George Jones

On 24/12/2022 at 15:17, Paul M said:

I'm struggling with the math.

1+3 spacetime? What exactly has 5 got to do with anything?

🤣

An n + m spacetime has n timelike dimensions and m spacelike dimensions. The spacetimes of standard special and general relativity are 1 + 3, i.e., each observer has 1 dimension of time and 3 dimensions of space. To simplify the maths, often 1 +1 spacetimes are considered (only 1 spatial dimension). Standard string theory operates in a 1 + 9 dimensional spacetime for which 6 of the 9 spatial dimensions are curled up so tightly that we perceive spacetime as being 1 + 3. This work uses a 6-dimensional spacetime but instead of using a 1 + 5 spacetime with 1 timelike dimension, it uses 3 + 3 spacetime, i.e., 3 dimensions for time, and 3 dimensions for space. The superluminal observers seem to occupy a 3 + 1 dimensional subspaces..

Interesting. I will work through this, possibly on Boxing Day as soon as the coffee shops open. I think, through work, I have access to the journal version of the manuscript. Daughter currently is playing Minecraft on home computer that has work software installed.

17 hours ago, saac said:

It's on Netflix and is titled A Trip To Infinity, runs for about 80 minutes or so.

I would love to watch this, but, unfortunately, we don't subscribe to Netflix. I think that my wife, a high school maths teacher, would like to watch it. If we were to watch, my wife likely would force our 16-year-old daughter to watch with us. I think that our daughter would treat it as an 80-minute prison sentence.

6 hours ago, iantaylor2uk said:

I'm quite into maths - it is fairly straightforward to show that the infinity of natural numbers (i.e. integers) is, somewhat surprisingly, the same as the infinity of fractions, and it is also possible to show that the infinity of irrational numbers is a bigger level of infinity. It is also possible to build even greater levels of infinity from these first two levels. One of the biggest questions in maths is whether there is any other infinity between that of the integers and that of the irrationals, which as far as I know is still unknown (it is called the "continuum hypothesis" if you want to do a google search). I'm travelling to London next week so this is something to watch on the train. Thanks for the heads up on the documentary.

I love this stuff. A book about this that I love is "Infinity and Mind the science and philosophy of the infinite" by Rudy Rucker. From the preface "Cantor's theory of higher infinities and his discovery of the Continuum Problem remain, at least for this mathematician, The Greatest Story Ever Told.."

I think this work illustrates how, more than we like to admit, part of science is a social construct. The senior author of the work, Paul Steinhardt, is a maverick  who previously developed a cyclic model of a universe that cyclically expands and contracts. Steinhardt realized that for his personal model to be viable, he needed a way to connect the expanding past and present universe to a universe that contracts in the future. Steinhardt and his collaborators, being clever people, found a mathematical function which does just that.

In spite of the way I have phrased the previous paragraph, this is not a bad thing. Suppose scientists X, Y, and Z all come up with personal theories that are consistent with present data. It is then up to science (not just X, Y, and Z) to find experiments that will produce data that that will differentiate between the models Until then scientists, being human, will different subjective opinions on the models. One scientist might say "I think X's model will turn out to be true" while another scientist says "X's model is crap; Y's model is just the ticket".

In this way, personal opinion helps to drive the progress of science. For this to work well, it is important to have enough scientists that a wide variety of opinions are represented.

My personal opinion on the work of Steinhardt et al.:

Is it plausible? No.

Is it possible? Yes.

Is it interesting? Very.

Is it worth funding as research? Absolutely.

I hope that some folks have the same answers to these questions; I hope that some folks have different answers to these questions.

I have my own personal time machine - pop music. Listening to the old songs takes me back to a time decades earlier.

8 minutes ago, saac said:

Would it be possible to even approach the event horizon, would the gravity tidal force on approach not be lethal ?  Assuming no accretion disc just how close could we come to the event horizon?

Jim 

If the black hole is large (larger than about 100,000 solar masses), spaghettification occurs inside the event horizon, i.e., crossing the event horizon of a large black hole is not problematic for a human. If a black hole is "small" (smaller than about 100,000 solar masses), spaghettification occurs outside the event horizon.

3 hours ago, cajen2 said:

Thanks for that, Andrew. I'll add it to the fat and rapidly-growing file: "Things that make no sense to Clive".

A few years ago, I wrote a (somewhat technical) elaboration on the ideas about which Andrew has posted.

On 22/08/2018 at 10:41, George Jones said:

Because of spacetime curvature between "over there" and "over here", it is difficult to define the speed of an object "over there" with respect to us "over here" in a way that respects all of our everyday experiences with speed. This leads to a first explanation for the possibility of recessional speeds greater than the speed of light.

