1

Just Chat! / Re: Theories to start a new world.

« on: 08/12/2024 15:11:02 »

Hi.

   That's a nice reply and can have the best answer award.

And of course the learning of new facts and understandings.

    I'm sure you're smart enough to realise that not everything in the forum is the gospel truth.

Best Wishes.

2

Just Chat! / Re: Theories to start a new world.

« on: 02/12/2024 22:58:48 »

Hi.

Religion is all about selling undisprovable hypotheses in order to control people's actions.

   No, that just sounds like human selfishness, corruption and some desire for power by a few people who have come to be in charge of the religion or else in charge of something else and just citing religion as supporting them.    Religion is about not wanting to be alone, left feeling pointless, worried about not existing any longer, seeking the answers to fundamental questions  etc. etc.   Things that you aren't going to make disappear just by not bringing any religion to the new world.
   c.f.   @paul cotter  's reply.
- - - - - - - - - -

I think you meant "non physics" science rather than "non physical" science.

    Physical sciences involve studying the inanimate stuff while Biological science is the study of the animate (living) stuff.   There's definitions on the web and it's reasonable to split science into these two broad categories.   I think I can say non-physical sciences and just hope people understood I meant Biological sciences rather than the study of nonsense with no physical basis.

- - - - - - - - -

It must be useful there and then

    Thanks for your time and response.   It's good and perfectly understandable.   It does have some comedy but is otherwise similar to the response first given by @paul cotter so probably won't get the best answer award.....  Which really doesn't matter anyway and would not change your life.

Best Wishes.

3

New Theories / Re: what is temperature?

« on: 25/11/2024 13:20:30 »

Hi.

    Thanks for your comments @alancalverd .    I think I can see what you're saying.
Although there isn't anything resembling an ensemble of particles inside your box, there is only one particle in the box, we can theoretically treat it as a confined ensemble of particles.    That theoretical ensemble of particles could have a temperature much as you described.
     
      I don't often sympathise with a lot of stuff that @hamdani yusuf  talks about.   However, temperature isn't the simple or completely objective thing that we may have thought it was after sutdying some school level science and there's not much wrong with discussing this in a science forum like this.   It can be fun to re-examine some of the stuff that you (we) thought we knew and understood.    All the words you may have heard at school like "we are only presenting some models and as you study more, you'll find that many of these models will be replaced with something else or possibly that the model is just wrong and fails to explain some things but there's no better model available at the moment" are actually true but the problem is that people do actually leave school and then there's no school reunion event to actually follow up with the discussion about this.   A forum like this one can usefully provide a place for such discussion.

      If you (we) end up teaching some young people then we'd better make sure we teach the syllabus but at least we can say those words in red   (.... we are only presenting some models..... ) with some genuine conviction.

      One of the things about "temperature" or "hotness" of an object that I don't think has been mentioned much so far is that it can depend on the place where the object is put.   I hope @hamdani yusuf  won't mind too much if I spend a moment discussing something about this here.

Let's start here:
        Temperature is a measure of a quality of a state of a material. The quality may be regarded as a more abstract entity than any particular temperature scale that measures it, and is called "hotness" by some writers.   The quality of hotness refers to the state of material only in a particular locality, and in general, apart from bodies held in a steady state of thermodynamic equilibrium, hotness varies from place to place....
     [Taken from Wikipedia under "Basic Theory" for temperature.   https://en.wikipedia.org/wiki/Temperature#Basic_theory  ]
     
What does it mean?    Well it could mean that some things, like a long rod, have a temperature of 100 deg C at one end and a temperature of 80 deg C at the other end -   temperature can be locally defined and vary from place to place.   However, that is almost certainly NOT what was meant.   They probably meant that even if the whole rod had a temperature of 100 deg C while it's measured in one place then if we pick the whole rod up and just put it in a different place, it's quality of hotness or temperature can change.

    An example:    The Zeeman effect describes how the energy levels occupied by electrons can split in the presecence of a magnetic field.
     We consider a theoretical object, a sink for all thermal energy, that we identify as being at 0 Kelvin (absolute zero).   If we put a test object into thermal contact with this sink then energy will flow from our test object to the sink   if and only if   the test object has a temperature above 0 kelvin.  So we have a way to know if our test object is at absolute zero.
    Now we have a test object that is a bit like a group I metal, with a single valence electron in its outer shell.  We'll have this test object in its lowest possible energy state.   In a place with no magnetic field, the electrons have no lower energy level they can fall to, we can make thermal contact with out idealised thermal sink and no energy will flow to that sink.   Our test object seems to have a temperature of 0 kelvin.
    If we move our test object to a region with a uniform but non-zero magnetic field, without deliberately adding or removing any energy to the test object, then the electrons wave functions become a superposition of the two energy levels available from the splitting by the Zeeman effect.   Alternatively, if you prefer to think of the electrons individually and more classically then you could say half of the electrons go into the lower energy level, half go into the higher energy level and overall the energy of the test object remains the same.    Either way you wish to consider it, there are some electrons capable of falling into a lower energy state.  So when we make thermal contact with the sink, some energy can flow into the sink and out of our test object.   It seems that the test object has a temperature above 0 kelvin now that it's in a place with a non-zero magnetic field.

   There's quite a few things about temperature that are strange and not as clear cut as we may have thought.   In General Relativity we don't often consider temperature as an important thing, it may not be fundamental or objective enough.   For example, the temperature of an ideal fluid doesn't appear in the stress-energy tensor but the pressure of that fluid does.   Similarly, cosmological "equations of state" relate energy density to pressure and don't involve temperature unlike the typical "equatons of state" that you may encounter in thermodynamics.   Pressure may be a bit more fundamental, objective, or useful as a property that an object can have rather than temperature.    I don't know.... it's all just a model but maybe a model where we recognise temperature as just an approximate and emergent property is a small step closer to the underlying nature of things.

    In that respect, I would differ from @hamdani yusuf 's  line of attack:   I'm not sure there is a lot of point trying to make a better or more precise definition for temperature.   It's just an approximation that is useful sometimes.

Best Wishes.

4

Question of the Week / Re: QotW - 24.11.08 - How effective are cancer screening tests?

« on: 14/11/2024 01:13:58 »

Hi.

    I'm sorry to hear that @Bored chemist  .     
There's a lot of treatments and different people repond differently.  We genuinely just don't know how long we've got.   

I don't suppose there's a lot I can say that you haven't already been told or researched for yourself.   Just assume I've said all the usual stuff and I'm sure everyone else that you've helped on the forum would also say it.

The good news is that you'll be on the forum for years.

(The very best of) Best Wishes to you.


