Light Hurts my Brain

[McC]

What is light? I'm not looking for the simplistic answer "an EM wave" or "a photon" or anything equally unhelpful -- I want deeper than that. EM waves are the only waves the propagate without a medium -- why? Why is anyone comfortable with that? You get an EM wave with perpendicular electric and magnetic fields -- how?

I've just spent the past couple hours combing through the SDN archives and reading articles on relativity, dark matter/energy, FTL, and so forth and all of it bugs the crap out of me. I don't "get" it, as it were. And I want to. Light is seems to be the fundamental underpinning of pretty much everything, but so far as I can tell, we don't actually know much about it -- just how it seems to behave.

So, please help.

(I'm sorry if this post comes off as anything other than innocently inquisitive. I'm just very frustrated by trying and failing to understand this topic time and again.)

[Starglider]

So, please help.

If you really want to know and are reasonably scientifically literate go and read QED by Richard Feynman. It's the best concise semi-technical overview of the subject available.

[Xenophobe3691]

I'll have to agree with Starglider. I have a copy of the book (and have read through it), and it does answer many a question.

Although if you're looking for understanding...dump every single preconception that's ever crossed through that porridge you call your brain. I won't do you any good, I promise.

[Kanastrous]

"The God Particle" by Leon Lederman is also an educational book on the subject; heavy on history, light on mathematics and a bit of an easier read than Feynman. More about concepts, than mechanical details.

[Johonebesus]

If I'm not mistaken, and I may be, what it really comes down to is that light is a thing that cannot be really understood, so the best we can do is try to describe it mathematically, and say that it "sometimes acts like a wave, and sometimes acts like a particle." You're right, we don't really understand what it is, because whatever it is is not something that can exist at the macro level, and so it's properties make absolutely no intuitive sense to us what-so-ever.

That's the way it is with quantum particles. Some of them seem to spin 545° in a single rotation. Some of them seem to exist in several places at once. It is all very counter intuitive and illogical.

[Kanastrous]

Whether light appears as a wave or particle phenomenon depends upon the set of observational tools you apply.

There's the Double Slit experiment: light is a wave.

There are spectral absorbtion lines: light is particles.

The most conscise layman's distillation I think I have seen, so far, is that photons (the quanta, or discreet packets of electromagnetic energy) are particles whose probabalistic distribution in space is described by a wave function.

I hope I didn't mangle that.

[sketerpot]

I think that your concern about EM waves being the only waves which propagate without a medium may be a red herring. Wave behavior arises naturally from a certain type of simple system: a system that satisfies a wave equation.

A plucked guitar string satisfies that equation. If you combine a couple of Maxwell's equations, you get a wave equation for EM waves. If you look at air pressure fronts, you get a wave equation for sound. Wave behavior arises all over the place because of simple mathematics.

(It occurs to me that studying engineering may be warping my view of what constitutes "simple mathematics". If so, I apologize for dumping vector calculus on you.)

[McC]

The notion of a 'massless' particle leaves me with an incredibly uneasy feeling that might be verbally expressed as "the universe must make more sense than this." It seems to me that the phrase "we don't really know" is pretty much the long and short of it, and thus far we're only sort of guessing at it.

It is all very counter intuitive and illogical.

This is part of what bugs me about it. I can't help but think that our models and the like are at best inadvertently accurate, and only reflect reality "by coincidence" (for lack of a better phrase). The idea that the universe is not logical really doesn't sit well with me.

Yes, I know I'm mixing 'gut feeling' with attempting to understand the nature of the universe.

Take special relativity, for instance. It can't possibly be wrong -- it's been experimentally verified and even utilized daily (GPSes are the most oft-cited examples). However, the notion of the speed of a particle (photons) being steadfastedly fixed regardless of moving reference frame itself seems to defy all sensibility. Obviously, based on our existing mathematical models, that is indeed what's happening. But I can't help but protest that something else must be at work than what we currently understand.

To clarify, though, SR doesn't bother me until we get to talking about FTL effects and SR/causality, at which point my brain rejects the notion and I can't help but cry out that SR must simply be incomplete -- its description of what happens at FTL is entirely nonsensical. I'm referring to the A sends ansible message to B, B is moving relativistically, B replies via ansible message to A, the message arriving in A's past. No matter how many times I read this example, I can't accept that that would actually happen. I have similar issues with the relativistically-accelerated end of the wormhole resulting in time travel and all that mess.

I will definitely look into both of the books mentioned, though.

[Joviwan]

Potentially risking a thread jack, but I hope it's close enough to the source:

Why is does going faster than light seem to provide so many problems? Speaking as someone who never did too well in science in highschool, mind, but I don't know what the apparently obvious and inherent impossibility involved. Or if it's even an impossibility, people seem mixed.