Special relativity prohibits speeds greater than the speed of light. Cosmology, however, is governed by the curved spacetime of general relativity, to which special relativity is a good *local* approximation. Consequently, we will never see anything moving faster than the speed of light in our local neighbourhood, where special relativity is a good approximation. Stuff at the edge of the universe is not in our local neighbourhood, and thus is not governed by the laws of special relativity.

Alternate (more technical) explanation for recessional speeds greater than the speed of light.

speed = distance/time, so if different definitions of distance and time are available, we can have have differing definitions of speed. The definitions of distance and time used in cosmology lead to cosmological recessional velocities that correspond not to velocity in special relativity, but to something different called rapidity (sometimes called the "velocity parameter"). In special relativity, there is a relationship between velocity and rapidity, which, for some reason is not used in cosmology. If this relationship were used in cosmology than a recession rapidity of 3.4 corresponds to a recessional speed of 0.998 times the speed of light.

On 05/12/2021 at 10:29, George Jones said:

In practice, some stuff that we see now will become dimmer and dimmer, and more and more redshifted

I hove been careful here. Other stuff that we now see will become brighter and less red as we watch over centuries. This is very counterintuitive! As we watch, the universe expands, which seem like it should make all object dimmer and more red, because expansion increases distance and "stretches" light. This other stuff, will become less red, less red, ...., more  red, more red, more red, ...

On 01/12/2021 at 00:50, Earl said:

As we have had the opportunity to measure expansion due to when we are, when would objects outside of our galaxy become invisible to us? im guess high end billions of years or is this a not really thing?

Do mean in principle, or in practice? In principle, nothing that we can see now will become invisible to us. In fact, some, but not all, stuff that now is invisible to us will become visible. In practice, some stuff that we see now will become dimmer and dimmer, and more and more redshifted (as Andrew has said).

On 01/12/2021 at 04:06, andrew s said:

This is not  a simple question 

If I have understood this correctly it is, for non gravitationally bound objects, the particle horizon which will get to more than 60 glyrs in the current cosmological model and as time tends to infinity.  So we will see objects fade away as they get more and more red shifted.

On 01/12/2021 at 09:12, vlaiv said:

If event lasted 1 second and was localized in single point - we will now see it lasting 1 second. 

In an expanding universe, I think that we will now see it lasting (1 second)*(a_now)/(a_then), where a_now is the present scale factor of the universe, a_then is the scale factor of the universe when the 1-second pulse was emitted.

The October issue of Astronomy Magazine, which arrived in the post a few days ago, has an excellent article on the James Webb Space telescope. The article was written by John Mather, who is the senior project scientist for James Webb, and who won the Nobel Prize for work that first observed the variations in the Cosmic Microwave Background (CMB).

16 hours ago, saac said:

Gosh computer science courses in the 80s did they even exist. 

I went to high school in the 1970s in a small (pop. 8000) industrial town in Canada, but the high school was first-rate, and many of its teachers were first-rate. I took computer science classes in high school from 1976 to 1978. By today's standards, the circumstances were strange: the programming language used was Fortran (in high school!); the programs (penciled-in computer cards)  were physically sent by Greyhound Bus to a university in another city, and the cards and hard-copy output were returned by bus. The turn-around time was a least 3 days per run, sometimes to find "execution suppressed" because of a syntax error. The teacher, a computer science graduate from the University of Waterloo, had us work on several projects simultaneously, so that we were always sending and receiving stuff. Good courses and teacher.

I keep my NexStar 8 SE on the tripod in my shed, so that I do not have any cool down time. Even though I am  5' 4'' tall, and weigh 130 pounds, I find it easy to carry out the whole unit into the garden. I do have to do an easy star alignment. About 5 minutes after I go out the door of my house, I am observing with my scope at ambient temperature.

On 20/07/2021 at 04:38, John said:

Is the trend towards imaging growing or am I imagining it ?

Here is a slightly different question: in terms of absolute numbers, has the number of manly visual astronomers gone up or down? Is it possible that, over the past 20 years or so, the number of astronomers of all sorts has gone up, the percentage of mainly visual astronomers has gone down, but the number of mainly visual astronomers has not gone down? I do not have a feel for the answers to these questions, and I am interested in hearing what people think.

Personally, I am a purely visual. I have have taken zero pictures using my scope, not even with a mobile held to the eyepiece to capture the moon. This may because I don't own a mobile. 😀

My work day consists of doing technical stuff with books, lab equipment, and computers. When i get home, it is a nice change of pace to go out with my scope, and do visual observing while pondering the universe.