     

5

Physics, Astronomy & Cosmology / Re: Are Atoms Perpetual Motion Machines ?

« on: 10/11/2024 19:57:13 »

Hi.

Is it not the case that, where the source is receding from the receiver,  the photon red shift equals the increase in gravitational potential  of the transmitter?

No.   For a start, in the most general situation where we have a dynamic and curved spacetime, there just isn't a sensible "gravitational potential" that we can identify.
     Gravitional potential is something that may seem like it should be there - but that's just what follows from Newtownian mechanics where gravity was a force.   

Reference:   An informal discussion appears here:      https://www.nature.com/articles/314129a0.pdf

Best Wishes.

6

Physics, Astronomy & Cosmology / Re: Are Atoms Perpetual Motion Machines ?

« on: 09/11/2024 19:53:16 »

Hi.

I am fairly sure the first law is sacrosanct and will never be broken.

    The first law is usually stated in terms of energy passing into or out of some system or body.   We might assume the universe is a suitable closed system.   However, General relativity only enforces a local conservation of energy principle on the universe,  globally the total amount of energy in the universe could change.
    E-m radiation travelling through an expanding universe increases in wavelength.   The energy doesn't appear to have gone somewhere else, it is just lost.  More generally, as per Noether's theorem, since an expanding universe does not posses time translation invariance, we would not expect to have a conserved quantity that we could call "Energy".
    I'm fairly sure you ( @paul cotter  ) already know and have heard this sort of thing before.   It makes no practical difference here on earth and over timescales of a human lifespan.  Over that span of time, our local bit of the universe hasn't changed very much and time-translation symmetry is a very reasonable approximation.   Hence, there is an associated quantity we can call "energy" and it is independant of time (it is "conserved"). 

   Anyway, if you ( @neilep  or anyone else)  want to defy the first law of thermodynamics then (I would think) the easiest approach is just to change time and space.   For example, adjust the expansion rate to suit your purposes.  So that's the theory bit all done and dusted.   All you need next is some engineers and technologists to do their bit and make the equipment.

Best Wishes.

7

Just Chat! / Re: What is the purpose of the conductor in an orchestra?

« on: 27/10/2024 23:38:25 »

Hi.

    I seem to recall what our conductor did....   To be honest, if our conductor made an impromptu change in something just on the night when a piece was being performed, then a lot of us would have missed it and played the thing as we did in rehersals anyway.   We would have eventually noticed but not before some of us had made timing mistakes or terrible noises.
     However, the conductor was there during rehersals and that's when they really did their thing - made some changes and generally managed the whole ensemble.   They were the master and overlord of this whole thing and so they do deserve their place right up at the very front of the group.
     So I think that the main contribution by the conductor is what has happened "behind the scenes" before the audience was there.   As @Halc suggested, there are going to be a few places where we did really have to see when we were supposed to start or end but even that would have been brought to our attention by the conductor and practised a few times during rehersals.

Best Wishes.

8

Physics, Astronomy & Cosmology / Re: Why does Quantum Stuff Still Move At Absolute Zero ?

« on: 19/10/2024 13:30:52 »

Hi.

    I'm fairly sure we've had some other threads discussing temperature and the sorts of issue that @neilep has raised.

This is a short summary of how I see the current situation.   Note that  (i)  I'm only one person and don't claim any special expertise in this field of study,    (ii)  It is apparent that our notion of temperature is changing, so that whatever is said here will probably change again in a few years.


Summary:

    Historically temperature was understood in many different ways.   One simple approach is that temperature was the thing you measure when you stick a thermometer into something.   Problems with this approach were too numerous to mention.  For example, how do you build a reliable thermometer?   Sticking a thermometer into some liquid is reasonably easy but sticking a thermometer into some solid is often impossible.  How do you ensure you're getting good thermal contact etc?      In the very old days, some things just didn't have a temperature if you couldn't stick a thermometer into it.
     At a later time,  let's say between 1848  (when Lord Kelvin was developing his Kelvin scale) and 2019 (when the weights and measures commitee redefined temperature) it was quite popular to define and consider temperature using a thermodynamic approach.   That actually worked quite well.
    Since May 2019,  international agreement has been to define temperature based on a Kinetic Theory approach.   That's not my fault, it's just what was done by the weights and measures committee and whoever establishes the SI units for Science.   They wanted to improve our definition and undertstanding of temperature and it was thought that basing temperature on this theoretical framework would make it more objective and absolute.   Just to be clear about this - the modern Kelvin scale is NOT the one that Lord Kelvin developed, we kept the name but ditched most of the notions that Lord Kelvin used.

     We have a great model and good understanding for this kinetic theory approach  IF  the substance is an ideal gas.   The trouble is that most things are NOT ideal gases.   We have a few other reasonable classical models for other things but by no means do we have a good classical model for the microscopic behaviour of everything.
     One very important thing to note is that the sort of microscopic mechanics that is used in these models and that the weights and measures committee have declared we MUST use,  are based on classical mechanics and NOT quantum mechanics.    This is a problem because some things are evidently quantum mechanical but we never-the-less have to do some translation into an equivalent classical system when we assign a temperature to that system.   As mentioned already, we have some good classical microscopic models for some things but sometimes we just do not have any such model that we can use.   Overall, the attempt by the weights and measures committee to make temperature more objective and absolute have caused as many problems as they may have hoped to solve.

   Reference:   Spend the time reading Wikipedia's entry for "temperature" and you'll get some idea of how temperature has been redefined.    https://en.wikipedia.org/wiki/Temperature.     That Wikipedia entry has been edited and adjusted since the last time I looked at it about 1 year ago.   As mentioned, it is very likely to keep changing in the near future.


   Anyway, one answer or reply we can offer to the OP is the following:
You just aren't allowed to use a quantum mechanical model for the substance you are trying to assign a temperature to.   You would HAVE to translate the behaviour of your substance into some classical microscopic model.    What you'll then end up with is a temperature that says something about the average kinetic energy per particle, even if it's a total abstraction to imagine that anything like individual classical particles could be found in this substance or thing.
    In your case, despite your assertion that this thing has temperature 0 k, it probably won't be and can't be 0 k.  You probably can't obtain an official temperature of precisely 0 Kelvin unless the zero point energy of your thing actually is 0 and the corresponding classical model would have particles with 0 kinetic energy.

Best Wishes.

(Last edit to fix some spelling errors).

9

New Theories / Re: Could quantum mechanics be wrong?

« on: 08/09/2024 02:49:37 »

Hi.

Can quantum mechanics explain the generation, propagation, and reception of radio waves?

