1

Physics, Astronomy & Cosmology / Re: Are Black Holes Real? Is Time Travel Possible?

« on: 17/01/2025 14:55:59 »

Hi.

Some good answers already...  but who doesn't love a bit of stuff about black holes, wormholes and Sci-Fi TV shows?  I've got a spare 20 minutes and intend to chip in to the discussion.

Jimbee said:   I was going to start a question here by first saying that there is a wormhole at the center of every black hole.
Halc replied:    This is false....

   Contrary to @Halc 's comments, there's quite a natural link between some notions of wormholes and some notions of black holes.   The maximally extended solution of the Schwarzschild metric allows for four regions of spacetime to be identified and these are creatively named regions I,  II,  III and IV in a Kruskal diagram.   I know that @Halc is aware of this but has possibly overlooked the bridge at the centre of the diagram.

[diagram taken from Wikipedia.    https://en.wikipedia.org/wiki/Kruskal%E2%80%93Szekeres_coordinates ]

Regions I and III are the exterior of the black hole.   By convention we consider ourselves to be in region I.  Meanwhile Region III is just another patch of spacetime, it's not necessarily the same bit of spacetime that we have been in or will ever be in but it is otherwise physically similar to the exterior region I.  For example region III could be associated with another universe  (or "parallel universe").     The bit right in the centre of the diagram (which actually marks a place in space and a whole range of times) can be called a wormhole but a more commonly used name is an "Einstein-Rosen Bridge".     Warning:   These two terms can be used interchangeably - but most commonly the term "wormhole" is kept for something that would require a negative energy density and the term "Einstein-Rosen bridge" for the thing at a black hole.

    Interestingly, the E-R bridge associated with a black hole is NOT at Schwarzschild radial co-ordinate r=0 or what we might naively treat as the centre of the black hole.   (Why did I say "naively" ?   .... because where r<1, it turns out that this radial co-ordinate is no longer telling us about spatial location, it's behaving like time instead).   It's actually at r=1 which would be identified as the event horizon around the black hole where there would be a path (a trajectory you can take) to stay on the border between region II (the interior of the black hole) and region III.   We are about to describe two different co-ordinate systems, so it's important to establish the symbols and terminology I will be using:   The Schwarzschild co-ordinates will always be lower case letters,  we have r (the radial co-ordinate) and t (the time co-ordinate).   The Kruskal co-ordinates will have upper case letters, T and X and it is these Kruskal co-ordinates that are on the axis of the diagram above.   Some lines of constant Schwarzschild co-ordinates (constant r or constant t) are shown on that diagram.   

