1

Physics, Astronomy & Cosmology / Re: Why isn't my salt water evaporating?

« on: 15/09/2023 20:13:10 »

Thanks, Evan_au. As I explained above I answered similarly in a more verbose but most likely less clear answer that evaporated.

2

Physics, Astronomy & Cosmology / Re: Does helium make all sounds higher pitched?

« on: 05/09/2023 23:04:38 »

length of the air column and the timbre of the harmonics are determined  by the wood (or metal in most cases nowadays) .

So, a brass flute would sound more  like a tin whistle than like a wooden flute.
And a wooden flute would, I guess, sound like a recorder, rather a metal flute.

Or... not.

I suspect that the reason why plastic musical instruments exist is that, to a first approximation, the timbre depends on the shape of the instrument.

It would certainly be interesting to  fill a flute with helium (or hydrogen, if you are concerned that He isn't a renewable resource) and close it with cling-film so you don't blow any air through it.

I suspect the tone would not be improved by a bit of thin plastic, but it would illustrate a point.

: if you filled your trombone or bagpipe with helium, would that alter its pitch?

One way to find out; check on YT and see if it's been done.

Apparently, if you put helium into a bagpipe it turns into laughing gas.

3

Chemistry / Re: Is nitrogen based liquid fuel a viable energy storage solution?

« on: 29/05/2023 11:24:19 »

toxicity is a problem at high concentrations

Not if you are comparing it to lead-acid or lithium batteries.

4

Physics, Astronomy & Cosmology / Re: BLC 1 Signal from Proxima Centauri?

« on: 18/05/2023 23:09:32 »

- Half of the time, the Sun is above the horizon for a radiotelescope on the far side of the Moon

True, but for on the "near" side of the moon, the earth is in line of site 100% of the time and on earth the earth is in line of site 100% of the time.

So having the sun in your eyes only 50% of the time looks like a really good option.

5

Plant Sciences, Zoology & Evolution / Re: Where would I find King Kong?

« on: 15/05/2023 18:32:56 »

Where would I find King Kong?
Alphabetically, between Harald V of Norway and  Letsie 3 of Lesotho.
https://en.wikipedia.org/wiki/List_of_current_monarchs_of_sovereign_states

6

Just Chat! / Re: Can a being with a strength like Superman exist?

« on: 01/04/2023 12:21:08 »

Reversing time by flying around the Earth is not compatible with our understanding of General Relativity,

But if you fly eastwards from Samoa you can arrive yesterday, and 20,000 miles to the west,  even in a light aircraft.

7

COVID-19 / Re: can i have a full run down on spike protiens please?

« on: 18/03/2023 22:10:46 »

I think Ebola has spike protein
https://www.reuters.com/article/health-ebola-dc-idUKN0937961120080709

, are they god/bad/neutral

From the viruss perspective they are very good.
From the patient's point of view, they aren't.

3, how are they made

The same way a virus makes any other protein; they hijack the biochemical pathways of the cells they infect.

4, do they occur naturally

Yes; they are part of many viruses.

5, if they are bad, how do we get rid of them

Our immune system destroys them (if we are lucky)
To completely eliminate them I think we need to wait until the sun goes red giant and bakes them all.

8

New Theories / Re: What is the real readshift in the Cosmic Microwave Background (CMB)

« on: 08/02/2023 13:10:02 »

My simplistic understanding of what they said is:
- Roughly 15 (or 14) billion light years is the distance from "where the source was when the light was emitted" to "where we are now"

It's actually about 1/10000th of that. In cosmic coordinates (the only coordinate system I know that describes the universe), the oldest light we see (that of the CMB) was emitted at a proper distance of about 1.5 million LY away. The reason it took 13.8 billion years to get here is due to the very high expansion rate of the universe back at the time of the recombination event, perhaps 3M km/sec/mpc compared to 70 km/sec/mpc today.

