potential interaction turned out not to happen. |
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[edit] I'll note that you could just as easily block the slit without using a detector, and it would still let photons hit Fred |
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Oh my. I just had a memory of the last scene of back to the future and I know exactly what's going to happen if you try this. It will look like an interference pattern, and then you'll download the data and it will start to change into a normal pattern and then Marty will start playing real bad on the guitar and someone will get punched in the face and falls on the 'erase' button! And the interference comes back and Marty freaks out on a killer solo but nobody really gets into it. |
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If I may throw my two small monetary units into the mishmash of discussion... |
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I deny that it is possible to make a device which can detect the passage of a photon without affecting it. The double-slit experiment is itself proof of this--if you put a detector at the slits themselves to watch the passing photons, interference no longer works, therefore the photons are being affected by the detectors. it doesn't matter if we actually see the output of those detectors, their presence is enough to throw things off. It's not a question of observation, but an interaction which forces the presence (or non-presence)of a single photon--there's nothing magical about the fact that we actually see what happened. |
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[MaxwellBuchanan]. You assert that interaction-free observation is possible, but don't provide any evidence beyond your own assertion and the claim that someone told you so. (links?) Until you do, I'm going to assume that what I know is correct. As for [Vernon] and the Afshar experiment, it's very intriguing but still being very hotly debated as to whether Afshar's interpretation is true or not. |
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Anyway, The common misconception with the two-slit experiment is that this distinction of interference/non interferences is made at the point where the photons are collected. This is false. The distinction is made at the slits--whether you have one or two slits open to (unimpeded) progress of the photons. |
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If one slit is open, you get no interference pattern. If two slits are open, you get an interference pattern. The weirdness is that if you fire photons at the two slit version one at a time, you still get an interference pattern. The photons arrive at the far side one at a time, in individually distinct events, but they arrive in locations that correspond to an interference pattern. If you fire one photon at a time several hundred times, you'll see the pattern emerging. Somehow, the photon is producing an interference pattern by itself, which most people rationalize by saying it's acting like a wave. But the fact of each arriving one a time belies this--something else (quantum mechanics!) is going on. |
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The way this applies to this experiment: If the detectors were physically possible (doubtful) you'd see an interference pattern every time, because both slits are open. End of story. The business with the detectors is irrelevant, because the relevant portion of the experiment is the slits, not the detectors. What I would like to know is what the detectors would show. |
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Anyway, everyone should just read Feynman's Q.E.D. (Quantum electrodynamics) It's actually pretty easy to understand, and explains the entirety of light without actually worrying about the wave or particle nature of light at all. The whole argument is just a case of people using two equally bad metaphors for the way light (and other quantum objects) works. |
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[edit: I've changed my mind. No interference pattern will appear. scroll down for explanation.] |
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Wasn't I supposed to remind someone of something?
"Facking par'icol fysiks! Dannah they're fackin' born. Iv ahrv said it once ahrv said i' a faaasand times, facking virtuul par'icols innit. Ferfacksake. Do you want to go by London Bridge, or Tower? Ken Livingstone? Don't facking tawk to me abaht Ken Fackin' Livingstone!" |
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[Bigsleep] I'm not really following your
argument here, but never mind. You
seem to think I'm winding you up. All
I'm doing is presenting a well-known
phenomenon and question in QM, and
suggesting an experiment which would
lay the question bare. Sorry if that
causes you a problem. |
|
| |
[5th earth] Re Interaction Free
Measurement - just Wikipedia for this
term, and you'll find plenty of
references therefore. We also had a
seminar here (at the cab-driving
laboratory) a while ago where the guy
showed images obtained in this
counterintuitive way. I was taking this
as a given rather than as a contentious
point. |
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| |
[others] I'll catch up with later. Got a
fare, have to dash, you know how it is. |
|
| |
MB - I'm head-hunting you as a driver
for my own Taxipedia Cab company. |
|
| |
[zen_tom], sorry, I was supposed to remind you. |
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| |
OK, let me know when you do. |
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| |
All this talk reminds me of a book I recently read on holiday - most reccomended; Will Self's "The Book of Dave" - set in a post-global warming future where sea levels have risen over most of the UK, and a whole culture and religion has formed based on the discovered writings of a London Cab driver called Dave. |
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| |
//I challenge you to read *and* understand theleopard's last anno on his idea.// |
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| |
Indeed, bit of a conversation stopper that one. |
|
| |
The one about curiosity killing the universe? Seems reasonable to me. |
|
| |
//If the detectors were physically
possible (doubtful) you'd see an
interference pattern every time, because
both slits are open. End of story.// Yes,
but that's not the case. The detectors
are physically possible, and you don't
see interference. This is pointed out in
Feynman's QED, and also in at least one
recorded lecture by him (and others).
