Physics – David Bradley

Creating large plasma bubble between electrodes

Occasionally, I like to mention some of the search terms that bring new visitors to the Sciencebase website. One of those phrases that intrigued me somewhat is this:

creating large plasma bubble between electrodes

Now, I can half imagine it’s someone looking for information about a physical phenomenon, perhaps for a school assignment or maybe a research project. So, what were they looking for?

Plasma is often referred to as the fourth state of matter – after solid, liquid, and gas. It is most similar to a gas, but rather than being made up of neutral atoms or molecules, it is fully ionised. This means that every atom in the gas has been stripped of its electrons. A plasma therefore comprises ions, charged particles, and free electrons (negatively charged particles) rather than atoms or molecules.

We think of it as being the fourth state of matter, but there is an argument for calling it the first state of matter given that plasma is the most common state of matter across the universe, such as the core of stars, nebulae in space, the aurora borealis. That said, we do not tend to encounter it in everyday lives except in specific small-scale circumstance because plasmas form only at extremely high temperatures or in very strong electric fields.

One of those small-scale situations involves the formation of a plasma between electrodes. More specifically, a plasma bubble can form in the electric field between two electrodes as the field strips away electrons away from the atoms of a gas. The size of the plasma bubble depends on a number of factors, including the voltage applied, the distance between the electrodes, and the type of gas that is being ionised by the electric field. Scientists experimenting with plasma bubbles can adjust these parameters, to create plasma bubbles of different sizes and shapes.

60-year laser theory overturned

Quantum theorists have overturned a 60-year old theory about how lasers work that could fundamentally remove the Heisenberg Limit on coherence in these important and ubiquitous devices.

The coherence of a laser beam is essentially the number of photons that are emitted consecutively into the beam with the same phase. It determines how well a laser performs in various precision tasks, such as controlling all the components of a quantum computer.

However, researchers from Griffith University and Macquarie University in Australia writing in the journal Nature Physics have now shown that new quantum technologies open the possibility of making this coherence vastly larger than was thought possible.

‘The conventional wisdom dates back to a famous 1958 paper by American physicists Arthur Schawlow and Charles Townes,’ explains Howard Wiseman. ‘They showed theoretically that the coherence of the beam cannot be greater than the square of the number of photons stored in the laser. But they made assumptions about how energy is added to the laser and how it is released to form the beam,’ adds Wiseman. ‘The assumptions made sense at the time, and still apply to most lasers today, but they are not required by quantum mechanics.’

The new work shows that the true limit imposed by quantum mechanics is that the coherence cannot be greater than the fourth power of the number of photons stored in the laser. The researchers have now demonstrated through numerical simulation how this might be achieved, which could lead us to a “super laser”.

‘Our work raises many interesting questions,’ said Wiseman, ‘such as whether it could allow more energy-efficient lasers. That would also be a great benefit, so we hope to able to investigate that in the future.’

Why do accelerating electrons emit photons?

TL:DR – Discussion explain why an accelerating electron emits photons.

My friend Alice Sheppard, known on Twitter and elsewhere as @PenguinGalaxy, asked her physics pals to explain why it is that an electron emits photons when it is accelerating/change direction?

There were several replies that suggested this has been thought about a lot but nobody could come up with a simple, solid explanation. There was a bit of hand-waving and a lot of obscure words that I only vaguely rememberd the meanings of. As I understand it, even the great Dick Feynman got it wrong in one of his famous lectures.

Now, I am a lowly chemist, with aspirations, as you all know, to being some kind of award-winning photographer by day and a rockstar by night, and the farthest I got with physics was to successfully pass the first year undergraduate university course after two fails. But, I did try hard, did a lot of background reading and as you also know I have now spent more than thirty years writing about all kinds of science, including a lot of physics and quantum mechanics. So, I had a go at an explanation. Please feel free to pick holes in it and explain its fatal flaws as an answer to Alice’s question.

Electrons have an electric field. If they're moving they have a magnetic field too. If you give an electron a flick you add energy, like flicking a rope, this produces a wave in the electric and magnetic fields together, a wave in an electromagnetic field is a photon. The electron cannot hold on to this photon, so it is emitted.

I asked a Prof of Physics if my lay explanation passed muster and thankfully he said yes it does ;-)

Weave your antiviral facemask from cotton and silk

If you’re wondering what materials to use to stitch together your antiviral mask, it seems it could be that you need a couple of different fabrics for it to work best – woven cotton and a piece of silk or chiffon…

Tightly woven cotton acts as a physical barrier to viral particles and droplets carrying the virus. Silk and chiffon can both build up quite a static charge and this will help trap viral particles electrostatically.