Roughly 100 billion light years is the distance from "where the source is now" to "where we are now"

That would put it beyond the size of the visible universe which means we could not see it. So around 45 billion light years is the proper distance from "where the source is now" to "where we are now".

- There is an even smaller number which represents the distance from "where the source was when the light was emitted" to "where we were back then"

Well since we have not moved significantly in that time, that distance is also that 1.5 MLY figure. Cosmic coordinates has the Earth at the center, unmoving. You have to assign the origin somewhere.

As relativity illustrated, all times and distances are relative to which frame of reference you are talking about

Yes, which is why I carefully specified the cosmic frame and not say some inertial frame, which isn't valid at all at large distances since spacetime isn't Minkowskian.

And anything outside your light cone is irrelevant to you (eg if some object is now 100 billion light-years away, light from that object will probably never reach us, due to the expansion of the universe).

Correct. Any recombination light emitted from what is currently over about 58 BLY away will never reach us. Any light emitted today from over 16 BLY away will also never reach us. That latter figure is the current distance to the event horizon.

9

Technology / Re: How were images transferred before the digital age?

« on: 08/01/2023 18:31:53 »

Ahh video, hours and hours of people recording their holidays on camcorders,

Before Facebook, you had to photograph your lunch, rush to the chemist, get the film developed, make 25 postcard prints,then go to the post office and send them to your friends, by which time the meal was cold.
But whilst Evan can't see his recent photos on  his new computer, I still have black and white prints of equipment I built nearly 60 years ago, and if you want a good picture of an extinct bison there are plenty of 40,000 year old cave paintings that show you how to hunt them with spears.

10

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

« on: 08/01/2023 10:26:07 »

Hey...  Does anyone have any comments on my back-of-the-envelope calculation of air being 99.85% vacuum?

Especially the part where I try to derive the refractive index of air from the refractive index of liquid nitrogen
- The method I used for that calculation used a lot of hand-waving
- What would be a better method of calculating it?

While your calculation isn't precise, it's a whole lot better than lots of the ideas put forward in this thread.
Ignoring the oxygen is not a serious problem because the refractive indices of O2 and N2 are pretty similar.


It's possible to measure the speed of light in a gas at varying pressures and then plot a graph and extrapolate to zero pressure, which will give you the speed of light in a vacuum.

11

Physics, Astronomy & Cosmology / Re: why do a lot of people confuse between interference and diffraction?

« on: 28/12/2022 09:50:26 »

(400 kHz homing beacon)...interference and diffraction are distinct phenomena, but both are effects of superposition

The behaviour of radio waves from a homing beacon can be described quite well by Maxwell's equations. This is "classical" physics.
- Superposition derives from quantum theory, and cannot be described by classical physics.
- You don't need superposition to describe radio waves

Superposition principle has been widely used long before quantum theory.
https://en.wikipedia.org/wiki/Superposition_principle

The superposition principle,[1] also known as superposition property, states that, for all linear systems, the net response caused by two or more stimuli is the sum of the responses that would have been caused by each stimulus individually. So that if input A produces response X and input B produces response Y then input (A + B) produces response (X + Y).

A function F(x) that satisfies the superposition principle is called a linear function. Superposition can be defined by two simpler properties: additivity


and homogeneity

for scalar a.
This principle has many applications in physics and engineering because many physical systems can be modeled as linear systems. For example, a beam can be modeled as a linear system where the input stimulus is the load on the beam and the output response is the deflection of the beam. The importance of linear systems is that they are easier to analyze mathematically; there is a large body of mathematical techniques, frequency domain linear transform methods such as Fourier and Laplace transforms, and linear operator theory, that are applicable. Because physical systems are generally only approximately linear, the superposition principle is only an approximation of the true physical behavior.

