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Old 03-08-2010, 05:36 PM #1
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Question An interesting thought...

I just had this strange thought pop up in my head... in a purely hypothetical situation, if one were to shine a hypothetically bright laser beam that could be seen for millions of miles straight out from the Earth and an observer was looking at this beam jet out from the planet from a great distance would that beam appear as a spiral (if the planet were already spinning at near light speeds)? Since the speed of light is fixed and the outer end of the beam would be travelling at a faster speed than the inner portion I would guess that the beam would start to curve into a spiral much like the stars around the galaxy. What do you guys think?


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Old 03-08-2010, 05:55 PM #2
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Default Re: An interesting thought...

I dont think so. Nothing can travel faster than the speed of light and thats not how light works anyway.
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Old 03-08-2010, 06:28 PM #3
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Default Re: An interesting thought...

Well you are trying to say that on grand proportion scale , looking at light beam would be like looking at water sprinkled from a hose.
Moving the hose causes the water to change shape...

It would sure be like that, but the problem is would we see it like that, since both the beam and light diffused from the beam we see is traveling at same speed.

It is better not to pull such hypothetical questions, as finding the answer is impossible, and even if it is found it would prove meaningless since we would never use it anywhere...

Use your imagination
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Old 03-08-2010, 07:04 PM #4
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Default Re: An interesting thought...

Such a question is not meaningless. Since when was it wrong to wonder about something?

If you shine a laser to the sky, yes, the beam will be bent. Another thing to note, is that if the dot gets far enough away from the source, it can travel along a surface faster than the speed of light. But don't worry, that's not breaking any laws by any means!
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Old 03-08-2010, 07:09 PM #5
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Default Re: An interesting thought...

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Originally Posted by Meatball View Post
Such a question is not meaningless. Since when was it wrong to wonder about something?

If you shine a laser to the sky, yes, the beam will be bent. Another thing to note, is that if the dot gets far enough away from the source, it can travel along a surface faster than the speed of light. But don't worry, that's not breaking any laws by any means!
Don't get me wrong, I am quite a SF fan, and wonderer myself (Ph.D in wondering around)

I find the 'Is the space infinite or finite' thread much more discussable...

I don't get your surface travel faster than speed of light you pointed out, though.
Care to enlighten me, so to speak?
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Old 03-08-2010, 11:04 PM #6
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Default Re: An interesting thought...

Since when did the earth rotate on it's axis at light peed anyhow??

If it spun that fast, gravity would be meaningless, we would all be flung at unimaginable speeds outward.

Earth's Circumference at the Equator: 24,901.55 miles

the speed of light—about 186,000 miles per second

the earth would be rotating nearly 6.5 times a second!

imagine the seasons changing at that rate Yoda.
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Old 03-08-2010, 11:30 PM #7
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Default Re: An interesting thought...

If you kept the beam pointed in some steady direction from the earths surface, there would be doppler shift matching the 24h rotational cycle of the earth. Also, your beam would go dark a certain portion of they day since you'd be on the wrong side to point the beam at all.

I dont see any reason for a spiral like effect - just a very small up and down shift in wavelength over the course of a day.
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Old 03-08-2010, 11:35 PM #8
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Default Re: An interesting thought...

A spiro motor rotates substantially faster than the earth does, and the beams don't bend from that....
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Old 03-09-2010, 12:26 AM #9
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Default Re: An interesting thought...

Ok, let me throw this out there. Look up the global positioning system. The radios operate at the speed of light because, well, they are in a vacuum and they are radio waves but the satellites are also in constant motion at orbital speeds. As such there must be a correction for relativistic time dilation due to their motion. It's the issue of speed of light of the radio waves plus the rate of orbit. Since you cannot exceed the speed of light time dilation occurs (the on board clocks have to be corrected for this dilation.) That is what you would see in space if you point a laser into space and be seen from some great distance. Rotational and orbital speed of the earth plus the speed of the beam cannot exceed the speed of light so either time dilates or mass changes (photons have no mass).

The GPS is such a good example of relativity I recommend anyone interested in the theories to understand GPS as the application of relativity.
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Old 03-09-2010, 12:41 AM #10
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Default Re: An interesting thought...

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Ok, let me throw this out there. Look up the global positioning system. The radios operate at the speed of light because, well, they are in a vacuum and they are radio waves but the satellites are also in constant motion at orbital speeds. As such there must be a correction for relativistic time dilation due to their motion. It's the issue of speed of light of the radio waves plus the rate of orbit. Since you cannot exceed the speed of light time dilation occurs (the on board clocks have to be corrected for this dilation.) That is what you would see in space if you point a laser into space and be seen from some great distance. Rotational and orbital speed of the earth plus the speed of the beam cannot exceed the speed of light so either time dilates or mass changes (photons have no mass).

The GPS is such a good example of relativity I recommend anyone interested in the theories to understand GPS as the application of relativity.

As a surveyor we have to have at least a general knowlege of GPS and how it works to apply it to RTK surveying and running our Static infills. I myself have taken some classes for surveyng ceu's required to keep a liscense at some symposiums. They actually talked about the corrections the gps systems both terrestrial and orbiting satellites must make to stay subcentimeter accurate during positioning. I must say it was some mind blowing calculations that make this all possible. They also touched on the speed of light and how it affects the timing of the signals. It's some very interesting stuff but I cant remember/comprehend half of the stuff they talked about lol.
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Old 03-09-2010, 10:53 PM #11
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Default Re: An interesting thought...

