Amateur Astronomy


#1301

Is it still the case* that gravity force is instantaneous and infinite? So in other words, if you move something on one end of the universe, it has an effect on something on the other end of the universe, the effect being faster than the speed of light?

*maybe I dreamt it


#1302

No. The “speed of gravity” is the same universal speed limit that all other communication must obey – the speed of light (which perhaps should be called the speed of causality).

As well as not being instantaneous, it may not be infinite either. In one practical sense it is not infinite – just as we can’t see the light from stars which are beyond our cosmic event horizon, the limited speed of gravity means that we can’t feel their gravitational influence either.

There may be another sense in which it is not infinite. The quantum nature of light means that extremely faint light sources can be measured in individual photon arrivals. We don’t have a quantum theory of gravity yet, but many scientists are convinced there must be one. If we find one, then it will mean that gravity is mediated by messenger particles called gravitons, the quanta of the gravitational force. An extremely faint source then would be such that the gap between graviton arrivals could be arbitrarily long.


#1303

Thank you,sir


#1304

The best and simplest paper I’ve come across deriving a universal invariant speed limit is one referenced in the description of that PBS video:

Folks will remember that Einstein’s theory of Special Relativity is based only on two postulates – the invariance of the laws of physics across inertial frames of reference (a.k.a. Galilean Relativity), and the constancy of the speed of light in all reference frames. It turns out that the second postulate is unnecessary as we only require the existence of some universal speed limit, and this can be derived from the first postulate without any other assumptions. You need to be familiar with the concept of inertial frames, and with as much of the basics of group theory and matrix operations as I reckon could be explained to the average intelligent school leaver in about half an hour (i.e. it’s pitched at about my level :blush: ).

Galilean Relativity is such a seemingly self-evident concept that it’s hard to even think about it as “a thing”. Imagine I told you that if I measure the dimensions of a sheet of paper on my desk with a ruler, that the measurement would be different when you measure the same piece of paper with the same ruler on your desk. Or that the dimensions would change if you rotated the piece of paper. Or that they would be different at different times. You’d think I was nuts.

And you’d be right … but only because that’s not the way our universe works. There’s nothing compelling it to be that way, although you will undoubtedly have difficulty conceiving of how it could be different. The three scenarios I mentioned are referred to as invariance under translation in space, invariance under rotation, and invariance under translation in time. Amazingly, these three invariants have a deep connection to three fundamental conservation laws in physics – conservation of linear momentum, conservation of angular momentum, and conservation of energy. That connection is derived in a very elegant piece of mathematics called Noether’s Theorem. In its very basic form (the only one I’m able to understand) it just needs an understanding of Lagrangian mechanics (a.k.a. analytical mechanics), which uses calculus to reformulate Newtonian mechanics in terms of trajectories.


#1305

So if you had a rigid pole one light year long and you poked a ship one light year away with it, it would still take them a year to hear you tapping your stick on their hull?


#1306

Yes, that’s exactly right. Well … except for the implication that there’s no such thing as a rigid pole. What makes things solid to begin with? It’s the electromagnetic interaction between electrons in adjacent atoms. What speed is that interaction communicated at? The speed of light, obviously. Moving one atom will not cause the next atom over to move instantaneously. The cumulative effect on your light year long pole is just the sum of all those delays.

As an aside, that’s one of the arguments for electrons (and some other fundamental particles) being dimensionless points. Suppose a single electron feels a Coulomb force from another charged particle. By exactly the same logic as for the rigid pole, the near side of the electron has to start moving before the far side. That means the electron can be deformed. And if it can be deformed enough it can be cut in half. Which means it’s divisible and not fundamental after all. As far as we know electrons are fundamental, and if they’re not then their constituents at some level must be. Somewhere we have to get to particles with no spatial extension.


#1307

Why, what is stopping an electron from being its own universe?


#1308

Nowt. There’s nothing stopping it from being a Mint Aero either … it just seems unlikely and there’s no science to support it.


#1309

I don’t look at physics apart from the odd look through New Scientist and I could be wrong but I haven’t heard of any science stating that there has to be an indivisible unit of matter. It seems that new particles are being discovered quite often and I won’t be shocked if and when someone announces that particles like electrons are in fact composed of smaller particles.


#1310

You may be a little out of date there. The “particle zoo” was the term used until the 1960s to describe the bewildering array of new particle discoveries. Gell-Mann and Zweig independently came up with quark theory in 1964 and Gell-Mann got the Nobel Prize for it in 1969 after accelerator experiments confirmed the existence of quarks. Suddenly all the new particles were explicable in terms of different quark configurations.