He also explains how, if the detectors
are "inefficient", you get an intermediate
case - a weak diffraction pattern. |
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| |
Incidentally, my understanding is that
the same experiment can be done with
electrons, which also diffract, and for
which the experimental setup is a bit
simpler. |
|
| |
I thought we were taking all this as the
starting point? |
|
| |
The key question here, to cut to the
chase, is this. As long as everything is
kept 'quantum', the system exists in a
superposition of states. Interaction
with the 'macro' world is supposed to
collapse it. So, at what point does the
transition occur from 'quantum'
interaction to 'macro' interaction? Is
this point the same from all
perspectives? This is the nub of why
there has been no satisfactory
interpretation of QM, and why there
may never be one. I'm just trying to
devise an experiment that makes this
weirdness even more explicit. |
|
| |
I'm still wondering why all these photons are all collapsing into a state. Neurotic photons? I suppose that explains why they don't know whether to wave or not. |
|
| |
Perhaps light rather 'prefers' to be quantized but can be coaxed to separate under the right circumstances... |
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| |
Well my theory, seeing as we seem to be doing theories for the moment, is that photons actually move in the opposite direction, temporally speaking, to us and therefore they know what you are going to do before you do it. Hence their ability to befuddle the experiment. Alarmingly, the corollary to this is that photons do not originate from a light source but from the observer and that they accumulate around what appears to us to be the lightsource afterwards, hence the reason that observation has an effect upon outcome. After that my rather dodgy theory becomes a bit sketchy as it's too complicated for my poor brain to work out the details. |
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| |
Your appellation of the term "dodgy" to this pot of crack is a grave insult to Dodge. |
|
| |
I've reviewed the Wikpedia articles I think we're just quibbling over definitions of "non-interaction". I've got a non-standard one, so I'll admit I'm sort of wrong. (well, more than sort of--I've reversed my opinion, as I will explain in excruciating detail) |
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| |
All the experiments basically rely on setting up a system where one of two events can occur, and then observing only one of them. If it doesn't occur then the other must have happened. Oddly enough, it's this exact principle which, I think, shows this experiment won't work. |
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| |
I may be wrong. Working this out was tricky. |
|
| |
With your hypothetical detectors, you are assuming, at the detectors, that the photon is acting as a particle. On the other hand, at the film at the back with an interference pattern, you are assuming it's a wave. So you have a two-outcome scenario: 1, the photon acts as a particle and is detected at one or the other of the slits, or 2, it is NOT detected, acts like a wave, and makes an interference pattern (IP). |
|
| |
If you observe an IP, then detection at the slits did not occur--you yourself state this in the description of the experiment. Conveniently, you don't need to actually look at the results of the slit detectors to know this--the presence of the IP proves they didn't work, they recorded nothing. |
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| |
Conversely, if you do not see an IP, you know that the slit detectors worked, and they recorded something. Conveniently, you don't need to actually look at the numerical results (which slit) to know that they worked--the absence of an IP proves it. |
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| |
So, (if I've worked this out right) the knowledge of which way the photon actually went is irrelevant to the presence or lack of an IP. If the slit detectors work, no IP will appear, regardless of whether or not we actually look at the data they record. |
|
| |
You'll note I've changed my opinion--you'll get no IP, no matter what. I think. Go find a quantum physicist, or get a government grant and actually do the thing. |
|
| |
"Each detector records, in itself, when a photon goes past"
...and therein lies the fundamental problem of quantum mechanics. You can't think of it the same way as you thing of things in a classical scale. The reason why the observer perturbs a quantum mechanical experiment is that the observer *must* participate in order to observe. You *cannot* detect a photon going past you. You detect a photon by stopping it dead and looking at where it hit. |
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| |
Think of it like this:
Classical mechanics: I open my front door and look down the street. I see a red car. It's red. I can blink and it's still there. It's still red. I shut my eyes, turn around and it's still red. I ask hippo over to look at the car. He agrees that it's red.
