Watch later, hope it is good

There is a strange and mysterious world that surrounds us, a world largely hidden from our senses. The quest to explain the true nature of reality is one of the great scientific detective stories.

It starts with Jacobo Konisberg talking about the discovery of the Top quark at Fermilab. Frank Wilceck then featured to explain some particle physics theory at his country shack using bits of fruit. Anton Zeilinger showed us the double slit experiment and then Seth Lloyd showed us the worlds most powerful quantum computer, which has some problems. Lloyd has some interesting ideas about the universe being like a quantum computer.

Lenny Susskind then made an appearance to tell us about how he had discovered the holographic principle after passing an interesting hologram in the corridor. The holgraphic principle was illustated by projecting an image of Lenny onto himself. Max Tegmark then draws some of his favourite equations onto a window and tell us that reality is maths before he himself dissolved into equations.

The most interesting part of the program was a feature about an experiment to construct a holometer at Fermilab described by one of the project leaders Craig Hogan. The holometer is a laser inteferometer inspired by the noise produced at the gravitational wave detectors such as LIGO. It is hoped that if the holographic principle is correct this experiment will detect its effects.

Clues have been pieced together from deep within the atom, from the event horizon of black holes, and from the far reaches of the cosmos. It may be that that we are part of a cosmic hologram, projected from the edge of the universe. Or that we exist in an infinity of parallel worlds. Your reality may never look quite the same again.
[youtube]http://www.youtube.com/watch?v=o4Z8CqAiYI8[/youtube]

I watched this one a couple of weeks ago. On the whole it was kind of OK, not earth shattering - the outstanding bit for me was where Zeilinger demonstrated the double slit experiment. It was fantastic to watch the interference fringes being built up in real time photon by photon, with everything which that implies......

twistor59 wrote:I watched this one a couple of weeks ago. On the whole it was kind of OK, not earth shattering - the outstanding bit for me was where Zeilinger demonstrated the double slit experiment. It was fantastic to watch the interference fringes being built up in real time photon by photon, with everything which that implies......

There was a bit of sleight of hand in the presentation, though. Although we saw the interference pattern build, the "this is what happens if we detect which slit was chosen" part was demonstrated by a graphic. I don't know how you can detect a photon without absorbing it, or at least interfering with it, and I'd like to have seen how that's done.

Downloaded to my media server, but so far it's behind another 20 Horizon episodes in my "to watch" queue

Allan Miller wrote:

twistor59 wrote:I watched this one a couple of weeks ago. On the whole it was kind of OK, not earth shattering - the outstanding bit for me was where Zeilinger demonstrated the double slit experiment. It was fantastic to watch the interference fringes being built up in real time photon by photon, with everything which that implies......

There was a bit of sleight of hand in the presentation, though. Although we saw the interference pattern build, the "this is what happens if we detect which slit was chosen" part was demonstrated by a graphic. I don't know how you can detect a photon without absorbing it, or at least interfering with it, and I'd like to have seen how that's done.

I'm not sure you can (there are so-called quantum non-demolition experiments, but I don't know if they're feasible here), but isn't that the point:

the fact that a which-slit measurement absorbs or destroys the photon lends weight to the photon being particle-like, so this just increases the mystery - how can this particle like thing which I can detect at one slit know about both slits when I choose not to detect it at the slit ?

twistor59 wrote:

Allan Miller wrote:

twistor59 wrote:I watched this one a couple of weeks ago. On the whole it was kind of OK, not earth shattering - the outstanding bit for me was where Zeilinger demonstrated the double slit experiment. It was fantastic to watch the interference fringes being built up in real time photon by photon, with everything which that implies......

There was a bit of sleight of hand in the presentation, though. Although we saw the interference pattern build, the "this is what happens if we detect which slit was chosen" part was demonstrated by a graphic. I don't know how you can detect a photon without absorbing it, or at least interfering with it, and I'd like to have seen how that's done.

