1

Physics, Astronomy & Cosmology / Re: Does anyone understand Roger Penose's view of cyclic universe?

« on: 20/01/2023 20:15:40 »

I stand corrected.
Penrose does talk about a continuously expanding universe, but assumes that by changing the coordinates of the universe, the near-zero density and near-zero temperature at the end of one universe becomes the near-infinite-density singularity and near-infinite temperatures at the start of the next universe.
https://en.wikipedia.org/wiki/Conformal_cyclic_cosmology

2

Physics, Astronomy & Cosmology / Re: Has the speed of light been tested in a vacuum?

« on: 05/01/2023 20:38:30 »

Has the speed of light been tested in a vacuum?

When it comes to the speed of light, air at sea level is a pretty good vacuum.

Light slows down in a medium, compared to its speed in a vacuum
- For example, in glass, light is slowed by a factor of about 1.5
- In air at sea level, light is slowed by a factor of 1.0003
- The amount of slowing is measured by the "Refractive Index"
- See the Refractive Index list at: https://en.wikipedia.org/wiki/List_of_refractive_indices#List

For non-magnetic materials, you can calculate the amount of slowing from the measured relative permittivity of the medium.
- For air at sea level, the relative permittivity is already close to 1 (almost the same as a vacuum)
- For achievable vacuums (eg at LHC), the relative permittivity is immeasurably close to 1
- See the list at https://en.wikipedia.org/wiki/Relative_permittivity

From a totally different viewpoint, gravitational waves also travel at "c" (commonly called "the speed of light in a vacuum", but its more fundamental than that).
- Gravitational waves travel almost unaffected through the high density of the Earth, the Sun and neutron stars
- The LIGO observatory observed a neutron star merger, which was accompanied by a gamma-ray burst starting 1.7 seconds later. The source was in a galaxy 130 million light years away.
- This suggests that light travels at a speed through intergalactic space at a speed that is reduced by at most 1.7 seconds in 130 million years, ie pretty much the speed of light in a vacuum.
- See description at: https://en.wikipedia.org/wiki/Neutron_star_merger#Observed_mergers

3

Chemistry / Re: Strange aroma off smokeless fuel or has covid ruined my snout?

« on: 04/01/2023 22:01:48 »

Is Covid never ever gonna go away?

Probably not.

We will just need to learn to live with it (and some people will continue to die from it...).

The best parallel is OC43 (another coronavirus) which is probably to blame for the 1889 "Russian flu" pandemic.
- This had continuing waves for about 6 years.
- Today, it is treated as a "Common Cold", with outbreaks mostly in the winter months.
- It can still be severe in babies, the elderly and immune-compromised individuals, but the death rate is much lower than for the early outbreaks

The early outbreaks of "Russian flu" were not aligned with the usual winter colds
- This is roughly where we are with COVID-19 now
- But we can hope that it later becomes a seasonal cold
- There is one significant difference: Anyone with a genetic susceptibility to OC43 died in the early outbreaks, so today's survivors are less susceptible
- Through masks, testing, lockdowns and now vaccines & anti-virals, we have managed to reduce the number of people who died from COVID-19. But these people (and their descendants) will still potentially have a genetic susceptibility to COVID-19, which may cause ongoing deaths in later years and decades.

https://en.wikipedia.org/wiki/Human_coronavirus_OC43
https://en.wikipedia.org/wiki/1889%E2%80%931890_pandemic

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Physics, Astronomy & Cosmology / Re: Has the speed of light been tested in a vacuum?

« on: 04/01/2023 21:20:34 »

I always thought a vacuum was a process of extracting the air from a containment . Are you saying the universe is contained or something similar ?

Actually, while it was forming, the baby Earth extracted any nearby atoms out of space by its gravitational field, interacting with the protoplanetary disk
- The same goes even more so for the Sun (mostly hydrogen and helium) and the large planets (Jupiter, Saturn, etc)

The Sun blows out a high-speed Solar Wind, but its density is far less than the density at the surface of the Sun.
- Since the surface of the Sun is strongly compressed by gravity into the sphere we see in the sky
- By the time this Solar Wind reaches the Voyager spacecraft, its density and speed have dropped enormously. In this region, the extremely low density of the interstellar medium dominates.

CERN uses very fancy vacuum pumps, but for most of space, gravity is the invisible "pump" which keeps the air out of space.

https://voyager.jpl.nasa.gov/mission/status/

5

Physiology & Medicine / Re: Why is stroke induced paralysis sometimes temporary or sometimes permanent?

