Post by: ron123456 on 18/02/2022 20:06:01
Post by: alancalverd on 18/02/2022 23:24:34
Post by: evan_au on 19/02/2022 07:04:34
Quote from: OP
Why one constant maximum speed?It comes down to Einstein's relativity.
- It would take an infinite amount of energy to accelerate a massive particle (eg an electron or a baseball) up to "c"
- Massless particles (eg photons or eth hypothetical graviton) which carry energy can only travel at "c"
- As far as we know, "c" is the ultimate speed limit of the universe.
See: https://en.wikipedia.org/wiki/Theory_of_relativity
Post by: alancalverd on 19/02/2022 09:36:02
As I see it, relativity begins with the hypothesis that c is constant and derives a number of observables, so it explains the consequences but not the mechanism.
No theory can control the universe, so relativity (which seems to be 100% accurately predictive), aether (once popular but demonstrably garbage) or any other theory, may correlate our observations and predict the behavior of the universe, but Maxwell's derivation of the mechanism (and thus the speed) of electromagnetic propagation is based on experimental observation of the relationship between electric and magnetic fields, with no axiomatic hypothesis.
Post by: ron123456 on 19/02/2022 17:41:56
Post by: Petrochemicals on 19/02/2022 18:25:13
Post by: Colin2B on 19/02/2022 22:13:49
.perhaps Maxwell's equations are just good for static conditions???Up to now they have been found to be valid for all conditions.
An increase in energy does not have to imply an increase in speed, that’s KE. An increase in heat energy results in an increase in atomic vibration (putting it simply), but no increase in the speed of the hot body.
Post by: Colin2B on 20/02/2022 07:38:47
@ron123456 It’s worth adding that we usually add the conditions that the measurements are made in a vacuum and locally..perhaps Maxwell's equations are just good for static conditions???Up to now they have been found to be valid for all conditions.
As you know, the speed of light in a medium eg glass differs from that in a vacuum. Non-local measurements can be subject to relativistic effects due to differences in gravitational potential.
The speed of light or "c" is effectively energy, faster is energy plus energy which is nonsensical.Not sure what you are saying here.
If a car is travelling at v we can calculate it’s KE, if we accelerate it to v’ we have added KE.
If you are saying light is pure energy then that is untrue.
Post by: evan_au on 20/02/2022 07:47:01
Quote from: alan calverd
the speed of electromagnetic propagation is based on experimental observation of the relationship between electric and magnetic fieldsMaxwell's equations were a brilliant synthesis of everything known about electric and magnetic fields, producing a surprisingly accurate prediction for what we now call "c"
- I understand it was the agreement of the experimentally measured speed of light with the prediction of Maxwell's equations that influenced some people to think that light had to be electromagnetic waves.
However, Maxwell's equations do not explain why hypothetical gravitons (or the more classical gravitational waves) should also travel at "c".
- So far as we know, oscillating electric or magnetic fields do not generate gravitational waves, but do generate electromagnetic waves
- So far as we know, oscillating masses do not generate electromagnetic waves, but do generate gravitational waves (provided the masses are electrically neutral)
- And yet Einstein predicted that both would travel at "c"
- And observations of colliding neutron stars suggest that they travel at identical speeds (within very tight limits)
Quote from: ron123456
why does the frequency increase instead of a compromise between frequency and speed?"c" is the velocity of light in a vacuum.
- However, in materials (eg water or optical fiber), there is a change in speed with wavelength, which is called "dispersion"
- This produces the familiar rainbow
- From Wikipedia: For red light (wavelength 750nm, η = 1.330 based on the dispersion relation of water), the radius angle is 42.5°; for blue light (wavelength 350nm, η = 1.343), the radius angle is 40.6°.
Since the refractive index η = c/v, the higher refractive index of blue light implies that its velocity is slightly lower than red light, when traveling through water. The questioner assumes that higher frequency would produce a higher velocity.
https://en.wikipedia.org/wiki/Rainbow#Mathematical_derivation
Oops - overlap with Colin2B
Post by: Bored chemist on 20/02/2022 10:23:57
It's the same with EM radiation.
Post by: Colin2B on 20/02/2022 10:43:37
- And yet Einstein predicted that both would travel at "c"I think it was Poincare who first suggested it and also predicted gravitational waves before Einstein. Apparently he didn’t think it worth pursuing.
- And observations of colliding neutron stars suggest that they travel at identical speeds (within very tight limits)There are some predicted anomalies, but for very high intensity waves, far higher than we are likely to see here. Some suggestions of echoes from inflation period that might be detectable.