Screengrab from OnlineKyne's facemask howto video linked below

Together the materials will reduce the risk of the wearer shedding virus from nose or mouth into the environment and on to other people or surfaces that others might touch. Conversely, the mask will, to some extent reduce the risk of you inhaling viral particles from the air. The researchers say that substituting chiffon or silk for flannel or using a cotton quilt with cotton-polyester backing could be just as effective. But, Sciencebase would add that it’s not so strong a fashion statement

There is also the added benefit of wearing a facemask in that it will reduce how often you touch your nose and mouth with your filthy, disease-ridden hands. Now go and wash them thoroughly with plenty of soap and warm water for at least 20 seconds!

https://www.acs.org/content/acs/en/pressroom/newsreleases/2020/april/the-best-material-for-homemade-face-masks-may-be-a-combination-of-two-fabrics.html

The research paper is in ACS Nano here. OnlineKyne howto video here

Last UK Blood Moon for a decade

There will be a total eclipse of the Moon visible from the UK on 21st January. This will be the last total lunar eclipse here until 2029. A total lunar eclipse occurs when the Earth passes exactly between the Sun and the Moon. The Sun is behind the Earth, and the Moon moves into the Earth’s shadow.

Sometimes an eclipsed Moon is a deep-red colour, other times it remains quite bright. The exact colour depends on how light from the sun is being scattered, Rayleigh scattering, by molecules and particles in the Earth’s atmosphere, blue light is scattered away more than red.

You only get to see a total solar eclipse if you are in the narrow path of the Moon’s shadow, but a lunar eclipse is visible wherever the Moon is above the horizon at the time, so each one can be seen from a large area of the Earth. For that reason, they are much more common from any given location. Lunar eclipses always happen at a full Moon as this is when it moves behind the Earth and into line with the Earth and Sun. A full Moon happens every month, but most of the time no eclipse takes place.

The lunar eclipse on 21st January begins with penumbra at 02h35 GMT, umbra at 03h33 GMT and totality from 04h40 GMT. Mid-eclipse is at 05h12 GMT, which is when the whole Moon will appear red. The red will fade by 05h43 GMT. Coming out of the other size penumbra ends at 07h49 GMT.

the lunar eclipse will also be visible from north-western France, north-western Spain, Portugal, a small part of West Africa, almost the whole of North and South America, the eastern Pacific, and the north-eastern tip of Russia.

Lunar eclipses are very easy to witness as no special equipment or safety precautions are required. To watch the lunar eclipse on 21st January all you have to do is get up early, wrap up warm and step outside, unless of course you’re lucky enough to have a bedroom window facing the moon at that time. If you can see the full Moon you will be able to observe the eclipse as it happens.

I double-checked that this is the last proper total lunar eclipse for the UK until 2020. Astrobuddy confirmed that is indeed the case: “That’s the next that is fully observable from the UK. There are others that we see parts of before then, e.g. May 16, 2022, when the Moon sets during totality,” he told me.

Airy discs and keeping it sharp

In optics (and thence photography, microscopy, and telescopy), the Airy disc is the optimally focused spot of light that a perfect lens with a circular aperture can make. It is the diffraction limit. It’s named after George Biddell Airy who wrote a detailed description although astronomer John Herschel had described the phenomenon when observing a bright star through his telescope.

The Airy disk is a bright spot of light surrounded by concentric diffraction rings, all together they are referred to as an Airy pattern. The wavelength of the light and aperture size of the lens is critical to the size of the resulting pattern.

The old pinhole camera is at the diffraction limit, almost a point-like circular aperture.Â Due to this diffraction effect, the smallest point to which light can be focused with lens (or mirror) is theÂ size of the Airy disk. Of course, lenses are not perfect and apertures are rarely circular, in a lens for an SLR and other types of camera they are usually made up of an array of six overlapping fins, (sometimes more, sometimes less. More means more circular and so better, and TV and cinematic film cameras often have 8 fins and so produce octagonal, rather than hexagonal bokeh. 8-finned cameras seem to have a much-improved image quality over more those with a more conventional 6-finned aperture.

Anyway, if Airy discs, light’s wavelength, and the lens aperture conspire to produce a diffraction limit, then it is pixel size in your camera’s sensor that “sees” this limit. It’s possible to calculate the diffraction limit. So, if your lens can be set to an aperture of f/2 (that’s the biggest aperture), the diffraction limit for green light with that aperture is aboutÂ 2.7 micrometres. Dave Haynie discusses this in more detail here.