I'm not sure you can (there are so-called quantum non-demolition experiments, but I don't know if they're feasible here), but isn't that the point:

the fact that a which-slit measurement absorbs or destroys the photon lends weight to the photon being particle-like, so this just increases the mystery - how can this particle like thing which I can detect at one slit know about both slits when I choose not to detect it at the slit ?

Well, it is possible to concoct a thought-experiment version of Young's slits which would give the observed behaviour with waves alone. It hinges upon the properties of the detector. We 'know' a particle has arrived at the detector because we see a flash - but does that really mean that a particle has traversed the apparatus? If the detector consisted of atoms at various states of closeness to firing, and you bathed it in a uniform wave which continually pushed each atom closer to firing, then the next 'flash' would be detected from the atom nearest to its firing energy when we started looking. When it fires, it drops back to the ground state. The detector is simply a randomised collection of atoms at various intermediate energy states, and the interference pattern builds up because the wave front is non-uniform - the interference is real. The probabilistic and particulate behaviour is then an artefact of the limitations on detection - no information comes out of the detector except in quantised form, and our lack of knowledge about the distributions of states forces us to use a probabilistic representation of a deterministic process. Detection at the slit is similarly constrained - you can only detect transitional events, which have a 50/50 chance of being at one slit or the other on any one occasion. However much of the wave you interact with to make that measurement, leaves that much less of the wave to form the interference pattern.

I realise this is simply 'hidden variable' nonsense, which has been dismissed by experimental work on Bell's inequality. But I don't know how you would tell the difference between such an imaginary setup and one involving transit of real particles. It certainly avoids that 'many worlds' crap!

Allan Miller wrote:

twistor59 wrote:

Allan Miller wrote:

There was a bit of sleight of hand in the presentation, though. Although we saw the interference pattern build, the "this is what happens if we detect which slit was chosen" part was demonstrated by a graphic. I don't know how you can detect a photon without absorbing it, or at least interfering with it, and I'd like to have seen how that's done.

I'm not sure you can (there are so-called quantum non-demolition experiments, but I don't know if they're feasible here), but isn't that the point:

the fact that a which-slit measurement absorbs or destroys the photon lends weight to the photon being particle-like, so this just increases the mystery - how can this particle like thing which I can detect at one slit know about both slits when I choose not to detect it at the slit ?

Well, it is possible to concoct a thought-experiment version of Young's slits which would give the observed behaviour with waves alone. It hinges upon the properties of the detector. We 'know' a particle has arrived at the detector because we see a flash - but does that really mean that a particle has traversed the apparatus? If the detector consisted of atoms at various states of closeness to firing, and you bathed it in a uniform wave which continually pushed each atom closer to firing, then the next 'flash' would be detected from the atom nearest to its firing energy when we started looking. When it fires, it drops back to the ground state. The detector is simply a randomised collection of atoms at various intermediate energy states, and the interference pattern builds up because the wave front is non-uniform - the interference is real. The probabilistic and particulate behaviour is then an artefact of the limitations on detection - no information comes out of the detector except in quantised form, and our lack of knowledge about the distributions of states forces us to use a probabilistic representation of a deterministic process. Detection at the slit is similarly constrained - you can only detect transitional events, which have a 50/50 chance of being at one slit or the other on any one occasion. However much of the wave you interact with to make that measurement, leaves that much less of the wave to form the interference pattern.

I realise this is simply 'hidden variable' nonsense, which has been dismissed by experimental work on Bell's inequality. But I don't know how you would tell the difference between such an imaginary setup and one involving transit of real particles. It certainly avoids that 'many worlds' crap!

I think I see what you're saying - even if light was purely wavelike, your could envision a mechanism for the type of behaviour seen in the double slit expt, so just seeing discrete flashes doesn't immediately tag photons as "particles".

Rightly or wrongly I tend to think of photons as just discrete excitations of the radiation field, so they're not localised, and can feel out both slits. When you put a detector at one of the slits, though, it absorbs the whole photon, so this thing which was non local suddenly somehow gets absorbed in one place, so it's still very strange.