Post by: alancalverd on 20/02/2022 10:59:35
In general, the speed of a wave- such as a sound wave, depends on the medium, rather than the wave itself.
And as Maxwell showed, we can ascribe and measure the properties μ and ε to any "medium", including a vacuum. Hence c is constant.
Examining any electromagnetic spectrum we find that the minimum wavelength and maximum frequency depends on the energy (temperature, accelerating voltage, whatever) of the source.
Post by: Bored chemist on 20/02/2022 11:08:56
Examining any electromagnetic spectrum we find that the minimum wavelength and maximum frequency depends on the energy (temperature, accelerating voltage, whatever) of the source.In the very real sense that my walkie talkie radio broadcasting a few milliwatts on 49 MHz has more energy than the BBC's Radio 4 long wave transmitter that sends 500 kilowatts of Radio 4 across much of Europe on 198 KHz.
Post by: alancalverd on 20/02/2022 11:28:03
But full marks for listening to R4 on LW. Test Match Special is the last vestige of civilisation on this planet.
Post by: Bored chemist on 20/02/2022 11:32:55
Energy, not power.I don't think the pp3 battery has the same energy as, for example, the smoothing capacitors on the Droitwich rig's power supply.
But full marks for listening to R4 on LW.I don't.
I just use it as a frequency standard. That 198KHz is pretty close to exactly right.
It would be easier to use if they stopped modulating it with rubbish about seedcake.
Post by: Petrochemicals on 20/02/2022 12:02:06
If you are saying light is pure energy then that is untrue.Could you point me in the direction of pure energy, I'd love to see some.
Post by: alancalverd on 20/02/2022 12:39:21
I don't think the pp3 battery has the same energy as, for example, the smoothing capacitors on the Droitwich rig's power supply.True, but neither station is throwing its power supply at the receiver. Both are generating photons, and the BBC's photons are of lower energy than yours. Quality, not quantity.
If you listened to TMS you would find the Greenwich Time Signal an entirely adequate frequency standard. Six pips for lunch, six pips before the closing overs (can they complete the required overs in 30 minutes with pace or is it time for spin bowling?). Nothing else matters. The occasional seventh pip reminds us that the Laws of Physics are mightier even than the Laws of Cricket, but only just.
Post by: Eternal Student on 20/02/2022 14:21:15
There's a whole raft of different things going on here but I'll address the original question first and hope we haven't just lost @ron123456 somewhere.
Hello.....What is determining the EM speed of photons, if not energy?......Why one constant maximum speed?...thx
There are two different ideas mentioned in that phrase.
1. You ( @ron123456 ) mentioned EM. The speed of Electro-magnetic waves is determined by Electrodynamics and Maxwell's Theory. I think @alancalverd was one of the first to describe this. People have given good summaries already and there are plenty of texts on the subject. It is perhaps a little unexpected but the frequency of oscillations and the amplitude of oscillations have no effect on the speed of the wave. It is much as @Bored chemist mentioned recently, the properties of the medium in which the wave travel (so that will be the permeability and permittivity) control the speed of wave propagation.
2. You also mentioned "photons", this is where light is modelled as a particle. The speed of photons can be determined using Special Relativity under certain assumptions. Also as @alancalverd mentioned, all you can really determine is that all these sorts of massless particles should move at the speed c, whatever that speed c is set to. Fortunately, the strange behaviour of light was a significant motivator for the development of Special Relativity so this speed, c, was set to the speed of light. However, as alancalverd implied, you can't then really claim that SR proves light travels at the speed of light since that was an assumption to begin with. So it's going to be automatic to just re-interpret your question as asking why massless particles (like photons) must travel at the speed c.
Let's explain this another way: It's reasonable to assume that the Lorentz transformations hold (which you could have established experimentally, for example) but you had no reason to assume light travels at the special speed c that appears in that theory. Using that theory of special relativity can you then show that light would have to travel at that special speed c? The answer is yes you can, under certain assumptions (for example, the assumption that light is just a collection of massless particles - but I reckon there are a few more assumptions tacitly made which I might discuss in another post).
Anyway, that's why you're getting several different kinds of answers - there are two different theories and two different ways of trying to get an answer for your question.
Best Wishes.
Post by: alancalverd on 20/02/2022 15:18:08
We have two mathematical models to describe the behavior of electromagnetic radiation: continuous waves and discrete particles. Neither description is complete for all circumstances, but equally, neither defines what em radiation is, and the idea of a massless particle transferring momentum really does confuse people.