Quantum mechanics: I blindfold myself and walk out of my front door until I trip over. I assume what I have tripped over is a car. I chip a bit of paint off the corner at which point the car spontaneously combusts and crumbles to a pile of ash. I return to my house and conclude that there probably had been a red car outside. |
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| |
The more you try to apply classical rules to the quantum world the deeper and more confusing these paradoxes will be. You won't crack them. You won't solve them. You'll just highlight and perpetuate the paradox that exists when you approach quantum mechanics with the laws of classical mechanics. |
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| |
If you realise that the two instructions: "Tell me the colour of the car outside my house" and "drive it down the shops and get a pint of milk" are not both possible in the quantum world your head will be less likely to explode. |
|
| |
Thank for the heads up, the brothers Kimm. |
|
| |
"So, at what point does the transition occur from 'quantum' interaction to 'macro' interaction?" ~ MaxwellBuchanan,
Well why don't you take a look-see and find out. :-) |
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| |
My favourite quote, in relation to this: "Delayed-choice experiments of this type have actually been performed, and the interference pattern detected when D (the detector) is up is not affected by the delayed choice. Somehow the photon can retroactively arrange to go through one slit or two depending on which measurement is ultimately made." |
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| |
You want the truth? You can't handle the truth! ~ Col. Jessop. |
|
| |
To make it a bit easier to understand why we don't/can't understand these phenomena, I propose the following story (originally to have been posted under the Herge's Twins Paradox (you know.... for massless twins)): |
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| |
It starts with a paradox proposed by Einstein. The Twins paradox. Those unfamiliar with it can wiki it, if needs be. It deals with the age difference of twins, one travelling at rest, with respect to a certain frame of reference, and the other travelling at relativistic speeds, with respect to the same frame of reference. When they eventually meet again the "rest" twin is older than the other. |
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| |
Now consider twin photons. One is fired distance d to a detector and the other (simultaneously (or not, as it turns out)) distance x.d to the same detector. The first casualty of this experiment is the frame of reference (which previously resided with the "rest" twin). So let us just sweep that under the carpet. Shirley, each photon will have its own frame of reference and we can compare those. Photon twin A arrives at the detector at time a and photon twin B arrives at the detector at time b=x.a (for us). We perceive no age difference, but what if we asked the twins their ages? Presumably twin photon B is slightly (or infinitely) older the photon twin A. However, at the speed of light , time has dilated to the extent that it becomes a meaningless entity mathematically, so a calculation as to their ages relative to one another becomes just as meaningless. To put it bluntly, although it may seem to you that your photons arrived one after the other, for them it was a peculiar coincidence that they both arrived at the same time. |
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| |
//Somehow the photon can retroactively
arrange to go through one slit or two
depending on which measurement is
ultimately made.// Where was that quote
from, [4whom]? What was the context? |
|
| |
If memory serves, it was by Cramer, can't remember the specific article. It stems from this earlier gem from John Wheeler:
"We have a strange inversion of
the normal order of time. We, now, by
moving the mirror in or out have an unavoidable
effect on what we have a right
to say about the already past history of
that photon."
