61

EDIT: In the interest of avoiding spreading misleading information, I have removed the portions of this answer that have been disputed or refuted in the comments and edits on this question. Specifically, the parts about the ACK/Distance shown on the screen at 42:47 and the calculation of the curvature have been removed. The rest of this answer, however, ...


52

I think it's fair to say that explainxkcd.com is the authoritative source for questions regarding xkcd. In this case, a detailed discussion (including formulas) is taking place on the page for xkcd 2027. Here's a quote from its current text: According to Wikipedia and other sources, refractive index of air at 0°C is about 1.000277, which equates ...


42

It's more like the walls were semi-transparent glass, if you want to imagine it as light (and even then, you neglect diffraction effects). It would actually be better to imagine it as sound! But this seems to be exactly what you're looking for: http://arstechnica.com/gadgets/2014/08/mapping-wi-fi-dead-zones-with-physics-and-gifs/


36

Picture the radio waves from the TV transmitter as flat horizontal sine waves. You want the antenna to pick up the full width of this wave in the horizontal bars. So you need to point the antenna as directly as possible towards the TV transmitter. Cell phones send the signal in a vertical wave (apologies for the oversimplification). This doesn't allow you ...


33

A result known as Birkhoff's theorem forbids spherical electromagnetic radiation. The statement of the theorem is that any spherically symmetric vacuum solution to Maxwell's equations must be static. It is rather simple to prove. In a spherically symmetric solution $\mathbf E$ and $\mathbf B$ must be radial. Make an Ansatz, $$\mathbf E = E_0 \exp(i(\mathbf k\...


25

We've sent three radio telescopes to space so far: Zond 3 (USSR) HALCA (Japan, 8 meter dish) Spektr-R (Russia, 10 meter dish) Currently, only Spektr-R is operating; the other two have long ceased operations. Zond-3 was sent into interplanetary space, and contact was eventually lost; the other two were sent into Earth orbit, a necessity for simultaneous use ...


24

Different molecules and different crystalline structures have frequency dependent absorption/reflection/transmission properties. In general, light in the human visible range can travel with little absorption through glass, but not through brick. UV can travel well through plastic, but not through silicate-based glass. Radio waves can travel through brick ...


24

Well, without researching this at all, I’m going to go out on a limb and say it’s due to the difference in refractive indexes between visible light and radio waves in air. Air has dispersion like everything else, and so electromagnetic waves of different frequencies travel at different speeds through it. If you know the difference in refractive index, you ...


22

Note that the explanation from explainxkcd.com is not entirely correct. Not completely wrong but they make the common error to confuse the group index with the refractive index. It is the group index that is responsible for the delay of a burst, not the refractive index! * While in air the group index differs only slightly from the refractive index, in ...


21

It's hard to access how 'accurate' an analogy is (i.e. how is this being quantified?). But, I think, there is a simple - better analogy: WiFi is more like sound in a house. The transmitter is a speaker. If its a good, loud speaker, you will still easily be able to hear it in the next room - through a wall. A few walls inbetween and it gets very faint. ...


18

@Kyle Kanos is correct, there is a "Radio Window": allowing ground based operations. Also: radio telescopes are huge. Arecibo's receiver platform is 900 tons, or 33 full Space Shuttle payloads. The cost would be ridiculous. I'm not sure of the power requirement, but that is always a challenge for space based platforms. Ground bases antenna allow for radar ...


16

$H_2$ contains 2 electrons in the same ground-state orbital; by Pauli exclusion, one must be spin-up and the other must be spin-down. The 21cm line is generated in a normal hydrogen atom when an electron's spin flips from being aligned with the proton to being anti-aligned with the proton. In $H_2$, an electron's spin cannot flip because it would then be ...


14

Small pocket radios usually have fairly poor antennas in them, because of their small size. But by placing your hand nearby the antenna of that small radio, you are creating a capacitor in which one terminal is the radio chassis and the other is your hand. Any radio frequency signal induced in your skin by radio broadcasts will be capacitively coupled from ...


13

From a physics perspective, the fundamental reason for this is something called the bandwidth theorem (and also the Fourier limit, bandwidth limit, and even the Heisenberg uncertainty principle). In essence, it says that the bandwidth $\Delta\omega$ of a pulse of signal and its duration $\Delta t$ are related: $$ \Delta\omega\,\Delta t\gtrsim 2\pi. $$ A ...