This bridge (at spatial location r=1) exists at all times, t, in the Schwarzschild co-ordinates.  In the more convenient Kruskal co-ordinate system, we would tend to think of it as just a single point at T =0 and X =0 which is right at the centre of this diagram.   Indeed, that is the only location on the diagram where a sensible Schwarzschild time co-ordinate (one that isn't + or - infinity) can be associated with this location.   However the situation is a bit more complicated, if you wish to make it so....   the spatial location with radial co-ordinate r=1 exists all along both dotted diagonal lines in this diagram (if we allow the Schwarschild t co-ordinate to be +/- ininfity  BUT  we'll leave this alone for a paragraph or two.  LATE EDITING: The post is already too long, we'll just leave it well alone for the whole of the post).   There is no sensible way to know what Schwarzschild t (time) co-ordinate should be associated with the spatial location r=1.  The degeneracy of the Schwarzschild co-ordinates, in particular that multiple finite times at r=1 specify precisely the same event,  i.e. that (r=1, t=1)  is no different to  (r=1, t=2) or (r=1, t= -100)  etc.  is essentially why Kruskal co-ordinates  (large T and large X)  are used and are better for describing the spacteime manifold around black holes.   The Schwarzschild co-ordinates fail to uniquely identify an event (a 4-dimensional space and time location) in this spacetime manifold but the Kruskal coordinates will uniquely identify an event in spacetime.
    So the simplest notion of an E-R bridge is obtained just by noting that on a Kruskal spacetime diagram, the paths you can take (usually called worldlines for the object) are at most 45 degree diagonal lines.   That's the path you would take if you're travelling at light speed radially inward or outward from the black hole.  Any slower speed produces a worldline that is more upright  (going more up the Kruskal T axis rather than along the Kruskal X axis).   So, if you start from the centre of the diagram (say having just come in from the region I almost directly along the line of constant time we can set as t=0),  that is to say you are spatially located right at the event horizon of the black hole when we set our clock to t=0,  then there is a path (a trajectory at light speed) you can take that will keep you on the border between region II (the interior of the black hole) and region III (some other universe possibly).   Travelling at less than the speed of light can only take you into region II (the interior of the black hole) or having an incorrect trajectory (direction) of travel will also usually take you into the interior of the black hole.   The two interesting directions of travel are directly radially outward from the black hole (that trajectory follows the dotted diagonal line on the top right of the diagram and keeps you on the border between region II and region I)  and  directly radially inward to the black hole (that trajectory puts you on the border between region II and region III).   That's also a little bit interesting when you think about it, travelling inward to the black hole at some sub-light speed can only get you into the interior of the black hole (region II) as you might expect and seems like the last thing you'd want to do.   However, if you want to find the border or path to region III then you really need to get brave, put your foot down and race directly into the black hole at light speed.
     Anyway, that's a simple explanation of an E-R bridge that exists at the event horizon of a black hole.   There is a realistic path or trajectory that would have you on the border of region III.   However, you cannot properly enter deeper into region III unless you were somehow able to travel just a bit faster than light (which is not mainstream physics) - then you can take a path at a lesser angle than 45 degrees and emerge somewhere properly inside region III.    This is sometimes paraphrased and pictured as if there is a bridge, a path or viable trajectory to reach region III for an instant when you at the event horizon but it shuts just as instantly, you would need to travel a bit faster than light to properly cross that bridge before it closes.
    Time is of the greatest importance to all of us (of course) but there's a big problem for any object that is travelling at light speed.  The path taken at light speed is refered to as a null path, which means that no proper time elapses for the object between any two points along its worldline.   So, you may have put yourself on the border of region III but you'll have no time to enjoy the view or scenery from over there.   No proper time will elapse for you between being at T=0 and X=0 and having raced along right to the end of the dotted diagonal line on the top left hand side of that diagram (the border of region II and region III) where you hit the singularity at r=0.  At Schwarzschild co-ordinate r=0, all worldlines terminate, so there will be no more experiences you would have.

   Black holes, they are interesting things.....

    Final comments:   The above description of a black hole is called the maximally extended Schwarzschild solution and it is associated with what we would call an "eternal black hole".   A black hole that started out as just some dense material and then underwent a dramatic collapse at some later time is NOT meeting the criteria of an "eternal" black hole, indeed there was a time when the black hole was not there.   These sorts of black holes will be associated with a different metric (not quite the Schwarzschild metric) so they will not have the same spacetime diagrams.   Specifically, they may not have an Einstein-Rosen bridge to a region III of spacetime and this explains why a few sources of information may state that real world black holes do not feature any sort of wormhole or E-R bridge.  So are there any genuine eternal black holes in existence or were they all originally just some matter that gradually became over-dense?  I don't know... but it seems that black holes are both too numerous and some also have a mass parameter that is just too large to explain away as if they were created at some non-zero cosmological time by matter coming together and forming over dense regions.   There hasn't been enough time since the big bang to get all the matter together to make these massive black holes, so it is possible that there are some black holes that have always been there, genuine eternal black holes that are well described by the Schwarzschild metric.

   

But, black holes are possible.....? Has one been ever observed?

