Doppler shifts

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dysfunction

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Doppler shifts
« on: August 29, 2006, 06:04:13 PM »
The Doppler shift is a phenomenon that occurs when an object is moving towars or away from an observer. You can observe this effect by watching a police car drive by on the road with its siren turned on; as the car approaches you, the pitch of the siren grows steadily higher, peaking when the car reaches you, then the pitch grows steadily lower as the car moves away from you. This is a consequence of the fact that sound is carried by waves; as the car approaches, the sound waves arw "squished" (a simplified explanation); as the car recedes, the waves are "stretched" (again, simplified). The same effect occurs with electromagnetic waves, such as light.

To measure the Doppler Shift of stars, we look at how the stars' color varies. Remember that with sound waves, the pitch grows higher as an object approaches and lower as it recedes? With light waves, the color of an object moves towards the blue end of the visible light spectrum as the object approaches an observer (a "blue shift") and towards the red end as the object recedes (a "red shift"). When we measure the Doppler effect on nearby stars, we see that the amount and direction of the shift changes with a regular period. This is easily explained by a heliocentric model of the solar system; as the Earth approaches the part of its orbit that is nearest a particular star, the star has a blue shift, and as the Earth moves away from the star, the star has a red shift. However, if we are being accelerated at 9.8m/s/s, we should be approaching lightspeed on an asymptotic curve, and be moving very, very nearly at lightspeed relative to external stars. Therefore, we should not see a periodic change in shifts, but rather a constant shift on all stars, at least until we pass by them- stars in front of our acceleration should be exceedingly heavily blue shifted, and stars behind us should be exceedingly heavily red shifted.

Another problem also arises; we should be racing past the nearby stars, unless of course FE posits that the entire galaxy is being accelerated. However, if the galaxy is indeed being accelerated we should measure massive blueshifts on galaxies in half the sky (the part that is in front of us) and massive redshifts on the other half. Instead, we measure large (but much smaller than would be expected if we were moving near lightspeed) redshifts on the part of the great majority of galaxies in the universe. Unless, of course, the entire universe is being accelerated, but then the question arises, accelerated relative to what?
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dysfunction

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Doppler shifts
« Reply #1 on: August 30, 2006, 08:38:46 PM »
Bump.
the cake is a lie

Re: Doppler shifts
« Reply #2 on: August 30, 2006, 09:07:41 PM »
Quote from: "dysfunction"
but then the question arises, accelerated relative to what?


An inertial reference frame perpendicular to your velocity.

As for the rest, it's too late for me to start this today. Now that uni started and my mentor is already pushing me to do this in real life, for hours :(, it makes it all less interesting to do online. I'll probably need to severly trim back my visiting this site if the email I just got from my mentor is any indication.

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Erasmus

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Doppler shifts
« Reply #3 on: August 31, 2006, 12:53:40 PM »
In special relativity, we can always determine absolute acceleration with respect to an inertial reference frame that we construct.  In general relativity, this is not possible, since we distinguish the effects of acceleration with the effects of gravitation.
Why did the chicken cross the Möbius strip?

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dysfunction

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Doppler shifts
« Reply #4 on: August 31, 2006, 12:54:25 PM »
Quote from: "Erasmus"
In special relativity, we can always determine absolute acceleration with respect to an inertial reference frame that we construct.  In general relativity, this is not possible, since we distinguish the effects of acceleration with the effects of gravitation.


Why are you guys nitpicking and avoiding the main question here?
the cake is a lie

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Erasmus

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Doppler shifts
« Reply #5 on: August 31, 2006, 12:56:46 PM »
Quote from: "dysfunction"
Why are you guys nitpicking and avoiding the main question here?


Rowbotham's model has the entire universe coupled to the Earth.  In other words, if the Earth is accelerating upwards, so is the rest of the universe.

In this case, the only question of yours left unanswered is "accelerated relative to what?" and it is this question which qwerty answered tersely and which I tried to go into slightly more detail on.
Why did the chicken cross the Möbius strip?

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dysfunction

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Doppler shifts
« Reply #6 on: August 31, 2006, 01:36:39 PM »
Then your problems of generating the power for this acceleration, never small in the first place, are now multiplied; e=mc^2 gives us a limit on the amount of energy that can be generated from a given amount of matter. If only the solar system is being accelerated, the power source could hypothetically harness an external fuel supply; however, while I haven't actually calculated this, I would bet that by the law of diminishing returns (meaning that the amount of energy required to accelerate a mass increases faster than the size of the mass does), the universe could not possibly accelerate itself at 9.8m/s/s for the last several billion years, and probably not for the last 6000 years (if you take a literal view of the Bible), either.
the cake is a lie

Doppler shifts
« Reply #7 on: August 31, 2006, 04:43:17 PM »
"To measure the Doppler Shift of stars, we look at how the stars' color varies."

As far as I remember, according to RE theory, "the stars' color varies"  not only because of "the Doppler Shift of stars". There are several classes of the stars on the main sequence: O, B, A, F, G, K, M, with many subdivisions, each with its special colour.

" the color of an object moves towards the blue end of the visible light spectrum as the object approaches an observer (a "blue shift") and towards the red end as the object recedes (a "red shift")."