The superposition principle applies to any linear system, including algebraic equations, linear differential equations, and systems of equations of those forms. The stimuli and responses could be numbers, functions, vectors, vector fields, time-varying signals, or any other object that satisfies certain axioms. Note that when vectors or vector fields are involved, a superposition is interpreted as a vector sum. If the superposition holds, then it automatically also holds for all linear operations applied on these functions (due to definition), such as gradients, differentials or integrals (if they exist).

12

New Theories / Re: Not-Quite-So Elementary, My Dear Fermion

« on: 28/12/2022 04:30:44 »

However, I hope you will not be offended. If I said, we could use a second opinion of a physicist with regards to this particular point. because I believe it's important, I don't remember you addressing it anyway

Sure, anyone who wants should feel free to chime in.

Literally existing!? partly existing!? There are no such things, and they have no meaning in this universe, it's either you exist, or you don't.
I think the name quasi-particles has nothing to do with them being real or imaginary or partly existing. They have mass and they interact, they are a form of matter.

I'm not trying to say that quasi-particles aren't real, only that they aren't the same as true elementary particles, and thus aren't "real" particles (in the same sense that "horny toads" aren't "real" toads): https://en.wikipedia.org/wiki/Quasiparticle

Although am not sure if they can split the positron in a different experiment into anti-quasi-particles.

In principle, it should be possible to do so, but you would need bulk antimatter in order to accomplish that. Put a positron in normal matter and it annihilates.

what mattered is the question: would there be any matter left of the Electron? which you did not address as well.

Assuming you could somehow get all three quasiparticles to interact with a positron at the same time, then I think it would annihilate and thus become radiation. In that case, nothing would be left of the electron.

Maybe, they don't need a medium like light, or maybe when they are bound, they are each other's medium, maybe if there is a force that governs their bound state (external factor) maybe, it would be their medium, can you say for sure it is impossible to be?

Being in the right medium is what gives rise to them. That's what makes them quasiparticles. An analogy is phonons, which are another type of quasiparticle. Phonons are quantized vibrational modes in a lattice of atoms. If I had a solid that was filled with phonons and then pulled it apart atom-by-atom, then all of the phonons would be gone because there is no longer any way for lattice vibrations to exist in individual atoms. That's what I mean by quasiparticles being an emergent phenomenon. Take away what's needed for emergence, and you can no longer have them. If you still think that quasiparticles are elementary particles, then I think you still have misconceptions of what a quasiparticle is. Here is a video that explains them:

The takeaway here is that spinons, orbitons and holons are not more fundamental than electrons. Rather, they are more complex than electrons.

I don't see that; they can't cease to exist in any sense other than "cease to exist"

If you pluck an electron out of a solid, then it's just an electron. If it was an orbiton, holon and spinon before you did that, then yes, you can choose to say those three quasiparticles ceased to exist when you did that.

13

Just Chat! / Merry Christmas

« on: 25/12/2022 00:56:53 »

Merry Christmas everyone.

   
  {Image from Wikimedia - should be copyright free, more or less}

Stay cheerful and we'll see you in the new year.

14

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

« on: 21/12/2022 05:28:08 »

Hi again.
    Looks like everyone was writing a post at the same time.
Somehow everyone forgot to mention the important point about degeneracy pressures.

From one of the very early posts:

There is space between the particles, held at distance by the nuclear strong force which is strong enough to resist the pressure due to the gravity.

   Which is not entirely true and also uses the term "pressure" in a very informal way (I think "compression due the gravity" might have been better.  You can see that calling it pressure instead of compression has started a bad trend because @evan_au has gone right down the same road).

(i)  To keep it simple, gravity produces a net inward force, pressure does not.  Pressure is a force in all directions and the gradient in pressure is what can give rise to an outward force that can help to fight against gravity and prevent further collapse.
(ii)   It isn't just the strong nuclear force that stops the collapse.   The degeneracy pressure is a significant contributor.   Just to be clear then for anyone else reading, the collapse of the star is not caused by pressure, quite the opposite, pressure is on our side fighting against the collapse.