« on: 30/12/2022 22:14:34 »

It depends on how extensive the damage is, and whether there are nearby areas of the brain that can take over the damaged areas (when given suitable training).
- A major factor in the extent of the damage is how quickly the patient can be taken to hospital, diagnosed and given suitable treatment. The brain is very oxygen-hungry, and an hour can make a big difference to the amount of damage.
- Physiotherapy and similar rehabilitation can help with recovery, if the damage is not too extensive, and the rehabilitation is affordable
https://en.wikipedia.org/wiki/Stroke#Prognosis

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General Science / Re: How would you explain or define calculus in a sentence or two?

« on: 27/12/2022 20:23:41 »

Historical answer: "calculus" originally comes from the Latin, and means a small stone or pebble. These were used in an abacus for calculations.

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Physics, Astronomy & Cosmology / Re: How dense is neutronium and how many stars are in the Milky Way?

« on: 24/12/2022 21:40:01 »

I haven't done the experiment but we can assume that if you removed it from a Neutron star then it expands, interacts with other particles etc.

Playing with neutron stars is also beyond my rating on the Kardashev scale: https://en.wikipedia.org/wiki/Kardashev_scale

However, we have observed the collision of two neutron stars.
- This cosmic collision blasted large amounts of neutronium into space, where it promptly decayed into more familiar elements.
- Detection of the colission was only possible through the unique gravitational wave signature https://en.wikipedia.org/wiki/Neutron_star_merger#Observed_mergers
- This lasted long enough, and was observed by 3 detectors, enabling a direction to be determined for follow-up observations by optical and X-Ray telescopes
- This obscure astronomical event (which would have passed almost unnoticed* just a few years before) made the Wall Street Journal, on the basis that it produced the mass of the Earth in gold.
https://www.wsj.com/articles/a-clash-of-neutron-stars-forges-gold-1508162400  (behind a paywall... I get 2 seconds!)

*PS: Apparently the gravitational wave event coincided with a Gamma-Ray Burst which was detected in the same general region of the sky by two gamma-ray satellites. It is now believed that many "short" Gamma Ray Bursts like this (lasting only 2 seconds) were caused by neutron star mergers.

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Physics, Astronomy & Cosmology / Re: How dense is neutronium and how many stars are in the Milky Way?

« on: 20/12/2022 21:08:42 »

How dense is neutronium?

Neutron star material is remarkably dense: a normal-sized matchbox containing neutron-star material would have a weight of approximately 3 billion tonnes, the same weight as a 0.5 cubic kilometre chunk of the Earth (a cube with edges of about 800 metres) from Earth's surface

https://en.wikipedia.org/wiki/Neutron_star

how many stars are in the Milky Way?

It is estimated to contain 100–400 billion stars and at least that number of planets

https://en.wikipedia.org/wiki/Milky_Way

But no fundamental particle has a meaningful volume

Just expanding on Halc's comment a little...
- All subatomic particles (and even atom and molecule-sized objects) are not hard little balls with a definite radius
- In quantum theory, all subatomic particles (like neutrons and protons) are a little "fuzzy" or "blurry", meaning that there is no definite boundary. There are just regions where they are very likely to be found, fading off into regions where they are less likely to be found.
- You can still talk about the "average distance between protons & neutrons in a Uranium nucleus", as the uncertainty in the size of a Uranium nucleus is smaller than the uncertainty in the size of an individual neutron (relatively speaking)
- The strong atomic force between neutrons becomes repulsive at shorter distances, as if they were bumping into each other; but applying more force (eg the gravitational field of a collapsed star) will push them a bit closer together
See the graph here: https://en.wikipedia.org/wiki/Nuclear_force#Description
- Neutron stars start off extremely hot (like 10¹¹ °C), but cool down in a few years to something more like 10⁶ °C.
- Like the temperatures we are more familiar with, things at a higher temperature "jiggle" more, and the average distance between particles is greater than between cooler things, which vibrate less. This leads to the general rule that things shrink as they cool (freezing water being one notable exception). I expect that this would also apply to neutron stars, too.
https://en.wikipedia.org/wiki/Neutron_star#Mass_and_temperature

so there is in theory no limit to how far it can be compressed given enough pressure

There is another limit: If the total mass of the neutron star exceeds about 2.5 times the mass of the Sun (in a ball only 10km across!), it is thought that even the Strong Nuclear Force will not be able to withstand the pressure, and it will collapse into a black hole.