     We have some images as stated by @Halc  but most of the evidence for their existence is a bit indirect.   For example, we have studied the motion of stars in our own galaxy and found that they move as if they are being acted on by the gravitational force from a massive but extremely small object at the centre of the galaxy.   Only a black hole would fit the characteristics required.    We also have a lot of recent evidence from gravitational wave analysis, such as from the LIGO site.   Here we have results indicating that some massive things have been merging together in space and for some of these events black hole mergers are the only thing that would seem to fit the data.

Best Wishes.

2

New Theories / Re: Origin of magnetic force

« on: 04/01/2025 13:29:55 »

Hi.

Drift velocity of electrons in copper wires, are small. Yet the force between two parallel current carrying wires can be significant.

     The force on each charge carrier is small  BUT  there are quite a lot of these you need to add together  (approximately 10²⁹  free electrons per cubic metre of copper wire).

According to relativistic explanation, positive charges on first wire see moving electrons in second wire length contracted,......

    This line of reasoning and many of the comments you copied-and-pasted from a YT video just a little earlier are all based on the notion that length contraction should apply to the free electrons just as it applies to the metal ions.   So that in a lab frame where the metal ions were stationary, the electrons are then drifting, so they would have higher density due to length contraction and thus the wire should be of net negative charge.    This was discussed in an earlier post  (post # 406):

.....it is just not possible for the (current carrying) wire to be of overall neutral charge in every frame.....    For some reason the frame of reference where the wire is overall neutrally charged is the usual one, the one where the wire (the positive metal atoms in it rather than the free electrons) is stationary....
NOTE:   text in blue italics added to the original quote for clarity

    I haven't read the original paper / discussion you cited by Jefimenko, who died in 2009.   At a guess, this work is quite old and from a time when we were still sorting out the details of electromagnetism and relativity.
    This sentence you quoted seems most relevant:

The true meaning of the calculations demonstrating the alleged relativistic nature of the magnetic field and of the calculations presented in this paper is, therefore, that the idea of a single force field, be it magnetic or electric, is incompatible with the relativity theory.

   Indeed it is helpful to consider that there probably is only a combined electro-magnetic interaction and we cannot sensibly declare that a thing like an electric field (or a magnetic field) exists in isolation.   For example, Maxwells equations where a separate E and B field appear, starts to look more like a "hack" or convenient way of performing calculations involving 2 fields described as ordinary 3-vector fields.   3-vectors are basically the ordinary vectors we might associate with Newtonian mechanics and 3D space.   However, these 2 separate fields may very well be just fictitious artifiacts, useful for calculations but not representing anything that really exists. 
    It may be that the underlying nature of the interaction requires description using 4-vectors such as the Electromagnetic four-potential   (Reference:  https://en.wikipedia.org/wiki/Electromagnetic_four-potential ).   Specifcally, it seems that the more important and presumably more physically real field that may exist is a single 4-vector valued field (rather than 2 seperate 3-vector valued fields).

     About a year ago, the Aharanov-Bohm effect was discussed in this forum.   Here it seems that something can be affected by an electromagnetic field even where the Electric and Magnetic field are both 0 valued.   (Reference:  https://en.wikipedia.org/wiki/Aharonov%E2%80%93Bohm_effect  .   There's also an old forum post which had some diagrams and animation that still seem to be working  https://www.thenakedscientists.com/forum/index.php?topic=86694.msg722832#msg722832 ).    Anyway, this provides some evidence to support the idea that what physically exists and can exert physical effects may very well be a 4-vector valued field such as the  Elctromagnetic four-potential  rather than just 2 separate 3-vector valued fields like the E and B fields.

Best Wishes.

