This one has puzzled me for quite a while. I learned all through school that light and electricity both travel at the speed of light, as well as radio waves. Obviously this is not just coincidence, but it seems that they are different enough not to be related.
We know that light travels in waves (also photons, but that’s a different subject). Radio also travels in waves, but so does sound and it’s much slower.
Electricity needs a conductor to travel, and can either be AC or DC. I know AC current travels in waves, but what about DC?
I’m sure there’s something to explain how these dissimilar things all travel at the same speed, but not being a physicist I don’t understand.
Are there other things that also travel at the speed of light? Gravity, maybe? Or something else?
I’m no physicist but, to my understanding, they don’t. Light travels at the speed of light (theoretically the fastest speed possible). With radio waves there is a delay, most of which is encountered in when using them around the world (although the necessity for relay stations naturally contributes significantly to that). Electricity (I’m assuming we’re talking via wires here) is impeded by the resistance of the impedence of the medium it is traversing.
Not sure about electricty, especially since there’s all kinds of different thing there, whether you’re talking about the speed of electric current, or of the transmission of a static electric charge, or the ‘speed’ of an electric attraction or repulsion effect. But…
Light and radio do travel at the same speed, as long as they’re travelling directly from point A to B and not being ‘slowed’ by being absorbed and re-emitted by the medium they’re travelling through. As different as they seem to us, they’re very much the same thing, just a slightly different variety - different wavelength of electromagnetic radiation, along with microwaves, infrared, ultraviolet, x-rays, gamma rays, and probably at least a few others. Travelling at c from point A to point B in a vacuum is one property that is shared by all of them… and any discussion of ‘why’ further than that takes you into discussion of einsteinian relativity and other stuff about the nature of the universe that we may still not understand.
Hope this was helpful.
I think the jury is out on whether gravity travels at the speed of light or not, partly because it’s hard to tell when the gravity that is acting on a particular body ‘left’ its source, since there’s nothing you can do to interrupt gravity like you can to interrupt visible light, say.
INANP but I read something related to this once that made me think about it differently than I had been. It said that calling it the “speed of light” is a little misleading. That isn’t strictly a property of light itself. Instead, the speed of light could be called the “speed limit of the universe”. Light just happens to be one of the things limited by it. Likewise, electricity and radio waves nearly bump against it too. As to why that is the speed limit of the universe, I don’t think anyone has a clear understanding.
OK, it makes more sense now that you put it in terms of the electromagnetic spectrum, which ranges all the way from 0 Hz (Direct Current) all the way up through the band of light and radio waves. I used to know this, but apparently forgot the relationship at some point.
The just do. The all travel at the speed of light in whatever medium the traveling takes place. As to why, I don’t think there is any deeper explanation than that they just do.
That speed is c/(ue)[sup]2[/sup]. c is the speed of light in a vacuum, and u and e are the electric permitivity and magnetic permeability relative to that of a vacuum. In wires and on things like coaxial cables the velocity is about 2.310[sup]8[/sup] m/sec as opposed to the speed in a vacuum of 2.9979245810[sup]8[/sup] m/sec.
I’m pretty sure that longer wavelengths are slowed less than shorter wavelengths - that’s how a prism works - so radio waves would travel faster in most media. Generally, the denser the medium is, the slower the speed of light but the faster the speed of sound.
That’s true, although there’s typically only a fraction of a percent difference.
As was previously mentioned, visible light, radio waves, and electric fields are all electromagnetic radiation, and so all travel at the speed of light. As are x-rays, gamma radiation, ultraviolet radiation, infrared radiation, microwaves, etc. The difference is in their frequencies. (The frequency times the wavelength is the speed of the wave, and for light, that speed is constant. I also prefer to talk in terms of frequencies because that doesn’t change depending on the medium, unlike the speed and wavelength.) The higher the frequency, the higher the energy, which is why UV, gamma rays and x-rays can be hazardous to your health. (They all have higher frequencies, i.e., shorter wavelengths, than visible light.)
Also, be careful when talking about the speed of electricity. In a current, the speed of the electrons is typically only a fraction of a meter per second. However, the electric field propagates at the speed of light. This is also why we get light as soon as we turn on the switch, instead of having to wait for the electrons to get there. (Admittedly, I’m simplifying a bit, but it’s essentially the idea.) Also, AC/DC is a bit different than the other stuff. Here, when we talk about frequencies, we’re talking about the electrons are oscillating direction (i.e., AC), so the electric field is oscillating in time, although it still propagates at the speed of light. For DC, there is no oscillation, that is the electrons just move in one direction. The field still propagates at the speed of light.
Eleusis: Elephant rhinoceros sin θ in a direction mutually perpendicular to the elephant and the rhinoceros as determined by the right hand rule.