Short Answer:
   Probably not in an easy way for human beings to understand. 
   It probably is possible to use QM and apply a suitable interpretation of probabilities or the frequency of detection of photons to re-create the usual gross-scale classical radio wave propagation results - but this would be complicated.
   At the fundamental level, in most QM models there aren't any "radio waves".   So the problem starts with the way the question was phrased.

Medium detail / further discussion:
     The name "radio wave" that you are using in the question describes a phenomena that we (human beings) have come to ascribe some meaning to.   This thing,  "a radio wave", is an essentially classical physical phenomena.
    We imagine a radio wave as a ...  (you know) .....  spread out and continuous wave thing.  We expect to be able to measure an Electric and a Magnetic field and a radio wave will be a suitable oscillation in these fields.  There are many versions of Quantum Mechanics but most of them don't have that sort of thing as their subjects or objects of interest.   The simplest QM models consider all objects as if they are particles.   For example, we might consider a particle of light, a photon, as a suitable QM object but we do not consider an e-m wave as a suitable QM object.    QM objects just happen to have a wave function associated to them BUT they are essentially discrete particles and would always be detected as a discrete particle.
      More advanced QM theories, such as the mainstream versions of QFT, will consider the most fundamental objects to be fields.  For example, there will be a photon field that exists everywhere in space.   The thing we call a photon would not be treated as a fundamental QM object, it is just how we may interpret a certain oscillation in that underlying photon field at some place.    So, in QFT, there is a photon field but there is NOT any fundamental Electric field or Magnetic field.   These things, the electric and magnetic fields are just gross-scale fields that can emerge from the more fundamental fields of the particles in the standard model of particle physics.   QFT doesn't aim to deal with oscillations in the Electric and Magnetic fields, it only deals with the fundamental particle fields.   You need to add a layer of interpretation on to the top to translate what happens in these fundamental particle fields into something that can be observed in the gross-scale Electric and Magnetic fields.   That can be done in many versions of QFT but is rarely simple.
    When you are asking about "a radio wave", you have moved attention to something in terms of the ordinary (or gross-scale) Electric and Magnetic fields and not on any fundamental object (a photon in simple QM, or the photon field in QFT).   So the question cannot be answered without translating the results of QM back into something classical involving the Electric and Magnetic fields.   One of the simplest or most naive ways of making this translation is to just analyse the probablities or frequencies with which a QM object would be found at some place and time.   So, we might imagine that an e-m wave is a collection of many photons or that an experiment involving one photon has been done many times and the e-m wave we observe is the combined average of all of these.

Example:    If you could fire just one photon through the usual double-slit experiment apparatus,   then you probably won't get an interference pattern on the screen at the end.   Instead we think you'll get just one spot to glow on the screen.   However, if we repeat the experiment many, many times then we expect to see that the places where the screen glows and how often that bit of the screen glows will correspond to the usual interference pattern for a classical wave being put through that apparatus.    So, one individual photon does NOT behave like an e-m wave - but a thousand or a million of them would seem to reproduce classical radio wave-like patterns and behaviour.

NOTE:   I'm really not convinced anyone has actually done a genuine version of Young's double slit experiment where only a single photon is fired with a clear interval of time before the next is fired.   However, this demonstration gets close and claims to be using such a thin and dim stream of light that only "a few" photons per second will be produced.
https://sciencedemonstrations.fas.harvard.edu/presentations/single-photon-interference


   So, the important point is simply that "a radio wave" isn't the normal subject for any QM model.  It demands some form of interpretation to put the results predicted by any QM model back into terms which would make sense when you consider "a radio wave".   
   If we wished to be controversial we would say "there are NO radio waves... just QM objects that get detected at certain times and places.  On a gross scale, that looks like a radio wave is there".    ... However... I'm not that controversial or convinced that a simple QM model is some ultimate truth....  there are many versions of QM, all have some use and classical e-m theory is also extremely useful.   

Another minor note:    Your (Hamdani) extracts produced by Gemini seem to be blending many different versions of Quantum Mechanics.    For example,  QFT is a specialised theory and there are still about 3 versions or flavours of QFT that are in current use.   I don't think Geminii is reliably identifying where things can and cannot be combined.   For example, the simplest QM models are not like QFT.   They will involve the original Schrodinger equation, which is shown here:



That has a mass term, m, in the Hamiltonian for the QM object under consideration    (it's in the square bracket, h squared over 2 m).   This means it's actually useless for photons which should have 0 rest mass.   None the less, we (physicists) do generalise a lot of results from such a QM theory for massive particles to massless particles like photons.  For example, the Youngs double slit experiment can be modelled for electrons or even bigger particles like carbon bucky-balls.  We can (and have) then done experiments with these things and obtained a good match with the predictions from the simple QM model.   It is natural to assume a similar thing for photons, even though the zero mass term would technically make the simple QM model invalid.   If you read what I've said (above) about firing single photons then you'll hopefully notice that I've got phrases like "we think".   
     I believe that how you reconcile a classical radio wave with a suitable QM object ("a photon") is subject to a lot of interpretation. 

[Disclaimer:     I'M NOT AN EXPERT.  ]

Best Wishes.

10

Just Chat! / Re: Fed up with the lack of reasoned discourse

« on: 29/06/2024 02:41:15 »

Hi.

         A spinning magnet - that can be interesting.   Indeed we had some discussion here (TNS forum) about a year ago.   You ( @paul cotter ) contributed to that old discussion but it was a long time ago.   I'm going to recap some of it.

       In general, a thing that is rotating should just keep rotating at the same rate, that's basic angular momentum conservation.   However, in outer space that doesn't seem to be what happens....    well... at least not if the spinning thing is actually a magnet.    In that situation it does seem that the object emits e-m radiation and MUST eventually stop spinning.

      Let's put conservation of momentum and angular momentum to one side for a moment.    If a spinning magnet emits e-m radiation then it is losing energy.   Energy conservation MUST apply.    If energy is being emitted or lost from the spinning object then it came from somewhere.   Either the spinning object is losing some mass over time, or else it is losing kinetic energy by slowing down its rate of spin.    It doesn't seem that a lot of mass is being lost, so the object must be slowing down its rate of rotation even though there is no obvious source of external torque being applied to the object.

     A long time ago, I started writing a new forum post based on this - but it became too long and had to be split into many parts.    In the end, I decided it was a monologue rather than a discussion so I never actually posted it.

   (Part II) had much of the mathematics in it.   Not a lot of that displays well now that LaTEX markup isn't available.     In words, it showed that the far field produced by a spinning magnetic dipole has the right characteristics to be considered as e-m radiation.        "Does a spinning magnet in space emit e-m radiation" --->  yes, it does.