[rhoenix]

As the saying goes: "Relativity, Causality, FTL Travel: Pick two."

For more reading, read the Atomic Rocket page on FTL travel or even Wikipedia's FTL page - both have further links.

[Academia Nut]

Well, the thing with EM is that the mathematics behind it were discovered "before" the waves, in that people knew about light and what not, but they didn't really know what it was.

Let's begin with an accelerating charged particle, say an electron in a wire. Because it is accelerating, the electric field around it is changing, which is simple enough to understand as electric field is related to distance from the charge, so obviously as the particle moves the E-field must change. However, a changing E-field causes a changing magnetic field, which you know because when you turn on your lights you're getting AC power from a generator designed on that principle.

But, not only does a changing E-field create a changing B-field (B="magnetic field" from now on, technically its not, but we'll ignore that bit). This means that the E-field and B-field will regenerate each other, which is why EM waves need no medium to travel. They are forces. Gravity requires no medium for its effects to be felt, and electromagnetic force doesn't either.

Now, when the various equations are plugged into the afforementioned wave equation, a very curious thing happens. We get that the velocity of propogation for the waves is equal to the inverse of the square root of the permitivity of free space (constant for electrostatic force) times the permeability of free space (constant for magnetic force), which works out to exactly 2.99792458*10^8 m/s (well, they probably rounded to 3*10^8 back in the day, but you get the point). This was a complete surprise, because this was what the current experiments showed the speed of light to be, approximately. A few experiments later, and the discovered that light was in fact an electromagnetic wave.

By the end of the 19th century, Lord Kelvin made his famous statement about physics being a dry well with only more precision and a few final details to be cleared up. Those few last details included why they couldn't discover the luminiferous ether, the medium for light to propogate through, the final structure of the atom, and the ultraviolet catastrophe in black-body radiation.

The ultraviolet catastrophe was a massive stumper. Basically, the scientists had a rough approximation at low temperatures (less than about 1500K) for the spectrum of light emitted by heated objects, but when they really cranked up the temperature, like say on the surface of the sun, everything full apart and they got nowhere near the amount of UV light they expected. They were scratching their heads for quite a while until a man named Planck finally, in frustration decided to do something he felt was stupid (he literally apologized for his "flight of madness" in the paper he published) and decided to treat light as "quanta" and divide it up into a bunch of individual packets of energy, and low and behold, the math came out perfect for their observations.

Now, this wasn't all that accepted until a young chap named Einstein wrote a paper in 1905 for which he received a Nobel Prize a good decade and half later for. In it, he used Planck's quanta to explain the photoelectric effect, which is current generated by light incident upon metal. Increasing the intensity increased the current, but only decreasing the wavelength of light would increase the voltage produced, and light above a certain wavelength would not produce any current at all. A classical wave could deliver an arbitrary amount of energy, but light didn't work that way. The only way it could work was if light was composed of packets of energy quantized at E=hf per photon, where h is Planck's constant, and f is the frequency of light. More so, the atoms being struck had a certain threshold of energy that had to be met by exactly one photon. Two of half the energy would not cut it.

If you want, I can explain more of the quantum mechanics that evolved from this, but that will tend to go beyond EM radiation. To summarize on EM, it's a E-field and B-field perpendicular from one another, taking energy from the back of the pulse and sticking it at the front to regenerate the waves. The wave-particle duality is a lot stickier, but a general rule of thumb is that lower energy EMR will tend to act more like a wave on interaction with matter, while higher energy EMR will tend to act more like a particle a slam into things like a billiard ball.

Today's rambling, incoherent lesson brought to you by 3 years of university level physics.

[Kuroneko]

EM waves are the only waves the propagate without a medium -- why? Why is anyone comfortable with that? You get an EM wave with perpendicular electric and magnetic fields -- how?

To make it short, light is kinky. That's all there is to it.

Once you accept STR and just the electric field, the classical conception of light is not too hard to wrap one's head around (actually calculating specific phenomena, however, is a different matter). Imagine an small stationary electric charge and its electric field, which radial and spherically symmetric around the charge. Nothing special. Now, kick the charge (giving an instantaneous impulse for simplicity), and wait some small amount of time t.

If STR is correct and there is a speed limit c to the propagation of information, the fact that you kicked the charge will be known at distances more than ct from its original position. Thus, the electric field outside this radius will still be the same old radial spherically-symmetric field. Inside, however, we get a Lorentz-contracted field: the components of the field lines in the direction of the charge's velocity will diminish, so they'll be displaced.