Wheeler made the statement without his experiment being ratified. Cramer made his statement post "ratification", as several experiments conducted showed this phenomenon of photons choosing their paths/natures post facto. |
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| |
There is an even bigger bug-bear to consider. In recent experiments (circa 2005-2006) certain arrangements have been detecting photons *before* they have been emmitted! |
|
| |
//certain arrangements have been
detecting photons *before* they have been
emmitted!// That sounds very much akin
to the "interaction free measurement", in
which a real image can be obtained as a
result of the _possibility_ of photons
interacting with an object, despite the fact
that they don't actually do so. |
|
| |
It's all enough to make a poor psychotic
trolling cab driver's head spin. |
|
| |
We are deeply dissapointed that this idea, however fascinating, contains no reference to forcing cats through narrow slits, nor placing them in boxes with vials of poison and radioactive sources. Therefore, though clever, it is of little practical benefit. |
|
| |
[1.142857..] I have to disagree. If I may
quote an esteemed cab driver's
annotation, from the foregoing: |
|
| |
//You could, for example, use a series of
cat-boxes to record the passage of each
photon, and then decide whether to open
the cat boxes and look inside.// |
|
| |
The cat sat on the matter. |
|
| |
By pure coincidence (or is it?), cats have vertical slits in their eyes... |
|
| |
// cats have vertical slits in their eyes.. |
|
| |
"Igor, bring me the laser .... no, stupid, not the little hundred-watt one, the good one ...." |
|
| |
<points at 8th of 7>
I know him!
Quickly, check Schrödinger's cat. There's a far greater than 50/50 chance that it's dead... or blasted into space on the back of a rocket...
<stops pointing> |
|
| |
large cats have round pupils, Ling. |
|
| |
I find it interesting that quantized third dimensional forces (read that "normal physical properties of the macro physical environment that can't be measured in subatomic milieu, therefore don't exist") defy probability and continue to manifest in the behavior of photons (link to "airy optical beams"). |
|
| |
The authors depict special photon behavior, under conditions that simulate classic 'falling frame of reference' observations but rather than moving a reference point in tandem to a photon, the photon travels unguided in an arc and is registered across points in a CCD field. How is this possible? Does the photon's 'spin' travel at a slower speed than its propagation wave? Does the less-than-ideal milieu impose on each undispersed photon a horizon of sorts, creating spin (third dimensional vertex) from an otherwise pristine 2D wavefront? Is this all consistent with Heisenberg, in that CCD fields are sufficiently weak to allow quantum level flux at a detectable distance but not so weak as to cause no interference if detection happens? |
|
| |
Must have the seer read some cat entrails. |
|
| |
Sounds like a Soliton (Saltire) wave propagation effect to us .... <link> |
|
| |
dog chasing tail .
An experiment of water by manipulating water with water in a box of water and trying isolate what the water is made of . outside leak in . |
|
| |
The Tao of Physics: An Exploration of
the Parallels Between Modern Physics
and Eastern Mysticism (1975) by Fritjof
Capra. Still a thoughtful gem. |
|
| |
The problem is, when something is
sufficiently vague, it has good analogies
with everything. Tao schmao. |
|
| |
//large cats have round pupils, Ling.// |
|
| |
But smaller cats are easier to get into boxes. |
|
| |
// But smaller cats are easier to get into boxes. // |
|
| |
Yea verily, for it is easier for a Camel to enter the Kingdom of heaven than it is to cram a large cat into a small box ..... |
|
| |
One is always south of the river. It is only
a question of which river. |
|
| |
[Bigsleep], I really don't follow this. If it's a
running joke, I don't get it. |
|
| |
Well, fair enough, and if everyone else
agrees I'll delete this. The idea was for
a photon detector which could store
information until it was either
downloaded or deleted, as a means of
understanding what's happening in the
twin-slit experiment. It's certainly
halfbaked, and I'm not professionally
qualified in quantum physics. I don't
see the philosophy side myself, but
there you go. |
|
| |
I still don't get the
spoon reference - perhaps you could
explain? |
|
| |
I have a theory and you may laugh - anything that reduces that little twiddly thing that you use to scroll down an idea - see to your right; If it's really weeny and you have to squint to see it at the top/bottom then its an idea is worth keeping! |
|
| |
Well, thanks [po]. However, I suspect
[bigsleep] is a physicist, and I've obviously
pressed a wrong button without realizing
it, so no worries. It can go. |
|
| |
// I've obviously pressed a wrong button without realizing it // |
|
| |
You are George W. Bush and we claim our five dollars ... |
|
| |
Ah well, there you go. Bless you for