Quarks have two types of “charge”, called colour and flavour, that govern their interactions with the strong and weak nuclear forces. They’re more complicated than electric charge, coming in three and six different varieties respectively. That’s basically why there are more configurations of them, leading to the particle zoo. (Quarks also have electric charge, in fact they’re the only particle which feels all four of the fundamental forces).

Once the quark theory arrived, the list of known (and assumed fundamental) particles started to settle down. You had six different predicted flavours of quarks (in three pairs), although it took a while to actually discover them all, and those may have been some of the new discoveries you’ve heard of. The last of them was discovered at Fermilab in 1995. You had the leptons (including electrons), which also came in three different flavours along with three associated neutrinos. (The electron was discovered by Thompson in 1897, the muon by Anderson in 1936, and the tauon by Perl in 1976).

Each of the types of particles has an associated antiparticle. Antiparticles were first posited on purely quantum theoretical grounds by Dirac and Weyl in the late 1920s, and the first of them – the positron – was discovered in 1932 by Anderson who got a Nobel for it in 1936, the same year he also discovered the muon.

Then there are the bosons which are the carriers or intermediaries of the known fundamental forces. There are photons for electromagnetism and gluons for the strong nuclear force. The picture was completed for the weak nuclear force by the discovery of electroweak theory for which Weinberg, Salaam and Glashow won the Nobel prize in 1979. This predicted three new type of bosons for the weak force. There is no known boson for the gravitational force because we don’t have a quantum theory for it, but the graviton has been named in advance in expectation of its arrival. Finally, there is one more boson – the Higgs – associated with mass.

And that’s it – all our known particles are organised into the so-called Standard Model, which reflects the beautiful symmetries that govern their arrangements and force interactions. (Actually, all “ordinary” matter of everyday experience is made out of a single flavour of quarks and leptons, the first generation column below).

So why do we think these particles and bosons are fundamental? There are a number of reasons. Firstly, we have theories dealing with a small set of quantised properties that seem to explain pretty much everything there is to explain about the fundamental particles.

Secondly, they show no sign of internal structure in high energy particle experiments. In the early 1900s, Geiger, Marsden and Rutherford demonstrated that atoms had internal structure by shooting alpha particles at a gold leaf target. The distribution of deflection angles clearly proved the existence of a tiny nucleus in a comparatively vast atom. Similar scattering experiments at much higher energies showed the existence of quarks inside nucleons in the late 1960s at SLAC. To date, the highest energy experiments show no sign of substructure in quarks or electrons. We went from from 5 MeV (million electron Volts) in the Rutherford-Geiger-Marsden experiments, to 20 GeV (giga- or billion electron Volts) in the electron-nucleon SLAC experiments. Today’s proton beam collisions at the LHC are at energies of 13 TeV (trillion eV) – a thousand times the original quark scattering energies. No additional structure has been detected, nor is expected on theoretical grounds.

Thirdly, the fundamental particles have invariant physical properties – definite charge to mass ratios, spin values, and so on. And fourthly they are stable against decay. Whereas composite particles can transmute into other particles under various force interactions, that’s not the case for the fundamental particles. Actually, it’s a bit more complicated than that – there are conservation rules which preserve the number of quarks and leptons in interactions, though certain flavour changes are allowed.

Does that mean the Standard Model is definitely the last word? Not quite. The Higgs was the final addition to the model in 2012, but different versions of it at higher energies are within the bounds of current theory. It’s conceivable – but considered very unlikely – that there is a fourth generation of quarks and leptons to be discovered at vastly higher energies. There are also various theories like supersymmetry that propose supersymmetric partners to each of the standard model particles. None have been discovered, though the proposed names are cute – squarks, selectrons, sneutrinos :smile: . Extensions to the Standard Model are required to explain how physics operated in the high energy environment close to the Big Bang, but all avenues have thus far drawn a blank. But any anticipated new particles will themselves be considered as fundamental as the existing ones.


#1311

Yes I was aware of quarks etc but thanks for taking the time out to write an informative summary of the whole particle landscape. But as I stated previously, as there seems to be a move forward in man’s thinking every generation or so, a lot of what is understood about particles is observed in experiments so perhaps its true there still isn’t an absolute theoretical certainty that leptons and quarks aren’t themselves sub-divisible. Perhaps a new Einstein will come along and begin a new physics that will unite all forces including gravity and propose a whole new bunch of elementary particles based around the elusive graviton, perhaps these are even hiding in plain sight inside current “fundamental” particles. If something is possible, regardless how far-fetched, you just cant rule it out.