    (Part III)  was based on an objection that @alancalverd raised in the (very old) original forum thread concerning the notion that the spinning object would ever slow down.   Where is the torque coming from to slow the magnet down if it's in outer space?     How is angular momentum conserved?   etc.
    The answer mainly seems to be that the e-m radiation that is emitted will be circularly polarised.   This allows the e-m radiation itself to have or "carry away" some angular momentum.   Well.... that explains "where" the angular momentum has gone but it still doesn't explain the source or cause of any actual torque acting on the spinning object.
      The torque acting on the spinning magnet seems to arise by consideration of the near field (near to the dipole object) as opposed to the far field approximation.   The mathematics here is just too complicated (for me anyway) and the other references I chased up seem to use only numerical (computational) approximations instead of finding exact analytical solutions.   In spirit, it seems that the near field changes at a rate slightly slower than the dipole is rotating,  so that the B field in the local space differs from the magnetisation, M vector, within the object itself.   As such the B field pulls on the magnetised object slightly and effectively puts a drag or retarding torque on it.   So, let's re-phrase this because it's quite weird when you think about it.......   What exerts a torque on the spinning magnet in free space?  Well... it seems that space does or whatever it is that we recognise as the B field in space.  Space isn't exactly "nothing", it has B fields in it.    The spinning object will spin at the speed it wants to spin, of course, and the Magnetisation vector M of that object should change at that rate - however space a few centimetres away cannot instantly change its B field when the magnetised object has moved.   The maximum propagation speed of a new B field is c,  so the B field just a small distance away is still the old B field.    This is a very simplified and wordy rather than mathematical version of what appears to happen but I hope it makes some sense and provides the gist of what seems to be going on.    It raises a number of other questions... space doesn't seem to be just empty space, it's almost as if the B fields in it are something tangible - but that's probably a whole other thread and not something we need to consider here.

You mentioned the following (which is why this example is relevant):
 

The discussion was about possible exemptions to the conservation of momentum because of the finite speed of em propogation.

     The spinning magnet in free space seems to be an example where instead of being a problem that may break the principle of conservation of (angular) momentum,  the finite speed of waves propagating through the E and B fields is actually NECESSARY, indeed it is ABSOLUTELY ESSENTIAL to maintain the principle of conservation of angular momentum.   It is the (tiny) delay between the magnetisation vector of the magnet changing its orientation and the changing of the B fields in nearby space that puts the retarding torque on the spinning magnet.   So, a magnet spinning in free space will slow down and eventually stop spinning and no laws or principles of conservation are violated.

Best Wishes.

11

Question of the Week / Re: QotW - 24.06.28 - Are there stars which aren't part of a galaxy?

« on: 21/06/2024 21:02:53 »

Hi.

Possibly.
     The definition of what is a galaxy, or exactly where one ends is vague.   Hence, under one definition of a galaxy, a given star may be in a given galaxy but under another definition it is oustide any galaxy.

One reasonable definition of a galaxy:
    Galaxies consist of stars, planets, and vast clouds of gas and dust, all bound together by gravity.

[Taken from NASA website.     https://science.nasa.gov/universe/galaxies/ ]

    The key here is that we have some understanding of what it means for gravity to be generally keeping a collecton of things (stars, gas, plantets etc.) together.    Gravity follows an inverse square law with distance.   This means that at some distance from the centre of what we would define as the galaxy, gravity will be too weak and the star(s) this far out are not considered to be part of this galaxy any longer.

The definition of a galaxy is then mired by the existance of dwarf galxies apparently inside or just on the periphery of a larger galaxy.   For example, our Galaxy (the Milky Way), has the (Large and Small) Magellanic Clouds as dwarf galaxies.   They can be though of as small galaxies in their own right but they have become trapped in the gravitational well of the Milky Way and now orbit around the periphery of our galaxy.    So, that they are gravitational bound to our galaxy and could reasonably be considered as just a part of one big galaxy instead of being called galaxies on their own.      We can consider ever larger structures (regions of space) until we identify regions we call Clusters or Super-Clusters.   All the mater in these regions is gravitationally bound together, they do not tend to drift apart, but they appear much as many seperate galaxies orbiting around each other in some complicated way (literally "a cluster of  galaxies").    However, since everything in this cluster is gravitationally bound together, it forces us to question whether we should just define the whole cluster as one really big galaxy.

-------
   So, there probably isn't going to be a definitive answer to the original question.   If there's a star in space somewhere then it is probably inside someones definition of some galaxy.
   None the less we can still try to address some of the spirit of the question:    Stars don't tend to form in complete isolation from other stellar bodies.    Based on our simulations of cosmology and the evolution of the universe,  matter tends to distribute itself into filaments spaning through space.   The overall structure is often referred to as a cosmic web   (you can Google the term Cosmic web to find more info if you wish).    Stars will form only where there is matter for them to be formed from.   So stars tend to form along these filaments rather than in the voids in between them.   Once stars have formed, gravity naturally tends to start attracting other material and vice versa (other material will attract the star), so that stars will usually end up with other stars orbiting around them and/or they are pulled into orbit around some existing cluster of stars.

Exceptions
   Can there be a star that is NOT gravitationally bound to any galaxy?   Yes.    Over enough time, it is likely that every star would eventually be attracted to some some galaxy and orbit in some stable fashion.    However, for a short while (in Astronomical terms that can be a million years), some stars can be "flung out" of a galaxy and fly through the voids for a while.
     Although stars cannot form in the voids between galaxies (since the density of matter is far too low), there are in fact large numbers of intergalactic stars. It has been estimated, for example, that 10 per cent of the mass of the Virgo galaxy cluster is in the form of these stellar interlopers........
     ..... stars can be expelled from their parent galaxy if it collides, merges or passes close to another galaxy.

[Taken from:   BBC Science Focus.    https://www.sciencefocus.com/space/are-there-any-stars-between-galaxies]

Best Wishes.

12

Technology / Re: Does regenerative braking have a hazardous complication?

« on: 27/04/2024 14:33:10 »

Hi.

In Ireland when changing down in a manual the recommended method is to lightly touch the brakes

    As far as I know that's not written in the "highway code" booklet (for the mainland UK) or generally expected - but I have once had such advice from my driving instructor.

