1

Physics, Astronomy & Cosmology / Re: Correlation vs association

« on: Today at 00:56:18 »

Hi.

    Good general discussion from @evan_au above.

is it wrong to say that they are correlated to each other

    No it's not "wrong" it's just a bit dangerous or could be misunderstood.
    Essentially it depends on your target audience  -  the people who you expect to read your statements.

The phrase  "X and Y are uncorrelated" has a precise meaning to a Statistician or Mathematician.   It means precisely r(X,Y) = 0    (the correlation coefficient = 0)  and nothing more.    They won't jump to any other conclusions, in particular they won't assume that X and Y are completely unrelated or independent variables.  They know that X could still be entirely determined by Y, they just aren't linearly related.

The phrase "X and Y are correlated"   would just mean that r(x,Y) equals anything else other than 0.   To be honest, that's a rare phrase to use for statisticians.  It would be more common to take more lines and state that r(X,Y) cannot be zero but its not clear that a linear relationship exists  or else just leave it written in symbols   r(X,Y) ≠ 0.    If you did leave that phrase "X and Y are correlated" as if it was some sort of final conclusion then they might reasonably assume you meant that  X and Y are strongly correlated,    or  that   |r(x,Y)| ≈ 1.   To say that in plain English - they might assume that X is (or is almost entirely explained by) a linear function of Y.

    If your target audience is not a group of statisticians,  then you "know" that when people hear the words   "correlated"  or  "uncorrelated"   they will jump to conclusions about whether X and Y are independent or unrelated.   They might make even bigger jumps than that and assume one thing is actually the cause of the other.    So if your target audience isn't a group of statisticians, then you really must do as @evan_au  suggested and choose your phrases more carefully.

Best Wishes.

2

Radio Show & Podcast Feedback / Re: Feedback on: Question Of The Week in Podcast vs Forum

« on: Yesterday at 23:39:49 »

Hi.   Perfectly good points you've raised @evan_au .

QOTW is a question, and the Forum is about answering questions, so it's still worthwhile posting QOTW on the forum.

    Is it about just finding something to do, evan_au?   Maybe it is and that's fine. 
    The QOTW can stay, just make it a bit clearer that no-one was actually seeking an answer and is likely to be interested in the replies there.   It's just something you can do for fun and to pass the time - which is fine.    Other forums have something like a "puzzle of the week" so this one might just as well have a QOTW.
 
    If it seems interesting to you (@evan_au) then, of course, you can and should spend your time on it.   There's never been any reason why you couldn't comment on a podcast and start a new post at any time, of course you should.

     I may be wrong, it's just that you (possibly "we") seem to be under the impression that it matters to the original person who asked the question.  It doesn't most of the time, they aren't going to read the forum and the podcast has already been made that answered that question.    If you were giving up your time under that assumption, then don't.    Your expertise would have been better spent working with a different thread on the forum.   Helping just one real person is important and something which this forum does well.

Best Wishes.

3

Radio Show & Podcast Feedback / Re: Feedback on: Question Of The Week in Podcast vs Forum

« on: Yesterday at 12:30:22 »

Hi.

Just wondering if this part of the discussion should be in the feedback section as it isn’t directly answering the question?

    I'll just remove it.

Best Wishes.   

4

Radio Show & Podcast Feedback / Re: Feedback on: Question Of The Week in Podcast vs Forum

« on: Yesterday at 02:34:08 »

(A minor complaint was removed).

  Note that it wasn't completely ignored,   the originator (me) received a reply by email from Chris of the NakedScientists.

5

Physics, Astronomy & Cosmology / Re: relativity paradox or misunderstanding?

« on: 23/05/2022 22:15:27 »

Hi.

   This is complicated   BUT  an excellent question to ask and think about.

When light passes through a region with high relative permittivity it no longer travels at C...

    That would be correct considering light as a classical electromagnetic wave.  It's more conventional to describe the slowing of light by referring to the "refractive index" of the medium instead of the "permittivity" of the medium - but the basic idea is right.    Using classical e-m theory, a change in either the permittivity (a thing affecting Electric fields)   OR  the permeability  (a thing affecting magnetic fields)  should change the speed of propagation of an e-m wave.
   The typical equation connecting these quantities is:
   v = velocity of wave propagation  =       with    ε = permittivity.  μ = permeability.

- - - - - - - -
   However, as @Halc stated,   that's the speed of a classical wave.   A photon is a quantum mechanical particle and is quite a different thing.   A photon should always travel at the speed c, in any reference frame and through any medium.