#1312

There’s certainly no absolute theoretical certainty, and never will be. Absolute certainty is simply not a “thing” in science. So yes, what are thought to be fundamental particles might turn out not to be.

I’m inclined to think, though, that we’ll end up realising we were asking the wrong question. Big revolutions in science tend to completely change our picture of the world. Just as Relativity did not simply replace Newtonian mechanics, it swept away the whole notion of objects with positions and momenta moving in a fixed space with a global time coordinate. The universe is not a giant 3D room with a universal ticking clock to keep track of time. There’s simply no such thing as independent space and time coordinates. Newton’s universe literally does not exist.

And so it may well be with particles. In fact, I think it’s already happened. The progression from de Broglie’s matter waves to Schrodinger’s wave mechanics to Dirac’s prediction of the positron in just five years was a kind of golden age for theorists. Dirac did not have some kind of crazy intuition about possible companions for the electron. No, it was the mathematics which told him there was an alternative negative energy solution to the wave describing the electron. When this turned out to actually exist, it seemed the mathematical waves were as real as the things they described. In the Quantum Field Theory that was later developed, particles are not the basic realities. Everything is a vibration in a set of fields that pervade spacetime.

Are there more fundamental particles? The answer seems to be that there’s no such thing as particles. The only reason we still talk about them is because QFT is so abstruse. We still teach Newtonian mechanics in elementary science classes because it’s a reasonable approximation to reality in low energy situations. We teach Bohr’s solar system model of the atom in chemistry because it’s a simple concept to grasp compared to the quantum mechanics and spherical harmonics that are actually needed. And we pretend that particles exist instead of excitations in universal fields because … well, because the universe turned out to be way weirder than most people are able to imagine.


#1313

Watch the unveiling of the beautiful camera lenses of the Large Synoptic Survey Telescope. The LSST camera is a bit like your DSLR except it’s absurdly superlative in every sense – it’s the size of a car, weighs 3 tonnes, has nearly 200 sensors totalling 3.2 gigapixels, and will collect more data in its first year of operation than every other telescope in the history of the world combined.


#1314

The LHC discovered the Higgs boson but was it was thought that it might discover a lot more besides – particles more fundamental than quarks and electrons, dark matter or energy, supersymmetry, extra dimensions of spacetime, microscopic black holes, and other exotica. It didn’t find any of them. Sabine Hossenfelder, never shy of criticising her fellow physicists in her wonderfully melancholy and frank style, puts it all down to wishful thinking. Her book “Lost in Math” (which I have yet to read) is about how scientists can be led astray by wrongheaded notions of mathematical elegance.

Bonus Sabine: Is light a wave or a particle? (Spoiler: “there are no particles, only wave functions”)


#1315

Thank you sir. Your indefatigable explanations.

I have a rye smile when I read a scientific text and the authors go straight into the mathematics , “…the maths says this…”. Usually there is <1% on the physical understanding. Quantum mechanics just looks like guided electromagnetic waves in waveguide structures.


#1316

Yesterday I passed this place – it’s 23 Longford Terrace, Monkstown … just on the Monkstown side of the Purty Kitchen in Dun Laoghaire.

The plaque on the wall is this:

image

I wrote previously about Agnes Mary Clerke another Irish astronomer. (Kicking myself that I missed a lecture in Armagh Observatory a few weeks ago about her.) Like Agnes, Margaret Lindsay Murray had the privilege but also the limitations of the home-schooled education available to a daughter of a middle class family. But also like Agnes she had a family member who was both a bank manager and a budding amateur astronomer. Although her formal education included only art, classics, literature, languages and music, she developed a huge interest in astronomy and the new technology of photography.

In Britain, the science of astronomical spectroscopy was in its infancy, trailblazed almost single-handledly by Sir William Huggins. The Royal Astronomical Society supported the endeavour with a long term loan of a telescope they commissioned in 1870 from the famous Dublin telescope maker Sir Howard Grubb (who I’ve also written about elsewhere in these pages). Margaret had read about Huggins work in a religious periodical that covered science subjects, and actually constructed a spectroscope herself. While Huggins was visiting Dublin in 1873 to inspect progress on the RAS scope, Grubb introduced Margaret to him, and a relationship blossomed in spite of a considerable age gap. The pair were married two years later. Margaret was 27 and Huggins was 51.

As Sir and Lady Huggins, the duo were a dynamic force in science with their observatory in Tulse Hill. As the crow flies it is only four kilometres south of Big Ben in the Borough of Lambeth. It’s amazing nowadays to think it could be the site of an astronomical observatory, but there was only a single farmhouse there in the early 19th century before a few detached dwellings were built, including the Huggins’ home.

