what propels light?

[ballaratdragons (Ken)]

I didn't understand a word of that

I must be tired because it wasn't going in, and about halfway through I wanted to stop reading. But I made myself read right through to the end, and it still made no sense to me.

I guess I just don't have a 'physics' mind even though I am fascinated by it

[Paduan (Brett)]

There is a great rubbish bin in the universe that the Physicists dump all of the "unexplainable" it is called the Quantum world, it is a place where the impossible meets the improbable and comes out as an answer that is indecipherable to 99.9% of the population. so when a physicist has a little hissy fit at your less responsive grey matter remember there will always be things that are alien to them. Things that you take for granted are in the Quantum world to most people bordering on Genius

[Jaybee (John)]

Thank you Mark for your answer....I'm afraid I'm with Ken though...I didn't understand one bloody word of it!

[barx1963 (Malcolm)]

Originally Posted by Peter Ward

Where did all the energy of the (very high frequency) photons from the big-bang go? since they have apparently now cooled to very low frequency in the CBR !!!

I am no expert but here goes. As the space through which light (or any electromagnetic radiation) expands it stretches the wavelength of that light. This is how we get redshift. The photons that were emitted at the time of the big bang are still all around us, but their wavelength has been stretched to microwaves.
Obviously the total energy that was in those photons is still there, its is filling a vastly greater volume of space, so its temperature is much lower.
The above is the explanation as I have read it but summarised in my own words. The thing I am not sure about is as follows. A percentage of these photons would have been absorbed by matter in the 13 billion years since the big bang. For example the snow you see on old TVs that were out of tune was partly the CMB. So the photons that hit your TV antenna have been absorbed and are no longer out there. So these photons have been happily hitting stars, planets, interstellar dust and gas, people, aliens and TV antennas for 13 billion years, give or take.
As the temp of the CMB is one of the main planks supporting the age of the universe, wouldn't that make these estimates a bit inaccurate?
I am aware that the age of the universe is supported by other lines of reasoning, but this is something I haven't been able to get an answer to.

Malcolm

PS Bert, there are a lot of people who don't understand basic physics(myself included, but I am getting better!). Getting angry is not the answer, save it for the genuine hoaxers and bull crap artists out there. I think the OPs question was reasonable and interesting and deserved a little more respect.

[marki]

Originally Posted by ballaratdragons

I didn't understand a word of that

Don't worry Ken and John, niether do I .

Mark

[KenGee (Kenith Gee)]

Bert may have called troll a bit early, but fully understand where he is coming from. Science is hard so if you wan to learn be ready to study and work hard. There are to many trolls trying to prentend they have done the work.

The fact that the answers don't come easy is the very reason these men and women are my hero's.

[Peter Ward]

Originally Posted by barx1963

I am no expert but here goes.........
Obviously the total energy that was in those photons is still there, its is filling a vastly greater volume of space, so its temperature is much lower.
..........

Yes...I have read pretty much the same. Problem I have with that is due the particle/wave nature of light.

As a particle, the photon's oscillation frequency slows from gamma-ray frequencies to micro-waves over time, yet, from a relativistic point of view, for the photon, no time at all has elapsed since the big-bang, and as far as it is concerned space hasn't got any bigger at all, yet its energy has decayed.

[Jaybee (John)]

Originally Posted by KenGee

Bert may have called troll a bit early, but fully understand where he is coming from. Science is hard so if you wan to learn be ready to study and work hard. There are to many trolls trying to prentend they have done the work.

The fact that the answers don't come easy is the very reason these men and women are my hero's.