    There are also alternatives, like double de-clutching   (double clutching in the U.S.):   The strongest slowing down effect is caused within moments of lifting your foot off the clutch pedal when you change to a lower gear.   This is when the engine, drive shafts on the engine side of the gear box and gear wheels in the gear box which were only spinning at (for example) 2000 rpm has to increase to 3000 rpm since you are initially at the same road speed but will now be in a lower gear.    An option then is to shift gear to neutral, ensure the clutch pedal is then released, briefly touch the accelerator pedal to spin everything (engine and the gears in the gear box) and then clutch and shift again to the new lower gear.   This isn't in the highway code either but it can be done and you will soon get quite good at it.
https://en.wikipedia.org/wiki/Double_clutching_(technique)       (that article talks about reducing wear-and-tear rather than application in racing).

    Another level up again is to just change gear without even using the clutch pedal.  You ease your foot slightly on the accelerator to reduce the pressure or contact force between gear teeth, then move the gear stick to neutral, then increase pressure on the accelator again (but it won't be any seconds, just milli-seconds because without any load the engine will pick up rpm almost instantly), then when the new gear is spinning at about the right rpm you'll push the gear stick again and engage the new gear while everything is still spinning.   With a lot of practice (which involves a lot of terrible noises from the gearbox while you fail) you can perfect this.   To practise this, the usual requirement is to use your dads car and be about 17 - 18 years old.  Your dad will have enough money to get their gearbox repaired and re-built after all the times you failed and have broken gear wheel teeth while trying to enage a gear wheel spinning for 4000 rpm to another gear wheel that was only spinning for 2000 rpm  etc.  (and after-all, new drivers do make mistakes even while making less extreme racing gear changes).   Once you own your own car, you won't try it again because the technique of double de-clutching is actually good enough and considerably cheaper.

    Anyway, all of these driving techniques are fast becoming obselete because most new cars will be fully automatic.   These days, they (automatics) are better and don't result in a loss of performance (e.g. fuel economy). 

     I suppose we should address the original question:
Does regenerative braking have a hazardous complication?
   Possibly but the driver behind shouldn't be following that close.   If they are then according to the highway code you must now adjust your own driving appropriately.  For example, reduce your speed so that they can stop in their braking distance.   It's not exactly what they deserve but it's what is safe and recommended.   Her Majesty's Secret Service takes a dim view of it if we keep deploying the special counter measures on ordinary civillian drivers who were just too close.

Best Wishes.
[ These days     Her = His       but all the films are based on Her Majesty's Secret Service ]

13

Physics, Astronomy & Cosmology / Re: Does time dilation and length contraction apply to light?

« on: 08/04/2024 01:56:22 »

Hi.

   I basically agreed with @Halc and therefore didn't need to say anything.   However, it's always pleasant to hear from a moderator (Kryptid) and since you give up a lot of time to answer others, we ought to give up some of our time to reply to your post.

I had been under the assumption that time is frozen for light because it travels at the speed of light.

    This is usually a result people obtain or propose just by extrapolating the Lorentz Transformation with v→c.   So it can be useful to have as many ways as possible to guard against it or see where it might fail.   It's especially helpful to go along with their line of reasoning and identify some potential flaws in it, which is what I'll try to do first (below).  It's also possible to go straight against the line of reasoning but then people will wander around trying to figure out why their line of reasoing wasn't right and whether you might just be wrong or trying to brush a potential flaw in physics under the carpet.

Situation 1:    People often personify light and assume that this result (extrapolating LT with v→c) tells you how things would look or seem to the light.
    Problem 1:  We shouldn't personify light but let's be tolerant.
    Problem 2:    Whenever people make the claim 'this is how it seems to "Fred" ',  they are always telling you how Fred reports things with co-ordinates of an inertial frame where Fred is at rest.    For example, no one would dream of telling you how Fred would report things or identify events in a co-ordinate system where Fred himself is still moving at a speed of  0.133 c,  or  c/2, or  -0.3c   why would any of those frames seem natural to Fred?   It is clear that we're interested in how Fred reports things with co-ordinates derived from a frame of reference where he is at rest and we would probably be prepared to accept that a frame where Fred is at rest is the one that seems most natural to him.   Provided Fred does use the co-ordinates of his own rest frame, then his wrist watch is measuring the co-ordinate time that he will use, that seems natural.   If he used any other frame then his wrist watch is not counting the co-ordinate time he would be using (and that isn't a natural choice for Fred, if he's anything like me and you).
     So the problem with light is that we have no guarantee that light is at rest in any inertial frame.    People have usually just tried to extrapolate  the LT  with v→c but if you start trying to do this, there is no evidence that light is ever slowing down.   Go along with a simple approach of just following a pattern....

      With boost parameter  v =0.9c   ,   light would move at speed c in the primed co-oridnates.
      with v = 0.99c,  light would move at c in the primed co-ordinates.
      with v =0.999c,  light moves at c in the primed co-ords.
      with v =0.9999c,  light moves at c in the primed...
      .....
      with v =0.999999999999c,  light still moves at c in the primed co-ordinates.

   The important point is:  There's nothing here suggesting that light is slowing down in the primed co-ordinates as you increase the parameter v towards c.
    Now you could even use some algebra and pull out the relativistic velocity addition formula.   You can easily show that light is always travelling at c in the primed co-ordinates, regardless of how close that LT boost parameter v  gets to c.

    This is very diferent to what happens with Fred.   If you tried the same game with Fred,  then as you increase the v parameter in the Lorentz transformation (LT),  his speed in the primed co-ordinates is slowing down.   Moreover you can even over-shoot the mark and have the parameter v set so high that Fred is travelling backwards.   Specifically, we know that we can always find an inertial frame where Fred would be at rest.    However, we have no such guarantee with light,  it never seems to slow down in the primed co-ordinates regardless of how high we set the v parameter in the LT.

     Now when people (who have just extrapolated the LT with v→c) suggest that   "this set of things"  (like time being frozen and length contacting to 0) is how it would seem to the light,   you at least have something you can come back with as a reply or counter-argument.    You'd say  "but this isn't how it seems to Fred, even though he could have chosen to use the same frame.  I'll assume that's because Fred would naturally choose to use a frame where he is at rest.    However, we have no guarantee that light would be at rest in this frame you are proposing for light and so you cannot claim that this frame would be the natural one for light to use or that this is in some way how things would seem to the personified light."