For thirty years the pair did ground-breaking work in astrophysics, and they also formed a wide network of notable scientists and science popularisers. Agnes Mary Clerke was one of their many acquaintances, and a friend of Lady Huggins. As a result you can find a good amount of information about spectroscopy in her book that I recommended previously, a history of astronomy in the 19th century. The Hugginses used spectroscopy to positively identify the stars as faraway suns, and also detected the radial motions of the stars by Doppler spectroscopy. They found that stars such as Sirius had velocities of hundreds of miles per second relative to the Sun. This was the same technique that would herald the discovery of the cosmic redshift of the galaxies some years later. They were also instrumental in classifying the spectral types of the stars, and put together an early catalogue of such types. Even today stars are still primarily classified in this way and spectral type is one axis of the Hertzsprung-Russell diagram, probably the most ubiquitous tool in all of astrophysics.

image

Sir William with spectroscope attached to the Grubb refractor at Tulse Hill.

image

Lady Margaret at the telescope

Margaret Lindsay Huggins became a member of the Royal Astronomical society in 1903 along with Mary Agnes Clerke, only the third and fourth female scientist members. I’ve started reading Barbara J. Becker’s Unravelling Starlight: William and Margaret Huggins and the Rise of the New Astronomy (2011, Cambridge University Press). It brings a very human element to the story, such as recounting the Hugginses touching goodbye to the Grubb refractor when it was given to Cambridge University during Sir William’s declining years in 1908. One of Becker’s sources for the story actually used the telescope when it was still there at Cambridge, 50 years ago.

If you don’t have time to read a book but would like a fascinating insight into being a female scientist at the turn of the last century have a read of the first article here (starts on page 10 of the pdf, The Wellesley News of January 1913). Sarah F. Whiting was the first professor of physics at Wellesley College for women in Massachusetts, and would later be teacher to astronomer Annie Jump Cannon, who we’ve also discussed before. In 1889 she did a tour of Europe’s science faculties, which she recounts in The Experiences of a Woman Physicist. On a second trip in 1896 she struck up an acquaintance with Lady Huggins who as a result would later leave the Tulse Hill diaries and other artifacts to Wellesley College. You’ll get a sense of how small the scientific world was just a hundred and thirty years ago when even a visiting lowly American female professor could meet Kelvin, Rayleigh, Fitzgerald, Crooks, JJ Thompson, and so on. It’s also funny to read of a university faculty objecting to electric lighting in the library for fear it would burn the students’ eyes out! :smile:


#1317

Did anyone catch the large meteor that seems to have traversed the country from north east to south west this evening, just before 7pm? Reports on boards.ie and Twitter from Newry to Killorglin in Kerry, also seen in Mullingar, Tipperary, Waterford, all over Cork, even Mayo in northwest and Cardiff in Wales! Exotic green bolide with yellow or orange tail that lit up the sky for up to seven seconds or more … resulting in some great dashcam footage. From the first clip below in which the dashcam registers it against bright streetlights I’d hazard a guess it might have rivaled the Moon in brightness, i.e. maybe magnitude -12.


#1318

Belated TESS update. NASA released this mosaic including the southern Milky Way and Magellanic clouds from the first half of the mission.

The spacecraft finished its year-long survey of the southern sky in July and has turned its attention to the north. The mission is due to finish in June 2020. On the first half of the survey TESS identified over a thousand candidate planets of which 29 have been confirmed so far.

TESS is also finding things other than planets:


#1319

I don’t mention meteor showers often, because most of them are a disappointment. The Leonid maximum tonight is unlikely to amount to much as the Leonids fizzled after the 1999 and 2001 storms (which I had the good fortune to see). But next week a much lesser known shower has an extremely sharp peak which could see a maximum hourly rate (ZHR) of 400, but only for about half an hour at 4.50 am next Friday morning.

Unlike the Leonids tonight, there won’t be a spoiling moon. And the shower radiant passes the meridian just before the predicted outburst, which means it’s as high in the sky as it gets – around 40 degrees above the southern horizon in this case. The shower name doesn’t exactly trip off the tongue – they’re the Alpha Monoceratids – but the radiant location is close to the brightish star Procyon (circled below). It’ll be south (left) of the top of Orion which will be in the southwest. The shower’s parent comet is unknown, but probably a long period comet returning only once every 500 years. The compact meteor stream is a mere 50,000 kilometres wide, which doesn’t take long for the Earth to plough through at 30 km/sec.

(line spacing 10 degrees)

#1320

Will this ruin astrophotography??