13

Some advantages of space over the moon: The Moon's a lot farther away than near Earth orbit. If you want to land something on the moon instead of just smashing into it, you need to match the moon's velocity, which requires slowing your thing down a lot. Getting into Earth orbit requires much less velocity change after the initial launch. Not sure if this is ...


12

It's definitely possible to modulate a laser beam to carry radio frequency signals, using any of several different methods. Amplitude modulation, frequency modulation, and phase modulation are all easy. An Acousto-Optic modulator can be used to modulate either amplitude or frequency at radio frequencies. Many decades ago I designed (and a friend built) a ...


10

The way light, radio waves or microwaves interact with matter is through electromagnetic interaction with the microscopic charged particles. Different types of excitation can happen with these charges depending on the energy of the photons constituting the radiation. With increasing energy the radiation can cause molecular rotations, molecular vibrations, ...


8

Using current technology (and by that I mean experiments and telescopes that are available now) we would probably be unable to detect radio signals from Earth even if observed from a distance of a few light years. Therefore there is currently no prospect of detecting such signals from (around) another star. If we are talking about detecting "Earth", and ...


8

The electromagnetic field in a medium gets attenuated exponentially. $$ \mathbf E = \mathbf E_0e^{-x/\delta} $$ Where $x$ is the distance the signal has traveled. Since the power of the signal is proportional to the square of the field, then the power will be attenuated by $P = P_0 e^{-2x/\delta}$. The quantity $\delta$ is called skin depth. Its the ...


8

Radio telescopes need to be big - the latest on earth is 1 square km (total area) ! There is no reason to put them in space, (most of) the wavelengths pass through the Earth's atmosphere so there is no real advantage to being in space and it would be expensive. The main advantage in space is you can have much longer baselines and so more resolution. There ...


7

Note: Emilio Pisanty wrote an answer that is probably a better fit for the question and site, but I'm leaving this answer around because I feel it can contribute to an understanding of how this works in practice. For one thing, you'd need to be able to differentiate between the signals inside the frequency band. As an example, I'm going to use a Morse code ...


7

"If that is possible, how do you produce a spherical EM radiation?" A spherically symmetric transverse field is topologically impossible - if it is required to be coherent and linearly polarized everywhere. This is the case for usual dipole or higher multipole radiation, as has already been pointed out in another answer. On the other hand, an incoherent ...


7

The question of "why" radio waves have a lower speed in air than light might be due to the interactions of radio waves with diatomic molecules ( O1 and N1 ). The radio photons energy are going to be closer to the "available" transition energy for rotations. The Hyperphysics page has a discussion. Think of it as the cumulative effect of many photons causing ...


6

For the first question, yes. Because the surface of the sun is close to a blackbody emitter, it radiates at all wavelengths below the peak. So radio waves are included. However, the longer the wavelength, the less the power that is put into that portion of the spectrum. Radio is so far from visible light on the EM spectrum that the solar radiation in ...


6

UHF (Ultra High Frequencies, i.e. microwaves) would be very harmful to us, if our body was subjected to enough UHF energy: it would fry our flesh in the same way it heats up that microwavable 'Chicken Tikka Massala' that you buy from your supermarket. Domestic microwave ovens however are designed to minimise UHF radiation leakage and thus are extremely safe....


6

tl;dr -- baseband it with beats. The Problem If you want to accurately record a high frequency time series, you need to sample the data at an even higher frequency, at least twice the target frequency to be exact. This is called the Nyquist frequency, and it is a general result of signal processing based on Fourier analysis. So you want to record a 1 mm ...


5

(All images in this answer were made by me, for wikipedia! Links here) Let's start with the simplest question: Voltage relative to what? It's the voltage from one line to the other. Now let's look at an animation of a transmission line. This one is terminated with an impedance-matched resistor, so we don't have to think about reflections yet. The dots ...


5

Stars can produce "sound" when they pulsate. Stellar oscillations produce waves of density and or pressure that propagate within the star. These sound waves cannot travel to us through space because the near vacuum does not allow them to propagate. Sound waves due to stellar oscillations can be detected by looking for periodic signals in a star's ...


5

Firstly, a smaller antenna is able to transmit/receive the signals due to the low wavelength of such waves. Secondly, operating using high frequency supports a greater bandwidth as compared to the low frequency operation. Thirdly, the noise levels are much reduced in this domain as compared to the other. There is one caveat however, you need more ...


Only top voted, non community-wiki answers of a minimum length are eligible