1st post, I love this site.
My question is, how do the various types of radiation in space travel through the vacuum of space? For instance, how does heat get from the sun to the earth? The analogy for waves that I usually see is waves or ripples on water, but in that case we’re talking about movement of particles. One water particle bumps another, and thus the wave moves through the medium of water. But in space, what is the medium?
Most waves that people think of travelling through space are types of light (this includes radio waves and infrared… Most “heat” from the Sun is in the form of infrared and visible light). With light, the “medium” that’s being disturbed is electromagnetic fields, which is why it’s called electromagnetic radiation. You can also produce waves in a gravitational field, but they’re much harder to detect. In other words, just because there’s no matter in a vacuum, doesn’t mean that there’s nothing at all.
Welcome, by the way, and may you enjoy posting as much as you did lurking.
No, it’s ether. I always liked ether. It makes so much stuff so much easier to explain.
mangoldm, let me be the first to welcome you to the SDMB. (Or, a little bit later, darn slow boards. 
)
Let’s assume you’re talking about electromagnetic waves (such as visible light, x-rays, radio waves). Scientists used to hypothesize that e.m. waves were oscillations in a universal aether. Aether theories are now discarded by mainstream physicists. Einstein’s theories of relativity, which give no special preference to any frame of reference, do away with the aether. But it’s easy to find fringe scientists who still favor it (try Theories of Aether).
My own interpretation is that e.m. waves are oscillations in the e.m. field. Of course the e.m. field is a function of your coordinate system, but so are the waves. The trouble is that the e.m. field is not exactly tangible, so many people find this explanation unsatisfying.
What are you trying to do, get Michaelson’s Nobel taken away?
Hey, this GQ. Please don’t confuse the issue. Ether my ass. Michelson and Morely proved that there is no ether. The “Space substance” type, I mean. The anesthetic type is a different thing altogether.
Wasn’t Michaelson’s Nobel for his work in precision standards for the meter?
Of course, there’s no space æther, but there might be a space quintessence, which, of course, means “æther”. Don’t even get started on phlogiston or caloric, now.
A friend of mine majoring in Physics at the time once explained light to me as an electric field inducing a magnetic field inducing an electric field etc…
I don’t know how closely this interpretation holds to the actually current theory, but it gave me a good conceptual idea that was different from the traditional particle / wave metaphors.
I’m an engineer, not a physicist, mangoldm, but as far as my education explained it, there is no medium, other than the fabric of space-time itself. But space-time is not a propagation medium in the way that air propagates sound – light does not distort space-time when passing through it.
Sorry if it’s not easy to understand, but light waves have a dual nature that mystified the best minds in the world a century ago, back when there was no one to teach them about it. It’s still not explained to the full satisfaction of many students; they’re just told “it’s a particle and a wave - deal with it.” And really, that’s all the answer I have to offer, too.
Physicists attend grad school for years before they find an answer that satisfies them - and by then, their minds work in ways completely foreign to the rest of ours, so good luck finding one to explain it to you in terms you can understand.
The Michelson interferometry experiment disproved the existance of ether as a medium for the transmission of EM waves (e.g., light). It pretty much nailed the coffin shut on any formally acceptable notion of ether.
Quintessence is another concept entirely and is inferred due to the evidence that the universe is “flat” despite the problems we’re having finding enough matter and energy within it to keep it from expanding forever. Think of it as a sort of tension in the fabric of space-time, like the tension in a compressed spring.
I’m sure Chronos knows the difference. He’s just playing with us…
I took an antennas course as an EE undergraduate. As we were immersed in some hairy equations, I raised my hand out-of-the-blue and asked the prof, “How does an electromagnetic wave propagate through a vacuum?”
He said in 500 words or less, “Well, uh, it, uh, just does.”
And to this day I still wonder how (or more precisely, why) it “all works.”
To me, “It’s a Propagating Electromagnetic Field” doesn’t cut it. Think about it: how do you generate an electric field? From charged bodies, right? But there aren’t any charged bodies in a vacuum! And how do you get a magnetic field? From moving charged bodies, right? Well, if there aren’t any charged bodies in space, then there certainly aren’t any moving charged bodies, either. So how in the hell can you generate electromagnetic fields a billion miles away when there ain’t any charged bodies around to produce them?