There are various theories about other subatomic particles (like the "Strange" particles and "Quarks") that are denser than neutrons, and may be able to withstand slightly higher pressures than neutrons.
- Evidence for this is being collected by the NICER X-Ray telescope mounted on the ISS, which is trying to measure the mass, size and density of neutron stars. This will give some clues about the density of neutron stars, and whether "Strange stars" or "Quark Stars" exist.
https://en.wikipedia.org/wiki/Neutron_Star_Interior_Composition_Explorer

Oops! Overlap with Halc's latest post. One comment:

otherwise heavily positively charged star.

I expect that a neutron star would be electrically neutral, overall. Electrons will be attracted into regions with protons by the electric force. Due to the size and temperature of the neutron star, I expect that atomic orbitals will not exclude electrons from proton-rich regions.

PS: In our normal experience, the electric attraction between electrons and protons is far greater than the gravitational attraction of electrons and protons. But in the extreme environment of a neutron star, the gravitational force is more comparable to the electric force.

9

Physics, Astronomy & Cosmology / Re: Is there a substance where entropy decreases at higher temperature?

« on: 16/12/2022 21:09:09 »

Is there a substance where entropy decreases at higher temperature?

Perhaps Rubik's Cube?

It is not a 2-state system, but a 20 state system, with the state representing the number of moves required to get it back to a fully-ordered system.

And the number of microstates (ie probability of finding the system in that state) decreases as you move to state 19 or 20.
- But up to state 18, the number of microstates increases if you add a random turn.

Does this fulfill the definition above?

In a cube with 19 or 20 moves to solve, inserting extra energy (random moves) decreases the entropy (the number of moves to solve).


Rubiks_Cube_Moves.png (30.34 kB . 471x587 - viewed 1178 times)
Table from: https://www.quora.com/How-many-permutations-does-a-Rubiks-Cube-have

This came up in https://www.thenakedscientists.com/forum/index.php?topic=85923

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Just Chat! / Re: How to access the symbols when correcting?

« on: 08/12/2022 19:49:29 »

Try using the Actions > Modify option, rather than "Back".

11

Physiology & Medicine / Re: Is strep a infection phage related?

« on: 07/12/2022 07:59:02 »

Bacteriophages actively set about destroying their bacterial targets.
- Your microbiome contains many types of bacteria, balanced by many types of phages which reduce the chance that one kind of bacteria will produce a monoculture in your gut.

A prophage differs from a "normal" phage in that it  is a retrovirus held latent within the bacterial DNA (just like HIV hides within the human genome). When triggered, the prophage starts multiplying in the host cell, bursting it open to spread more phage particles.
https://en.wikipedia.org/wiki/Prophage

In Diptheria, the toxin is encoded on the prophage genome. This toxin affects the human host.
https://en.wikipedia.org/wiki/Diphtheria_toxin

Overall, I expect it is more typical that a phage attack would reduce the quantity of a bacterial pathogen (and thus reduce the severity of a bacterial infection). Diptheria appears to be an exception...

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Physics, Astronomy & Cosmology / Re: Can we detect coloured objects wavelength's by device?

« on: 06/12/2022 20:23:47 »

the monitor displays white noise

This sounds like a very old experiment. We don't use direct analogue modulation any more.

If you point your camera at a blank blue wall, your movie camera's electronics examine the pixels and categorise them as "a lot of blue, a bit of green and hardly any red; this is spread uniformly across the wall (not a gradient); and it's not moving over time". (That's my verbal description of MPEG encoding; other video encodings are available...)
- You transmit this to a TV screen (optionally via a satellite), and the TV electronics decodes this as "a lot of blue, a bit of green and hardly any red; this is spread uniformly across the wall (not a gradient); and it's not moving over time".
- If you point your satellite dish at a random point in the sky, it will be picking up the cosmic microwave background radiation in the GHz region of the spectrum, which is random noise. It will not pick up the colour of visible light, which is around 400 THz.
- If you point your satellite dish at a wall, it will be picking up thermal noise in the GHz region of the spectrum, which is random noise.  It will not pick up the colour of visible light, which is around 400 THz.
- None of this random noise has the highly ordered structure of a modulated carrier, carrying the highly ordered structure of an MPEG stream and encoded with the highly ordered structure of an error-correcting code.
- A modern digital TV would not show white noise - it would detect that the highly ordered input is missing, and display a message like "no signal".

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Physics, Astronomy & Cosmology / Re: Is there an experiment that shows the oscillation in the E field of light?

