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Physics, Astronomy & Cosmology / Re: What is the relationship between an object and its field?

« on: 13/03/2024 20:59:28 »

Hi.

   Original title question:
What is the relationship between an object and its field?

A reasonable reply is much as you ( @geordief ) have already stated.    For QFT,  a particle is just an excitation in the underlying field corresponding to that particle.

In QFT the former (an electron) is described as an excitation of the latter (the electric field),I believe.

   Not quite.   The electron is an oscillation in the electron field, not an oscillation in the electric field.  The electron field is NOT the same as the electric field,   the start of the words looks the same but the ending is very different.    In QFT there isn't an electric field, or at least it's not anything that you need to assume exists as a fundamental object.    The Electric field will just be something that appears on a macroscopic scale.   However, the electron field is fundamental - you must have an underlying field for electrons if you wish to have electrons as fundamental particles in your model.

As an example an electron and its electric field?

    That's possibly where the confusion may start.   In QFT,  an electric field is not something that belongs just to an electron.   On a macroscopic scale, an electrostatic interaction actually involves a coupling between two fundamental particle fields  (the electron field and the photon field).   The overall interaction does look like or emerge as something we can describe as if there was a classical field, the Electric field, in existance.

Are there other ways of looking at it?

    I'm sure there are.    If it's just a change of pictures or vocabulary,  then it's just window dressing.   These sorts of changes are only useful or important if the new vocabulary and pictures are better for human understanding - even then the changes are not always made.
   Examples:  Methanoic acid is a 1 Carbon organic acid you can make in a lab.   Formic acid is a thing you can extract from ants (Formic comes from the Latin name for an ant, Formica).  It turned out that they were the same thing but both names still persist, we never insisted that we all start calling it the same thing. 
     In Physics, we thought that current was the movement of positive charges.  It turned out it's usually electrons that are negatively charged and moving the other way.   We didn't change the vocabulary or diagrams.

    There may also be some other ways of looking at things that would really involve changing the mathematics or mechanics of how things work instead of just changing some words or pictures.

Can we imagine the field as an extension of the object(the two being aspects of the greater whole)?

    Yes ?   I guess that's the opposite way around to how we usually imagine or phrase things with QFT.  The thing is, does it help in some way and will it change the mathematics or mechanics at all?   Is it just a change of vocabulary?

Could there be (is there?) a mathematical equation  where the object is defined in terms of the field? (along the lines of energy and mass in relativity)

     Does it have to be one equation?   QFT is a collection of results, mainly equations, that quite sucessfully treats objects that look like particles as oscillations in fields.   
     If you mean is there some simple and short equation that does the job - well, I don't know but I don't think so.   If we try hard we could fit most of QFT on a T-shirt but it may be a bit longer than E=mc².   Looks like some people have already tried.

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Physics, Astronomy & Cosmology / Re: Can Light Experience 'Time'

« on: 16/05/2023 23:55:30 »

Hi.

Is it just a convention that c should be a very large number?

   Yes, more or less.  As far as physics is concerned, yes.  As far as human evolution is concerned, no.
Most units of measurement are going to be based on what seems sensible to a human being, the things we experience and the things we can do.   For example, we can't throw a stone or a spear all that far and people probably wanted to have vocabulary that is useful to tell others how far they should throw their stones.   If we had a notion of length where 1 unit = the diameter of our planet, then everything you can see is (approximately) 0 distance from everything else, it would be useless information.  The evolution of language was bound to be such that we would be able to describe smaller distances more easily.   Similarly, language would evolve with some notion or units of measuring time that would be useful instead of having 1 unit of time = 1 average lifetime of a galaxy.
   So recognising that our units of speed are based on what we can do, then c = 300 000 000  (in m/s) is telling you something - it's really fast.   If you were hunting an animal that moved that fast, it's gone, hunt something else.
   

I think that sometimes it is given the value 1.

  Yes.   (It's understood that this will put everything else we might be working with into different units as well).

Would it be equally possible  to give it a very small number so that  in the expression e=mc^2 we might have the impression that it would take a numerically  huge  amount of mass to  render a numerically tiny amount of energy?

