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Domain: mathpages.com

Stories and comments across the archive that link to mathpages.com.

Comments · 52

  1. Re:The universe exists because God created it by UnknownSoldier · · Score: 2 · on Why Does The Universe Exist?
    > the entire four-dimensional space-time can, in fact, be continuous.

    That's not quite entirely correct:

    1. The energies of an atom is quantized.

    2. Three-dimensional space is also quantized. See Zeno's Paradox.

    Does anyone know if time is similiary quantized?

    --

    "The issue today is the same as it has been throughout all history, whether man shall be allowed to govern himself or be ruled by a small elite." -- Thomas Jefferson

  2. Re:Redshifting radio? by Matt_Bennett · · Score: 2 · on VMSK/2 Promises 5 Times More Bandwidth

    Correct in respect to Doppler but wrong in its application. A radar gun actually does not measure anything but Delta D over Delta T (change of distance over change of time). Granted it does this quickly but it does not use the doppler effect to measure the speed. For doppler to really become an issue I would think that you would really need to be traveling around 1/10000th the speed of light or even 1/100000th for something like this.

    Sorry, *absolutely* correct in its application. The only sort of police "radar" that uses timing to determine distance is a laser radar gun. The traditional RF police speed radar (known as Doppler radar) mixes a sample of the outgoing signal with a received signal and makes a beat between the two. Generally this beat is in the audio frequency range. By beating the outgoing with the incoming it doesn't really matter how much your main oscillator drifts, because over the microsecond or so that the signal takes to make a round trip it won't have drifted far. This sort of radar (as I have described it) can't distinguish between incoming and outgoing targets, but there are DSP based police doppler radars that have this ability, with a different downconversion technique.

    Time to go back to basic physics- for a speed much less than c the doppler shift of a signal will be a factor of 1/(1 +/- (v/c)) where v is the radial velocity to or from the observer (- when approaching the observer, + when going away).

    For example, with a radar operating at 18GHz, the doppler shift of a car moving at 100 ft/s is about 1.85 kHz. Check out this page for more information on doppler. Things do change as you approach the speed of light, but doppler still does matter, and I think it matters in this situation because of the extreme timing accuracy that this modulation requires. If the timing accuracy goes down, so will the bit-rate.