On 23/07/2021 at 06:45, Cosmic Geoff said:

The Celestron C8 SE outfit (same aperture as a 8" Meade SCT) is light enough to be picked up as one lump and carried through a standard doorway.  I have mine as a quick-deploy outfit with the addition of a Starsense kit.

I easily do the same, even though I am 61 years old, 5' 4'' tall, and weigh 130 pounds. I keep mine set up in a shed, so I do not have to wait for it to cool down, I do the star alignment, as I don't have Starsense. About 5 minutes after I go out my door, I am aligned and observing.

4 hours ago, Astro Waves said:

Not sure if this was the right part of the forum to post this in.

I was just wanting to know if any one had any good book recommendations on space/astrophysics/time/blackholes etc and that fun kinda stuff and also the philosophy of science? My physics knowledge isn't exactly the greatest so anything that's really technical I might slightly struggle with but I've been really trying to learn anything I don't know and more than happy to put in the time to get to grips with it. The kind of things Ive been reading to give you an idea are:

The universe in your hand - Christophe Galfard

The consolations of physics - Tim Radford

Reality is not what it seems: The journey into quantum gravity - Carlos Rovelli 

There's a couple others as well but I'm sure you get the idea. I've been enjoying watching stuff on the philosophy of science as well but would prefer some things to read as well as its easier to make notes on get to grips with a concept. 

Hopefully this isn't all too vague, if it is pop us any questions. 

There are too many books to read. Here are some examples, going form newest to oldest by publication date.

"The End of Everything (Astrophysically Speaking)" by Katie Mack https://www.astrokatie.com/book

"Our Mathematical Universe: My Quest for the Ultimate Nature of Reality" by Max Tegmark https://en.wikipedia.org/wiki/Our_Mathematical_Universe

Going back years, both John Barrow and Paul Davies wrote many interesting books. Example include

"The Universe that Discovered Itself" by John Barrow

"About Time" by Paul Davies.

Steven Weinberg, who I considered the greatest living physicist, has died.

His non-mathematical "The First Three Minutes" is a wonderful account of the evolution of the universe during the first three minutes after the Big Bang, Weinberg also wrote a series of wonderful very advanced advanced books.

Some personal thoughts.

I twice saw Weinberg give technical talks.

Even though I love Weinberg's advanced books, I say the following. I am glad that: 1) they are on my shelf; 2) I am not taking a course that uses any of them as the text; 3) I am not teaching a course that uses any of them as the text. Weinberg explains (at a technical level) some things better than anyone. Sometimes I read a passage in Weinberg, and I think to myself "Wow, I finally understand what is really going on." There are, however, passages in Weinberg's books that I find difficult to understand. Also, sometimes it is difficult to see past the clutter of Weinberg's notation.

The lecture notes for a course  that Weinberg taught in 2017 (at the age of 84!) were turned into his second to last  book, Lectures on Astrophysics. The section on white dwarfs and neutron stars helped me when I had to scramble to prepare an upper-level COVID-related and unexpected teaching overload last fall.

I like the personal aspect of the final paragraph of the Preface,
 

3 hours ago, saac said:

There is a section in the video - the 2nd part about Nothing - where Prof Al Kahili is explaining that the void (the true vacuum of space) isn't actually "nothing".  Randomly, so called virtual particles,will appear as he explains "borrowing energy" from the void. These particles only exist for the briefest of moments of time, almost instantly annihilating so returning the energy back to the void.  This results in quantum fluctuations in the void energy which in turn manifests as variations in density throughout space . These sites of varying density become the nucleation sites for baryonic matter to form and are impressed on space at the macro level.  Perhaps think of these fluctuations in energy as ripples in space around which dust settles. 

Jim 

The way I like to put it: when we look at galaxies through our telescopes, we are looking at quantum fluctuations blown up to galactic proportions by the expansion of the universe! Mind blowing!

19 hours ago, bomberbaz said:

In most cases this is true but apparently there is always something that comes along and opens up a tin of worms to spoil or make theories hard to prove.

Mystery of Galaxy's Missing Dark Matter Deepens | NASA

Pieter van Dokkum, the lead author of the research explained at this link, takes this as evidence for dark matter and against MOND (MOdified Newtonian Dynamics). From the link

Quote

Nevertheless, van Dokkum thinks finding a galaxy lacking dark matter tells astronomers something about the invisible substance. "In our 2018 paper, we suggested that if you have a galaxy without dark matter, and other similar galaxies seem to have it, that means that dark matter is actually real and it exists," van Dokkum said. "It's not a mirage."

