Re: Doppler

[Marcus D. Leech]

On 01/01/2024 16:11, Marcus Müller wrote:

Liya,

Doppler shift Δf is proportional to both speed and carrier frequency f₀

Δf = f₀ · v/c₀,

where v is the relative speed of your thing, and c₀ is the speed of light.

The highest frequencies we can, so far, do radio communications on, are in the range of f₀=150 GHz.

So, assuming you do communications on 150 GHz, for your Doppler shift to be Δf=10 GHz higher after 1s, your acceleration must been

a = Δf / f₀ · c₀ / 1s = 10 GHz / 150 GHz · 3·10⁸ m/s / s = 2/30 · 3·10⁸ m/s² = 1/15 c₀/s.

The fastest object mankind has ever built is the Parker Solar Probe, which will burn up while it spirals into the sun, at a maximum velocity of ca 1/15 of the speed of light. It takes it years to reach that speed, not 1s.

So, you're assuming you're seeing a doppler from a satellite rotating around earth that sees a relative acceleration higher than a "satellite" around the sun actively being pulled into the sun by the sun's immense gravity.

That sadly makes no physical sense!

Best regards,
Marcus

    I, ahem, can't math tonight!
    
    Parker solar probe in its death-spiral will achieve 692000km/h
    (Note: per *hour*)  C=299792 km/s (Note: *second*).
    
    So, 692,000/3600 = 192km/s
    
    Which is 0.0006C, or about two orders of magnitude slower than my
    previous calculation...
    
    However, that does NOT negate the point that simulating doppler of
    10GHz/s for "satellites" makes no physical sense at
      all.   We don't know how to move physical objects anywhere close
    to that fast, and we sure as heck don't know how to
      *accelerate* massy physical objects like that.   Objects in earth
    orbit have quite leisurely (relative to C) velocities.  The same is
      true of any of the deep-space probes humans have launched.  The
    one exception is the Parker Solar probe, but it will
      only reach nail-biting speeds near the end of its mission, and
    even *those* are decidedly leisurely compared to C.
    
    Now, maybe this is all about searching for aliens making little
    tourist trips into near-earth space or something.  I dunno.
      I have to imagine that any object with non-trivial mass
    accelerating at those rates would leave signatures other
      that a silly little doppler signature, but what do I know?