[geordief (OP)]

I understand that em waves propagate as disturbances in the em field (correct?)as an expanding sphere centred on the point of emission.

What is the actual mechanism that causes this propagation effect?(I "know" that the electric field  creates a magnetic field and vice versa)

Is it connected to the action and reaction effect that allows rockets to move in a vacuum?

Is it valid to ask what happens if there is no pre-existing em field for the  wave to propagate through?

Would that be nonsensical ?

[evan_au]

what happens if there is no pre-existing em field for the  wave to propagate through?

A field consists of a parameter which is measurable at every point.
- Atmospheric pressure is a scalar field, having a value, but no direction. This field extends into outer space, where the value of the field is 0.
- The electromagnetic field is a vector field, where the field has a strength and direction (and this may vary at different frequencies or at different times). This field extends inside the shield of a quantum computer, where the value of the field is 0 (or as close to 0 as they can manage).

If you create a change in electrical current inside a quantum computer, this will cause a change in the strength and direction of the electromagnetic field, which will propagate at the speed of light through the (normally 0) electromagnetic field inside the shield, until it is absorbed.

[Colin2B]

I understand that em waves propagate as disturbances in the em field (correct?)as an expanding sphere centred on the point of emission.

It is only a sphere if there is a point source radiating in all directions.
There are many sources that don’t eg microwaves from a dish, headlights, lasers, directional radio antenna.

What is the actual mechanism that causes this propagation effect?(I "know" that the electric field  creates a magnetic field and vice versa)

It is as you say

Is it connected to the action and reaction effect that allows rockets to move in a vacuum?

no, nothing like that, although such waves can transfer a small amount of momentum.
 

Is it valid to ask what happens if there is no pre-existing em field for the  wave to propagate through?

Would that be nonsensical ?

as we’ve explained in many previous threads about fields, a field is described by a series of measurements at particular points in space and time. So a temperature field describes the temperature at various points in a room, a wind field describes the speed and direction of the wind over the earth, so an em field describes the strength of the electric and magnetic field at various points. So it is fairly nonsensical to talk about a preexisting em field in the absence of the wave, don’t think of the wave propagating ‘through’ something.
However, it is useful shorthand to treat fields as objects in modelling, hence you will hear the electron described as a disturbance of the electron field - which might be taken as “1” = electron, “0” = no electron, or you might get a little more detailed and talk about the presence or absence of a group of properties having certain values eg mass, charge, spin.


Whoops, looks as though Evan has answered while I was typing

[geordief (OP)]

If you create a change in electrical current inside a quantum computer, this will cause a change in the strength and direction of the electromagnetic field, which will propagate at the speed of light through the (normally 0) electromagnetic field inside the shield, until it is absorbed.

Are the values of the points in the field probability values? Does one have to distinguish between classical fields and fields  that are modeled in a quantum way?

[Colin2B]

Are the values of the points in the field probability values?

Not unless you are trying to predict or model the values. Most of the time you can take actual measurements eg radio wave

Does one have to distinguish between classical fields and fields  that are modeled in a quantum way?

We differentiate between fields that require a medium eg sound and those that do not and are termed relativistic fields - they behave as light does. Note here that I am refering to the field as if it were an object, but remember it is only a mathematical description of what we measure.

[geordief (OP)]

Note here that I am referring to the field as if it were an object, but remember it is only a mathematical description of what we measure

Would it be correct to say that ,while the field itself is a mathematical abstraction it does nevertheless model something  that can be described as objects(can a physical action-or potential physical action- be described as an "object" -or ,at least require an "object"?

[Colin2B]

Would it be correct to say that ,while the field itself is a mathematical abstraction it does nevertheless model something  that can be described as objects(can a physical action-or potential physical action- be described as an "object" -or ,at least require an "object"?

No.
We can treat a number of things as objects that do not exist as physical objects or phenomenon. For example, in object based programming we can treat a string of numbers or an array as an object.
To ask “can a physical action-or potential physical action ......... require an "object"?” Is a little like asking whether a spiders web requires an object to support it’s span; no it doesn’t, it is self supporting. In the same way em radiation is self propagating; as far as we know it requires no supporting object.