Now, f/2 is a low f-stop for most lenses. The Sigma 150-600mm with which I have photographed birds recently using my Canon 6D, can be set to f/5 when it’s at 150mm focal length, but only as low as f/6.3 at 600mm. That’s the biggest apertures it can manage. A larger aperture means more light in and exposure balanced against shutter speed and ISO number. My 90mm Tamron macro lens with which I have been photographing moths, has its biggest aperture at f/2.3.

Larger aperture means a smaller depth of field. The parts of the image that are closer or further away than the point at which youÂ have focused the camera will be out of focus with a larger aperture (smaller f-stop). If you want a larger depth of field, then you need a smaller aperture, which means less light and critically from the perspective of sharpness, you begin to approach the diffraction limit. This occurs because of the Airy disc effect and how that coincides with the pixels on your camera’s sensor. To get a sharp image,Â you need the Airy Disc to be smaller than a pixel, realistically smaller than 2-3 pixels, explains Haynie. If you make the aperture bigger the Airy disc becomes bigger and so each perfect point of light will inevitably traverse a larger number of pixels, which is not what you want. Rather, you want each pinpoint of light from the object you are photographing to impinge on a single pixel.

This is where balance and compromise must come into play.Â A smaller aperture gives a larger depth of field, which is more important when doing close-up macro photography of small objects such as moths. So, you push theÂ f-stop to a higher number to get a greater depth of field. Now, with small aperture, you are approaching that Airy problem. If you have a point-and-shoot camera, the sensor is only a few millimetres across, a two-thirds sensor (common on consumer-level dSLRs) is a lot bigger, although still smaller than the full-frame (35mm) sensor of professional dSLRs, and of course even that is a whole lot smaller than a medium-format digital back. (All of this feeds into why the highest quality photography, even in terms of film cameras) is often most associated with medium and large format.

Okay. So smaller aperture means a larger depth of field, but that means a bigger Airy disc, which means you need larger pixels (and the same number) to overcome the diffraction limit and get a sharper, better quality image.

Haynie has a Canon 60D, which he says has 4.3 micrometre pixels size. The Airy problem doesn’t arise at f/2.0, which such a camera. However, the pixels in older Smart Phone cameras are a lot smaller, perhaps 1 micrometre on a tiny sensor chip in order to cram as many as the market demands on such a small area. This means sharpness can be very limiting in older phones and many modern ones too. HTC and Apple have actually increased the size of the pixels on their sensors rather than increasing the megapixel count to overcome the Airy problem to some effect. Megapixel count always was marketing BS, anyway, because of all of the above and many other factors. Cheaper cameras (point and shot and/or phone) don’t have an aperture control or if they do it’s f/2 to a minimum aperture size of about f/4. You won’t be able to push it to f/8 or anywhere useful for depth of field. The size of the sensor and the pixels crammed in always mean passing the Airy border.

For that Canon 60D, stopping down to f/8 approaches the boundary as the Airy disc is about 10.7 micrometres at this aperture, stop to f/11 and it is 14.3 micrometres which is definitely larger than the width of 3 pixels on this camera. Contrast this with my Canon 6D, which has a full-frame (35mm) sensor. The pixels are a little over 6.5 micrometres and so I will be safe from Airy up to f/11. Take it to f/16 and it crosses the boundary. I reckon f/8 or f/9.5 would be the sweet spot for my moth macro setup. Assuming there’s sufficient light to keep the ISO low to avoid noise and the shutter speed short enough to avoid camera shake. I could use a tripod and remote shutter release with mirror lockup but that’s quite cumbersome when chasing small moving targets like moths. I do have the option of using Tamron’s onboard image stabilisation, which is worth two stops of shutter speed, so I can keep shutter long enough to let sufficient light in to avoid high ISO without introducing too much camera shake.

Your mileage will vary depending on what camera you are using. The Cambridge in Colour site has a more detailed explanation of Airy discs and a table to help you work out the optimal f-stop for your camera model. When you fill the form in it also simulates an Airy Pattern on your sensor, so you can see whether you’re at the limit with your camera for a given f-stop. f/8 is often considered a sweet spot, balancing reasonably large depth of field with minimal aberration due to Airy disc effect. The diagram below generated on the Cambridge site shows why this is the case.

LHC short primer

A lot of people were recently reaching Sciencebase using the search phrase “LHC short primer”. I assume they’re after information about the Large Hadron Collider. So here’s a quick executive summary:

The Large Hadron Collider (LHC) is a particle collider, in some ways the most complex experimental facility ever built, and the largest single machine in the world. It was constructed for the European Organization for Nuclear Research (CERN) between 1998 and 2008 with 10,000 scientists and engineers involved from more than 100 countries. Hundreds of universities and laboratories were involved in its design, construction and implementation. Its construction budget was 7.5 billion Euros.