   Yes but see above.   The amount of mass would now have to be measured in different units.  You can't change reality just by assigning c a small value,  all you will do is change the numerical description of the amount of mass that is equivalent to it.
 
Best Wishes.

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Physics, Astronomy & Cosmology / Re: Can Light Experience 'Time'

« on: 15/05/2023 20:59:22 »

Hi.

@geordief   asked about  the speed c.
LaTex isn't working so we can't have mathematical symbols here in the post,  sorry.

    Basically, yes starting from the Lorentz transformation you would quickly obtain the relativistic velocity addition formula - see  https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity

   That will show you that a velocity with magnitude c (which is just what appears in the Lorentz transformation and not necessarily the speed of light) is mapped to another velocity with the same magnitude in any other inertial reference frame.

    The entire discussion was about assuming light may not travel at the speed c.  So, that's why @hamdani yusuf  said what they said (I would think).   In most textbook developments of special relativity it is common to start from an assumption about light and the invariance of its speed.  In this situation, the whole point is that you don't - we assume light has some other speed.

    You could still obtain the Lorentz transformation even if nature had been very unkind to physicists and never given them a massless particle that could be detected.   We were lucky, we had light, it did exhibit properties that strongly pointed us to the development of special relativity.   Assume light wasn't available or did not behave like that.  Provided special relativity is still a rule in nature, then there would have been signposts to it.
   See https://en.wikipedia.org/wiki/Experimental_testing_of_time_dilation   which describes how muons can be produced in earths upper atmosphere when cosmic rays come in.    We use this as a test of special relativity, specifically we suspect that muons moving fast relative to the lab frame would live long enough to reach the surface of the earth before decaying.   The main point is that the effects of relativity like this would still be there, in nature, and eventually we would notice:    Someone would have studied the half-life and decay of particles and they would have noticed that fast moving particles seem to live longer,  they would have said "that's weird, it's as if their clock is ticking slowly".   We now also have particle accelerators and with equipment like that to play with it's almost certain that we would have noticed strange effects when objects have high velocities relative to each other.    Basically, unless the important speed which we are calling c and appears in the Lorentz transformation was many orders of magnitude greater than it actually is, then there would have been signposts to relativity and eventually we would have noticed them.
        With some LaTex mathematical symbols (which, as I mentioned, we don't have the luxury of), we could demonstrate that the Poincare group of transformations is the only set of transformations between reference frames we should consider.   Non-linear transformations are also possible but the words "non-linear" should strike fear into the hearts of anyone who has ever studied some mathematics.   So you can rest assured that every linear transformation between reference frames would have been proposed and examined first.   The basic Poincare transformations like a rotation of the space frame won't explain the results you were getting from experiments so before too long the subset of the Poincare transformations which are just the Lorentz boosts would be all you have left before you move to non-linear transformations.   While mathematicians would be beginning to sweat and fear that  non-linear transformations would be needed, the last set of linear transformations would turn out to be the charm, they would work.  Obviously a Lorentz boost will work, we know that a Lorentz transformation was precisely what we needed.   There would be a constant which we can call c  in those transformations, it would have the dimensions of a speed etc.    The value of c would be chosen to match the experimental results we were obtaining.  Once you have the Lorentz transformations, the rest of the theory of special relativity follows.
   So, summarising all of that,  we could reasonably have obtained the Lorentz transformations just from empirical observations of strange effects when two objects have high velocities relative to each other.   Having a massless particle like light which did travel at c was a great help and it is a clear signpost to relativity.   It probably speeded up the recognition and development of relativity by many years.  Indeed taking, as an axiom, that the speed of light is an invariant will allow you (a lecturer) to develop the theory of relativity on the blackboard in front your students in 1 hour rather than over several lectures.   The key is that it wasn't essential, special relativity is just all the physics which you can obtain from the Lorentz transformation.   You can get to that (the Lorentz transformation) by other routes, you do not need to assume the speed of light is an invariant.   (However, in the world in which we do live, the speed of light is c, so you aren't doing any harm by taking that as an axiom and you will see it done in many textbooks and hear it suggested by many people.   You need this much space on a forum to explain why it isn't quite like that). 