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That is what you would see in space if you point a laser into space and be seen from some great distance. Rotational and orbital speed of the earth plus the speed of the beam cannot exceed the speed of light so either time dilates or mass changes (photons have no mass).
This still just explains why one would observe a doppler shift, not any spiralling or odd effects otherwise.

I suppose this is the beauty of relativity: it doesnt matter if the obsever moves to (or from) the earth, or the earth moves from (or towards) the observer, the effect is identical.

The nicest effects of doppler shifts are observed with radio signals since their frequency is often measured very accurately (1 ppm or so being no problem on the short run). This causes the effects to be very observable at ordinary speeds, such as a transmitter mounted on a moving car.

Obviously the effect is exactly the same for any other type of light including visible, but that is not often measured to such accuracy. A one-in-a-million deviation on a 100 MHz signal is easy to measure with affordable equipment, but doing the same at 532 nm would be a more difficult feat.
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Old 03-09-2010, 11:01 PM #12
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Default Re: An interesting thought...

wow RADAR.........
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Old 03-09-2010, 11:09 PM #13
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Default Re: An interesting thought...

Quote:
Originally Posted by Cenobite9 View Post
I just had this strange thought pop up in my head... in a purely hypothetical situation, if one were to shine a hypothetically bright laser beam that could be seen for millions of miles straight out from the Earth and an observer was looking at this beam jet out from the planet from a great distance would that beam appear as a spiral (if the planet were already spinning at near light speeds)? Since the speed of light is fixed and the outer end of the beam would be travelling at a faster speed than the inner portion I would guess that the beam would start to curve into a spiral much like the stars around the galaxy. What do you guys think?
I'm not sure I entirely understand this situation...but if you had an incredibly bright beam of light emitted from a rotating body, any observer would just detect this as "on" and "off" signals, from when the emitting planet is facing them and away from them respectively. This is the case with pulsars, which are basically neutron stars that spin at obscenely high rates, (with rotational periods ranging from 1.4 MILIseconds to 8.5 seconds).

Granted the Earth doesn't spin anywhere nearly as fast as these odd stars, but I think the end effect would be about the same, long periods of detected light followed by long periods
without detection.
See: Pulsar - Wikipedia, the free encyclopedia
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Old 03-09-2010, 11:39 PM #14
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Default Re: An interesting thought...

You would have a slow light house. So on the plane of the beam (say on the plane of the earth's equator, asusming a stationary rotaqting earth) from an arbitrary distance away, you would get a pulsing light. With a perfectly collimated light source and zero divergence, you would get one short pulse followed by a longer time of darkness. With real light sources, but obscenely bright, you would get a fade-in, then a fade out, then a period of darkness. Just light with a light house when viewed from sea. There would also be a Doppler shift in the wavelength in this case, since the light source would be moving toward the observer during the "fade-in" and away from the observer during the "fade-out".

If viewing this situation from a very long distance above the plane of rotation (somehow magically seeing the location of the photons, say from an arbitrarily long distance above the north pole of the earth), then I suppose one could see photons forming some sort of spiral outward from the earth. A photon emitted at noon will have traveled roughly 8 billions miles away from earth in a straight line when, at midnight, another photon is emitted in the exact opposite direction of the first one. Another 12 hours later, the first photon has gone 16billion miles, the second has gone 8 billion miles in the opposite direction, and a third is emitted in the exact same direction as the first. You can see how, if this situation was expanded to a constant stream of photons and could somehow be "viewed" from above, and the locations of photons at a given time were plotted, the locations of the photons at a given time would look like a spiral.


I think part of the OPs question comes from a confusion on how a rotating body and light would interplay. Light's speed doesn't change, so it doesn't depend on how fast the earth was going when the light was emitted. It's not like a tennis ball being spun around an orbit on a string: when you let go of the string, the tennis ball's energy and momentum are conserved, so the tennis ball travels on a tangent to its rotation with the speed it had when you let it go. With a photon, the speed of the photon is independent of the motion of the object emitting the photon. The direction of the photon could depend slightly I suppose (hmm, that's another problem to think about for a minute), but the speed is 100% independent of the lasers motion when the photon is released.

Last edited by pullbangdead; 03-09-2010 at 11:46 PM.
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Old 03-10-2010, 12:25 AM #15
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Default Re: An interesting thought...

Quote:
Originally Posted by pullbangdead View Post
If viewing this situation from a very long distance above the plane of rotation (somehow magically seeing the location of the photons, say from an arbitrarily long distance above the north pole of the earth), then I suppose one could see photons forming some sort of spiral outward from the earth.
Thats your problem right there, you cannot magically see those photons from any angle where they do not travel towards you. This could be 'remedied' by placing the eath in some kind of mist or fog, where you get scattering that would allow you to observe the whole ordeal from the 'side'.

The question would still be if it would look like a spiral from that perspective. I'm inclined to think that it would not, since the beams would actually point out in a straight line, and still be observable as such. They might very well look like 'pieces of a pie' radiating out, but there would be no bending to observe.
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Old 03-10-2010, 01:02 AM #16
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Default Re: An interesting thought...

if the beam were to go near a black hole it would bend because black holes have so much mass (gravity) they actually bend light
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