If you try to adopt a continuous wave triggering a transition sort of picture, don't you run into the problem that if you turn the frequency down, and wait long enough, the atom should eventually trigger because over time it will have absorbed enough energy. Whereas it doesn't (a la photoelectric effect).

twistor59 wrote:

I think I see what you're saying - even if light was purely wavelike, your could envision a mechanism for the type of behaviour seen in the double slit expt, so just seeing discrete flashes doesn't immediately tag photons as "particles".

Yep - although my scheme does fall down somewhat when you get the same behaviour out of things that are more definitely particle-like - electrons, atoms, even molecules. I could get round this (in my tiny mind) by envisioning quantum strangeness as associated in some way with motion. Setting an object in motion turns up its wave-like quality and turns down its 'particleness', to a degree that is related to its mass. When you have a lot of mass, there is very little 'waviness', which is why objects in the world we perceive directly don't display it. (I realise that the only things waving at the moment are my hands! This idea hits the roadblock when you look at accelerators, which are all about particles in motion).

twistor59 wrote:Rightly or wrongly I tend to think of photons as just discrete excitations of the radiation field, so they're not localised, and can feel out both slits. When you put a detector at one of the slits, though, it absorbs the whole photon, so this thing which was non local suddenly somehow gets absorbed in one place, so it's still very strange.

Yes, but I guess if the detector absorbs a whole photon, it can't tell which slit it went through (because it didn't get past the detector). It knows which slit it was going to go through, but in that instance it's no surprise that slit detection turns off the interference pattern - which is why I'd like to have seen that demonstrated, and not just explained.

twistor59 wrote:
If you try to adopt a continuous wave triggering a transition sort of picture, don't you run into the problem that if you turn the frequency down, and wait long enough, the atom should eventually trigger because over time it will have absorbed enough energy. Whereas it doesn't (a la photoelectric effect).

Hmmm. The photoelectric effect requires some frequency-dependent interaction between the incident wave (or particle) and the electron-emitting atoms. Below the threshold frequency, in my hidden-variable version, the light wave imparts no energy to the electrons, and they remain at their current level. When the frequency passes the threshold, there is transfer of energy (continuously) and hence emission of electrons (discontinuously) - a bit like the opera singer and the wine-glass: she has to hit the right note. In a particulate model of the PE, we have the idea that discrete photons somewhow zoom straight to the heart of a target atom and knock out an electron, but it has a similar problem with the threshold frequency - why do photons in the right range of frequency hit the target, and what happens to the rest?

[Edit: Just found this linked from the Wikipedia article on the photoelectric effect, predating me by about 40 years!]

Allan Miller wrote:
[Edit: Just found this linked from the Wikipedia article on the photoelectric effect, predating me by about 40 years!]

There is a famous paper by W E Lamb (the shifty guy) called "Anti Photon" - I just had a look for it but I can't find it online any more - I'm sure it's around somewhere. He gives his objections to the whole "photon" concept, and makes the statement that it would have been better if the term had never been invented. Photons are particularly awkward to interpret in a particle like way because of the lack of a photon wavefunction, in turn caused by the lack of a photon position operator.

twistor59 wrote:

Allan Miller wrote:
[Edit: Just found this linked from the Wikipedia article on the photoelectric effect, predating me by about 40 years!]

There is a famous paper by W E Lamb (the shifty guy) called "Anti Photon" - I just had a look for it but I can't find it online any more - I'm sure it's around somewhere. He gives his objections to the whole "photon" concept, and makes the statement that it would have been better if the term had never been invented. Photons are particularly awkward to interpret in a particle like way because of the lack of a photon wavefunction, in turn caused by the lack of a photon position operator.

Yes, Lamb's the lead on the paper I linked. It is intriguing that EM radiation's wave-particle duality is exhibited only via interaction with particles - the only means we have to detect light is matter, which feeds us information in quanta.

Still, the whole "0-c in zero seconds" behaviour of photons is very odd. And how such a massless particle - or a wave - is gravitationally lensed?