LATE EDITING:   I re-read the old forum post and it may cause some confusion when people read a sentence that said  ..... but back in the day it was important to imagine that magnetism (and possibly every other force) may demand that two fields existed throughout all of space..   That may seem to oppose the notion of needing to consider only ONE combined electro-magnetic object.
    The old forum thread emphasised that considering only a magnetic field (a B field) is not enough, we also need to consider a magnetic vector potential field.    That's why it was convenient to say we needed to consider 2 things (B field and a potential field).   There would be these two things to consider.   Note that both of these things (a B field and a magnetic vector potential field) are just ordinary 3-vector fields.   We get the next layer of complexity by considering 4-vector fields.
    In this post (this thread), you just need to be aware that we can recover ALL the information about EVERYTHING   (the E field, the scalar electric potential field, the B field and the magnetic vector potential field)  from just a single suitably defined 4-vector valued field we call the electromagnetic four-potential.   So if we're looking for the most fundamental description of electro-magnetism then we may need to be considering this object.  Specifically, the electromagnetic four-potential  seems to be a sufficiently integrated object to describe all the electro-magnetic effects that I am aware of.   Meanwhile a lesser set of objects such as:
  (i)  the B field on it's own.
   or (ii)  The B field plus a suitable potential for that B field,
 would NOT be sufficient.

    I hope that makes some sense.    Let's go back to your ( @hamdani yusuf ) original question and paraphrase all of the above comments.

What is the Origin of magnetic force?

I don't know but maybe the entire notion of a magnetic field existing as some sort of thing on its own is just a bit fictitious.
(i) We seem to need to have a E field with it.   We cannot adequately explain all effects with just a magnetic field.
(ii) We seem to need to consider potentials in addition to the forces.
(iii)  Overall we need to have an object like the electro-magnetic four-potential field in existance.   All of physics is just a model but considering the electromagnetic four-potential field as something that exists may be a good step closer to the truth rather than considering that we could attempt to identify and define a magnetic (or electric) field as something that could exist separately.

3

General Science / Re: Can Human Consciousness Be Uploaded?

« on: 26/12/2024 10:26:51 »

Hi.

    Other than @evan_au 's link to a wikipedia article,  I can't see any mention of the term "connectome".    That's a keyword people will want if they're looking for more information.

Otherwise we will have recreated someone with "completely locked-in" syndrome: conscious, able to think, but in total isolation would quickly go mad.

     Even this much is not certain.    A real-life connectome is never without some input and would always have some syanpses within it that are firing, i.e. it is half-way through processing something.    It is never completely "off" and inactive.
     If you built a perfect copy of a connectome, it still isn't known if it would spontaneously start to work or ever get into some sort of normal firing pattern,  it may just stay basically in the "off" position.     Since so much of the brain structure involves feedback loops, arbitrarily firing a synapse here or there is as likely to set up something like an epileptic seizure rather than switch the connectome on and get it into a normal firing pattern.
    So, ideally, you don't just want to copy the connectome,  you'd aso want to copy the exact firing pattern that was happening at that time etc.


Para-phrasing:    It's not enough just to copy the hardware.   The whole thing is more like a syphon pump.  It must have the fluid in it and be working now in order for a copy of it to also be a syphon pump.    Otherwise all you have is just an empty tube rather than a syphon pump.   Put one end of this tube into some water with the other end lower down and it does nothing.

Best Wishes.

4

Just Chat! / Re: A holiday wish.

« on: 22/12/2024 12:55:05 »

Hi.

     Thank you, merry christmas and/or best wishes to everyone.


(that's a copyright free animated GIF as far far as I could tell).

5

New Theories / Re: Origin of magnetic force

« on: 21/12/2024 06:09:12 »

Hi.

    I think I can see what you ( @paul cotter ) are saying and yes, you do seem to have the right idea.

I can only assume a relativistic effect of the movement of + and- charges in the conductor produces an electric field with force on the charge

Yes.
In post #406 I said the following:

....the same force and overall effect should appear.   However, it doesn't have to appear as a magnetic force and in this case it will be partly a force due to an electric field....