- - - - - - - - - - - -

Situation 2:     You just try to remind them of the limits of SR.
    SR can be derived in various ways,  however the most common method is just to assume light has a speed c in all inertial reference frames.
    So it is clear that any frame where light is at rest cannot be an inertial frame and therefore isn't a situation that SR ever aimed to or claimed to say anything about.
     They probably will have arguments about extrapolating the LT with v→c and suggest or insist that it ought to mean something or be valid.   You could try to remind them that it's valid while v<c but abruptly stops being valid when v=c.   They won't like it but you can remind them that this happens often in physics.   
   There's a whole set of favourite examples you might have,  here's just a few you can use:
  (i)  A position on planet earth can be uniquely defined by spherical polar co-ordinates everywhere and perfectly continuously right up until you hit the North or South poles.   At these poles, the situation abruptly changes,  the azimuthal angle can be anything you like and yet you're still describing the same position and not a different position.   You can't do this anywhere else, a different azimuthal angle would certainly be describing a different position but at the poles, it's fine.   This isn't a continuous change, it's a discontinuous or abrupt one. 
   (ii) A graph of  y=1/x  behaves perfectly sensibly with x gettting as small as you like.   However at x=0 it's undefined,  worse than this it's very badly behaved.    It might seem that it's becoming positive infinity but you only have to look at small negative values of x and you realise it's a strange and slippery thing.   It somehow doesn't come back from positive infinity, it seems to come back from  negative infinity.   Any pattern you may try to follow as x → 0 has got to break at x=0.
   (iii)  The helicity of a Dirac particle will be a fixed unvarying quantity (+/-  1 for an electron)  in all frames of reference right up until you overtake the particle, in those frames the helicity changes sign (would be - or resp. + 1).  It's an abrupt change, it doesn't slowly slide down to +1 from -1.  Extrapolating the behaviour for frames of reference with boost parameter v < particle speed (in the lab frame) would not have predicted this behaviour.  That'll do as an example and you could leave it there.   If you know what helicity is then it's apparent that it must change sign when you overtake the particle.  However, this is why I picked this example because we can push this one a bit further:  Light is an interesting particle in that it really does always have a constant helicity in any inertial frame of reference.   This is a result you can obtain from Quantum Mechanics.  We can only understand that result classically by recognising that light cannot be overtaken (or even brought to rest) in any inertial reference frame.

- - - - - - - - - - - - -

Situation 3:    Again you go straight against any attempt to extrapolate the LT with v→c and their claim that this is an indication of what would happen in a "frame" where v=c and light might be at rest.   However, this time you target the tacit or implicit assumption that you would have anything you can call a valid frame.
    A co-ordinate system can be almost anything but it MUST serve to uniquely identify different events, this is part of the formal definition of what a co-ordinate system must be and is non-negotiable.   Just take all of their claims at face value.    So they claim that light starting it's journey over here and the event where the light ended it's journey over there (which could clearly be identified as different events in the lab frame) are given the same  x,y,z,t co-ordinates in the rest "frame" of the light.   That automatically and necessarily dictates that this "co-ordinate system" obtained from the "rest frame" of the light is actually NOT a valid co-ordinate system.
     We have no experience of using a degenerate pseudo-co-ordinate system and it is not a natural choice for Fred to try and use a degenerate pseudo-co-ordinate system.   There are plenty of degenerate systems Fred could have chosen to use but he didn't,  he chose a co-ordinate system where time was the thing on his watch and distance was something which would also help to keep different events separated, only that seemed like a natural choice of co-ordinate systems to Fred.   So it's not sensible to suggest that anything reported in the degenerate pseudo-co-ordinate system you propose for the "rest frame" of light indicates how things would seem for the light.
     We then have two possibilities:
     (i) There may be some co-ordinate system for light where its notion of time is the thing that passes on its hypothetical wrist watch and its notion of space stuff also helps to seperate different events in some way.   However, the Lorentz Transformation does not seem to be the thing connecting these co-ordinates for the rest frame of light to the co-ordinates of space and time that we use.  (We can see that precisely because it would suggest that light is left with a set of numbers or pseudo-co-ordinates that fail to separate different events).
    (ii) There just may not be any such co-ordinate system or a suitable rest frame for light.

- -  - - - - - - -
   That's too long, even I'm fed up with listening to myself.   I hope some of it is of some use.

Best Wishes.

14

Question of the Week / Re: QotW - 24.04.12 - What law governs the distance of an electron from the nucleus?

« on: 06/04/2024 16:41:20 »

Hi.

     Why are we arguing over fine details when the basics haven't been esatblished?

'What law governs the distance of an electron from the nucleus of an atom?'

     The basic answer is that it depends on the model(s) you are using.

    In a very simple model, the electron is attracted by electrostatic forces to the nucleus but the electron is assumed to be in some motion.   So the electron orbits the nucleus like a planet orbiting a star.    The laws governing everything are similar to Kepler's laws for planetary motion but where Newtonian gravity is replaced by Couloumb's law of electrostatic attraction.

     There is now a whole set of more advanced models we can work through to suggest slightly different answers to the question....
     Eventually, you reach a level of sophistication in the models where the basic question needs to be completely re-examined.   For example, an electron may not be an ordinary particle as we might imagine, it may be a wave in an underlying field and thus "spread out" all over the place.   The question becomes impossible to answer because there are no definite positions for any of the things mentioned in the question and hence no way to determine a distance as the difference between these two positions.

     I propose that the best answer for the question is to take things only as far as you can so that words like "electron", "nucleus" and  the distance between them   continue to have the same meaning as that which is suggested in the question.
    In practice, a fairly classical model of an atom works quite well and adequately describes the macroscopic or bulk properties of most materials.   We can determine the typical distance between atoms by performing experiments involving X-ray crystallography or surface electron scattering (often called  LEED = Low Energy Electron Diffraction).   We can reasonably assume that in the solid state of this material, the atoms pack together as closely as they can so that they are effectively in contact with each other.   Indeed we find that they are only a few crystal lattice structures for solids and simple cubic crystals   (such as  b.c.c. [body centred cubic] ,  f.c.c. and just  s.c.) are prevalent.   
    It is found that most atoms have an effective radius in the order of 1 Angstrom  =  10^-10 metres.   For example,  Copper has a crystal lattice constant ≈  3.6 A   (Editing:   The website won't support an A with a circle over it,  A means Angstrom  and any ? character is probably an Angstrom that I didn't edit)    =>   atomic radius ≈ 1.8 A  (note that there are two atomic radii between the centre of two atoms that are in contact).     
   Glance through this website (   https://periodictable.com/Properties/A/LatticeConstants.html   -   I'm not guaranteeing the reliability, it's just the first site that came to hand and readily displays the info we want).   Although there is a little variability the lattice constants are all between 2 - 7 Angstrom.
     Back this up with our favourite model of an atom,  the Schrodinger Hydrogen atom solution,  where the tpyical distance of the electron from the nucleus is the Bohr radius ≈  0.5 A  and note that this is one of the smallest atoms around.
     Overall, it seems that as regards the bulk properties of materials,  all atoms tend to have a radius of about 1 Angstrom.    At first consideration, this is a bit surprising,  e.g. Iodine has 53 electrons so you would have thought they occupy electron shells that are much further out than the  1s orbital of a Hydrogen atom.   However, we need to balance this against the idea that the total charges involved are 53 times greater than that in Hydrogen and all the orbitals are effectively pulled in and are of much smaller radius.    Indeed we get a fair explanation of the result by just considering the last electron of any atom.   In a simple (often called a Hydrogenic model), the last electron is essentially outside all the other electrons and the nucleus,  so it "sees" a total charge (in units of e) interior to its orbit that we can express in terms of Z, the atomic number:
  (Total charge seen, in units of e)  =   (charge from the nucleus)   +    (charge from the other electrons)
                                                     =   Z   +   -(Z-1)     =   1.
   We can approximate the behaviour of the outermost electron by modeling it as though it sees a total charge of just +1e at the centre of the atom.    To say that more simply,  the outermost electron would behave like an electron around a Hydrogen nucleus and so it's not at all that surprising it always has a typical distance from the centre of the atom equivalent to the distance displayed in the simple Hydrogen atom.