From the Introduction section of the actual 2021 research paper,

Quote

The EFE, unique to MOND, causes a low mass galaxy in orbit around a massive galaxy to have a lower velocity dispersion than the same object in isolation. Thus, the low velocity dispersions of NGC 1052–DF2 and NGC 1052–DF4 may be consistent with the expectations from MOND if both galaxies are in close proximity to NGC 1052 (Kroupa et al. 2018).

and, from the paper's Conclusion,

Quote

The relative distance also places a new constraint on the interpretation of NGC 1052–DF2 and NGC 1052–DF4 in the context of MOND (see§1). ... the observed velocity dispersions of both NGC 1052–DF2 and NGC 1052–DF4 are consistent with MOND if both galaxies are physically close (<300 kpc; Kroupaet al. 2018) to a massive galaxy. The most obvious candidate for this massive galaxy is NGC 1052 itself, but our relative distance measurement of ∆D= 2.1±0.5 Mpc rules out NGC 1052–DF2 and NGC 1052–DF4 both being within 300 kpc of NGC 1052.

A report on a Zoom talk about dark matter research that I "attended" last fall:

On 09/12/2020 at 10:19, George Jones said:

I meant to get to this earlier, but I have been doing a COVID-related overload, and things have been somewhat hectic.

The talk included theoretical results of the paper to which I linked in my previous post. The authors of the paper take the view that gravity is okay, and that new unseen and transparent dark matter is needed to account for the motions of stars in galaxies, and galaxies in clusters of galaxies. Observations of the relative abundances of primordial elements, and of the Cosmic Microwave Background (CMB), indicate that dark matter is not made of the particles that account for normal matter's mass, protons and neutrons.

To date, we only have evidence that dark matter interacts via gravity, but many physicist think/hope that dark matter interacts via other (quantum) forces. They think that dark matter has quantum interactions, because this would give scenarios for dark matter production in the early universe similar to the production of normal matter, e.g., matter antimatter annihilation/creation that continues until the universe expands enough to stop these processes. Physicists hope that dark matter interacts with normal matter, as this gives ways for experimentally seeing signatures of dark matter.

If there are other interactions (besides gravity) involving dark matter interactions, they could be between: 1) dark matter and dark matter (just as there are interactions between normal matter and normal matter), and/or 2) dark matter and normal matter.

The very interesting talk was about theoretical models that have both 1) and 2), where 1) is used to generate the masses of dark matter particles, and 2) is used to predict observable effects of the models.

Two scenarios were considered for generation dark matter mass. One was a possible a dark electromagnetic-like interaction between dark matter and dark matter. The analogy is not exact, as, unlike normal photons, the dark photons have mass that they acquire from a proposed dark Higgs-like particle. Another possibility is that there is a dark colour-like force. In the normal colour nucleon force, gluons carry the colour force, and thus gluons can interact with gluons. In the proposed dark colour-like interactions, dark gluons interact to form massive dark glueballls.

In both types of models, it is proposed that there is a "feeble" interaction between dark matter and normal matter. This feeble interaction can inject energy into the normal universe. If his injection is early enough, it can affect the relative primordial abundances of elements. If this energy injection is later, it can affect the the thermal spectrum of the CMB. If this injection is later still, it can affect the anisotropy spectrum of the CMB. High precision cosmological measurements could reveal these effects.

Possible results for the LHC and for high precision cosmological measurements probe complementary interaction strengths. The cosmological results are for weaker interactions.

5 hours ago, Marvin Jenkins said:

I don’t believe a word of it! Solid state universe

 

And before it was a solid state universe, it was a valve universe.

Entangled particles cannot be used to transmit information faster than the speed of light.

I had great fun giving the talk (wife: "Don't get him started on physics!"), and people asked great questions. I hope soon to elaborate a bit on some of my answers.

1 hour ago, saac said:

I follow the argument regarding how we have measured c but I don't understand what supports the premise that the speed of light may/could be anisotropic.  The speed of light being a property arising from  the nature of the particle (photon, massless) and the two constants which describe the permittivity of free space (the electric constant) and the permeability (the magnetic constant).  So what I don't understand is that are we saying the values of these two constants change for a particular part of the return trip?  Or have I missed something, quite possible, and usually often   

from Maxwell's equation's   C = 1/ (ϵ_oµ_o)^1/2

And my students do just this. I teach a second-year e&m lab course that performs one experiment per week. In consecutive weeks, the students measure ϵ_o and µ_o using

https://media.vwr.com/interactive/p...h_2014/files/assets/basic-html/index.html#289

and then compare to the speed of light. For µ_o , we send up to 20 amps through the metal rods.

Welcome to SGL. My in-laws live north of Toronto, near Canada's Wonderland.