Grumpy old men like me say that Maxwells' equations (wave model) predict the speed of propagation of EMR, and photon statistics (particle model) predict such things as photographic and ionisation phenomena where we can count discrete interactions with threshold energies.
Post by: Eternal Student on 20/02/2022 15:35:57
....and the idea of a massless particle transferring momentum really does confuse people.I'm definitely with you, or at least sympathetic to, a lot of what you've said alancalverd. It connects with something I was trying to get together and tidy up to make a post. I'm just going to post it now, scruffy as it is...
I would like to target something that @evan_au mentioned first but lots of other people have also added to.
Massless particles (eg photons or the hypothetical graviton) which carry energy can only travel at "c"
(ES adjusted the spelling of the word "the")
Go for it, evan_au, or anyone else reading this. Present your proof or provide links to a reference etc.
I've seen a lot of partial proofs and plausibility arguments but nothing that I'm completely happy with. It doesn't mean it's wrong, I don't know everything - I'm just old enough and stubborn enough to question every little detail these days.
A lot of arguments will start from this equation
E2 = m2c4 + p2c2
[Eqn.1]
(with the usual meanings, p = 3-momentum, m = rest mass etc.)....because I would stop you there and ask exactly how you derived that equation to start with. For massive particles that equation is easily obtained by considering the 4-momentum, Pμ, of a particle. It's simple enough since a 4-velocity, Uμ =
is well defined for particles with timelike paths and then we can define Pμ = m Uμ. However, for particles that follow null paths we can't use the same tricks: The 4-velocity is not defined since
= 0 on a null path. It seems common to just introduce a new definition for the 4-momentum of a photon,Pμ = ( E/c , VxE/c2 , VyE/c2 , VzE/c2 ) with Vx = x-component of velocity (w.r.t. co-ordinate time) for the photon etc.
... but the question is how did you come to decide on putting those particular components into the 4-momentum?
The only "proof" or plasuibility argument I've ever seen for that definition of the 4-momentum of a photon is based on using some additional infomation about the behaviour of photons. Something that you could observe in experiments on photons but is not otherwise obviously true for all massless particles.
For example, assuming straight away that the momentum and energy are related by p = E/c. Alternatively, you can derive that form for the 4-momentum under the assumption that the 3-momentum in any spatial direction will be proportional to the velocity (w.r.t. co-ordinate time t and not proper time Tau) of the particle in that direction. Another "proof" I've seen utilises a 4-wave vector for the photon (which assumed the photon had wave-like properties and propagated at speed c to begin with). Anyway, all of these things are extra pieces of information you could obtain by experiment on light but I don't see why they're necessarily true for arbitary massless particles.
Best Wishes.
LATE EDITING: This article, https://www.wtamu.edu/~cbaird/sq/2014/04/01/light-has-no-mass-so-it-also-has-no-energy-according-to-einstein-but-how-can-sunlight-warm-the-earth-without-energy/
is quite a good one, it's easily readable by non-specialists, if anyone wants a general guide for why massless particles travel at the speed c. It's typical of many Popular Science articles where E2 = m2c4 + p2c2 [Eqn 1] is the basis or starting point for the argument.
Post by: evan_au on 20/02/2022 20:20:30
Quote from: alancalverd
Test Match Special is the last vestige of civilisation on this planet.Do any Europeans care about cricket?
- Or is it just for British expats with a Long-Wave receiver?
Post by: alancalverd on 21/02/2022 09:22:07
Do any Europeans care about cricket?I was talking about civilisation, not Europe.
Post by: syhprum on 21/02/2022 12:38:31
Post by: alancalverd on 21/02/2022 12:59:57
Post by: Bored chemist on 21/02/2022 13:20:51
Do any Europeans care about cricket?Yes.
Notably quite a lot of Europeans who live in England care about Cricket.
Ask a geography teacher for further clarification.
(Don't ask a politician; they don't seem to understand).
Post by: alancalverd on 21/02/2022 16:24:36
Post by: evan_au on 21/02/2022 21:16:51
Both nations have subsequently drifted somewhat away from their colonial roots, and now prefer summer games with an axially-symmetric bat.
Post by: ron123456 on 24/02/2022 20:42:35
If a dipole antenna /atom (decreasing in length in time) emits a photon....yeah, perplexing, I guess?....The problem is with the quantum jump......Planck has the highest authority.....Sorry about his son....
Post by: yor_on on 02/03/2022 23:56:41