   What is degeneracy pressure and why would it form a gradient (With P decreasing from the centre to the outer edge of the star)?
[LATE EDITING - surplus discussion removed]  ...Well no-one asked and it's not entirely relevant to the OP, so I won't bore everyone with that.

If the electrons could get to the protons, they'd likely be turned into neutrons as had occurred to most of the proton/electron matter, but the picture below has electrons quite deep where the protons are, so go figure.

     The Chandrasekhar limit describes the point where the density of electrons would exceed that permitted by the Pauli exclusion principle.  If the star has mass above the Chandrasekhar mass, then gravity is too strong and the pressure due to electron degeneracy is insufficient to stop it.  At that point, either the temperature has to increase, so that the degeneracy of the electron gas is lifted and higher energy states are available to the electrons  (which doesn't happen*)  OR ELSE  the usual thing happens.
   *I'm going to pause here and just mention that there really was a good way out of the problem - electrons could have just occupied ever-higher energy states (which is just saying they could have higher momenta).   However, there was already high enough temperatures and pressures so that electrons and protons can fuse and that is the escape route from the Pauli exclusion principle that will be taken.   Pop Sci articles often skip that and just imply that violating the Pauli exclusion principle was inevitable at this point.
   As the star continues to collapse (and since promoting electrons to even higher energy states isn't done) the Pauli exclusion principle is about to be violated and that just cannot happen,  that number density of electrons has to be brought down somehow.   What seems to happen is that protons in the region pick up some electrons and form neutrons.  This is where or why we get a Neutron star,  the electrons are being depleted and only Neutrons tend to persist.   However, there is no need to deplete ALL the electrons,  Pauli is quite happy with their being some density of electrons,  it just can't exceed a certain density.
   In a simple model we have:    At any place in the universe,
n_e(p) dp  ≤     
  where n_e(p) dp =  number density of electrons (number per unit volume) with momentum between p and p+dp.
   There's nothing really special about a Neutron star in this respect, it abides by the same limits imposed by the Pauli exclusion principle.   It's just that the density can be so high that the limit of n_e(p) dp  was reached.
   To say this another way,  an ordinary lump of iron on your desk at home can have a free electron density of about 10²⁸  electrons per cubic metre.   So a Neutron star can also have an electron density of 10²⁸ electrons per cubic metre anywhere in it, even right at the centre.   
    Actually the momentum distribution of the free electrons in the lump of iron on your desk followed a Maxwell-Boltzmann distribution (more or less) which meant that the temperature had to be quite high to support that density (room temperature say, otherwise at some value of momentum p, the quantity  n_e(p) dp  would have exceeded the limit).  Meanwhile the electron gas in a Neutron star is almost fully degenerate (which means all of the lower energy states are filled only) and so that has a very different probability distribution for momenta.  Being fully degenerate is the optimum way to accomodate all of the electrons while having the lowest total energy.  This means, a degenerate gas of electrons would support a total electron density of 10²⁸ at a much lower temperature  (an equivalent way of saying this is that an electron gas with that total number density of electrons would not be fully degenerate unless the temperature is close to 0 kelvin).  I'm mentioning this because a simple model would treat the electrons in a Neutron star as an electron gas which is assumed to be fully degenerate - and yet the temperature is many thosuands of kelvin.  So that the total number density of electrons is necessarily way above 10²⁸ electrons/m³ everywhere.

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.

   That comment from evan_au was just another example where it reads as if "pressure" is what was causing the inward collapse and not the thing opposing it.  Additionally, degeneracy pressure has to be mentioned again.  It's not just the strong force that is keeping the Neutrons apart.  Neutrons are also fermions - they behave much like electrons and must comply with the Pauli exclusion principle.  The degeneracy pressure of the Neutron soup is the last bastion that gravity has to overwhelm.   This one is a much more genuine last great barrier where degeneracy pressure and the Pauli exclusion principle really is everything because there is no alternative this time.  The electrons could combine with protons and be removed.  The neutrons don't seem to fuse with anything and be removed.