  In this situation described by @hamdani yusuf ,  the velocity of the test particle became 0 exactly and so we have the most extreme case.    Using the Lorentz force law and putting v=0 we would have that   F = q(E + v x B)  =  qE  =  a force entirely due to an Electric field.
This was mentioned in reply #408 by Hamdani:

In the reference frame where the test particle is stationary, v is 0. Thus special theory of relativity interpretes that the force is purely electric.

     In this frame of reference, the distance between the -ve charges (the free electrons) and between the +ve charges (the metal atoms in the lattice) has been affected (by length contraction under SR) such that the wire had an overall +ve charge of exactly the right magnitude to repel the positive test charge as required.   Although a B field would still be there circling the wire, it plays no part in exerting a force on the particle (in this frame).

---------
Minor notes:    You said the following (with red text added by me):

and as the (test)  charge is moving in this frame it also produces a magnetic field...

.
    Yes that's actually completely true  BUT  you are over-thinking this. 
     We were just considering it as a test charge.   They do not create or alter the field(s) around them, they just experience the field(s) created by other things    https://en.wikipedia.org/wiki/Test_particle .       
     In the real world, any moving charge would create a magnetic field in the space around it and this would combine with the magnetic field from the wire  -  but  we're talking about 1 small charge  (maybe just a proton) compared to billions  (~10²⁴ atoms per cm³)  of free electrons and metal ions in the wire.   The alteration in the magnetic (or electric) field caused by our little postive charge is utterly insignificant and we can consider it as an idealised test charge.
   
Best Wishes.

6

New Theories / Re: Origin of magnetic force

« on: 20/12/2024 02:03:39 »

Hi.

Sorry to hear you're poorly @paul cotter .

As we know a static B field will have no effect on a static charge.

   Yes.   Stationary test charge implies no force.

Yet if we change frames of reference as you have done we get a force - what am I missing?

    Well, the "static" test charge won't be static (stationary) in both frames.   It's the motion of the test charge that undergoes an important change when you switch frames.   The B field doesn't need to change, the test particle now has motion.

    Just to be clear,  @hamdani yusuf   was discussing a situation with this sort of diagram:



      ------------------------------------------------------------ this is the wire


                                   *                this is the test charge

The test charge is something outside the wire,  we can move the wire while leaving the test charge static in the lab frame.    Of course, the moment you switch frames, so that the wire is stationary, then you see the test charge will be moving.       

Well that's a short answer anyway.
----------

I've read what you've written a couple of times now.   It seems that you think it would be important to know if a B field is moving or not.   However, it is NOT important to know.

You said:

Looked at from the frame of the moving wire the test charge and the B field will move in unison.

It's not meaningful to talk about the B field as if it can have motion and therefore move in unison with the test particle.   All that matters is that the test charge has motion in this frame.
 
The Magnetic force law (Lorentz force law where E=0 ) is often mis-interpreted in some fashion similar to the following:
   There's a force when a charge moves through a magnetic field   OR  when the magnetic field moves past the charge.

But that's just pop-sci and sloppy speech.   The Lorentz force law (or just Magnetic force law) actually says nothing at all about a magnetic field moving past a static charge because it's not even clear if a magnetic field is some sort of "stuff" that can move through space.   All we need is that the magnetic field, B, is something that has a value (this one has magnitude and direction) at every point in space and at every time.

     Much as you seem to suggest, quite often we just couldn't tell if the B field was moving or staying still.   For convenience we'll align our straight wire with the x-axis, then the current carrying wire has a B field that depends only on y and z co-ordinates of a point in space.   At any time, the B field at x=a is no different to the B field at x=b.   We can't tell if the B field at x=b,t=now   has just come from position x=a,  a second ago, or if the B field just wasn't moving.
   Fortunately, the Lorentz force law does not ask about how the test charge was moving relative to the B field.  It just asks how the test charge is moving   (the implication being this is relative to the frame you have chosen).

    Read perhaps just the first section of the Wikipedia article about the Lorentz force law to yourself:   https://en.wikipedia.org/wiki/Lorentz_force
    This time pay carefull attention to the LACK of something:   The lack of any need to know about the movement of the B field.

