The Department of Theoretical Physics at Maynooth University invites applications for a PhD in Theoretical Astrophysics starting in September 2023. The successful applicant will work in the group led by Dr. John Regan on a project examining the formation processes of massive black holes in the early Universe. Massive black holes populate the centres of all massive galaxies and are now also observed in both the centres and in off-centre locations in less massive dwarf galaxies.
For more details and instructions on how to apply, see here.
Posted in Uncategorized on December 8, 2022 by telescoper
I know quite a few people these days are asking what Mastodon is. Fortunately, I see in the latest Private Eye that some leading experts have given their opinion on the matter:
I’ve recently moved on to the part about Special Relativity in my module on Mechanics and Special Relativity and this afternoon I’m going to talk about the Lorentz-Fitzgerald contraction or, as it’s properly called here in Ireland, the Fitzgerald-Lorentz contraction.
The first thing to point out is that the physicists George Francis Fitzgerald and Hendrik Lorentz, though of different nationality (the former Irish, the latter Dutch), both had fine beards:
George Francis Fitzgerald (1851-1901)Hendrik Lorentz (1853-1928)
One of the interesting things you find if you read about the history of physics just before Albert Einstein introduced his theory of special relativity in 1905 was how many people seemed to be on the verge of getting the idea around about the same time. Fitzgerald and Lorentz were two who were almost there; Poincaré was another. It was as if special relativity was `in the air’ at the time. It did, however, take a special genius like Einstein to crystallize all that thinking into a definite theory.
Special relativity is fun to teach, not least because it throws up interesting yet informative paradoxes (i.e. apparent logical contradictions) arising from that you can use to start a discussion. They’re not really logical contradictions, of course. They just challenge `common sense’ notions, which is a good thing to do to get people thinking.
Anyway, I thought I’d mention one of my favorite such paradoxes arising from a simple Gedankenerfahrung (thought experiment) here.
Imagine you are in a railway carriage moving along a track at constant speed relative to the track. The carriage is dark, but at the centre of the carriage is a flash bulb. At one end (say the front) of the carriage is a portrait of Lorentz and at the other (say the back) a portrait of Fitzgerald; the pictures are equidistant from the bulb and next to each portrait is a clock.The two clocks are synchronized in the rest frame of the carriage.
At a particular time the flash bulb goes off, illuminating both portraits and both clocks for an instant.
It is an essential postulate of special relativity that the speed of light is the same to observers in any inertial frame, so that an observer at rest in the centre of the carriage sees both portraits illuminated simultaneously as indicated by the adjacent clocks. This is because the symmetry of the situation means that light has to travel the same distance to each portrait and back.
Now suppose we view the action from the point of view of a different inertial observer, at rest by the trackside rather than on the train, who is positioned right next to the centre of the carriage as the flash goes off. The light flash travels with the same speed in the second observer’s frame, but this observer sees* the back of the carriage moving towards the light signal and the front moving away. The result is therefore that this observer sees the two portraits light up at different times. In this case the portrait of Fitzgerald is lit up before the portrait of Lorentz.
Had the train been going in the opposite direction, Lorentz would have appeared before Fitzgerald. That just shows that whether its Lorentz-Fitzgerald contraction or Fitzgerald-Lorentz contraction is just a matter of your frame of reference…
But that’s not the paradoxical thing. The paradox is although the two portraits appear at different times to the trackside observer, the clocks nevertheless display the same time….
*You have to use your imagination a bit here, as the train has to be travelling at a decent fraction of the speed of light. It’s certainly not an Irish train.
Quite a few people have been playing around with a new-fangled AI tool called ChatGPT the developers of which say this:
We’ve trained a model called ChatGPT which interacts in a conversational way. The dialogue format makes it possible for ChatGPT to answer followup questions, admit its mistakes, challenge incorrect premises, and reject inappropriate requests. ChatGPT is a sibling model to InstructGPT, which is trained to follow an instruction in a prompt and provide a detailed response.
Here is an example (stolen from here) wherein this “model” creates the abstract of a scientific paper on a suggested topic:
This makes me wonder how many abstracts on astro-ph are actually written this way!
Please note that no papers of mine involved the use of any form of Artificial Insemination. I hope this clarifies the situation.
