The Secrets of Quantum Physics BBC4

[Richard Tarleton]

They were having lots of fun with the special effects last night. Wasn't that Caerlaverock Castle (I missed the credits)? - handy for the barnacle geese.

The section on bird migration - I felt sorry for those robins - was fascinating. The phenomenon of reverse migration - whereby birds setting off from Central Asia on migration in late summer end up on Fair Isle instead of in SE Asia, having set off in diametrically the wrong direction - fascinates ornithologists (and gives twitchers their biggest thrills). This seemed to offer new lines of enquiry.

[Quarky]

I am often the first to complain about trendy presentation and ditto presenters, but I have to say that in the case of Quantum physics, ordinary mortals like me need a load of analogies and visual props even to glean a smattering of the subject. And to be fair, Jim did stress that no analogy is a perfect representation. Looking forward to tonight's prog.

:

In my view, a lot of the problems in understanding of Quantum Mechanics arise from "useful" analogies that persons without a science/ maths background may find helpful.

Quantum mechanics was devised to explain atomic and sub-atomic phenomena, but it does not migrate very easily to the day-to-day phenomena we see around us. How can a cat be alive and dead at the same time? Obviously it cannot. A cat is made up of billions of atoms and molecules and these interesting atomic "quirks" get smoothed out in this huge complex assembly of a living organism. I understand Schroedinger himself became disillusioned with his work, and turned to biology - so I guess he came to the conclusion that the cat was alive.

In my view the principle of superposition is all that is needed to get to grips with this matter. That is a principle that is widely used in science and technology. Going one or more steps further and applying this principle to day to day matters is fraught with pitfalls.

[Gordon]

...maybe the 'aether' needs to be reconsidered (even if it has been proved conclusively not to exist).

The Michelson-Morley experiments as well as Maxwell's equations for EM waves indicate that there is no need for an "ether" ie the medum through which that wave propagates. The M-M experiment was set up to see if the ether could be found to influence the speed of lght.

The only parameters in the Maxwell wave equation that do have an effect are the permittivity and permeability of the medium in which the light travels eg air, glass. These parameters reflect the effect of the Electric field and Magnetic field elements in the medium. In purely empty space, ie a complete vacuum, there is nothing material to influence the light, no atoms or molecules to interact with - except that space is not a passive player in the progress of a beam of light. Space itself has basic permittivity and permeability, IOW Electric and Magnetic properties, and if you take the values of these parameters and multiply them together, then take the square root of the product and invert it - you get the speed of light. IOW that speed is a feature of space itself. You could say that space is the "ether" and you could say that the wave travels along because space is aiding it IOW space is part of the phenomenon not a passive player.

Maxwell cracked the maths of EM waves by introducing a feature called Displacement Current which is a kind of distortion of space caused by the E and M fields and that current generates a magnetic field even though it is not a conventional moving charge. At the time some critics thought this was a trick but, somewhat like Planck's quantum, the idea works but intuitively it is not easy to understand and still flummoxes students of EM. It's a current that has no conductor - for a magnetic field to propagate there must be some kind of current flowing. Dirac did entertain the idea of an aether but never really made a convincing case.

Fascinatiing second programme drawing attention to some aspects of QM that would not normally come to mind. Not quite as mind boggling as the first but much food for thought. Tunnelling is an observed phenomenon. Many moons ago a semiconductor device called a Tunnel or Esaki Diode appeared alongside transistors. It had some interesting properties like having two discrete states of forward conduction and a region of negative resistance [!] which is its most useful feature.

[ardcarp]

What does anyone think about linking Quantum Mechanics to bird migration and other biological phenomena? I tried hard to twig, but I'm afraid I'm much thicker than a Planck.

It was said quite categorically that the earth's magnetic field is too weak to be perceived by a bird's physiology. I thought some non-quantum experiments somewhere had discovered features (concrete features, that is) in a bird's brain which could do just that.
[from Wiki]
Migratory birds may use two electromagnetic tools to find their destinations: one that is entirely innate and another that relies on experience. A young bird on its first migration flies in the correct direction according to the Earth's magnetic field, but does not know how far the journey will be. It does this through a radical pair mechanism whereby chemical reactions in special photo pigments sensitive to long wavelengths are affected by the field. Although this only works during daylight hours, it does not use the position of the sun in any way. At this stage the bird is in the position of a boy scout with a compass but no map, until it grows accustomed to the journey and can put its other capabilities to use. With experience it learns various landmarks and this "mapping" is done by magnetites in the trigeminal system, which tell the bird how strong the field is. Because birds migrate between northern and southern regions, the magnetic field strengths at different latitudes let it interpret the radical pair mechanism more accurately and let it know when it has reached its destination.[60] There is a neural connection between the eye and "Cluster N", the part of the forebrain that is active during migrational orientation, suggesting that birds may actually be able to see the magnetic field of the earth.[61][62]

I enjoyed last nights programme, but I'm not sure throwing darts at spinning wheels (and getting a predetermined result) moved us on much further than last week.

[Richard Tarleton]

Here's the link containing ardcarp's quote

Yes I thought it was well established that [some] birds used the earth's magnetic field - though as this article explains very well there are many different types of migration, across many species, and migration evolved separately many times.

[aeolium]

Is this a significant development in the search to unravel the mysteries of quantum mechanics, or is it merely an interesting phenomenon?:

http://www.bbc.co.uk/news/uk-scotland-glasgow-west-30944584

[french frank]

Is this a significant development in the search to unravel the mysteries of quantum mechanics ...

If the question is directed at me, I'm still bewildered as to how one single photon can be isolated from the mass of other photons, still more by how the photon can be slowed down. After 'light', will they turn to 'time'?

[Serial_Apologist]

They will - in time.

[Gordon]

Einstein postulated that things cannot travel faster so more interesting would have been the photons travelling FASTER than normal light? Remember that business some months ago about neutrinos travelling faster than light? Turned out to be a measurement inaccuracy.

So speaking of measurement, the Uncertainty Principle may have something to say about knowing where the photon is and how fast it's travelling? I assume they've done all that - their conclusion is arrived at by comparing transit times of photon pairs travelling down two different paths. And what about the energy of the photon? Did it lose some by slowing down in the masking device and thus shift its frequency ie its "colour"?

They say that the amount of slowing is small - "a few millionths of a metre" - and the "race track" is a metre long. Light travels a metre in about 3.3 nanoseconds in free space and so a few millionths of that is a few femtoseconds. A bit tricky to meaure that sort of time difference.

Maybe that's why CDs don't sound so good - there are some slow photons coming off the disc.