    How can the wave propagate slower than c, when a photon is always moving at c?   Probably too complicated for me to explain,   I'd only get bits of it wrong anyway.   Most texts start by carefully defining and examining the difference between "group velocity",  "phase velocity" and the speed of a wavefront.

Here's someone else trying to explain it.  It's still a simplification and therefore quite accessible but equally it just jumps over and avoids discussing some complications.
"Why does light slow down in water",  FermiLab ,  available on YouTube.

Or does one use a reduced value of C such that the Lorenz expression still is indeterminate ie 1/0 ? 

   Definitely not.   c = the speed of light in a vacuum ≈ 3 x 10⁸ m/s   is always the constant that appears in the Lorentz transformations,  regardless of the medium in which a particle or thing you are considering is located.    To say that another way,  this value, c, has importance in special relativity.   Changing frames of reference won't be any different in some medium even if light has a different phase velocity in that medium.

Best Wishes.

6

Radio Show & Podcast Feedback / Feedback on: Question Of The Week in Podcast vs Forum

« on: 23/05/2022 20:19:38 »

Hi.

  Sometimes I wonder if something strange is happening.
There's a short gap (maybe 10 minutes?) between releasing a podcast with the answer and asking the question on the forum.     Sometimes the question precedes the answer but not always.   It's a bit random.

   What seems less random is that putting the question in the forum is almost a  complete waste of everyones time.   @chiralSPO has just spent time creating a well formatted and detailed reply.   The radioshow / podcast people paid it no attention and ultimately it has done nothing more than repeat the podcast.   Even if the person who asked the question does join the forum - which only happens about 1 time in 20 - that reply has wasted their time in addition to Chiral's time.

    I've asked what the point of running these "Questions of the Week" is before,  so I'm not even going to bother asking it again.   All I might do is offer some advice and sympathy to @chiralSPO  -  always look for the podcast first before you spend your own time.   If the podcast isn't there yet, wait 10 minutes.    It was a good answer @chiralSPO  but all you're likely to get is a "thanks" icon at the end of your post from people like me.

Reference:
https://www.thenakedscientists.com/podcasts/question-week/can-acid-remove-springs-potential-energy
   Officially released  23 May 2022.

Best Wishes.

7

Physics, Astronomy & Cosmology / Re: How can I pass a large DC current through mercury?

« on: 23/05/2022 10:47:33 »

Hi.

I would take the advice here with lumps of salt!

   No, please don't.
   Mercury is extremely dangerous.   There are regulations in place in most countries concerning the use and handling of mercury.   
    You wouldn't take a rock into a busy street and throw it a random direction under the assumption that it probably won't hit anyone, even if it usually doesn't.   Similarly you must not expose yourself, others who live in close proximity with you, or the pregnant lady who comes to your door delivering a package  to mercury vapour by heating the stuff in the way you have suggested.

Thank you and Best Wishes.     

8

Just Chat! / Re: a suitable pseudonym

« on: 22/05/2022 01:35:32 »

Hi.

Since virtually all scientists are adults, they could not be autistic, by definition.

    It's a moving definition.   They weren't diagnosed as being on the spectrum but they might be now.   There's also no obligation for someone to be diagnosed, or any reason it would be useful to be so diagnosed.
    Presumably, you don't have a diagnosis that you're not on the spectrum and even if, for some reason you do, is it updated every four years?

The rest of your post is good.   Let's just be clear that this all about neurodiversity not disability.  All human beings are unique and it is very good thing.

Best Wishes.

9

Just Chat! / Re: a suitable pseudonym

« on: 22/05/2022 00:45:38 »

Hi.

I had assumed it was HAL from the space odyssey books, but version c  - so not trying to kill everyone.
NOAX   would have been hard to work out.  No-one would have known it was Non-Oxide Adhesive eXperimental,  or a pop singer.   Best guess -    "No Axe to grind".

a long name just means a lot of typing.

   Presumably you do know that if you start with an  @ symbol  and just type something like @ET  a drop-down list replaces it with  @Eternal Student in no time at all?     There's a risk it also notifies the person but almost everyone has turned that feature off.

Best Wishes.

10

Just Chat! / Re: a suitable pseudonym

« on: 21/05/2022 23:21:35 »

Hi.

How safe is that assumption?

    It's safe in that it does no harm to assume.   It's helpful to choose a username that is helpful.

I can see no reason to adopt a pseudonym

     The single biggest reason is to achieve some minimal security.   Using a genuine name and being too precise about where you live is clearly dangerous in an online setting.    You're a moderator, you can't recommend people use their genuine names.
     Science forums have risks that are too numerous to mention:   People who can't stand to see the word of god challenged,  Animal rights activists,  Double Glazing Salesman,   Partners who might find out you aren't cleaning the house and doing the laundry  .... no end of risks.