    That's where I'd leave the answer.   Summary follows:

1.    The standard rule of thumb is that atoms have a radius ≈ 1 Angstrom   irrespective of the atomic number of that atom, i.e. what that atom actually is.   If you want a "simple law", you can't get much simpler than this.   The distance between an outer electron and the nucleus is about 1 A.
2.    This is what we see when we perform practical experiments on bulk materials  (e.g. LEED experiments on crystals).  We have the real-world evidence.
3.    A simple explanation is that the outermost electron of any atom will behave and therefore take an orbit as if it is orbiting a Hydrogen nucleus.
4.    It's possible to estimate the radius of orbit of an electron around a Hydrogen nucleus.   This depends on the model you might use but most of them do suggest a radius of about 1 Angstrom.   So the theory agrees reasonably well with the evidence.   You could use models (of varying sophistication) to estimate the distance of any other (more interior) electrons from the nucleus, if you wish.   However, this has negligible relevance for any properties of the bulk material.   We could leave all interior electrons as being "some cloud" rather than assuming they must have a definite radius of orbit.

Best Wishes.

15

General Science / Re: Is Absolute Zero Possible?

« on: 04/04/2024 20:34:16 »

Hi.

    The simple answer is the one your High School chemistry teacher gave you:   No.   That would be sufficient for most exams at most levels.

   It looks like you've already had a few comments or read some replies in other places (you mention other forum boards). 

    A lot of arguments will consider temperature with definitions based on kinetic theory (hot things have fast moving particles in them,  cool the thing and the particles slow down,  at zero Kelvin all motion of the particles should have stopped).   This is not the only way you could define "temperature", for example we can take a thermodynamic approach like the one suggested by @alancalverd involving Carnot engines.  We can also take an approach based on statistical mechanics:  Entropy can be directly defined as proportional to the number of microstates a system can occupy at a given energy   and temperature can then be defined as a derivative of entropy with respect to a change in energy.

The details are not important, it's just important to note that "temperature" is NOT a simple thing to define and there are several approaches that are all useful and equally valid.

    In typical situations, all of these definitions of "temperature" will be equivalent, they will all be measuring and describing the same thing.   However, they each describe what it would mean to be at zero Kelvin slightly differently.
The Kinetic Theory definition of temperature is often the easiest one to work with and has become the approach incorporated into the standard set of units for describing quantities  - the S.I. units.   S.I.  = "Syst?me International d'Unit?s"   [French.  Late editing: this website doesn't want to reproduce accents over the e].    So this is the definition of "temperature" that we would normally assume but it is not without problems.    For example, the basic models associated with this kinetic theory are all based on classical mechanics and not quantum mechanics.    So,  when you obtain a numerical value, say a temperature of 100 K in the SI units, for a substance it's telling you about some properties related to a classical model of the substance only.

     I expect you've already seen lots of discussion and arguments to state that a tempertaure of 0 kelvin could never be reached.  So I won't repeat that here.   It's true enough when you recognise that temperature was always set up to tell us only about a classical property of a substance under a conceptualised (and fully classical) kinetic theory model.   Instead we'll try to go around the problem and consider a situation where quantum mechanics is going to win the day.

     There are some things and situations where classical models just aren't useful or relevant any more.   In particular there are some substances (like a simple gas) that can form a Bose-Einsten condensate.   This is a situation where all the particles are occupying the lowest possible energy state.   This can happen at what we describe as a low "temperature" using the SI units for quantifying temperature   (e.g. close to but NOT exactly at 0 Kelvin).   When the gas has reached this state,  trying to cool it further is irrelevant, everything is in the lowest energy state, there is no lower energy condition it can have.     The particles do not have a "kinetic energy" that varies in proportion to the "temperature" in the region of space that some other instruments or thermometers you may have are reporting this "temperature" to be.   Whether it was 1 K  or  0.1 K  doesn't make the slightest bit of difference to the condensate, nothing about it can change, all particles are in the lowest energy state.
     So we are in a situation where the conventional definition of "temperature" just doesn't apply to the condensate.   It doesn't have this temperature as any sort of intrinsic property.   For example, lowering this "temperature" doesn't make the particles move more slowly.  Similarly, increasing the temperature (provided you keep it below the critical condesation temperature) doesn't make the particles move any faster.
     The "temperature" which your other equipment in the region is reporting and we nominally assign as the temperature of the condensate is just an artefact of the way we define "temperature" in SI units.  It's saying  "  if the condensate was represented by a classical system in kinetic theory, then the particles would have this much energy"  -  but the key point is that the condensate is NOT in any way behaving like a classical system any longer.   It is absurd to assign a value for the temperature of the condensate as if it was a classical system, because it just isn't one.    There is no physical difference in the condensate at 1 K,  compared to the condensate at 0.1 K.   Whatever this nominal temperature is describing it isn't any physical property that the condensate has.   I have probably laboured the point but I just wanted to try and make it clear.   When we consider quantum mechanical behaviour, very low temperatures can lose their meaning and just be artefacts of something that would happen if the system was a classical one.

    Anyway, I hope that's a positive note to end on.   We have cooled some things down enough to form a Bose-Einstein condensate and so they have achieved their absolute zero temperature  - the point at which no lower energy or physically meaningful internal temperature could be assigned to the substance.

Best Wishes.

16

Just Chat! / Re: Can an interest in space exploration lead to a greater interest in science?

« on: 02/04/2024 00:41:02 »

Hi  and welcome.