« on: 05/12/2022 20:39:22 »

Alan & I regularly use beat frequencies to tune musical instruments. I agree that the individual component frequencies still exist, but it doesn’t change the fact that the beat frequency exists and has a physical effect, you can for example get it to excite a tuned resonator.

I have also used beat frequencies to tune a guitar (in my distant youth)...
- If you pluck an open string, you will get a (decaying) sine wave (with a few harmonics).
- If you shorten the adjacent string, you will also get a (decaying) sine wave of almost the same frequency (also with a few harmonics).
- If you now pluck both together, your ears (a logarithmic sound power detector) will detect increasing and reducing power, as the two waves go into and out of phase (the beat frequency at say, 1Hz).
- If you measured this with an FFT over a complete beat cycle (eg 1 second), you would see just the two string frequencies, but not the beat frequencies. You would not see the difference frequency because the microphone is linear.
- The guitar does not resonate at 1Hz, as it doesn't really work much below 50Hz. What you hear are the two audio frequencies exciting the sounding board more and less, cyclically.
- Your ear does not resonate at 1Hz, as it doesn't really work much below 20-50Hz. What you hear are the two audio frequencies exciting your eardrum more and less, cyclically.

This is very different than if you:
- Wiggled your finger tension on the string while playing a string (vibrato - more commonly used on violins = non-linear), which creates a series of frequency-modulated sidebands, close to the natural string frequency.
- Wiggled the volume control while playing a string (amplitude modulation = nonlinear), which creates a pair of amplitude-modulated sidebands, close to the natural string frequency.

Really, I don't see that beat frequencies in a linear medium can excite events at the beat frequency (unless the pianist gets excited because the piano isn't tuned properly!).

PS: People don't tend to use this method of tuning guitars these days - it is too easy to use a crystal-locked electronic tuner which doesn't just tell you that the frequencies differ by 1 Hz, but tells you whether the string is too high or too low in frequency (to 1% of a semitone...). ...assuming you want an equal-tempered scale.

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Physics, Astronomy & Cosmology / Re: Is there an experiment that shows the oscillation in the E field of light?

« on: 04/12/2022 19:54:01 »

Describe the detector that shows (radio frequencies) to be discrete or non-discrete.

Adding to Paul Cotter's comment, your mobile phone (or your DSL modem) uses numerous closely-spaced frequencies, which can all be used to transmit signals.
They are separated out by a Fast Fourier transform (and are generated by an Inverse Fourier Transform), usually with some error-correcting code to overcome noise.

Around here, we had problems with VDSL2 broadband and telephone pillars which had "dry" joints or oxidised wire-wrap joints, which can be nonlinear. This would cause intermodulation products across the band, corrupting the signal with the dry joint, and also cross-talking into adjacent copper pairs. Linear crosstalk can be cancelled by (linear) "vectoring", but non-linear crosstalk would take far more processing power to analyse & correct. They resolved it by soldering all of the joints in the noisy pillar.
- This reinforced my view that optical fiber is far superior to copper wires - partly because the fiber (mostly SiO₂) is already fully oxidised.

all the equipment we use for detecting lower energy EM radiation is based on its wavy nature.

The point about lower-frequency EM radiation (eg radio-frequency) is that the individual photons ("radons"?) have ultra-low energy. That means you don't transmit individual photons, but instead a coherent wave consisting of trillions of photons. You can easily modulate or demodulate this wave (eg using FFT/IFFT).

However, as you go to higher frequencies, the energy of individual photons increases, and you are more likely to detect them as individual events, and less likely to detect them as a coherent wave.
- It is possible to produce a coherent wave of light (using a laser), but this gets increasingly difficult at higher frequencies like X-Rays and gamma rays.
- Large astronomical telescopes use photon-counting detectors (cooled to near absolute zero), where you can practically count how many photons struck each pixel of the detector. But the photons are still reflected/refracted as a wave,  the very large mirror localising the photon on an individual pixel of the detector, in a way that a smaller mirror would not.

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New Theories / Re: How Many Numbers Exist?

« on: 01/12/2022 21:31:57 »

When I was a kid, the concept of infinity did my brain in.

But even finite numbers can do your brain in. "Graham's Number" is so mind-bogglingly huge that if you tried to hold all the digits of it in your head, your head would collapse into a black hole.
- The proof of this comes from the observation that information has a certain entropy, and the surface of a black hole is the smallest area with a certain entropy
- Apparently, the Schwarzschild radius of Graham's number is bigger than your head... (Note: I haven't tested this calculation myself)
https://en.wikipedia.org/wiki/Graham%27s_number

Podcast (1 hour:15 minutes): https://www.preposterousuniverse.com/podcast/2022/10/17/214-antonio-padilla-on-large-numbers-and-the-scope-of-the-universe/

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New Theories / Re: How Many Numbers Exist?