[geordief (OP)]

We can treat a number of things as objects that do not exist as physical objects or phenomenon. For example, in object based programming we can treat a string of numbers or an array as an object.
To ask “can a physical action-or potential physical action ......... require an "object"?” Is a little like asking whether a spiders web requires an object to support it’s span; no it doesn’t, it is self supporting. In the same way em radiation is self propagating; as far as we know it requires no supporting object

Thanks.

To go back to this self propagation that I was having a problem with understanding,  could I ask you where I could learn  about the mechanics of the interaction between two moving electric charges as explained in quantum mechanics?

And how this interaction leads to the em propagation.

Or is em radiation perhaps only described in Classical physics?

[evan_au]

To go back to this self propagation that I was having a problem with understanding,

Light is not the only thing that self-propagates....

Looking at the universe from a wave viewpoint:
- Waves on a pond (or sound-waves in air or vibrations in a guitar string) self-propagate as an interchange between kinetic and potential energy
- Light self-propagates as an interchange between electric and magnetic fields.
- Gravitational fields self-propagate as a distortion of space and time
- The difference is that light and gravitational waves don't need a physical medium to support their oscillation.

 Looking at the universe from a particle viewpoint:
- Electrons, baseballs and the Earth self-propagate through a vacuum
- just like photons and (hypothetical) gravitons.
- None of them need a medium to move.

As quantum theory has shown, every particle acts a bit like a wave, and every wave acts a bit like a particle...
See: https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

[geordief (OP)]

To go back to this self propagation that I was having a problem with understanding,

Light is not the only thing that self-propagates....

Looking at the universe from a wave viewpoint:
- Waves on a pond (or sound-waves in air or vibrations in a guitar string) self-propagate as an interchange between kinetic and potential energy
- Light self-propagates as an interchange between electric and magnetic fields.
- Gravitational fields self-propagate as a distortion of space and time
- The difference is that light and gravitational waves don't need a physical medium to support their oscillation.

 Looking at the universe from a particle viewpoint:
- Electrons, baseballs and the Earth self-propagate through a vacuum
- just like photons and (hypothetical) gravitons.
- None of them need a medium to move.

As quantum theory has shown, every particle acts a bit like a wave, and every wave acts a bit like a particle...
See: https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

Does the relativity of motion add anything to the points you made?

I have been thinking along those lines recently as I have been pondering the well repeated saying that we all "travel" through space-time at the "speed" of c ,even when "at rest":it seems to me that the relativity of motion itself may put puts a spanner in the apparent sense of that saying)

Hope the OP gets a little  license to go a bit off piste on his own thread 

[Halc]

Does the relativity of motion add anything to the points you made?

Relativity of Motion is a Galilean concept, and just means that all motion of ordinary things (like that bird over there) is relative to to some arbitrary reference, and thus any massive thing is stationary relative to itself. This relativity of motion has little to do with what Evan has been posting since massless things like light and gravitational waves propagate at c relative to any inertial reference frame. It is said to be frame invariant.

I have been thinking along those lines recently as I have been pondering the well repeated saying that we all "travel" through space-time at the "speed" of c ,even when "at rest"

That's pretty poorly worded, so I hope it isn't as well repeated as you claim. Nothing 'travels' through spacetime. Things trace static worldlines through spacetime, and the time between a pair of events at the same location but separated by one second happens to be the same as the interval for 300,000 km. It seems improper to represent that as travel since the object in question is stationary and 'travel' implies motion from one spatial location to another. It is just a statement of how to convert spatial distance to time.
Again, it seems to have little if any to do with propagation rates of light and gravitational waves.

[charles1948]

If everything is "relative", why does light have a fixed speed of 300,000  kms per second. Why that particular value, and not say, 500,000 kms per second?  Or any other value?  Such as 0, or infinite?

[evan_au]

Why that particular value, and not say... 0, or infinite?

