The Math Forum

Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Math Forum » Discussions » sci.math.* » sci.math

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Replies: 2   Last Post: Aug 15, 2014 11:02 AM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
Pentcho Valev

Posts: 6,212
Registered: 12/13/04
Posted: Aug 15, 2014 11:02 AM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

A light source emits a series of pulses the distance between which is d (e.g. d=300000km). A stationary receiver/observer measures the frequency of the pulses to be f=c/d:

The receiver starts moving with (small) speed v towards the light source - the measured frequency shifts from f=c/d to f'=(c+v)/d:

Why does the frequency shift from f=c/d to f'=(c+v)/d ?

Answer 1: Because the speed of the pulses relative to the receiver shifts from c to c'=c+v:
"The Doppler effect is the shift in frequency of a wave that occurs when the wave source, or the detector of the wave, is moving. Applications of the Doppler effect range from medical tests using ultrasound to radar detectors and astronomy (with electromagnetic waves). (...) We will focus on sound waves in describing the Doppler effect, but it works for other waves too. (...) Let's say you, the observer, now move toward the source with velocity vO. You encounter more waves per unit time than you did before. Relative to you, the waves travel at a higher speed: v'=v+vO. The frequency of the waves you detect is higher, and is given by: f'=v'/(lambda)=(v+vO)/(lambda)."
"vO is the velocity of an observer moving towards the source. This velocity is independent of the motion of the source. Hence, the velocity of waves relative to the observer is c + vO. (...) The motion of an observer does not alter the wavelength. The increase in frequency is a result of the observer encountering more wavelengths in a given time."

If Answer 1 is correct, both Einstein's relativity and modern physics as a whole collapse:
Faster Than the Speed of Light, Joao Magueijo: "If there's one thing every schoolboy knows about Einstein and his theory of relativity, it is that the speed of light in vacuum is constant. No matter what the circumstances, light in vacuum travels at the same speed - a constant that physicists denote by the letter c: 300,000 km per second, or as Americans refer to it, 186,000 miles per second. The speed of light is the very keystone of physics, the seemingly sure foundation upon which every modern cosmological theory is built, the yardstick by which everything in the universe is measured. (...) The only aspect of the universe that didn't change was the speed of light. And ever since, the constancy of the speed of light has been woven into the very fabric of physics, into the way physics equations are written, even into the notation used. Nowadays, to "vary" the speed of light is not even a swear word: It is simply not present in the vocabulary of physics."
Bryan Wallace: "Einstein's special relativity theory with his second postulate that the speed of light in space is constant is the linchpin that holds the whole range of modern physics theories together. Shatter this postulate, and modern physics becomes an elaborate farce! (...) The speed of light is c+v."

Answer 2 (possibly preventing the collapse of modern physics): Because...

There is no reasonable statement that can become Answer 2. Yet there is an idiotic thesis that Einsteinians sometimes use as Answer 2: The motion of the receiver/observer changes either the incoming light's wavelength (the distance between subsequent pulses) or some miraculous "MEASURING INSTRUMENTS" of the receiver/observer - the result in both cases is that the moving observer sees a light with a different wavelength (measures the distance between subsequent pulses to be different) but the same speed c, Divine Einstein, yes we all believe in relativity, relativity, relativity, that's the way ahah ahah we like it, ahah ahah:
Professor Martin White, UC Berkeley: "...the sound waves have a fixed wavelength (distance between two crests or two troughs) only if you're not moving relative to the source of the sound. If you are moving away from the source (or equivalently it is receding from you) then each crest will take a little longer to reach you, and so you'll perceive a longer wavelength. Similarly if you're approaching the source, then you'll be meeting each crest a little earlier, and so you'll perceive a shorter wavelength. (...) The same principle applies for light as well as for sound. In detail the amount of shift depends a little differently on the speed, since we have to do the calculation in the context of special relativity. But in general it's just the same: if you're approaching a light source you see shorter wavelengths (a blue-shift), while if you're moving away you see longer wavelengths (a red-shift)."
"The observer moves closer to the source. The wave received has a shorter wavelength (higher frequency) than that emitted by the source. The observer moves away from the source. The wave received has a longer wavelength (lower frequency) than that emitted by the source."
John Norton: "Every sound or light wave has a particular frequency and wavelength. In sound, they determine the pitch; in light they determine the color. Here's a light wave and an observer. If the observer were to hurry towards the source of the light, the observer would now pass wavecrests more frequently than the resting observer. That would mean that moving observer would find the frequency of the light to have increased (AND CORRESPONDINGLY FOR THE WAVELENGTH - THE DISTANCE BETWEEN CRESTS - TO HAVE DECREASED)."
Tom Roberts: "Wavelength is not an intrinsic property of light, so it cannot be discussed independent of how it is measured. But it is clear that in vacuum the light ray itself is unchanged as it propagates. Differently moving observers will measure different wavelengths for a given light ray, because their MEASURING INSTRUMENTS are oriented differently in spacetime, and such a measurement inherently PROJECTS the light ray onto the measuring instrument."

Pentcho Valev

Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2018. All Rights Reserved.