pointing out my mistake, [Bigs]. |
|
| |
And //Its the whole Community/
Halfbakery dilemma.// - marked for
tagline, shirley? |
|
| |
Still not following the spoon business,
though. |
|
| |
// Still not following the spoon // |
|
| |
Good, because follwing a spoon would be Idolatry, which is a cotravention of the first or second commandment, depending on which version you go for (link). |
|
| |
[po], the slider thingy in the scrollbar is called a "thumb". I don't usually use it, myself. When scrolling a long page, I usually use the Page Down key on the keyboard. Sometimes it doesn't work, and then I position the mouse cursor near the bottom of the scrollbar, not on the button at the very bottom, and left-click that spot. It's an alternate way to get a page-down effect, one screen-full at a time. Either way, I don't have to worry about how teensy the "thumb" is. |
|
| |
[8th], I'm not sure, but I presume [Bigs]
was using the spoon in a more Gellerian
sense. |
|
| |
// I presume [Bigs] was using the spoon in a more Gellerian sense // |
|
| |
Possibly, but we hold to the view that what a person does with their own spoon in the privacy of their own home is their business, and we shoud not interfere or criticise. Do we, after all, hold the moral high ground where spoons are concerned ? |
|
| |
Can we have our five dollars now ? |
|
| |
//Can I have my five dollars now ?// I
strongly refutiate any insemination that I
am or ever have been George Bush. To
suggestify such a preposition is beneath
content. |
|
| |
I didn't remember the bent-spoon-in-the-
ass reference from the Matrix, but it was
an OK film. The whole "reality is just a
computer simulation" thing is a bit
hackneyed, but each to their own. So,
what
exactly was the significance of the spoon
in the film? |
|
| |
Walk into any intro to philosophy class and you'll be suprised to find a topic that some blindly led, beer fed college soon-to-be-dropout won't respond to with: "Oh, like in the Matrix!" |
|
| |
That's why I hate the movie. |
|
| |
// Scriptwriters needed a device to
reinforce Neo's disassociation from
"reality" before introducing "the vase"
paradox by the oracle.// They don't write
'em like that any more. |
|
| |
//if everyone else agrees I'll delete this. // |
|
| |
No, don't delete it. I'd like some more time to reconcile the actual experiment in [4whom]'s link with the stuff in the annotations. |
|
| |
By the way, whose spoon is this? |
|
| |
What frustrates me as a non physicist is the lack of good solid common public metaphors that are non mathematical for atomic objects .
What is an electron - A negatively charged particle that whizzes around the nucleus of an atom .
What is a negative charge ? - A charge that has more electrons than protons .
See my point . |
|
| |
//What frustrates me as a non physicist
is the lack of good solid common public
metaphors that are non mathematical
for atomic objects .// |
|
| |
Physicists have the same problem. But
the basic problem is, quite possibly,
that there *is* no metaphor for the
behaviour of objects on that scale. To
paraphrase Richard Feynman, they just
don't behave like anything you're
familiar with. A photon really isn't like
a little bullet, and it really isn't like a
ripple on a pond - it's a photon, and it
behaves like a photon. |
|
| |
Quantum mechanics gives a set of
mathematical rules which describe, with
greater accuracy than any other theory,
how things actually behave at that
scale. It really works, and you can do
wonderful things with it. But translating
that into analogies or metaphors that
we can "understand" is, so far,
impossible. Most physicists have given
up trying to find a "classical"
interpretation that fits with quantum
mechanics, and just accept that the
equations work even though they don't
fit with "common sense". Other
physicists (quite a few) argue that there
are underlying mechanisms which, if we
could find them, *would* make "sense",
and would explain quantum behaviour.
For example, the "parallel worlds"
school of thought says that the photon
always goes one way or the other, but
makes different choices in different
worlds - we see the combined influence
of all these parallel worlds until we
make a measurement that "catches" one
world or another - collapsing things
into a classical outcome. |
|
| |
The idea of all these quantum
experiments (including, for instance,
Wheeler's Delayed Choice experiment -
which you should google - it's fun) is to
try to lay bare the paradoxical
behaviour of things at the quantum
level, in an attempt to make the
paradox understandable. But we may
well be faced with the fact that quantum
behaviour is not analogous to macro-
scale behaviour, any more than
imaginary numbers can be represented
as a number of beans. |
|
| |
//Wheeler's Delayed Choice experiment - which you should google //
Later, maybe. |
|
| |
//charm the water from the taps// |
|
| |
Maybe we could halfbake that idea and rebuff claims of [mf*]:magic with a "Quantum Mechanics Defence". |
|
| |
[Jinbish], "Any sufficiently advanced technology is indistinguishable from magic.