    A lot of us are cautious over first posts because there are a lot of instances where things just become advertising.
However, your post looks interesting and looks like you've put some time into it rather than just copying some text from somewhere.

Can the space program help inspire people to become supportive of other science or has the evolution of my beliefs been due to other things besides the revival of the space program and a TV show?

    Obviously, I don't know what caused your interest in science better than you would know yourself.
A lot of people are interested in space and the science fiction genre for books and TV shows usually attracts a big audience.   I'm reasonably certain this stirs some interest in science among many people quite often.

Looking back I am embarrassed.....

   It's less important what you did some years ago and perfectly fine to acquire or re-acquire an interest in science now.

Best Wishes.

17

New Theories / Re: How does light propagate? By what means does light get from Point A To Point B?

« on: 01/04/2024 17:24:42 »

Hi.

Some minor questions from @Zer0  seem to have been missed.

Please do Allow a few randomly absurd thoughts...

   I'll assume you want short replies.

Can " Space " bend over itself?

    I'm not sure what you were asking.    Here are 2 possibilities:
1.     Can space act to bend space?   Space isn't as empty as you might think,  there is always vaccum energy in every cubic metre of space.    All energy contributes to gravitation (the curvature of spacetime).   So space will bend spacetime and some of that curvature will be purely in the spatial part of spacetime,   so we could reasonably say "yes, space can bend space".  Indeed "vaccum energy" is an important component in modern cosmological models.
2.      Can space roll up like a carpet or fold over itself somehow?    It's best that you don't think of space as some 2-dimensional rubber sheet that curves in 3-dimensional space,   so that whatever image you have of space bending over itself, it probably isn't like that.   Black Holes are an example of a region where curvature becomes extremely degenerate as you approach the singularity.   You might declare this as a region where space has folded over on itself but you'd need to be working in higher dimensional flat space to "see" that.
     String theory sometimes considers the existence of additional dimensions that are described as being tightly rolled up and generally go un-noticed.  However, this isn't really a consequence of space doing anything to roll these other dimensions up,  they just are tightly curled up for some reason.

Can ' Relativistic mass ' not produce a similar, but not the same, effect such as Gravity?

     Yes and it essentially does produce a gravitational effect broadly equivalent to having more mass.
    1.   We don't really use "relativistic mass", it's an old term and discouraged - but we don't need to, we capture the important information about the movement in other ways.
    2.   In GR various things are sources of gravitation.   Sources of gravitation are all the forms of stress-energy that exist and all this information appears in the stress-energy tensor.   So, mass is certainly a source of gravitation but so is energy, momentum and pressure.    So a moving particle contributes a lot more to gravitation (the curvature of spacetime) then a stationary particle.    A moving particle has mass, kinetic energy and momentum.   It can also elevate the pressure that exists in a region of space  (e.g. a gas has pressure due to the movement of particles in it).    Overall, the curvature of spacetime will be more significant when a moving particle is in that region than when a stationary particle is in that region.   
     If you have Newton's ideas of gravity in the back of your mind then it is broadly equivalent to a moving particle acting as a source of gravity according to its relativistic mass rather than its rest mass.   (Why only "broadly equivalent"?  At relativistic speeds, where relativistic mass would be significantly different to rest mass, we'd be better off abandoning Newtonian mechanics and Newtonian gravity entirely and using General Relativity.   Patching up Newtonian gravity with relativistic mass is only going to be a rough or crude adjustment you could make).

Could the ' Higgs field ' twist, tangle, entangle in on itself?

   I don't know.   The Higgs field isn't an ordinary physical thing and I don't suppose it's very useful to think of it as a thing that can twist and tangle in any ordinary sense.   However, such terminology may exist in some very specialised models and areas of Physics that I haven't seen.

Best Wishes.

18

Just Chat! / Re: My History.

« on: 30/03/2024 15:01:26 »

Hi.

You also have my sympathy and I'm sure the same applies from a great many people.

"Your history" isn't just a collection of medical things that may have happened to you.  You won't be judged (or shouldn't be judged) on your medical history.   You've been involved in several discussions here.   I would have thought you're welcome to continue asking questions or join discussions in this website.

Best Wishes.

19

Physics, Astronomy & Cosmology / Re: Why do the Earth's satellites orbit longer than the Moon's?

« on: 21/03/2024 23:29:10 »

Hi.

planetary satellites do not have satellites

    There are many moons around Saturn  (146 according to the NASA website today - but new bodies get identified often).   Of those some were thought to be moons that were in a fairly stable orbit around other another moon - but carefull examinations seem to have shown they weren't.     (Reference:  https://en.wikipedia.org/wiki/Subsatellite    under discussion of "possible natural instances" of sub-satellites).
     None-the-less  there are so many things up there around Saturn that some moon will have some other body that will orbit it,  even if that sub-satellite isn't much bigger than a dust particle.

    This article  ("Can a moon have a moon?",   BBC sky at night magazine),   https://www.skyatnightmagazine.com/space-science/can-moon-have-moon       also discusses the issue.    Their main conclusion was:.......

    ....the moons probably don?t have a large enough gravitational field to control the motions of a smaller object and compel it into orbit.   The planet would likely still be the controlling influence and create an unstable orbit. So the answer is probably no: planetary moons cannot have natural satellites.

    In general, it is known that the mechanics of a gravitational 2-body system will usually result in the formation of a stable orbit of the two bodies around a common barycentre.   However, the 3-body problem is much more complicated.   For example, one of the bodies is often ejected or thrown out of the system over enough time.  It is presumably rare for a moon to retain a sub-satellite in a stable orbit around itself.

Why do asteroids have satellites,

    Many of them stay far enough away from large sources of gravitation (like a planet) that they are effectively only two-body problems,  the asteroid and its satellite.

Best Wishes.

20

Physics, Astronomy & Cosmology / Re: How many spots are produced from Stern-Gerlach apparatus that rotates?

« on: 16/03/2024 14:54:29 »

Hi.

how are the silver atoms entrained in a beam?

    Put silver in a furnace chamber,  it will eventually start to vaporise.   Put a small hole in the side of the chamber and a beam of essentially atomised silver streams out of it.
    Probably not something that passes health and safety standards but it was all done in a vaccum chamber that was well sealed.   I don't know the full history but I'm sure there are some full explanations and accounts of what was done.   There are a few critera they wanted to meet  (e.g. I think they wanted a thin flat beam so they used slits inside instead of round holes,  probably also ensured the beam was reasonable collimated by passing it through two slits etc.)

on Earth- which is rotating.

   It is,  although not in the way we (possiby just I) want to rotate it.   The centre of rotation and axis are in the wrong places.

Best Wishes.