Best Wishes.

11

Just Chat! / Re: a suitable pseudonym

« on: 21/05/2022 18:47:08 »

Hi.

   Choosing a username is a fine art.   I'm happy to send people to sleep with some advice:

Consider the abbreviations
People often tend to shorten names online, which I leant by experience.   So if your username is  "Dickson" that will be shortened.    Initials are also used quite often,  so    "Brave Understated Man"   will be an unfortunate acronym.

First Appearances
    Next, online your username is one of the first things people see,  it's like your appearance in real life.   You can shift that through time and interaction, this is a forum for science and hopefully the average user cares more about what is said rather than who said it.   However, your name is what you start with and why make people start with an impression that is miles away from where you are?

Help don't hinder identification
    People can't remember most of the interactions or comments other people have made.   When I started using this forum I thought all "moderators" were the same person.  It wasn't until I looked more carefully that I saw they were not one person and sometimes they contradicted themselves.   Even when I noticed, it's not like I can stop being human at the flick of a switch.  I'm flawed and I can't easily remove my concept that the moderator is one person, it was a useful category to have.  You find yourself reasoning that "they" will all generally be speaking from much the same point of view.  I mean, after all they are the moderator...  they just uphold the law and speak from the general policies that the whole forum should follow.   You see what I'm saying?  It's just about being human and recognising that a forum is not like seeing people in 3-dimensional real life.   I even started a thread to find out what the moderators do or who they are etc.    @chiralSPO still hasn't described themselves so they will remain in the category  "Moderator", with a mental side-note "Physical Chemistry most likely".

Seriously help
   Since you can't see the person at all, there is very little human identification happening naturally.   You can safely assume that many scientists are borderline Autistic, so you've really got to go out of your way to call yourself something that rings in a message and has utility.     "My name" isn't going to work  but  "Bored Chemist" means something, we know who that is and what they are about.

Wearing your heart on your sleeve
    So how do you see yourself?   How does a human being read it?     
    The name "Struggles with English" should be warmly received while calling yourself "Professor King" says something completely different when the psychoanalysis is all done and finished.

Best Wishes.

12

Physics, Astronomy & Cosmology / Re: Does charge contribute to mass?

« on: 21/05/2022 16:08:22 »

Hi.

   I'm sorry to add something to what was my own post but I've just found something that does seem relevant.  I don't have anyone else to tell and it seemed so interesting that I've got to tell someone.

Does charge contribute to mass?
    Yes, absolutely, no doubt, 100%, yes, yes it does.   At least certainly in classical physics (including relativity - just not Quantum theories).

Relevant examples:   
1.   Capacitors have already been discussed.   They really should have more inertia, more resistance to an applied force, when they are charged.   Obviously the difference is small and there's no realistic hope of measuring it.

2.   A more extreme example is obtained by the electron.  Here the charge is significant compared to the mass and as you can imagine it now matters a whole lot whether you include the energy in the E field created by the electron as something that contributes to its mass.
    The main result, in words, no equations:   Classically,   ALL of the mass of an electron is due to the energy in the E field.   Let's say that another way - because it seemed interesting to me and can't be overstated:
     Not only does the energy stored in the E field of an electron contribute to its mass,   every scrap of the mass can be fully explained and accounted for as being due to the energy stored in the E field it creates.

Consequences:  I fell off my chair.

Let's just be clear about a few things first:  This isn't a new result in the world of physics, it's been known for a while.  It's just well hidden.   If you want to lower its importance, you can.   You can argue straight away that this is ONLY a classical result; our best estimates are that electrons are actually best considered as genuine point particles..... there's stuff you could point at to suggest why the result isn't given much attention.
   However, we're not going to do any of that.   For the purposes of classical physics - the inertia or mass of an electron is significantly due to dragging around its E field when you apply a force to the thing you classically identify as being "the particle".

I'll skip all equations, I don't think anyone will want that.

References:   It all hinges around "Compton Radius" as the radius of a uniformly charged spherical electron such that its rest mass is entirely due to the energy in the E field.
  https://www.chemeurope.com/en/encyclopedia/Classical_electron_radius.html

Best Wishes.

13

Physics, Astronomy & Cosmology / Re: relativity paradox or misunderstanding?

« on: 20/05/2022 21:02:28 »

Hi.

Looks like a good discussion, I hope you won't mind if I join.