" (Arthur C. Clarke, "Profiles of The Future", 1961 {Clarke's third law}), English physicist & science fiction author (1917 - ) |
|
| |
"Any sufficiently advanced technology is
impossible to repair." (Buchanan's Second
Law; Maxwell Buchanan, freelance
consultant topologist, 1962-). |
|
| |
"If it ain't broke, it don't have enough features yet." (Scott Adams) |
|
| |
// it needs more monkeys. // |
|
| |
An infinite number thereof ... but who will provide the typewriters ? |
|
| |
PLease, don't badger us ... |
|
| |
[Bigs] I've finished with your spoon if you'd
like it back. |
|
| |
I hope there are no cases where the formula works but points the understanding in the wrong direction . |
|
| |
Things can be done without understanding .
You don't really need to know about magnetism to generate electricity in a wire just a magnet and a wire .
I suppose theorising allows leaping of all that time consuming trial and error experimental reality stuff . |
|
| |
// I suppose theorising allows leaping of
all that time consuming trial and error
experimental reality stuff .// Also, people
need to understand how things are. |
|
| |
//people need to understand how things are// |
|
| |
<saddles up weary old hobby horse>
Actually, that's not universally true (though it's probably true of most half-bakers). People who are good at 'living in the moment' don't really have the same felt need for this.
</suwohh> |
|
| |
I think a hand fondue is a gross idea. I
certainly wouldn't want a second one. |
|
| |
// a hand fondue is a gross idea // |
|
| |
Maybe not gross, but a pocket fondue would be much more practical. |
|
| |
I think a fondued pocket would taste even
worse than a fondued hand, frankly. |
|
| |
That's a kind offer, [custard], but one
which I shall politely decline. |
|
| |
Long ago, [hippo] said //You can't detect the presence or absence of a photon without affecting the photon itself.// |
|
| |
Can't detect the presence of a photon without affecting the photon itself? Fine. Can't detect the absence of a photon without affecting the (nonexistent) photon itself? That doesn't make any sense to me. Mind you, quantum mechanics is weird enough that it could still be true. Something to do with virtual photons maybe. And the twin slit experiment might suggest it's true. |
|
| |
// That doesn't make any sense to me // |
|
| |
It makse sense to everyone else. When you "observe" the photon, its wave function collapses; energy is transferred, causality is conserved. |
|
| |
When there is no photon there is no signal. This is a presence/absence system, not a logical system where zero is an included value. |
|
| |
The system is still determinate, even though one of its states id defined in terms of the absence of the other state. It is still possible to disambiguate the state of the system at any given Planck interval. |
|
| |
Two things I'm not convinced about here: (not that I know much about QED): |
|
| |
1. (Already mentioned) - can you really measure a 'passing photon' without interfering with it? |
|
| |
2. Won't the 'erasure' operation also interact with the data? |
|
| |
What's really the difference between downloading the information and never looking at it and erasing it before downloading it ? (I've not quite followed your logic, but I assume you had to put in the 'erase' device to make this work). |
|
| |
Another way of putting it: isn't your 'eraser' an idealized device:in practical terms would it be possible to build such a machine ? Would it not have to interact with the stored information ? |
|
| |
//observation is not just a large part of things like this, but it really is everything.// |
|
| |
//Everything doesn’t exist until we sensory it.// |
|
| |
What you are doing there, [Ian] old bean, is making a fundamental error. It is the
same error that continues to be made by physicists and philosophers. |
|
| |
People tend to assume that quantum states "collapse" at some point - either through
"observation" or interaction with a larger system (which is the same thing). This is
silly and very Newtonian. |
|
| |
What actually collapses is the _combination_ of the "observed" event and the
_observer_ collapse. Nobody seems to appreciate that the observer gets collapsed
as much as the observee does. |
|
| |
Moreover, this dovetails nicely into the other little-appreciated truth, namely that
quantum collapse is relative. The state of the cat and the state of the observer
both collapse when the box is opened, but only relative to one another. Relative to
someone outside the room, both are still uncollapsed. |
|
| |
This also (if you follow it through sensibly with a glass of something agreeable)
explains all the tombollockry of "spooky action at a distance", which of course isn't. |
|
| |
// Nobody seems to appreciate that the observer gets collapsed as much as the observee does. // |
|
| |
That's odd: even though I can't pinpoint [8th]'s
location precisely, nor tell his exact mass, I knew for
a fact that he would say that. |
|
| |
I just thought of a way to really complicate things...