One explanation I heard was that the charged bodies back at the source are producing them. I guess I can accept that. But the direction of the fields are perpendicular to the source! How in the hell do the (local) charged bodies produce perpendicular electric and magnetic fields a billion miles away? Yea, yea, Maxwell’s Equations. But do they really tell you why it’s happening? I’ll go look at them again and let you know.
Do photons correspond with the peaks of either the electric or magnetic waves?
The wave is a probability wave. The chance of finding a photon at any position is the square of the height of the wave. The probability of finding the photon where the wave crosses the axis is zero. Between observations, a photon’s position spreads out into a probability wave. Actually, this is true of all particles: Between observations, a particle’s position spreads out into a probability wave.
There’s two ways to produce an electric field, and two ways to produce a magnetic field. The first way for each is with charges, as Crafter_Man points out. The second way, though, is from other fields: A changing electric field produces a magnetic field, and a changing magnetic field produces an electric field, regardless of whether there’s any charges or what they’re doing.
Actually, there’s three ways to produce either field, but let’s leave magnetic monopoles out of this.
Quoth bughunter:
Interestingly enough, due to the special nature of gravity, the gravitational waves I mentioned earlier actually do distort spacetime itself. Of course, that’s not the same as light, but worth mentioning nonetheless.
And yes, I was just having fun about quintessence, but the name does mean “æther”. Quintessence = fifth element = æther (the first four are earth, air, fire, water). Another bit of irony here is that æther, as originally described by the Greeks, meant “ever-moving”, since it was the stuff of the stars and planets, which are continually circling the Earth (Ancient Greek worldview). By the time of Michaelson and Morley (never forget the grad student!), though, the defining property of æther was that it was always at rest.
I can’t think of any easy way to explain how an EM wave breaks away from its source in the far field, but once it does the time varying electric and magnetic fields are self-sustaining. In other words the wave doesn’t need a medium to propagate. The time varying E field produces a B field and vice versa.
http://didaktik.physik.uni-wuerzburg.de/~pkrahmer/ntnujava/emWave/emWave.html
Sheesh. I’m a Physics grad and an engineer and the best I can really tell you is that “they just do.” It helps to not think of EM waves as being physically propogated, but as being something inherent to <science mumbo jumbo> the very fabric of space-time itself </science mumbo>. Relativity and classical E&M describe the observed behavior of radiation, but the deeper “why” of it all isn’t something that can be explained in any intuitive way. I’ve thought I’ve almost understood it all, life the universe and everything, on many occasions but it always ultimately slips away
“Ether” is the intuitive answer of course, but it’s wrong
I must step back in to clarify the statement of DrMatrix.
The photon particle itself is indeed a probability wave… more accurately, the sum of a multitude of probability waves that add up to a localized packet of nonzero probabilties - the photon’s position in space-time.
But the EM wave of a light ray (or heat ray, or radio wave, or whatever wavelength you desire) is not a probability wave, it is a cross product of electrical and magnetic waves. Maxwell was the first to express this identity in explicit terms.
Might I remind the other esteemed posters of the Poynting Vector?
Yowza. Thanks for all the replies. I still don’t understand, but at least I don’t feel stupid for not understanding.
This discussion has led me to another question, if I may. Since radio waves do not require a physical medium in which to travel, how do they affect matter when they do encounter it (by what mechanism)? For example, how does a roof-mounted TV antenna become affected by radio waves so that it is receiving a signal? Do they smack it, thus causing it to vibrate? How is the energy transferred?
A photon does not exist except when it is being observed. But whenever it is observed it is most definitely a particle. The electromagnetic wave is the probability wave for a photon, and the probability of finding the photon in a small volume is equal to the square of the electric field vector.
There are a number of named ways that a photon interacts with matter, but almost all of them involve the photon transferring its energy to an atom’s electron, causing it to jump to a higher energy level - or even completely freeing it from the atom. (By the way, this is the true meaning of the term “quantum jump,” and it’s typically a very small change, not an immense one as some people try to apply it.)
In the case of the antenna, which you specifically ask about, you can describe the interaction using either the particle or wave nature of the radio signal.
In the particle version, the photon does indeed “smack” something… namely its energy is absorbed by an atom in the antenna, freeing an electron.
In the wave version, the changing electric and magnetic field components of the electromagnetic wave induce a electric signal in the conductor that makes up the antenna.