The LHC sits in a roughly circular tunnel with a circumference of 27 kilometres some 175 metres beneath land on the French-Swiss border near Geneva. Scientists began using it for scientific research in March 2010, running countless experiments until early 2013. The energy levels of particulars in the collider reach 3.5 to 4 teraelectronvolts (TeV) per beam (7 to 8 TeV total), which bust the previous collider record by approximately four times. It was then temporarily shut down for maintenance and upgrades and fired up again in early 2015 at which point it could operate at 6.5 TeV per beam (13 TeV total).

Basically, the collider fires beams of protons in opposite directions around the circuit and watches what happens when they collide. At these high energies, the collisions release other particles and energy that give scientists clues as to the fundamental nature of matter. One of the biggest results was the detection of the Higgs boson, which was nicknamed the “God Particle” by some observers. This particle generates a field that fills the universe and gives particles their mass.

Why has the sun gone red today?

Odd weather we’re having right now. It’s 23 Celsius outside, albeit with a stiff windchill. The wind is apparently down to the ex-hurricane we know as Ophelia. The heat…definitely not what you’d expect for mid-October, more like late July, but probably a jet stream phenomenon combined with that tropical storm pushing warm air towards us (here in the South of England, anyway; your mileage may vary).

But, it’s 3 pm and the sun is looking distinctly like it’s a sunset but too high in the sky. The fact that the cars are all covered in desiccated, dusty raindrops from last night suggests we’ve had a load of dust blow northwards from the Sahara Desert. A quick Google confirms this. That said, there are forest fires in Spain and/or Portugal that would also generate plenty of dust.

Ophelia has stirred up a storm and carried megatonnes of dust into the atmosphere of the British Isles and elsewhere. As we know from high school science lessons (you were listening, weren’t you?) tiny particles of dust in the atmosphere scatter light of different wavelength to different degrees. So, the blue end of the spectrum of the white light from the sun is scattered away from your line of vision while the lower energy red is scattered so little it passes straight to your viewpoint.

Anyway, the fat ol’ sun, the hurricane sun, above was snapped at 3 pm on my Canon dSLR with a 600mm lens #nofilter. (Sunset isn’t for another 3 hours).

All that desert/fire dust might also explain the sore eyes Mrs Sciencebase and myself are both suffering today.

UPDATE: 17:25, half an hour before sunset, this is how it looks:

Double slit experiment rides a wave

In the infamous double-slit experiment of quantum mechanics, it appears thatÂ particles whether massless particles of light, photons, or charged electrons, fired at a pair of slits will pass through and form interference pattern on the other side, as if they are behaving likeÂ a wave, even when only one particle is passing through a slit at a time. It’s as if, so the Copenhagen interpretation of QM goes, the particle is in both places at once, passing through both slits and a “decision” only being made at the point the observer looks/measures the interference pattern.

It really always seemed silly that the observation itself could alter the “choice” made by a subatomic particle passing through one of a pair of slits. Click the diagram for details of the experiment.

However, an alternative explanation might be that it is not the particles but aÂ pilot wave, an energy field exuded by the particles is what creates the interference pattern, carrying particle after particle through in such a way that the pattern emerges naturally without an observer effect. This is the Bohmian interpretation of QM. It sounds a lot more plausible to thisÂ chemist’s brain and much more common sensible than the Copenhagen ideaÂ mocked by Einstein. And, now, there is experimental evidence as reported in Quanta magazine.

As Dan Falk puts it in his excellent article:

“The electrons act like actual particles, their velocities at any moment fully determined by the pilot wave, which in turn depends on the wave function. In this view, each electron is like a surfer: It occupies a particular place at every specific moment in time, yet its motion is dictated by the motion of a spread-out wave. Although each electron takes a fully determined path through just one slit, the pilot wave passes through both slits. The end result exactly matches the pattern one sees in standard quantum mechanics.”

Nobel Prize in Physics 2012

UPDATE: Serge Haroche, CollÃ¨ge de France and Ecole Normale SupÃ©rieure, Paris, France and David J. Wineland, National Institute of Standards and Technology (NIST) and University of Colorado Boulder, CO, USA “for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems”.

The Physics Nobel Prize was just announced this morning, Tuesday 9th October (at about 10:45 UK time). The announcement will be made by Staffan Normark, Permanent Secretary of the Royal Swedish Academy of Sciences. Is it too soon for Peter Higgs et al, re this year’s results from the Large Hadron Collider at CERN? Of course, the Nobel committee will have had nominations much earlier in the year than results emerged from LHC, but one never knows…they could easily accommodate it if they wanted to. Not sure whether Higgs would be the first Geordie Nobel Laureate…fairly sure that Marie Curie and Erwin Schrodinger were not.

Press release here.