It's closely related to this, so I don't think it merits a new post: |
|
| |
Instead of detecting which slit the photon passes by measuring at the
slit, add a pocket on the receiving plate. The pocket is designed so a
photon which reaches the bottom can only originate from one slit.
The rest of the plate is the same as before. |
|
| |
Oooh, but I did just think of a variant.... Probably easier
using particles (electrons probably) than photons. |
|
| |
Instead of trying to detect the photon/particle without
affecting it as it passes through the slits - do something
different at the target. Put the target plane at a non-
orthogonal angle to the slits and use particle detectors
on the target to precisely measure the time of the
impact. |
|
| |
By knowing when the particle was emitted, and knowing
when the particle strikes the target, and by having the
target plane at an angle (different distance to any point
on the target from each slit) - you can infer which slit the
particle went through. |
|
| |
[Limpnotes] - I don't think that has anything to do with
what I suggested. |
|
| |
I'm perfectly happy with the notion that the waveform
collapses when observed. My contention is that it is the
observation that causes it - bouncing a photon off, or
measuring infinitesimally changes to a magnetic field
from, etc the wave is probably what does it. I contend
that if you do the observing after the fact - by geometry
make it so you can tell which slit the particle went
through - you haven't interacted with the particle until
after the experiment is over. |
|
| |
A very simple way to do it is have the "target" as a W-
shape - angled so that a particle coming from slot A hits a
different part of the target than a particle from slot B,
etc. |
|
| |
Anyhow, it's been said that Quantum stuff is not meant to
be understood anyway, and anyone that pretends like
they do is either crazy or lying. Certainly our brains are
wired to instinctively think in terms of causality and
Newtonian physics, neither of which are of any concern
to quantum stuff. |
|
| |
// anyone that pretends like they do is either crazy or lying // |
|
| |
Why don't you try arguing that one with Stephen Hawking ? Let us know how that works out for you ... |
|
| |
To quote Jeremy Hardy, you shouldn't trust Stephen
Hawking too much as he's subject to interference
from minicab radios. |
|
| |
// Why don't you try arguing that one with Stephen
Hawking // |
|
| |
I'm relatively familiar with some of his more popular works.
You find me a passage where he says he, or anyone else for
that matter fully understands quantum physics well - and I'll
eat my hat. I'll eat your hat. The point is we have some
equations that seem to to work most of the time, and the
beginnings of some technologies that utilise some aspects,
but the research and experimentation is hardly over, is it? |
|
| |
//equations that seem to to work most of the time, and the beginnings of some technologies
that utilise some aspects// |
|
| |
That's a huge understatement. We have equations that, in all but a very few peculiar
situations, work to a higher degree of precision than any other equations used to describe
the physical world - the only limit on their precision is our ability to measure the physical
world for comparison. They have been tested to far greater precision than relativity; and
Newtonian mechanics is an order of magnitudes of orders of magnitudes less true. |
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And as for "the beginnings of some technologies that utilise some aspects" - pretty much
every electronic device made in the last 50 years not only depends on quantum mechanics
but includes materials and structures designed on the basis of quantum mechanics. Fission
reactors are designed from quantum mechanics upward. Many chemical processes were
designed on the basis of our understanding of quantum mechanics. Every modern
microscope and telescope is designed with an element of quantum mechanicry in mind. |
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Biology is the only major field that has been slow to go quantum. But it turns out that
photosynthesis, respiration, magnetoception and probably most other enzyme reactions
work by specifically exploiting quantum mechanical tricks. |
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