At least for the "red shift", there are alternative explanations even from the RE enthusiasts. One of them is, so to speak, "ageing" of photons. Yet another suggests that the light might became red after passing through interstellar clouds of gases and/or dust.
"It is not necessary that hypotheses should be true, or even probable; it is sufficient that they lead to results of calculation which agree with calculation".
Copernicus

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dysfunction

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Doppler shifts
« Reply #8 on: August 31, 2006, 05:30:58 PM »
Quote from: "Humble_Scientist"
At least for the "red shift", there are alternative explanations even from the RE enthusiasts. One of them is, so to speak, "ageing" of photons. Yet another suggests that the light might became red after passing through interstellar clouds of gases and/or dust.


Those ideas are from creationists, and were completely refuted decades ago.
the cake is a lie

Doppler shifts
« Reply #9 on: August 31, 2006, 05:48:56 PM »
"Those ideas are from creationists"

Your point?

"and were completely refuted decades ago."

I am not so sure. Anyway, this is true. When light is passing through gas, the rays with a longer wavelength have much better chances to go through. As far as I remember, the intensity of light dissipation is proportional to (the frequency)^4, and that is the very reason why the sky is blue.  This effect is enhanced by the presence of small particles and often explains why the Sun appears red at the sunset. So the light that have passed through a cloud of gas and dust would demonstrate a "red shift".
"It is not necessary that hypotheses should be true, or even probable; it is sufficient that they lead to results of calculation which agree with calculation".
Copernicus

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dysfunction

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Doppler shifts
« Reply #10 on: August 31, 2006, 06:14:56 PM »
No, that's not true- because the explanation I gave was simplified. Astronomers don't simply look at the apparent colors of stars, that was a simplified explanation, they look at the positions of specific lines in stellar spectra. Astronomers use stars with constant composition to calibrate; stellar spectra are dependent on a star's chemical composition, so when they see that these specific lines have shifted away from where they know the lines should be for a given chemical composition, they know there has been a 'Doppler' shift.
the cake is a lie

Doppler shifts
« Reply #11 on: August 31, 2006, 06:15:46 PM »
LALALALALA!!!! IT'S ALL A CONSPIRACY!!!!!!!!

Doppler shifts
« Reply #12 on: August 31, 2006, 06:53:38 PM »
"No, that's not true"

What exactly is not true?

"they look at the positions of specific lines in stellar spectra."
"Astronomers use stars with constant composition to calibrate;" etc.

This explanation is also simplified. The procedure of such a calibration is quite complicated. They are also using some other methods to estimate the distance to those stars, comparing the "Doppler shift" of those "standard" stars with other stars etc. Interesting enough, they often found celestial bodies whose "Doppler shift" can not be explained. Then the astronomers start talking about "ageing" of photons or suddenly remember that intensity of light dissipation is proportional to the (frequency)^4. One of the most extraordinary explanations I have met was that between us and the star was an invisible "black" hole whose gravity changed the path of the light ray in such a way that the light of the star was coming to us from a different part of the Universe, as if the distance to the star was much longer than it actually was. Grab a bunch of something like "Scientific American", you could find plenty of interesting details. Remember, it is not they are trying to deceive you, the matter is really very complex.
"It is not necessary that hypotheses should be true, or even probable; it is sufficient that they lead to results of calculation which agree with calculation".
Copernicus

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Erasmus

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Doppler shifts
« Reply #13 on: August 31, 2006, 07:06:59 PM »
Quote from: "dysfunction"
Then your problems of generating the power for this acceleration, never small in the first place, are now multiplied;


Suppose I'm "not moving".  Meaning, I wake up in a spaceship (in the RE universe) in the middle of nowhere.  Can't see any stars, for whatever reason.  Engines off.

Now I turn the engines on to whatever power rating (according to the little dials marked on the controls) is required to accerate at, say, 40 m/s^2.  The engine turns on and the dials indeed read that the power output is what's expected, given I'm burning fuel at such-and-such a rate, etc.  At what rate do I accelerate?

I claim: 100 m/s^s.

I let this go on for a while.  Suppose I have quite a lot of fuel.  I let this process continue for a full year.  My speed should have asymptotically approached the speed of light, right?

Well,now I turn off the engines.  Can I move about the spaceship?  How much energy is required to move me from the back of the spaceship to the front?

I claim: no more than it takes when the spaceship was "stopped" at the beginning of the experiment.  When the engines are off, I'm allowed to pretend the ship is not moving.  In other words, I'm not moving just under the speed of light: my velocity is zero, so my relativistic mass is zero, so the force required to accelerate me across the spaceship is just that given by my rest mass.

In other words, it doesn't take more power to continue accelerating at a given rate from the perspective of the thing being accelerated, but it does take more power from the perspective of an inertial observer.
Why did the chicken cross the Möbius strip?

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dysfunction

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Doppler shifts
« Reply #14 on: August 31, 2006, 09:31:00 PM »
You misunderstand what I'm saying. I'm not talking about mass increasing due to relativity here- you took me too literally when I talked about mass increasing. I realize the mass remains constant from the perspective of that which is accelerating, and that more fuel is not required as it approaches lightspeed. What I meant was, the larger a mass you are accelerating, the more energy required to accelerate it- but that the two values are not linearly proportional. This is why there is an upper size limit on a spacecraft utilizing any given propulsiuon technology, depending specifically on the efficiency of that technology's conversion of matter to energy- the more massive the craft is, the more fuel is required to propel it, and the fuel adds mass as well, so by the law of diminishing returns there is an upper limit at which the spacecraft is practical. Unless the conversion of matter to energy by which the universe's hypothetical acceleration is powered is very near 100% efficiency, the universe should be much too massive to accelerate itself at 9.8m/s/s.
the cake is a lie