But as to the behaviour in Young's slits, I think that some of the more fanciful interpretations of 'reality' (many-worlds, Wheeler and the self-observed universe) could do with a dose of Occam's Razor!

Allan Miller wrote:

twistor59 wrote:

Allan Miller wrote:
[Edit: Just found this linked from the Wikipedia article on the photoelectric effect, predating me by about 40 years!]

There is a famous paper by W E Lamb (the shifty guy) called "Anti Photon" - I just had a look for it but I can't find it online any more - I'm sure it's around somewhere. He gives his objections to the whole "photon" concept, and makes the statement that it would have been better if the term had never been invented. Photons are particularly awkward to interpret in a particle like way because of the lack of a photon wavefunction, in turn caused by the lack of a photon position operator.

Yes, Lamb's the lead on the paper I linked. It is intriguing that EM radiation's wave-particle duality is exhibited only via interaction with particles - the only means we have to detect light is matter, which feeds us information in quanta.

But not entirely unexpected - we need matter (meaning fermions really) to build anything, including detectors.

Allan Miller wrote:
Still, the whole "0-c in zero seconds" behaviour of photons is very odd. And how such a massless particle - or a wave - is gravitationally lensed?

Yes, physics is odd. That's why I like it Gravitational lensing mm - well, don't think of it as a particle, think of it as a null geodesic (like the lawd meant us to !)

Allan Miller wrote:
But as to the behaviour in Young's slits, I think that some of the more fanciful interpretations of 'reality' (many-worlds, Wheeler and the self-observed universe) could do with a dose of Occam's Razor!

Yeah, not a big many worlder myself.

It was an interesting episode I guess, but I was it a wee bit disappointed by it. I'm a layman when it comes to theoretical physics and I was hoping for an education, but nothing in this episode was anything I hadn't heard before.

twistor59 wrote:

Allan Miller wrote: It is intriguing that EM radiation's wave-particle duality is exhibited only via interaction with particles - the only means we have to detect light is matter, which feeds us information in quanta.

But not entirely unexpected - we need matter (meaning fermions really) to build anything, including detectors.

And eyeballs. But it does leave open the possibility that light really is just a wave, and photons an artefact. But I soon wander out of my depth, and Nobel prize-winners are no bleeding help! We have Einstein who demonstrated the quantum nature of light while clinging to hidden variables, Lamb saying he was right about hidden variables but wrong about photons, and Bohr who favoured accepting all the implications of complementarity. Even Lamb seems to dimiss photons only some of the time.

twistor59 wrote:

Allan Miller wrote:
Still, the whole "0-c in zero seconds" behaviour of photons is very odd. And how such a massless particle - or a wave - is gravitationally lensed?

Yes, physics is odd. That's why I like it

Maths, though!

twistor59 wrote:
Gravitational lensing mm - well, don't think of it as a particle, think of it as a null geodesic (like the lawd meant us to !)

Classical gravity is odd. Start with two equal masses, move one molecule at a time from one to the other, and the force between the two masses changes with each movement - every molecule is influencing every other molecule in the two masses. So yes, maybe relativity helps - each molecule is doing its bit to help curve spacetime. Get a bunch of them together and they can really get some curving done.

I found Lamb's anti-photon paper. You can read it here.

twistor59 wrote:I found Lamb's anti-photon paper. You can read it here.

Very interesting, ta!

He's a card -

I suggested that a license be required for use of the word "photon", and offered to give such a license to properly qualified people. My records show that nobody working in Rochester, and very few other people elsewhere, ever took out a license to use the word "photon".

At times, he seems almost to be muttering to himself - but he does offer a cogent argument.

I'd be interested to know how similar arguments pan out with, say, electrons. There, the wave-particle duality seems a little more concrete (if that's not a contradiction in terms!): we are taught to view electrons as 'true' particles. The Young's-slit electron wave retains the properties of the atom-bound electron - you can deflect it with a magnet, for example. But perhaps it's wrong to think of it as a stream of bullets, each 'choosing' one slit or the other - as with photons, just because you detect an electron at the detector, does not mean that a whole particle must have come through the apparatus. The source becomes deficient in electrons in exact complement to a detector's enrichment in them, by conservation of mass/energy, but (perhaps) it is not the same, whole, electron that passes, but sufficient 'electron-wave' energy to cause an electron-detecting event. No stranger, to us denizens of the macro-world, but it evades the 'which-slit' paradox.