« on: 01/12/2022 21:08:39 »

there are more (numbers) than you could ever need, and they are all free

In AI (Artificial Intelligence) work, numbers are not free. It takes serious electrical energy to train a large AI model, and even more to run it.
- The more resolution in your calculations (eg using 64 bit floating point instead of 32 point floating point), the faster it converges, but the more electricity it takes to train the model.
- An AI model which is to be used by millions of people (eg a self-driving car algorithm) might use far more electricity to execute than it took to train, so researchers are looking at bringing AI execution down to 16 bits, 8 bits or perhaps even less?
- Some of the recent gaming video chips turned into supercomputers (eg NVIDEA*) have partitioned their computation hardware so it can do 1 x 32-bit operation or 4 x 8-bit operations at once.

..which leads to a debate about "how many numbers do you need?". It's a bit simplisitic to say that the nerves in our brains only have 2 states: firing or not firing.
- There is a complex modulation with rates of firing
- and a poorly-understood overall modulation by brain waves (alpha, delta, etc).
- but rough calculations suggest that neurons on the brain have a marvellous energy economy in terms of energy per transition, which is the envy of engineers designing computers out of silicon...

* other gaming chips are available, and other supercomputers are available, but not many that are both...

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Physics, Astronomy & Cosmology / Re: ELI5: Diffraction grating

« on: 28/11/2022 20:16:14 »

You could start here: https://en.wikipedia.org/wiki/Diffraction_grating

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Physics, Astronomy & Cosmology / Re: Orbiting or descending into the black hole at the centre of the Milky Way?

« on: 27/11/2022 20:49:35 »

Mainly the MW is in orbit around itself.

And the mass of the Milky way is dominated by the (so far) invisible Dark Matter halo.

it looks like the stars are swirling down into the black hole

Out in the fringes (where we live), the stars are well-separated, most of the stars are in a fairly thin disk, have fairly circular orbits, and and have roughly the same angular velocity. So they won't interact very strongly with each other.

However, in the central bulge, stars are closely spaced, they have wildly different orbital planes, and the ones we can see near the central black hole have rather elliptical orbits, so the stars will transfer angular momentum between each other.

Some of these gravitational interactions will "cancel" angular momentum (if the stars have different orbital axes).
I expect that this will result in the central bulge flattening out over time - but the average distance from the central black hole will then be less than it is now. When stars are moving in a disk, they will have fairly stable orbits.

...that is, until the central black hole merges with another black hole; the incoming black hole will add angular momentum into the system on an entirely different axis, and once again send the stars off in all directions.
https://en.wikipedia.org/wiki/Sagittarius_A*_cluster

it looks like the stars are swirling down into the black hole

You inferred this from looking at a still image, and imagining the galaxy as a drain emptying in a whirlpool. This is not what "Whirlpool Galaxy" means.
https://www.nasa.gov/feature/goddard/2017/messier-51-the-whirlpool-galaxy

The Gaia space probe has been able to measure the velocity of stars in the Large Magellanic Cloud, and it is showing the stretching associated with being spaghettified by its close approach to the Milky Way. But the orbits are still roughly elliptical, rather than converging on the central black hole (if it has one...)
https://www.esa.int/ESA_Multimedia/Images/2018/04/Rotation_of_the_Large_Magellanic_Cloud#.Y4PLHulNzqU.link

19

Physics, Astronomy & Cosmology / Re: LHC costs, is it worth it?

« on: 19/11/2022 20:08:57 »

Is mathematics truly capable of explaining the laws of the Universe?

Newton's equation F=ma explains a lot about the universe, when combined with his equation about universal gravitation.

Are we capable of explaining everything? No. Much of the universe is outside our light cone, so we would have no way of verifying such a theory, let alone generating it.
- Are we capable of explaining the physics principles underlying everything we see? Maybe - we see with light (electromagnetic radiation), and physics has explored many interactions involving electromagnetic radiation
- Are we capable of explaining the physics principles underlying everything we feel? Definitely not yet! In our galactic orbit, the Sun feels the tug of Dark Matter, and we don't know what that is. The LHC has not yet turned up any likely particles as candidates.

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Just Chat! / Re: Anyone here have any good penis jokes?

« on: 19/11/2022 19:59:03 »

This is not the place for them. We aim to keep the forum "family friendly", so children can safely come here for answers to scientific questions.