I hope that helps.

Best Wishes.

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Physics, Astronomy & Cosmology / Re: Acceleration of light?

« on: 25/04/2023 03:38:03 »

Hi.

I completely missed the bit about transitions to different mediums, well spotted @geordief .

Here's a pleasant Physics expert talking about why or how light slows down in glass for a YouTube video produced by Sixty Symbols,  it lasts about 15 minutes  and you might as well listen to him rather than read some waffle I produce.

So here's 4 different answers, which are are more or less bullet points summarising that video as I saw it:

 1.   Light can travel more slowly in different mediums but photons don't.   Don't ask, live with it. 
Be aware that many of the old and popular explanations are wrong.

 2.   A photon is a particle that is only readily identified in a vaccum.   Inside a dense medium which is some regular lattice structure of atoms, what you will actually have is a different particle (sometimes called a quasiparticle) which is known as a "polariton".   These polaritons travel at less than the speed of light.
     So the photon wasn't accelerated or ever travelling at less than the speed c,   instead it was just changed into a polariton once inside the dense medium.

  3.  The photon is a quantum mechanical object.   It can take all possible paths through the medium and some of those involve an interaction with other QM objects like electrons and nucleons that are present in the dense medium.  The sum of all the possible paths is such that the overall wave description looks like a photon that has been delayed (travelling at less than c).
     There are similarities between the Quantum Mechanical model and the explanation using classical electromagnetic waves.  With the classical model, an e-m wave passing close to an atom will cause charged particles like the electrons of the atom to oscillate.  However, oscillating electrons will produce their own electromagnetic waves.   So these will interfere with the main wave that was passing through.   (For the QM model, we have that one photon takes mutiple paths so it is all the wave you need on its own and also the interactions with electrons are "like" the classical interaction in some broad sense).

4.   I'm not actually certain I can really articulate a 4th answer.   I'm just fairly sure you could find one if you tried.

Best Wishes.

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Physics, Astronomy & Cosmology / Re: What images do you most want to see from JWST?

« on: 04/08/2022 22:54:04 »

I would like to see stars circling the supermassive black hole at the center of our galaxy.
- At infra-red wavelengths, it should be possible to see more stars than we can see with ground-based telescopes
- and see how active the accretion disk is now
- By watching nearby stars move over a period of years, we should be able to predict when the accretion disk might fire up again.

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Physics, Astronomy & Cosmology / Re: What would happen to the Solar System's gravitational connections if ....?

« on: 14/07/2022 20:30:13 »

while, on a occasion, a comet does crash into the Sun, the newly formed Black hole would be an extremely small target to hit.

So the Sun turning into a BH would affect the orbit of a comet but not (as @Halc  says) that of the planets?

No, it would not alter the orbit of a comet or any other object. The sun squashed down to a 3 km black hole merely becomes a much smaller target, so a comet that might have hit the sun would miss the black hole.

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Physics, Astronomy & Cosmology / Re: How long does a gravitational wave last?

« on: 24/11/2021 03:25:56 »

I am still  wondering if we can still say  that these very asymmetric  "rings" carry away their  energy without  any loss of power at all  as they encounter obstacles in their path.

A spiral is still symmetric, just not spherically symmetric.
By time-symmetry, if matter (say a binary star) can emit gravitational waves and lose energy in doing so, then such a system can hypothetically absorb them, gaining energy in the process, but the passing wave is not going to be focused in such a way that any measurable effect will occur. It would have to look like the time-reversed wave spiraling in, which isn't how it looks when it was emitted from afar.

So would a neutron star or another black hole absorb their energy?

A lone mass like a  neutron star which seems no more capable of absorbing them as emitting them. Any mass will however deflect the waves, breaking the symmetry like water waves crossing a shallow spot. A black hole must absorb the energy as there is no worldline through them. They're also the only things that absorb dark matter.

Or does the gravitational wave go through these objects as if they were not there?

Like dark matter, it goes through them like they were transparent, but still deflected by the gravity.

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Physics, Astronomy & Cosmology / Re: A particle in 2 places at once?