Allan Miller wrote:

At times, he seems almost to be muttering to himself - but he does offer a cogent argument.

LOL ! yes I can imagine him in a big armchair with a glass of port whining about the confusion....

Allan Miller wrote:
I'd be interested to know how similar arguments pan out with, say, electrons. There, the wave-particle duality seems a little more concrete (if that's not a contradiction in terms!): we are taught to view electrons as 'true' particles. The Young's-slit electron wave retains the properties of the atom-bound electron - you can deflect it with a magnet, for example. But perhaps it's wrong to think of it as a stream of bullets, each 'choosing' one slit or the other - as with photons, just because you detect an electron at the detector, does not mean that a whole particle must have come through the apparatus. The source becomes deficient in electrons in exact complement to a detector's enrichment in them, by conservation of mass/energy, but (perhaps) it is not the same, whole, electron that passes, but sufficient 'electron-wave' energy to cause an electron-detecting event. No stranger, to us denizens of the macro-world, but it evades the 'which-slit' paradox.

I believe it's possible to say (loosely) that electrons are more easily seen as particle-like than photons. I think there is a reasonably good position operator (the Newton-Wigner operator) defined for them, but this does not work for photons, although I'm a bit hazy about the details. It appears that people have, over the years, tried to define position operators and wavefunctions for photons, but there doesn't appear to be anything universally accepted.

I don't really have a good way to visualize a quantum field. I can do the maths, calculate the amplitudes (well trivially simple ones anyway!), but I can't say that I can really picture what's going on in quantum land. But then as someone said in another thread, why should apes have evolved the ability to picture such things ?

twistor59 wrote:I found Lamb's anti-photon paper. You can read it here.

Very interesting. He might be on to something (particularly looking back on my "photon" thread). However, he says:

Objects like electrons, neutrinos of finite rest mass, or helium atoms can, under suitable conditions, be considered to be particles, since their theories then have viable non-relativistic and non-quantum limits.

That seems to me to be an arbitrary criterion. Why is having "viable non-relativistic and non-quantum limits" a necessary and sufficient criterion for calling something a "particle"? Particularly since there is not much to choose between the interference pattern created by an electron and a (ahem) photon? Perhaps his discomfort arises from his imagining a "particle" as something that has mass?

twistor59 wrote:Rightly or wrongly I tend to think of photons as just discrete excitations of the radiation field, so they're not localised, and can feel out both slits. When you put a detector at one of the slits, though, it absorbs the whole photon, so this thing which was non local suddenly somehow gets absorbed in one place, so it's still very strange.

Interesting hypothesis, but how does this reconcile with the "delayed choice" experiments, where the "which-path" information is determined post-hoc? I mean, if the entire photon is actually absorbed at one of the slits, then how can this information change after the photon passes the slit?

iamthereforeithink wrote:

Objects like electrons, neutrinos of finite rest mass, or helium atoms can, under suitable conditions, be considered to be particles, since their theories then have viable non-relativistic and non-quantum limits.

That seems to me to be an arbitrary criterion. Why is having "viable non-relativistic and non-quantum limits" a necessary and sufficient criterion for calling something a "particle"? Particularly since there is not much to choose between the interference pattern created by an electron and a (ahem) photon? Perhaps his discomfort arises from his imagining a "particle" as something that has mass?

I think he just means that, for these entities, you can transform to a frame where they're moving at a reasonable speed (non relativistic), and they have a reasonably well -defined location (non quantum), so they "look" like classical particles to some approximation. Whereas for photons (licence application pending), of course you can't do this because (1) you can't bring them to rest and (2) there's no position operator.

iamthereforeithink wrote:

twistor59 wrote:Rightly or wrongly I tend to think of photons as just discrete excitations of the radiation field, so they're not localised, and can feel out both slits. When you put a detector at one of the slits, though, it absorbs the whole photon, so this thing which was non local suddenly somehow gets absorbed in one place, so it's still very strange.