The most intuition-bending fundamental properties of EM waves can be deduced from just the following observation: the "perturbed" field lines inside the radius ct must join with the "unperturbed" field lines outside of it, which are just the same old field lines we had before the charge was kicked. Picture it--a field line inside will have to make a sharp turn, go roughly along the circle with radius ct, and then make another sharp turn to join with the corresponding external field line.

Electromagnetic radiation is a kink in the electric field. Notice that
(1) accelerating the charge yields radiation;
(2) Lorentz-contraction will not disturb the field line in the direction of the charge's velocity, so we do not get radiation in that direction;
(3) the kink in the field lines propagates outward at a constant velocity (at STR's universal speed limit), but because of the turn in the direction of the field line there, any particle intercepting it will experience a force perpendicular to the direction of propagation;
(4) as time goes on, the bent parts of the field lines will get less "dense" proportionally to distance, so the "field strength" is inversely proportional with distance (rather than inverse-squared!).
All of these predict the qualitative behavior of EM waves exactly. To make a magnetic field, we extract the perpendicular force in (3) and attribute it to this field. It's a very useful trick--the Maxwell formulation is simply much more powerful, although obviously was not derived in this manner.

I hope this answers your question adequately.

--

As for STR, I'm afraid if you're asking for an intuitive answer without basis in mathematics, you'll be disappointed. However, at least for STR, there's no reason that we can't use the mathematical framework to make it fairly intuitive. Fortunately, the learning curve isn't steep at all--if you're competent in both coordinate geometry and trigonometry, it doesn't take much work to make STR at least comprehendable.

First, STR describes physics from the point of view of spacetime; since physical events occur both in space and time, this isn't too much to swallow. However, the time direction has a fundamentally different nature than the three spatial direction. For example, we can't "turn around" in time as well can in space. So it makes some sense for there to be something geometrically special about it. In terms of coordinate geometry for the (t,x)-plane, the Euclidean metric (distance formula) and trigonometry would have
distance d² = t² + x², angles from the t-axis satisfying tan θ = x/t.
Thus, the constant-distance loci from the origin are simply circles, and we have the standard trigonometry--in particular, rotations about the origin stay on the original circle. The way STR makes the time direction special is by having space and time be of different signatures:
distance τ² = t² - x², angles from the t-axis satisfying tanh θ = x/t.
Therefore, the constant-distance loci are hyperbolas, the trigonometry is hyperbolic, and and rotations about the origin trace out hyperbolas instead.

Physically, all we need for basic STR is the physical interpretation of (1) rotation from the t-axis = acceleration, (2) tanh θ = velocity, (3) spacetime distance τ = elapsed time for a lock traveling along a straight path between those two events ("proper time"). Pretty much everything in STR makes sense if you imagine geometry with rotations tracing out hyperbolas instead of circles. Hyperbolas have asymptotes, so we do have a speed we can approach but not surpass (|tanh θ| < 1); the velocity addition formula u⊕v = (u+v)/(1+uv) is found directly by using the hyperbolic angle-addition formula, tanh(α+β) = [tanh α + tanh β]/[1 + tanh α tanh β]; the Lorentz transformation is really nothing but a hyperbolic version of the standard Euclidean rotation (v = tanh α, γ = cosh α, γv = sinh α), and so on.

It takes a bit of forcing to visualize geometry in which rotations stay along hyperbolas, but it's quite possible. And personally, since time is "not like" the spatial dimensions, having a different signature for time never really bothered me. Now, quantum theory--there's something to be bother one's intuition.

[Winston Blake]

The ultraviolet catastrophe was a massive stumper. Basically, the scientists had a rough approximation at low temperatures (less than about 1500K) for the spectrum of light emitted by heated objects, but when they really cranked up the temperature, like say on the surface of the sun, everything full apart and they got nowhere near the amount of UV light they expected.

Nitpick: I thought it was a problem of expecting extremely large amounts of UV light, going up to infinite energy with higher frequency, hence 'catastrophe'.

BTW Kuroneko, that's the most intuitive explanation I've ever seen for EM waves, specifically the magnetic part. Also, I don't know much about all this, but I imagine the next question's going to be 'Well, what's an electric field then?'. The answer of 'particles exchange virtual photons...' would then be circular.

[Ariphaos]

BTW Kuroneko, that's the most intuitive explanation I've ever seen for EM waves, specifically the magnetic part. Also, I don't know much about all this, but I imagine the next question's going to be 'Well, what's an electric field then?'. The answer of 'particles exchange virtual photons...' would then be circular.

Ignoring electronuclear stuff, my physics professor simply stated "No one knows what Force is. Force just is. That said, we know a bit about how Force affects us, so that's what we're going to be talking about."

[Kanastrous]

You just have to let the Force flow through you.