Kenny
I must be as thick as two short planks, because I seriously do not understand why the OP's question got such a tirade of abuse. Isn't the foundation of this forum based on the unity and generosity of its members? The one thing I found here is how quickly members come to the aid of one another. If one does not know the answer, they only have to ask, and information comes from those that know and who are only too happy to share the knowledge.
From what I can gather the OP asked a simple question that deserved a simple answer. Well an answer of sorts, I don't think this question even has a simple answer...He had limited resources available so with what he had, chose a well respected and trusted source, but got a slap in the face. I agree that Science is hard, and needs to be worked at and studied, but isn't it up to the ones that we would call teachers, to teach, if asked? If he had asked this question in a University Lecture hall, would he be berated, or would the Teacher try to the best of his ability, to provide an answer that would not only benefit the OP but the rest of the class? Isn't 'study' seeking answers to things which we do not know? In any way we can? Nowhere in the OP's original post does it give the impression that he has pretended to have done the work. Clearly he hasn't done any, otherwise he wouldn't have needed to ask the question in the first place.

[JB80 (Jarrod)]

I must be a bit thick too John, don't worry it's not just you.
Here I was thinking an interesting discussion was in danger of breaking out but I shouldn't of been worried.

Thankyou to those who have treated the question seriously.

[shane.mcneil]

Hi all.

This is still called the Astronomy and Amateur Science form isn't it? I probably don't even rate as an amateur. I've learnt heaps from the friendly people hear. Still not sure what the answer to the question is though. I'll re-read the answer again.

[avandonk]

I am sorry maybe this will help.

It was James Clerk Maxwell that published his seminal work of four equations that related magnetic fields to electric fields in the late 1800's.

A dynamically changing magnetic field produces a changing electric field orthogonal to itself which then produces a changing magnetic field also orthogonal to itself and so on ad infinitum. We call this Electromagnetic Radiation or light or radio waves or xrays or gamma rays or lots of other labels. They are all the same thing only the frequency of oscillation or wavelength is different.

Quantum effects only control the means of production. For example an electron orbiting a nucleus even though it is a charge that is accelerating does not emit EM radiation. It is only when the electron changes from one quantised orbit to another that it emits or absorbs a quantum or photon of EM radiation.

Large masses like the Sun or Galaxies can affect the trajectory of photons by the localised distortion of space time due to the large mass present. The photon merely follows the straight line path though space time. It is not deflected by the gravitational field.

The curious structure of our Universe is such that even when there is no matter present or even photons virtual particles appear and disappear. The laws of physics seem to be independent of any localised matter or photons or anything else.

The real scary thing is these laws exert their 'authority' even when there is nothing to detect. Each of the infinite number of possible Universes are thought to all behave like this but only the basic laws are different. If the laws of our Universe were only slightly different we would not be here to observe it.

We can only tell the passing of time by observing events such as Sunrise and Sunset. Or the vibration of atoms in our atomic clocks and Pulsars etc.

I could write a book but that has been done and it is still happening.

Bert

[GrampianStars (Rob)]

Originally Posted by avandonk

,,,,,
We can only tell the passing of time by observing events such as Sunrise and Sunset. Or the vibration of atoms in our atomic clocks and Pulsars etc.
''''''
Bert

Ah! "OB1" left out chin stubble & nose hair

[ballaratdragons (Ken)]

Originally Posted by avandonk

Large masses like the Sun or Galaxies can affect the trajectory of photons by the localised distortion of space time due to the large mass present. The photon merely follows the straight line path though space time. It is not deflected by the gravitational field. . .
Bert

So, the effect of Einstein Rings using the term 'Gravitational Lensing' is innaccurate.
Would it be 'Space-Time' Lensing'?

[Dave2042 (Dave)]

Originally Posted by ballaratdragons

So, the effect of Einstein Rings using the term 'Gravitational Lensing' is innaccurate.
Would it be 'Space-Time' Lensing'?

I think the point is that you get into all kinds of semantic pitfalls when you try to be precise in your language, but while using everyday terms. So in my experience, physicists tend to not talk of 'straight lines' in GR, as the term just confuses. They talk, instead, of 'geodesics', which are the paths that light follows. In a sense geodesics are like straight lines - absent matter they are straight lines. Once you add matter, space-time bends, and the geodesics change accordingly. Does this mean the geodesics are now bent, or are they still straight lines? I don't know that the question even means anything.

As for gravitational lensing, yes it's arguably a classically-based term that doesn't properly reflect the phenomenon, but my view is that it's just a label, and a neat one.