Best Wishes.

LATE EDITING:  Changed notation   n(e) dp   to    n_e(p) dp     which is more sensible to describe the number density of electrons at a given momentum p.

15

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

« on: 28/11/2022 16:30:16 »

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.

Though a great bit of that halo extends beyond the Galactic disk, and thus, due to the shell theorem, has no effect on the stars' orbits around the Sun.  Only that DM at a lesser distance from the center than the star is question would.
 So for the Sun, DM has an effect on its orbit, but doesn't dominate, as the DM closer to the center than we are, while a significant fraction of the total mass affecting our orbit, it isn't the majority of it.
If you look at the predicted and observed rotation curves here:
https://www.astronomy.ohio-state.edu/thompson.1847/1101/RotCurve2.gif
Note that close in to the center, you see little to no difference, as the mass of visible matter heavily dominates there. You don't see the deviation until you get out of the central bulge region and into the disk.

16

General Science / Re: Would a pipe made of synthetic diamond erode and be corroded with seawater?

« on: 24/11/2022 23:24:36 »

if the tetrahedral diamond carbon structure could coated with something like a graphene carbon surface,

The graphene would act as kindling.
https://en.wikipedia.org/wiki/Graphene#Chemical

graphene carbon surface, which has no loose bonds

Graphene is entirely covered with loose bonds and is rather reactive.

But at the surface are "loose" carbon bonds, which are much more reactive. Dissolved oxygen will react with these bonds, exposing yet more unpaired carbon bonds, and slowly eroding the diamond.

If that was significant, the same process would occur in the absence of water  and indeed, it does. Diamonds will burn in air.
But the reaction is so slow that you need to work on it.

Adding water will slow down the oxidation.

Diamonds in seawater are pretty much "forever".
The sun will boil the oceans before the diamonds dissolve (unless you make them impractically thin).

17

General Science / Re: Why is gold gold?

« on: 19/11/2022 23:56:52 »

Multiple effects can be blamed for the colors of metals--sometimes it is from the electronic structure of the metal itself (as in the case of gold, freshly polished copper, bronze, etc. see below), and sometimes it is due to the electronic structure of what is on the surface (often dull colors due to "tarnish" "rust" "patina" etc., like red iron, green copper/bronze, dark silver), and sometimes it is due to the thickness of an otherwise transparent oxide layer on the surface, creating colors due to interference, just like with soap bubbles or petroleum residues on the surface of water (creating the rainbow colors observed on bismuth crystals, or can be done intentionally, as when anodizing aluminum to achieve certain desired colors). There can be even more unusual phenomena involved when the metal is in very small particles (nanoparticles), where the electronic structure can become non-metallic (at least at "defect sites", which become more densely present in smaller and smaller particles, and then there's plasmonic resonance, which is how gold or silver particles in stained glass can adopt so many colors)

So, with that out of the way, I will now just focus on answering why gold is gold:

Gold is metallic because it has a partially-filled "conduction band" that spans the entire lattice. The electrons in this band can be considered delocalized across the whole "piece" of gold, and are very much free to be anywhere, or "moving" anywhere given a potential. There are ALSO bands that are completely filled (lower in energy than the conduction band) AND bands that are completely empty (higher in energy than the conduction band). Photons of suitable energies (frequencies, wavelengths) will be absorbed, promoting an electron from one band to another. And in the case of gold there are a few absorbtions in the blue part of the visible spectrum, so we perceive it as yellow.

(being able to calculate the electronic structure of gold is no small feat--it requires QM principles, modified by general relativity, because the electrons close to the gold nucleus experience both relativistic time dilation and mass dilation, compared to electrons far from the nucleus!)

18

Technology / Re: How to learn process control system in practice?