Interesting hypothesis, but how does this reconcile with the "delayed choice" experiments, where the "which-path" information is determined post-hoc? I mean, if the entire photon is actually absorbed at one of the slits, then how can this information change after the photon passes the slit?

When I looked at it last time, I think I was of the opinion that such a description was interpreting the delayed choice experiments a bit too far. The delayed choice experiments don't directly deal with the two slit scenario and the results can be explained without resorting to causality violations. I'd have to look at the papers again to be sure - my memory is hopeless these days !

twistor59 wrote:

iamthereforeithink wrote:

Objects like electrons, neutrinos of finite rest mass, or helium atoms can, under suitable conditions, be considered to be particles, since their theories then have viable non-relativistic and non-quantum limits.

That seems to me to be an arbitrary criterion. Why is having "viable non-relativistic and non-quantum limits" a necessary and sufficient criterion for calling something a "particle"? Particularly since there is not much to choose between the interference pattern created by an electron and a (ahem) photon? Perhaps his discomfort arises from his imagining a "particle" as something that has mass?

I think he just means that, for these entities, you can transform to a frame where they're moving at a reasonable speed (non relativistic), and they have a reasonably well -defined location (non quantum), so they "look" like classical particles to some approximation. Whereas for photons (licence application pending), of course you can't do this because (1) you can't bring them to rest and (2) there's no position operator.

iamthereforeithink wrote:

twistor59 wrote:Rightly or wrongly I tend to think of photons as just discrete excitations of the radiation field, so they're not localised, and can feel out both slits. When you put a detector at one of the slits, though, it absorbs the whole photon, so this thing which was non local suddenly somehow gets absorbed in one place, so it's still very strange.

Interesting hypothesis, but how does this reconcile with the "delayed choice" experiments, where the "which-path" information is determined post-hoc? I mean, if the entire photon is actually absorbed at one of the slits, then how can this information change after the photon passes the slit?

An interesting thought experiment occurs to me. If you take an electron version of the Young's slit apparatus, it exhibits all the properties of the photon version (except that you can interfere with the beam in some way, eg by a magnetic field). On the other hand, the particulate nature of electrons is well shown by tracks in cloud chambers or the 3D wire frames that are used in more modern devices that detect the debris from collisions. So what would happen if we combined the two?

Start with a detector screen that shows interference fringes. Then punch holes in it, such that 50% of the screen is unchanged, and 50% is holes. We would expect the interference pattern to still be displayed by the 50% of the screen that is not occupied by holes. Now, behind the holes, we place a wire frame track detector. What we would expect from that are tracks that must be traceable back through a specific slit, back to the source. Instead of interference fringes, this detector must show the 'bullet' version - we are detecting "which-way" information, and there can be no interference.

Now, it is clear that when we switch the apparatus on we would expect both patterns simultaneously. The "wave detector" detects waves (and fringes), and the "particle detector" detects particles (no fringes). To me, that suggests that the detectors are detecting different things. If you shift this mesh by the width of a hole, without changing the source-slit-detector beam in any way, electron paths that were hitting holes (and going into the particle-detector) are (seemingly) now hitting surface (and contributing to an interference fringe) and vice versa. Maybe there are simply two kinds of state for electrons? In particulate form, they require a line of sight (and are invisible to electron-wave-detectors); in wave-form, they do not travel as indivisible entities, but really are waves (and are invisible to particle-detectors).

You mean like this:

What we would expect from that are tracks that must be traceable back through a specific slit

Is that really true though ? You wouldn't be able to do that with 100% certainty. For example the bottom cloud chamber track could have come from an electron that came through the lower slit, line of sight to the source, or one that scattered off the top surface of the upper slit. To know which slit an electron came through, wouldn't I need a detector at the slit itself ?