Wednesday, April 26, 2006

World: Bloody Photons

As one might expect intuitively, the history of physics discovery is like peeling back layers of an onion: there’s another layer beneath, and eventually you have to cry.

Newtonian physics makes sense. But when you hit the 20th century, everything becomes counter-intuitive. That is, it’s susceptible to logic, but within particular, peculiar frameworks, and with peculiar exceptions – especially at the fringes, the very small (subatomic) or very large (close to light speed). The weeping begins when you hit counter-intuitive critical mass.

This discussion was brought about by C’s efforts to analyse photons in a Newtonian framework, and subject it to theoretical situations, such as sub-light travel. But there are limits to what you can do with photons – or, at least, what’s been done with them so far. Maybe if someone slowed down a photon to sub-light speed, they’d also discover time travel. We just don’t know what other frameworks are yet to be discovered; we just know that what we have so far accounts for most phenomena.

Photons, as we know, are discrete quanta of light, ie the smallest indivisible unit quantity when light is treated as a particle (in fact, all electromagnetic radiation exists as photons). They can be created or destroyed by interactions with other particles. Whereas most matter exists at sub-light speeds, and can never reach the speed of light because its mass would increase infinitely, photons are the flipside, which (to my knowledge) exist only at light speeds. They have momentum and energy, yet have no mass. In that respect, Newtonian analysis at sub-light speeds makes about as little sense as speeding up a finite mass to light speed, then analysing it. You can do the analysis, but it doesn’t have meaning (at least in our universe).

Photons are the carrier of electromagnetic force, one of the fundamental forces of nature. (Along with gravitational, it is a long-range force; the other two, strong and weak nuclear forces, act only within an atomic nucleus.) A photon’s energy is calculated somewhat differently from sub-light physics:
E = hu
Where h is Planck’s constant and u is the frequency of the light (more correctly, of the radiation). This is the photon equivalent of
E = m c(squared)
Here, an object with greater mass has greater energy; with photons, one with a higher electromagnetic frequency has greater energy.

So, the framework for photons is such that they:
a) can’t be slowed down to sub-light speeds
b) have no mass, yet have momentum and energy
Although their velocity is always constant, large gravitational forces - such as stars – can affect them in two ways: bending the path of the photon (and thus momentum), and stealing energy – this results in a gravitational redshift effect.

Photons can, of course, be swallowed by black holes, which are massive gravitational forces, where practically nothing can escape. Being photons, they would continue to travel at light speed until engulfed. (Read this for further detail on the interaction of photons and black holes, according to General Relativity theory. In particular, Relativity accounts for the fact that photon paths are deflected more by gravitational fields than could be accounted for by Newtonian physics.)

Of course, all this opens up a number of other questions, for C and others to ponder. I’m happy to field those questions, since I want to understand better myself, and this is a learning process for me.

Lafferty P & Rowe J: The Hutchinson Dictionary of Science, Helicon, Oxford, 1994.
Black Holes (from Astronomy 162 at University of Tennessee):


Anonymous said...

Hi C here

If the speed of the photon is a constant - the speed of light, and its energy is downshifted into the infrared spectrum, this would imply that the speed of light is adjusted by the presence of a gravitational well.

This would imply that the speed of light is not the same within a gravitational well.

Other complications: A photon has zero mass, thus is undetectible by us using currently available technologies, as a photon can (according to you) be destroyed or created by interaction with other sub atomic matter.
Would this not be a direct contradiction to the formula E=MC(squared) in that the sum total of mass and energy must remain constant.

Would it not be easier to assume that photons do exist outside of the speed of light, and the problem is that we cannot detect them with our current understanding of technologies.

From the law that the sum total of energy and mass must remain constant, it makes more logical sense to say we simply cant detect photons traveling slower than the speed of light.

S Simmonds said...

Hi Chris,

The best way for me to conceptualise this is to accept that there are two different frameworks, where matter interacts but doesn't cross over (except in the context of that interaction, which generally constitutes exchange of other subatomic particles.

Matter at our speed cannot reach light speeds. Likewise, matter (ie photons) at light speeds cannot normally slow down or be dealt with at sub-light speeds. (black holes - singularities - probably excepted.)

Photons are affected by gravity, but are not slowed down as such. Their paths (direction)can be altered as can their energy (that's the downshift). I believe the actual energy change happens through an exchange of subatomic particles...

A photon has zero mass, yes, and we can observe/detect it, but not weigh it as such. We can observe its frequency and path.

Newtonian conservation of energy in a Einsteinian context becomes conservation of enery+mass, since there is an equivalence in the E=MC(squared). But in the lightspeed framework, that would be conservation of energy+frequency. And that would not be violated. So if a gravitational field changes the path of a photon, I would say it has absorbed some of its angular momentum - however angular momentum is described in a lightspeed context. And if a gravitational field reduces that energy, I guess the energy would be released somehow - possibly absorbed by the corresponding mass. Through exchange of subatomic particles. Here my understanding gets murky; I'm just working from general principles. But conservation of energy/mass/frequency as a whole should still hold. Hmm, do you want me to investigate how that happens?
Conservation of energy is integral to the analysis of even a black hole.