[geordief (OP)]

I appreciate that the speed of a massless object in a vacuum is invariant (c) but ,with em radiation the frequency  does depend on the frame of reference.

If one approaches the source the intensity of the radiation increases.

Can one say that the energy used to create the radiation at source is transmitted more quickly to the observer in this frame of reference than it  would be to an observer in a frame of reference  that was moving away from the source?

Can we talk about a speed of transmission of energy  ( units being expressed as energy x distance per second perhaps)?

[Halc]

I appreciate that the speed of a massless object in a vacuum is invariant (c) but ,with em radiation the frequency  does depend on the frame of reference.

Sound also has frame dependent frequency.

If one approaches the source the intensity of the radiation increases.

A function of distance, not of speed.  Sound again has this property.  Not saying light is like sound, but in some ways it is.

Can one say that the energy used to create the radiation at source is transmitted more quickly to the observer in this frame of reference than it  would be to an observer in a frame of reference  that was moving away from the source?

The frame of reference does make a difference with EM energy.  Let's suppose that Earth is bathed in X amount of energy per second from the sun in the frame of the solar system.  In some other frame (.87c relative to the first, along the line between the two), it would I think be 0.5X per second, because if there is a meter on earth measuring it coming in, it would take 2 seconds to get to X.  But that moving meter is measuring energy in a different frame, and energy itself (like Kinetic energy) is very frame dependent, so that might be the wrong way to go about computing it for this alternate frame.
The distance is halved (quadrupling the brightness) but also changing the frequency of the light (depending on which of the two is in the lead) and halving the rate the photons are emitted.  Those seem to all cancel out.  Perhaps I am doing something wrong.

Can we talk about a speed of transmission of energy  ( units being expressed as energy x distance per second perhaps)?

Energy times speed isn't a transmission rate.  Speed of transmission would be energy per second, which is known as 'power'.  Are you talking about something other than frame dependent power?

[geordief (OP)]

Energy times speed isn't a transmission rate.  Speed of transmission would be energy per second, which is known as 'power'.  Are you talking about something other than frame dependent power?

I think you are right (I was very unclear/muddled  about that).

Suppose we have a "power sensor" moving away from a power source at a relative speed approaching c...

Would it reading not drop to  zero  (as a limit and not simply as a function of increasing distance-as a result of the Doppler effect)?

Is there a "propagation of energy" in that scenario which is also dropping towards zero?

And conversely if  the sensor approaches the source  at a relative speed approaching c ,does this "speed of propagation of energy" approach  infinity?(Does it increase with both decreasing distance AND  increasing speed  or is it just the former as you said in your reply?)

[Halc]

Are you talking about something other than frame dependent power?

I think you are right (I was very unclear/muddled  about that).

I am right about some things perhaps, but I am admittedly out of my field here, so I am not responding with any sort of authority.  I'm exploring this issue just like you are. Perhaps somebody who knows their EM might make corrections for both of us.

Suppose we have a "power sensor" moving away from a power source at a relative speed approaching c...

OK, that's easy.  I have a pair of solar panels in the same place, one stationary relative to the sun, and one moving away at .87c.  The meter on the stationary one will read '1' kilowatt (our control figure), and the one moving away will read say 60 watts (as a guess).  That's a frame independent answer since the meters read the same number in any frame.  Those are proper measurements of the power received by each device.
Where did I get 60?  Not only is that panel receiving only about an 8th of the number of photons per Earth second, but the wavelength of those photons is much higher, so it is probably considerably less than 1/7th of a kw.  The panel might not even be particularly sensitive to this alternate spectrum light, and put out a small fraction of that, say 30 watts in Earth frame, an illustrative guess.  The 60 figure is twice that since it gets two hours of proper energy for every Earth hour.

But the power meter is measuring energy per hour, and while energy is frame independent, time is not.  So in the Earth frame, the moving one actually gets only 30 watts since it takes two hours to pull in that 60 watt-hours of energy.  In the frame of the other meter, it is getting 60 watts and the moving one gets 500 watts, a different ratio.  So it seems that power is indeed frame dependent.

Would it reading not drop to  zero  (as a limit and not simply as a function of increasing distance-as a result of the Doppler effect)?

Your topic concerned speed, not distance.
Distance has no effect on Doppler.  Of course I get less energy from more distant (but equally bright) stars.  My speed relative to them has little to do with that.

Power over distance depends on how the energy is transmitted. If I have superconducting wire, I can transmit power over any distance without loss.

Is there a "propagation of energy" in that scenario which is also dropping towards zero?

If the energy is from a radiant source (one with an inverse square relationship), then obviously energy from that source approaches zero with arbitrary large distance.  Not so with the superconductor, but that isn't a radiant source.  There is a 1-1 law there, not inverse squared.

I think you wanted to focus on the effects of speed, not distance.  I have an electric vehicle on superconducting rails moving relative to the power source for the rails.  Is the power from that source frame dependent?  I say yes.

And conversely if  the sensor approaches the source  at a relative speed approaching c ,does this "speed of propagation of energy" approach  infinity?(Does it increase with both decreasing distance AND  increasing speed  or is it just the former as you said in your reply?)

Now you're talking speed again, not distance.  OK, so consider a 3rd panel moving at .87c, but towards the sun.  It 'counts' photons at 3.74 the proper rate of the 'stationary' one.  The blue shift of the light may or may not contribute positively.  Sure, the blue light is more energetic, but the panel again may not be optimized for that spectrum.  So our power meter reads anywhere from say 2 to 5 kw.  In Earth frame, that's 1-2.5 kw of power, and in the panel frame, the Earth one again produces 500W of power.

[yor_on]

No, it's just the 'magnitude' that differs it. The speed is always 'c'. You need to introduce a source and a sink, then you need to introduce how they move relative each other. The force you measure this light to have is a function of both, but the speed is a invariant as far as I know. Another way to look at it would be to state that a photon 'intrinsically' always have a same energy, the rest is just a function of your 'systems' relative motion (and gravity).

[jeffreyH]

Light is defined by the properties of wavelength and frequency. These are variable. What is not variable is the speed of light in a vacuum.

Frequency and wavelength define the energy of the photon. Wavelength times frequency equals c. Planck's constant times frequency gives the energy of the photon.

So the propagation speed for this energy is c, in a vacuum. Other media have refractive index that determine the speed of the photon.

Kinetic energy for other particle types have the magnitude of the velocity of the particle as a "propagation" speed. Although the full energy of the system includes the rest energy of the particle.