« on: 05/11/2021 22:45:25 »

You seem to me to be be saying or implying that ,if measurements were finer and more efficient that the position and the momentum of a particle could be determined separately and not as a pair that are joined at the  hip

No, I’m not saying that, as Alan has explained very clearly.
What I’m saying is that the analogy @Harri was giving is inappropriate. There are two factors at work here, one is our state of knowledge of a particle because we can’t see it in the same way as looking at a player on a field. We see the player on the field because photons bounce off him and hit our eyes, those photons don’t move him or alter his momentum significantly, but they do if the player is an electron. One way we can detect an electron is to have it hit a detector eg phosphor screen, but then it’s stopped moving. Even if we use a speed gun on the player s/he has to move in order to get a doppler reading. These are the physical and practical problems.
The second problem is answered by Alan.
Reread Alan’s reply https://www.thenakedscientists.com/forum/index.php?topic=83459.msg659621#msg659621
There is an intrinsic limit to how accurately we can know these 2 properties at the same time and is quite different from the measurement problems already described. This is a limit set by the way the universe works. Reread both posts by Alan as he has put down very clearly what is an endless source of confusion to most non physicists, and unfortunately a few physicists!
For a large object like a ball kicked by a player the difference is so small that it is irrelevant around 10^-30m.

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Physics, Astronomy & Cosmology / Re: A particle in 2 places at once?

« on: 05/11/2021 22:14:24 »

You seem to me to be be saying or implying that ,if measurements were finer and more efficient that the position and the momentum of a particle could be determined separately and not as a pair that are joined at the  hip

Quite the opposite. If you know the position of a particle to infinitesimal precision, you have no information as to its momentum. People look different when they are running compared with standing still, but if you photograph a car with a very short flash, you can't tell whether it is moving forwards, backwards, or stationary Cameras have advanced to the point that you can now get "propellor disc blur" software so that photos of classic aircraft in flight look different from stationary models, but a true "snapshot" gives you no clue as to its speed.

So far, so intuitive. But intuition breaks down if you know that an electron is absolutely stationary, Heisenberg says in that case, you can have no idea where it is!

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Physics, Astronomy & Cosmology / Re: Could mass be considered as an excitation of the gravitational field?

« on: 09/10/2021 16:47:44 »

way of looking at the relationship between mass and the gravitational field

You'll have to describe what gravitational field you're speaking of. There is a gravitational potential field, which is a relative (not absolute) scalar at any point, and there is the gravitational strength field, which is the derrivative of the potential field. This is not relative, and is a vector at any given point.

Anyway ,does my question have any merit?

I think so. Mass certainly effects both, but expressing it that way differs in some ways from the typical 'excitation of field' description. Most importantly, excitations imply positive energy relative to the non-excited state, but it is negative energy with mass present. Empty space with no mass in it has more (zero) energy than does the negative energy of the same space with an object in it.

The 'excitation' doesn't move, but changes to the distribution of it is something that travels at light speed, hence gravitons and gravitational waves, which very much are excitations and fields. So there is a field, but excitations of it move at lightspeed, and gravity doesn't do that any more than gravitons are responsible for the attractions between planets.

Also,however defined it, can we imagine the gravitational field to have been established in its initial form  after the BB and to be metamorphosing (changing shape) ever since

Sounds good. The potential has been going up ever since the initial maximum negative value of the big bang. The strength on the other hand has been increasing as the energy distribution changed from completely uniform to today's very localized concentrations of mass/energy.

I think Einstein  may have used the "mollusk" description about it.

Einstein seemed to use the word to describe arbitrary (abstract) coordinate systems (ones that are not inertial or not in Minkowskian spacetime) and not about physical fields, gravitational or otherwise. That's how I read it at least.

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Physics, Astronomy & Cosmology / Re: Are possibly all massless particles just travelling waves in particle fields?

« on: 06/08/2021 09:50:48 »

Who ,I wonder was or were  the  inventors of QFT? Some unholy collaboration,I'd guess.

If science were a religion then this would be a holy collaboration.
In fact as ES says it has been a progressive development over many years, starting as far back as the 1920s. What is now QFT is the modelling that underlies what we call the standard model so you will find that it is built on the work of some very famous names, each contributing a part, and some of those parts are famous in their own right eg QED. That’s how science often works.