      The Lorentz force law states that a particle of charge q moving with a velocity v in an electric field E and a magnetic field B experiences a force (in SI units) of    F = q ( E + v x B ) .

    The velocity, v, that appears in these formulae is never stated as being the velocity of the test charge relative to the B field.   It is just the velocity of the test charge  (in whatever frame you have chosen to use).   Specifically, we start by picking a frame of reference,  then the test particle has a velocity v in that frame.... that's the only v we're interested in.... we don't care if the B field is something that might also be moving, it doesn't matter, we don't ever need its velocity. 

Best Wishes.

7

New Theories / Re: Origin of magnetic force

« on: 19/12/2024 03:16:05 »

Hi.

Let me simplify the question. A long straight metal wire moves to the right at speed 1 m/s relative to the lab. Electrons in the wire move to the left 1 m/s relative to the lab. A positively charged test particle is stationary relative to the lab, 1 cm below the moving wire. Will it experience a force by the wire? Which way?

   Yes, it should do.  It will be a force pushing the charged test particle directly away from the wire.

   Why?   Translate to the more typical frame of reference.    So we'll have the wire stationary, so we can assume the positively charged atoms of it are stationary.   Adjust the velocities of everything else appropriately.
    The free electrons of the metal now move with velocity 2 m/s to the left   ( my left direction is    < ---   ).   Actually slightly less if we use the the velocity addition of special relativity but  2 m/s will be OK.
     By conventional electromagnetism stuff, we have a magnetic field around the wire.   Conventional current is going  to the right  ( ---> that way ),   right hand rule shows the B field circles around the wire.
    The test charge, in this frame, is now moving at  1 m/s   (  <-- that way), or more importantly in the oppositie direction to the current.   A moving charge does feel force in a magnetic field,  we use the other right hand rule   for  F  = q  v x B.    We see the test charge is accelerated downwards,  i.e. directly away from the wire   (if I twisted my hands around in all the right directions).

    In the atypical frame where your situation applies   ( the wire moves ---> that way),   the same force and overall effect should appear.   However, it doesn't have to appear as a magnetic force and in this case it will be partly a force due to an electric field.
    We could draw the diagrams and do the maths - to show that due to relativisitic effects,  the average distance between charges has altered and the wire is no longer of overall neutral charge as far your test particle is concerned.

    The smart person would then ask a follow up question:   Aren't the electrons and positively charged ions affected equally?   Aren't the distances between +ve charges and between -ve charges equally contracted and the charge does remain overall neutral?    Indeed, in your original situation where the test paticle was at rest, it does seem that both the positively charged ions and the electrons are moving with equal speed, albeit in opposing directions.
    That's a good question.   The answer tends to be that the free electrons are not like the positively charged atoms in the metal wire.   The free electrons are, as in their name, "free",  while the metal atoms are much more locked in place in the crytsal lattice of the overall structure.
      An approximation is made here.   Here's the step-by-step reasoning:
1.   It does seem that when a current is passed along a wire, the free electrons do start drifting. 
2.   So there would be a corresponding length contraction for the distance between them, which would make the density of negative charges exceed the density of positive charges in the lab frame.
3.    However, the electrons are free to move.   This is the important approximation step.    By electrostatics, they will re-distribute themselves and the wire remains overall neutral in the lab frame, to a good approximation.    Note that the metal atoms could NOT do this, they are not like the free electrons.   We know that a static test charge placed next to this wire which has a current in it, doesn't seem to experience any force, that's the experimental evidence that we've got.  So it seems that the wire has remained overall neutral in charge, even when a current is made to pass through it.     