1.     Got to support  @Origin  and then @Halc.    There isn't an inertial rest frame for a photon.   Light can never be at rest in any inertial frame, instead it must move with velocity c.    So, you can construct a rest frame for light (yes, that can be done) but there is no way it is an inertial frame of reference - just straight from the basic properties of inertial frames and light.  As it hapens, in an accelerated (and thus non-inertial) frame you can have light travelling at any velocity you want, including being stationary.

2. 

Einstein was fond of thought experiments and he mused on what travel with a photon would be like but I don't know what his conclusions were.

   Yes, that seems correct.   To the best of my knowledge, no formal results were published in anything like a journal or research paper.  It was apparently an interesting or useful thing to consider but not easily expressed or useful to what was finally distilled and ultimately became Special Relativity.

3.   

It is postulated that a horizon exists or will exist in the future whereby remote galaxies are receding so fast that the light from them will never be seen....

    Yes.   It is often stated something like this.  It's not your ( @paul cotter ) fault, this just is how it is usually stated.
    The "recession" and it being "so fast" is not to be understood as if they have an ordinary sort of velocity through flat space.   On average they don't, they're stationary, it's just that space is behaving strangely.

4.  We have already suggested that you don't push the limits of imagining an inertial rest frame for a photon too far (because, and it's worth saying again, there isn't one).  However, if you must do this, then note that the Lorentz transformations only tell you what happens in flat Minkowski space.  So considering limits of the Lorentz transformation as  v/c ---> 1   is barely relevant.  We already have good reason to believe that the universe does not exhibit Minkowski geometry on large scales.   Even if the Lorentz transformations suggests that a flat Minkowski space becomes degenerate and reduces to single point when v/c -->1, the real universe does not have to do that and there can still be regions of space that the photon cannot reach.

Best Wishes.

14

General Science / Re: Why would boiling water explode out of a pan?

« on: 19/05/2022 21:42:15 »

Hi.

     Just to be safer, don't boil a pan of just water.  Leave a wooden spoon in the pan (straight away, i.e. from cold as you start to heat it).  Otherwise just add some peas.  Shooting blobs of water due to a rapid growth of gas bubbles is often called "bumping".  Peas make good anti-bumping granules but rough edges on a wooden spoon will also often be enough to reduce the risk of bumping.

Best Wishes.

15

Physics, Astronomy & Cosmology / Re: What happens when photons leave the sun?

« on: 18/05/2022 18:37:57 »

Hi.

   It's not the same photon that exits from the sun as the one that might have started in the core.   That's the main thing that explains why the journey has taken so long.   Also there are widely varying estimates of the average journey time -  from a few thousand years to a million years.

In the core of the sun the protons and helium nuclei are so thick that an emitted gamma ray can't get very far before it is absorbed. If you imagine that a gamma ray is emitted right at the center of the sun then it will start out heading right for the surface. When it crashes into a proton the result of the collision is a proton with extra energy. The proton gives up that extra energy by emitting another gamma ray photon. But this one could head in any direction -- even right back where it started from. And so it goes, with the gamma ray heading from one collision to another, changing its direction each time it is absorbed and re-emitted.

https://sciencing.com/long-photons-emerge-suns-core-outside-10063.html

     Although they make the whole thing sound poetical by referring "the gamma ray",  it's clearly been absorbed  (it has gone, there is no gamma photon to be found for a while)  and  some other photon was re-emitted - this has happened several times over.

    In between collisions the gamma rays travel at a speed that is either c (the speed of light in a vaccum),  or else the speed of light through the medium (generally slower).   Exactly what speed it has travelled at depends on how complicated you want to get and whether you wish to consider a group velocity or a phase velocity for light waves.  Explaining group and phase velocity is beyond the scope of one thread.   
     The propagation of light in plasma is complicated and apparently still under research.  The older thinking is that plasma should slow light down and indeed it's almost completely opaque to EM radiation in the visible spectrum.   However, just to turn this upside down, there are some new articles suggesting that you can actually get the group velocity (but not the phase velocity) to go higher than c in some situations.   (https://www.sciencealert.com/pulses-of-light-can-break-the-universal-speed-limit-and-it-s-been-seen-inside-plasma ).

   Finally, the gravitational potential and the over-all redshift issue is also complicated.   As @Halc  mentioned the change in potential is what will be important    BUT....   the potential energy at the centre of a sphere (the sun) is a topic which might spill over to another thread.   So we probably do need to know exactly how deep within the sun the photon was when it was created.

Best Wishes.