[GeoffW1 (Geoff)]

Originally Posted by ballaratdragons

So, the effect of Einstein Rings using the term 'Gravitational Lensing' is innaccurate.
Would it be 'Space-Time' Lensing'?

Hi,

That wouldn't be a bad way of putting it. Bert meant (I think) that these photons don't feel gravity directly, but just follow their normal path through space-time, the very fabric of which has been distorted by the presence of matter.

Howzat?

[GeoffW1 (Geoff)]

BTW,

You know that at conferences and seminars and drinking sessions between front-rank physicists, working at the very edge of all this theory, they also get upset and hysterical and abusive, and throw chairs and things?

Not Prof Pearlywhites though

Cheers

[Kal (Andrew)]

Originally Posted by Peter Ward

Which leads me to my next question: condsider E=cv (energy of a photon is directional proportional to its frequency )

Where did all the energy of the (very high frequency) photons from the big-bang go? since they have apparently now cooled to very low frequency in the CBR !!!

I think that when someone clearly explains the whats and hows regarding the expansion of the universe you might get the answer you seek

[GeoffW1 (Geoff)]

Originally Posted by Jaybee

Kenny
I must be as thick as two short planks,

We did not want to say anything

[Jaybee (John)]

Thanks Bert...


and yeh....thanks Geoff!

[cwjohn (Chris)]

De Brogle showed that all particles exhibit a wavelike motion. Bohr cointed the bord "Complementarity" as an all purpose methodology to cover the particle/wave debates. In short whether something exhibits behaviour as a particle or wave will depend on it method of measurement.

Along these lines one can look at the world as composed of waves, or at the very least define it by wave functions mathematically. Simplistically, one could say that the natural state of any wave in a vacuum and sans mass will be to travel at the speed of light in all directions unless prevented otherwise. Thus goes the way of all photons which in common parlance when grouped together we call light. This is the general case. When we add rest mass the wave slows down and when we combine particles the wavefunctions combine in a complex arrangement which generally are beyond our computation capacity to fully appreciate, but which in the macro sense lead to classical concepts.

A photon being massless does not require classical descriptions such as inertia and therefore does not require a compelling force. Upon creation a photon will entangle with others and in bulk they will generate the electromagnetic phenomena defined by Maxwells equations. A photon however does have to be created in the first instance as discussed here. The creation of photons by "quantum jumps" has been hotly debated for a century and defies any classical analysis. The semantics we use are probably insufficient to describe the phenomena. For instance to say an electron orbits the nucleus is incorrect as the physics of this do not work. To say it is a standing wave, vis a vis Schrodinger's equation is more mathematically pleasing but nevertheless fails closer scrutiny. The best we can say is that the electron, whateve that is, has a probability of being in a certain place at a certain time, with a certain momentum or energy, but both cannot be exactly defined together, the basis of this being of course Heisenberg's uncertainty principle. Dirac made a clear point of this when he stated that his Theory of Electrodynamics assumed electrons behaved in a certain way for all "intents and purposes". Perhaps one day we will understand the actual mechanism of quantum jumps when we have a more stingent theory of everything.

As to a photon's frame of reference from the point of view of special relativity the main point is that energy is conserved. In our frame of reference energy is conserved by the balance of the expansion of the universe and the stretching of wavelength. I believe that is all that the basic physics requires.

As to the stupidity or otherwise of this question I believe the problem is the intermingling of classical and quantum mechanical concepts that creates the problem. Most classical descriptions in physics can boil down to an explanation that you can proverbially describe to your mother. Quantum mechanics however is counter intuitive. Therefore when one talks about light with a little bit or reading and some dumbing down of the calculus one can get a basic feel for Maxwells equations and their general context. However, once one delves into particles which "exist" in the order of the plank length then one has to deal with an epistomological and potentially metaphysical mindfield. Unfortunately modern media deals with a mix of classical and quantum mechanical phenomena in a mish mash of short statements deviod of any contextual basis and the result is mass confusion.

In this situation I am afraid the only way forward is either to leave the subject alone or undergo a serious amount of self education.