    One consequence of this is that the wire cannot be of overall neutral charge in other frames of reference.   In other frames, there must be length contraction as usual.   So, the easiest one to consider is where we move to a frame where the electrons are stationary and the metal atoms were the things moving with (the negative or direction reversal of)  the electron drift velocity.   In this frame, the electrons are not moving, while they were in the first frame, so the average distance between electrons is now bigger,   -ve charge density is therefore lower.   Meanwhile, the metal atoms are now moving whereas they were stationary, so the average distance between them has decreased,  hence positive charge density has increased.   So overall,  the positive charge density now exceeds the -ve charge density and the wire has a net positive charge.

    So the irritating and relentless person may then ask - but the electrons are still "free" in this new frame, why don't they just re-distribute themselves again?   Why isn't a current carrying wire of overall neutral charge in any arbitrary frame I wish to use?
One half of that we can answer well:   Since the electrons do drift when there's a current in the wire, it is just not possible for the wire to be of overall neutral charge in every frame.   Do the maths carefully, there is only one frame where that can apply.    The second half... well I don't have a satisfactory answer for it.... 
    For some reason the frame of reference where the wire is overall neutrally charged is the usual one, the one where the wire (the positive metal atoms in it rather than the free electrons) is stationary.  You can try and dig up a few reasons, for example a current just does not flow unless there's a cell (a battery) or something that is maintaining a potential difference across the wire.  You can attempt to argue that as part of that setting up procedure,  just the right number of electrons are initially pulled out of the wire at the +ve terminal of this battery,  so the slight over-density of -ve charges that should appear in the wire when the electrons do start drifting along does not emerge.  However, we can't make too much speculation.   Let's just leave it as  "for some reason" the frame where the current carrying wire is overall neutrally charged is the one where the wire is stationary.

Best Wishes.

8

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.

9

New Theories / Re: In four dimensions, must an object move at c in at least one of them?

« on: 05/12/2024 22:17:16 »

Hi.

"c" has units of m/s and time has units of s, how can one have a speed(m/s) in a dimension of s?

    If you skip all the details, as is often done in PopSci,  it's summarised like this:

If you use a conversion factor c to convert the proper time elapsed to a space equivalent, then.....

   A more rigorous mathematical formulation would require us to be careful about defining 4-vectors and not simply treat them as if they are just ordinary Newtonian 3-velocties with a 4th time-like component strapped on.   For example, we would specify an objects position 4-vector as   (ct, x, y, z)   rather than (t, x, y, z).   That decision alone means that the time co-ordinate for an objects 4-position has units of  metres not seconds.  Next, we would also define the velocity 4-vectors  as a derivative w.r.t. proper time,  d/dτ  of the 4-position vector,   instead of using co-ordinate time t as in newtonian mechanics where the 3-velocity is just d/dt of the Newtonian position 3-vector.  [Reference:   https://en.wikipedia.org/wiki/Four-vector#Fundamental_four-vectors ]
    Note that the last decision makes very little difference at low speeds,   the proper time elapsed for an object is almost exactly the same as the co-ordinate time elpased.   So, it's not too criminal to allow a reader to imagine that a 4-velocity has 3 spatial components which are just the ordinary Newtonian 3-velocity with a 4th component, a velocity in the time direction, strapped on as an extra.   In truth the 3 spatial components of the 4-velocity are not quite the same as the Newtonian 3-velocity and that will become more apparent at higher speeds.
    Anyway, with a proper consideration of 4-vectors we can make all the ideas more rigorous and there is no conflict in the units of measurement for the time component or spatial components of the 4-velocity.
    It is really only the formal 4-velocity of an object that must always have a magnitude of c and it's convenient that the 3 spatial components are a bit like the ordinary Newtonian 3-velocity of the object.

- - - - - - - -

Have you seen Penrose's description of two observers walking past each other?

    I'm not sure that I have, sorry.   Is there any online reference?
I can hazard a guess as to what it was about.

Best Wishes.

10

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.

11

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.

12

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.


     

13

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.

14

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.

15

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.

16

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).

17

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.

18

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.

19

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.

20

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 ]