What do you get when you cross an elephant with a mountain climber?
You can’t; a mountain climber is a scalar.
FBG: The relationship between energy and wavelength is E = hν, where h is Planck’s constant and ν (the Greek letter nu) = c / λ (the speed of light divided by the wavelength). If the frequency is 0 s[sup]-1[/sup] (i.e. 0 Hz), you’ll get E = 0; in order to get such a frequency, though, you’d need λ to be infinity. Also, all particles (ranging from a photon to an electron to a baseball to the Earth) have some wavelength, called their de Broglie wavelength, which is Planck’s constant divided by their relativistic wavelength. So you can’t have a frequency of 0 Hz for electromagnetic radiation. As has been mentioned, frequency for electricity is a different concept.
Heller Highwater: In case there’s anyone out there who thinks radio waves might cause cancer, it should be made clear that gamma rays, x-rays and UV rays all have shorter wavelengths and higher energies than visible light. Visible light can cause some of the same electronic transitions that UV light causes, but generally the energy is not high enough to cause transitions that break molecules apart. Infrared light can make bonds in molecules stretch and bend, but they do not break. Radio waves have longer wavelengths and lower energies than visible light; thus, if we know that visible light doesn’t cause cancer, it’s irrational to think that radio waves could cause cancer (mostly because they’re related to technology).
I thought that electrons in a conductor moved much faster (I was thinking on the order of 0.3c) than they actually do; I searched for some sites and the answer appears to be that electrons flow at a rate of a few cm per second at most. Considering how electrons travel in a conductor, this isn’t altogether surprising. But why do potential differences propagate at the speed of light through a conductor as soon as the switch is turned on? Is any change observed when the electrons actually reach the load, and what happens to them when the circuit is turned off?
Yeah, I meant to mention that, Roches. The same holds true for the waves emitted by cell phones, as I recall, so there’s not really any big danger from them, as far as I’m aware. For anyone who’s interested, visible light has wavelengths on the order of 400-700 nanometers (1 nm=10[sup]-9[/sup]m, i.e., 1 billionth of a meter), while radio waves can be as long as 1 meter.
Some interesting chemistry to go with the physics (pardon the hijack):
Now, I’m no biologist, but I believe it is precisely those electronic transitions that allow us to see: certain molecules in our eyes can absorb photons of visible light, which causes them to become electronically excited, which lets them change their structure slightly. This in turn gets interpreted by the brain as, well, light. Photons outside the visible range don’t cause these transitions, and so we don’t “see” photons with those frequencies.
Also, I believe microwave ovens work by emitting microwaves at the right frequency to cause water molecules to vibrate, which is how the rest of the food cooks (presumably by transfer of energy from colliding with water molecules). Someone please correct me if I’m mistaken here.
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Yeah, I meant to mention that, Roches. The same holds true for the waves emitted by cell phones, as I recall, so there’s not really any big danger from them, as far as I’m aware. For anyone who’s interested, visible light has wavelengths on the order of 400-700 nanometers (1 nm=10[sup]-9[/sup]m, i.e., 1 billionth of a meter), while radio waves can be as long as 1 meter.
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I’d have expected better from a fellow East Lansing resident. Radio waves can be a LOT longer than a meter. At a frequency of 1 MHz (in the middle of the AM broadcast radio band), the wavelength is 30 meters.
I’m pretty sure this isn’t rigorous but think of electrons as comprising an almost incompressible fluid. As soon as you push on one end a pressure wave travels through the fluid and reaches the other end almost instantaneously even though the fluid particles move quite slowly.
Also, as to electron flow and em waves and conductors. A potential difference between two points will cause all charged particles in the vicinity to start moving. If there is a conductor between the points with the potential difference that’s where nearly all of the charged particles, mostly electrons, are. That’s why the approximations of electric circuit theory work for frequencies up into the hundreds of megahertz.
Yes. To be specific, photons with too low a frequency (e.g. IR) don’t excite the molecules or don’t excite them efficiently enough. Too high a frequency (UV) and they excite molecules in the lens or cornea, which blocks them from reaching the retina. At a higher frequency yet (X-rays, gamma rays) they start knocking inner electrons loose from atoms, producing highly reactive and damaging ions.
Electrical energy propagates outside the conductor as an electromagnetic wave at pretty close to the speed of light.
As an example the speed of a 1 MHz radio wave in a copper conductor is only 400 meters per second. So it’s pretty clear that the energy must propagate via the fields outside the conductor.
Radio waves can be a LOT longer than a meter. At a frequency of 1 MHz (in the middle of the AM broadcast radio band), the wavelength is 30 meters.