16

Physics, Astronomy & Cosmology / Does a gyroscope still fall in opposition to the direction of the missing force?

« on: 18/05/2022 12:01:08 »

Hi.

    I was looking through some of the old Feynman lectures and came across this section which attempts to explain what makes a gyroscope start to precess instead of falling.   Just to be clear, it does not suggest this is what sustains precession, just what starts it.   You have a situation with a spinning gyro supported on a pedestal at the centre and you are holding the (soon to be) free end stationary with your hand until it is suddenly released.

gyro.JPG (28.45 kB . 583x312 - viewed 206 times)

This is what the lecture states:
     Some people like to say that when one exerts a torque on a gyroscope, it turns and it precesses, and that the torque produces the precession. ........it does not fall under the action of gravity, but moves sidewise instead!  .....( but ).........
    .....The gyro actually does fall, as we would expect. But as soon as it falls, it is then turning, and if this turning were to continue, a torque would be required. In the absence of a torque in this direction, the gyro begins to “fall” in the direction opposite that of the missing force.....

[End of section 20-3,  "Rotation in Space", Feynman lectures.
https://www.feynmanlectures.caltech.edu/I_20.html ]

      I can't be the only one who finds the idea of a "missing force" a bit odd.   To say that things accelerate in the direction of a net force is one thing,   to say that they would accelerate in the opposite direction to a missing force is much harder to live with.
     There's a computer on my desk and it is missing a force to move it left but it does not accelerate to the right.

    Anyway, is this explanation with "missing force" still popular?   At the moment I'm inclined not to recommend that explanation.   Maybe Feynman tried to oversimplify and has ended up with something that isn't all that usefull for understanding?   i.d.k.   I do note that he has quotation marks around "fall" as if he was just making an analogy to objects falling when they are missing a force to support them. 

Best Wishes.

LATE EDITING:  Fixed various spelling errors.   Title changed from "in the direction" to "in opposition to" etc.

17

Just Chat! / Re: Should we report all people to the police if we find them with child porn?

« on: 18/05/2022 00:57:51 »

Hi.

You seem to have answered your own question.   I'm not sure what further discussion you were hoping for.
You have a similar civic duty to report this as you would to report other crimes.

Best Wishes.

18

Physics, Astronomy & Cosmology / Re: Is there a limit to how hot things can get?

« on: 17/05/2022 01:19:51 »

Hi.

Then you have discovered an insect that does not obey Stefan's Law,...

    I'm not sure about Steafn's law.   I'm familiar with the Stefan-Boltzmann law but I've only seen that proven for Black bodies.  There are some things that are not black bodies.

   Wikipedia uses this notation for the Stefan-Boltzmann law:
              j* = total power radiated (per unit surface area) =   ε.σ.T⁴ .     
with σ = constant;    T = temperature in kelvin     but  noteably  ε = emissivity   with  0 ≤ ε ≤ 1   and the following comment....

In the still more general (and realistic) case, the emissivity depends on the wavelength, ε = ε (λ).

   I'm not sure what wavelength they were talking about,  I guess it's the peak wavelength of the whole spectrum of emissions.  Anyway, if it is that then something approximating Wien's law implies  λ_peak ~ 1/T.   Hence, ε = ε(λ(T) )  =   a function of Temperature in disguise.
    So, all I'm asking is that  ε(T)  ~  1/T⁴    over a small range of T,   then the power radiated does lose all of it's dependence on T for that range of temperatures.

Best Wishes.

19

Chemistry / Re: How well understood is the Chemistry of the trans-uranic elements?

« on: 17/05/2022 00:49:49 »

Hi.

Thanks @Bored chemist .  It might have been more realistic to think about ligands that just offer a significant polarisation,  a  δ+  on the ligand etc.

   I agree that the main problem is (or was) finding some external factor that rivals the size of the nuclear energy changes.   As I mentioned a little earlier,  when I was a schoolchild I recall being taught that nuclear reactions are completely unaffected by any physical (or chemical) conditions external to the nucleus.  The decay of a nucleus was the textbook example of being as random a process as you will ever find in nature.
     I don't think that's on a school syllabus any longer.   At least some Nuclear reactions are not as random as we once thought.   On a side note, I wonder if Schrodinger's cat thought experiment needs to be re-written.  They usually have a radioactive substance decaying (or not decaying) as the random process determining if the poison is released.   Are there any more good examples of a process we believe to be genuinely random left in science?

Best Wishes.

20

Chemistry / Re: How well understood is the Chemistry of the trans-uranic elements?

« on: 16/05/2022 18:47:05 »

Thanks @chiralSPO .