« on: 17/11/2022 17:33:53 »

bends in the road can take the car in front out of the beam for a few seconds. So it appears to "remember" the speed of the car in front for perhaps 5 seconds

But the car in front could do an emergency stop in the 5 seconds it's out of view, can the assisted braking respond in the time left after the car reappears?

The more complexity in the control system, the more responsibility on the driver to understand it in detail, and to understand (and take action) when it hits corner cases where it doesn't do what you intend.

The more complexity in the control system, the more opaque it generally gets.

if the car in front is slowing to turn a corner, and I can see that it will be safely out of the way before I get close; the radar system (configured for sensitive mode) sees the car in front slowing to a near stop in the line of sight, and rapidly slows down my car. Flicking off the speed control for a second resolves this problem.

The problem with automation is fighting it to stop it doing what you don't want can often make more work than doing the job yourself.

19

Physics, Astronomy & Cosmology / Re: Complex or real wave function?

« on: 15/11/2022 12:18:14 »

Hi.

As I understand it, the magnitude of a wave function indicates the probability of detecting the particle in a particular place.

   Yes.      At a particular place and also a particular time  if  ψ = ψ(x,t) = a function of space AND time,    which is how  CompuAI seemed to be representing the wave function.   
    Also,   it's a probability density rather than a probability on it's own.        Given a small element of space  δx    and  of time δt    then  you do need to multiply by the space-time volume element  δt δx,    so that    |ψ(x,t)|² . δx . δt    =  probability of finding the particle in the element of space  between  x and x + δx    and  at a time between   t and t + δt.
     Similar idea  if  it's 3-dimensional space instead of just one dimensional along the x-axis.    Then  we use the space element δ³x  =  δx δy δz.

    Anyway,  the first part of what CompuAI had written looks OK.    You CAN consider the Schrodinger equation as a some differential equations involving only a real valued  function(s) if you want to.    In that case,  you have a PAIR  of simulatenous differential equations linking time derivatives of  Ψ₁ (x,t)  to  space derivatives of Ψ₂ (x,t)    (and vice versa....  time derivatives of Ψ₂   are linked to space derivatives of Ψ₁ )   but   both  Ψ₁    and  Ψ₂    are  real-valued functions.

     In general, it's NOT easier that way.    In solving that pair of simultaneous differential equations you probably would start by just writing it as one combined differential equation with a complex valued function.    However, it's of some relevance because there is always the discussion point about whether Complex numbers are in some sense "real",  i.e. that they have some deep real world meaning because they are essential in Quantum Mechanics.    The sort of thing where you replace the Schrodinger equation with a pair of real valued differential equations suggests that,  no you don't need Complex numbers,   you can keep all wave functions real valued  BUT it's harder that way.

Best Wishes.

20

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

« on: 13/11/2022 15:27:39 »

Hi.

   Yes that's the right idea @evan_au .   Except that you need to take squares, so the probability of a photon passing the filter is 0.5 not 0.71...    Under classical field theory (so that is E and B fields) the intensity of light received is proportional to the square of the amplitude of the wave that is transmitted.   You resolve the incoming wave into components and get the fraction Cos 45°  for the amplitude that is transmitted exactly as you stated.  That means  Cos² 45°  =  ½  is the fraction in Intensity.   Then you can jump back to the particle and photon interpretation where  intensity  is just proportional to the number of photons received (per unit time and per unit area of the receiver).  Hence, you get the probability ½ for each photon.

Many compounds with chiral asymmetry that exist in right and left handed configurations will rotate the plane of polarisation...

   I completely agree.  It's not clear that polarising lenses based on crystalline macromolecules really are just blockers rather than rotators of polarised light.   (I also agree with the other statement but that's why I'm asking if anyone knows of a suitable experiment).

Best Wishes.

LATE EDITING:   Overlap with Colin2B who just posted.  At a glance, looks OK and this post doesn't need adjustment.