Some suggestions from a video I saw posted in this forum recently:
- Since nothing can go faster than c, if c=0 then you have an immobile universe which cannot sustain life. So if there was such a universe, there would be no intelligent inhabitants to understand it.
- Due to the E=mc² , if c=∞, it takes an infinite amount of energy to create mass, so there can only be massless particles traveling infinitely fast, and no matter.

Another comment from that video: c is actually the speed of causality. if c=∞, then we would be potentially affected by everything else going on in the universe, and it would be hard to tell what event causes other events(ie the concept of time would be very different).

[Bill S]

- Since nothing can go faster than c, if c=0 then you have an immobile universe which cannot sustain life

Or possibly a universe composed of tachyons, and whatever might evolve from that.  Might be good for sci-fi.

  Due to the E=mc2 , if c=∞, it takes an infinite amount of energy to create mass, so there can only be massless particles traveling infinitely fast, and no matter.

If we are postulating that c could be infinite, we could also have an infinite cosmos, with infinite energy available, so an infinite amount of matter would be inevitable. 

Another comment from that video: c is actually the speed of causality. if c=∞, then we would be potentially affected by everything else going on in the universe, and it would be hard to tell what event causes other events(ie the concept of time would be very different).

Come back David Bohm, all is forgiven!

[RobC]

If everything is "relative", why does light have a fixed speed of 300,000  kms per second. Why that particular value, and not say, 500,000 kms per second?  Or any other value?  Such as 0, or infinite?

Both charles1948 and I in the past have asked this and a complete answer has not been given.
Why 300,000 km/s and not another value?
Can an explanation be given that derives the 300,000 km/s?

[evan_au]

Both charles1948 and I in the past have asked this and a complete answer has not been given.
Why 300,000 km/s and not another value?

The value of the speed of light just pops out of Maxwell's equations for electromagnetism: c= 1/√ε₀μ₀
- Where ε₀ is the permittivity of a vacuum
- and μ₀ is the permeability of a vacuum
- Both can be measured in a lab
- Both light and radio waves were known to travel at this speed, which led to the conclusion that they were both electromagnetic waves.
- If light were to travel at a different speed, then the properties of a vacuum would have to be different

See: https://en.wikipedia.org/wiki/Maxwell%27s_equations#Vacuum_equations,_electromagnetic_waves_and_speed_of_light

[RobC]

The complexity of the mathematics overwhelmed me especially when I found permittivity being defined in terms of 'c'.

Could you offer a simplified derivation of 300,000 km/s for 'c'?

[Colin2B]

Could you offer a simplified derivation of 300,000 km/s for 'c'?

What @evan_au is describing is about as simple as it gets. Permittivity and permeability are derived from the basic behaviour of electric and magnetic fields and an electromagnetic field/radiation is a combination of these 2 phenomena.

[geordief (OP)]

Could you offer a simplified derivation of 300,000 km/s for 'c'?

What @evan_au is describing is about as simple as it gets. Permittivity and permeability are derived from the basic behaviour of electric and magnetic fields and an electromagnetic field/radiation is a combination of these 2 phenomena.

So the speed of propagation of the wave based on Maxwell's equations is a function of the permittivity and permeability of the medium ?
And when that medium is a vacuum , that measured speed  is the same as the speed of light in a vacuum?

[Colin2B]

Could you offer a simplified derivation of 300,000 km/s for 'c'?

What @evan_au is describing is about as simple as it gets. Permittivity and permeability are derived from the basic behaviour of electric and magnetic fields and an electromagnetic field/radiation is a combination of these 2 phenomena.

And when that medium is a vacuum , that measured speed  is the same as the speed of light in a vacuum?

Except the vacuum is not a medium. As soon as you have a medium then the light hits the atoms in that medium, is absorbed and re-emitted hence a delay, it travels at c between the atoms (vacuum). The atoms in the medium will have specific energy levels which is why the re-emitted light might not be the same frequency as that entering.

There is a simplified summary here https://www.sparknotes.com/physics/optics/light/section3/