The other day I came across an old clipping from the December 2005 issue of Physics World. It’s from an article called What will they think in 2105? looking forward from 2005 at likely developments in the next 100 years of physics, given the context of the centenary of Einstein’s “year of miracles” (1905) in which he came up with, among other things, Special Relativity which I start teaching today.
The article asks what present-day discoveries would be remembered in a hundred years. Many of those asked the question said string theory. My response was somewhat less enthusiastic:
I got quite a lot of stick at the time from senior physicists for this statement! My use of the phrase “dead again” was based on the observation that the popularity of string theory has waxed and waned several times over the years. It may not have died in 2015 as I predicted, but it does seem to me to be in a moribund state, in terms of its impact (or lack thereof) on physics.
I’m mindful of the fact that many mathematicians think string theory is great. I’ve had it pointed out to me that it has a really big influence on for example geometry, especially non-commutative geometry, and even some number theory research in the past few decades. It has even inspired work that has led to Fields medals. That’s all very well and good, but it’s not physics. It’s mathematics.
Of course physicists have long relied on mathematics for the formulation of theoretical ideas. Riemannian geometry was `just’ mathematics before its ideas began to be used in the formulation of the general theory of relativity, a theory that has since been subjected to numerous experimental tests. It may be the case that string theory will at some point provide us with predictions that enable it to be tested in the way that general relativity did. But it hasn’t done that yet and until it does it is not a scientifically valid physical theory.
I remember a quote from Alfred North Whitehead that I put in my PhD DPhil thesis many years ago. I wasn’t thinking of string theory at the time, but it seems relevant:
There is no more common error that to assume that, because prolonged and accurate mathematical calculations have been made, the application of the result to some fact of nature is absolutely certain.
My problem is not with string theory itself but with the fact that so many string theorists have become so attached to it that it has become a universe in its own right, with very little to do with the natural universe which is – or at least used to be – the subject of theoretical physics. I find it quite alarming, actually, that in the world outside academia you will find many people who think theoretical physics and string theory are more-or-less synonymous.
The most disturbing manifestation of this tendency is the lack of interest shown by some exponents of string theory in the issue of whether or not it is testable. By this I don’t mean whether we have the technology at the moment to test it (which we clearly don’t). After all, many predictions of the standard model of particle physics had to wait decades before accelerators got big enough to reach the required energies. The question is whether string theory can be testable in principle, and surely this is something any physicist worthy of the name should consider to be of fundamental importance?
I was reminded today that 4th December is the anniversary of the death, in 1131, of the Persian astronomer, mathematician and poet Omar Khayyam. That in turn reminded me that just over year ago I received a gift of a sumptuously illustrated multi-lingual edition of the Rubáiyát of Omar Khayyám:
Edward Fitzgerald‘s famous English translation of these verses is very familiar, but it seems there’s a more of Fitzgerald than Khayyam in many of the poems and the attribution of many of the original texts to Khayyam is dubious in any case. Whatever you think about this collection, I think it’s a bit unfortunate that Khayyam is not more widely recognized for his scientific work, which you can read about in more detail here.
Anyway, as we approach the end of 2022 many of us will be remembering people we have lost during the year so here is a sequence of three quatrains (XXII-XXIV) with an appropriately elegiac theme:
For some we loved, the loveliest and the best That from his Vintage rolling Time hath pressed, Have drunk their Cup a Round or two before, And one by one crept silently to rest.
And we, that now make merry in the Room They left, and Summer dresses in new bloom, Ourselves must we beneath the Couch of Earth Descend–ourselves to make a Couch–for whom?
Ah, make the most of what we yet may spend, Before we too into the Dust descend; Dust into Dust, and under Dust to lie, Sans Wine, sans Song, sans Singer, and–sans End!
The little paradox in probability that I posted earlier in the week seemed to go down quite well so I thought I’d try a different paradox on a different topic from the same book of paradoxes, which is this one:
It’s quite old. I have the first edition, published in 1945, but many of the “riddles” are still interesting.
Here is one which you might describe as being about “knot theory”…
It’s probably best not to ask why, but the two gentlemen in the picture, A and B, are tied together in the following way: one end of a piece of rope is tied about A’s right wrist, the other about his left wrist. A second rope is passed around the first and its ends are tied to B’s wrists.
Can A and B free each other without cutting either rope, performing amputations, or untying the knots at either person’s wrists?