I didn't realise it could be used to predict scenarios in the real world
I thought  this was ivory tower territory.

Although often called theoretical physics, much of the theory is in response to the results of experiments. If the theory didn’t fit the experimental results and predict accurately, it would be discarded or changed; and that really is how science works.
It’s worth repeating what has been said, that the current theory is very successful at predictions.

I think by particle fields you may have meant multiple fields, which is correct and as ES says, each particle has its own field. The reason for this is that each particle (the interaction we detect) has its own set of properties so if you are going to describe them and their interactions using a field model then each model will have its own set of properties and the particle will be a quantum of that field.

Waves? The field model supersedes the old idea of wave particle duality (which is still used for elementary teaching) and gets over questions like which slit did the photon go through. However, particles have wave properties and this has been demonstrated for those with mass as well as massless eg electrons. The quantum objects we describe as particles have wave properties (eg they interfere) and particle properties (eg mass or momentum) but they are neither. It’s important to remember that these are models and it is important not to assign a classical reality to them. However, if it helps you to imagine them as waves there is little harm as long as you bear in mind what Stephen Hawking said about virtual particles “just don’t take them too literally”.

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Physics, Astronomy & Cosmology / Re: What happens to the spacetime interval between two events as they approach a BH?

« on: 20/02/2021 16:19:26 »

Is it fair or noteworthy  to say all these events in GR are treated as point objects even though they are actually spatio-temporally extensive?

Events are by definition mathematical points in spacetime, say the event of a photon being emitted and such.  In practice, yes, they're spread out, so one can speak of the event of the sinking of the Titanic even though the ship is fairly large and moved around quite a distance during the hours-long process. It just means we lose precision when we talk about that event.
Billiard balls are modeled as point events (the event of the contact between two balls) when in fact the contact takes finite time, involves finite (but very large) acceleration, and finite surface area of contact.

Is that where a theory of  quantum gravity might be useful?

It's just geometry. No, quantum gravity solves other issues.

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Physics, Astronomy & Cosmology / Re: How do electromagnetic waves propagate through a vacuum?

« on: 30/09/2017 19:11:07 »

I thought fields extended indefinitely in space.That image seems to show the two fields  extending a finite distance .Do they actually extend indefinitely in the direction at right angles to the direction of propagation? (the image being for illustrative purposes)

Electromagnetic fields do extend indefinitely into space. What you are seeing in the image is the amplitude of the photon, which is different from the range of the field that makes up the photon. Amplitudes are finite. Even for a hypothetical, infinite ocean, the waves in the ocean will have a limited amplitude.

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Physics, Astronomy & Cosmology / Re: Is spacetime real?

« on: 26/09/2016 22:07:20 »

geordief, your skepticism of Atkhenaken's claims are well-warranted. I would recommend being highly skeptical of those who make such extreme claims (look up some of their other posts, and you might see a pattern develop) Atkhenaken does not believe that viruses exist, or that medicine is anything other than a scam...

To those who are interested in this thread, I would also recommend paying attention to PmbPhy. He is very knowledgeable regarding physics, especially relativity (we have our disagreements too, but on this particular subject, he is an expert!) evan_au is also a knowledgeable and well-intentioned member of this forum.

Thanks ,yes of course . I look forward to PmbPhy's contributions down the line hopefully.

The phrase "logical certainly " certainly rings alarm bells  as the stand up performer  Tim Vine  might put it

https    ://www.youtube.com/watch?v=HcFd5j1cios

Perhaps I don't have enough posts to post a link on this site yet?

Yeah, I don't remember what the required number of posts is before you can link urls directly, but I think you are close. Maybe 20?

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Physics, Astronomy & Cosmology / Re: Is spacetime real?

« on: 26/09/2016 20:16:36 »

Having drifted into talking about particle spin, this might be worth looking at.  I’m not always happy with the answers here, but this seems not bad.

http://www.askamathematician.com/2011/10/q-what-is-spin-in-particle-physics-why-is-it-different-from-just-ordinary-rotation/