I’m glad I was too busy today to respond earlier to a junk science story that has been doing the rounds, in the Guardian, in Quantaand even in Physics World to name but a few. Had I had time to write something as soon as I’d seen these pieces of tripe I would probably have responded with more expletives than would be seemly even for this blog. This sort of crap makes me rather angry, you see.
Meaningless Illustration
The story is basically that a group of scientists have created a “wormhole in space-time” that enables quantum teleportation.
Of course they have done no such thing. The paper, like so many stories hyped beyond the bounds of reason, is published in Nature. There are some interesting things in this publication, but nothing to justify the absurd claims that have propagated into the media. The authors must take some of the blame for allowing such tosh to be spread about in their names. I don’t think it will do them any good in the long run.
At least I hope it doesn’t.
You can read it for yourself and make your own , but my take is the following:
Did the authors create a wormhole (even a baby one) in a laboratory? Definitely not.
Did they discover anything whatsoever to do with quantum gravity? No way.
Did they even simulate a wormhole in a lab? Not even close.
Did they even make progress towards simulating a wormhole in a lab? Still no.
Apart from all that it’s fine.
The author of the Quanta article, Natalie Wolchover, writes:
Researchers were able to send a signal through the open wormhole, though it’s not clear in what sense the wormhole can be said to exist.
Au contraire, it’s absolutely clear that no wormhole can be said to exist in any sense whatsoever.
I hope this clarifies the situation.
UPDATE: I see that Peter Woit has gone to town on this on his blog here.
Just a quick post to advertise the fact that the Department of Theoretical Physics at Maynooth University is inviting applications for a Postdoctoral Fellowship Position in Computational and Theoretical Astrophysics. The successful applicant will join the Research Group led by Dr John Regan and is expected to develop their own independent research program within the confines of a research project investigating the formation, growth, and demographics of Black Holes in the early Universe. The group, currently consisting of four PhD students and one additional postdoctoral researcher, is currently engaged in numerous research topics with the goal of understanding early black hole formation. In line with this we are currently implementing an ambitious research project using the EnzoE exascale class code to run large volume, high resolution simulations focused on the first billion years of black hole formation. The successful candidate will be expected to contribute significantly to this research effort but are free to pursue their own research lines under this remit.
For more information, including deadlines and the applications procedure, please see the AAS Jobs register advertisement here.
Now as we approach the last fortnight of term, I am nearing the end of both my modules, MP110 Mechanics 1 and Special Relativity and MP201 Vector Calculus and Fourier Series, and in each case am about to start the bit following the “and”…
In particular, having covered just about everything I need to do on Vector Calculus for MP201, tomorrow I start doing a block of lectures on Fourier Series. I have to wait until Monday to start doing Special Relativity with the first years.
As I have observed periodically, the two topics mentioned in the title of the module MP201 (Vector Calculs and Fourier Series) are not disconnected, but are linked via the heat equation, the solution of which led Joseph Fourier to devise his series in Mémoire sur la propagation de la chaleur dans les corps solides (1807), a truly remarkable work for its time that inspired so many subsequent developments.
Anyway I was looking for nice demonstrations of Fourier series to help my class get to grips with them when I remembered this little video recommended to me some time ago by esteemed Professor George Ellis. It’s a nice illustration of the principles of Fourier series, by which any periodic function can be decomposed into a series of sine and cosine functions.
This reminds me of a point I’ve made a few times in popular talks about astronomy. It’s a common view that Kepler’s laws of planetary motion according to which which the planets move in elliptical motion around the Sun, is a completely different formulation from the previous Ptolemaic system which involved epicycles and deferents and which is generally held to have been much more complicated.
The video demonstrates however that epicycles and deferents can be viewed as the elements used in the construction of a Fourier series. Since elliptical orbits are periodic, it is perfectly valid to present them in the form a Fourier series. Therefore, in a sense, there’s nothing so very wrong with epicycles. I admit, however, that a closed-form expression for such an orbit is considerably more compact and elegant than a Fourier representation, and also encapsulates a deeper level of physical understanding. What makes for a good physical theory is, in my view, largely a matter of economy: if two theories have equal predictive power, the one that takes less chalk to write it on a blackboard is the better one!
The views presented here are personal and not necessarily those of my employer (or anyone else for that matter).
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In the Dark 