Posted on 07/02/2003 5:51:48 PM PDT by NormsRevenge
1 second = 760 pulsecs
Maybe Han Solo meant to say 'pulsec' instead of 'parsec.'
Suppose you are on a distant planet "watching" earth with a radio telescope. You just happened to be tuned to the frequency that the International Space Station transmits a spot beam down to Houston on. You don't see it when it is pointed away from you towards earth, nor do you see it when the earth blocks the signal. In fact, you see it only for a moment as it transits around the limb of earth just before it is blocked by the earth itself. You also see it as it reappears around the other limb before it points away again. Since the ISS orbits about once every 90 minutes, you see the signal twice in each orbit or about once every 45 minutes. You of course, know nothing about the ISS nor the fact that it is in orbit so you assume the observed signal is attached to the surface of the object being observed -- earth in this case. Would you conclude that the earth rotates once every 45 minutes or 32 times a day based on your observations? If so, you would be wrong.
What data fixes the observed thermonuclear "flicker" to the pulsar's surface? Are you sure the gasses being compressed into a thermonuclear explosion aren't "in orbit" around the pulsar as they spiral down towards the pulsar's surface?
Well, you probably wouldn't, because you'd see that every other pulse was doppler-shifted in an opposite direction.
This effect would be far more pronounced in the case of a neutron star, where the velocities are hugely greater, and you may also see the orbits precess. The most telling thing, however, is that radio objects in orbit around a neutron star will have different orbits, and therefore different periods, whereas the observed periodicity of pulsar pulses is stable to one part in 10^8!
I might also point out that these rotational periods are at the limit of what the strong nuclear force can hang onto; the period of an object in free-fall at a larger radius is bound to be much smaller.
Which leads to my next question: could a black hole revolve faster than 760 rps?
The article says: "What stops them [from disintegrating at faster than 760 rps] is the phenomenon predicted by Einstein's theory of relativity -- the rippling of the fabric of space and time. Known to scientists as gravitational radiation ... " Could someone explain, for a humble layman, what that has to do with the speed of rotation?
Which leads to my next question: could a black hole revolve faster than 760 rps?
I'm not sure the question has meaning as it applies to a Black Hole; it doesn't have a solid surface, and besides, I don't know how you'd measure it's rotation with the event horizon prevent anything other than Hawking radiation from getting out. In any case, I assume that the "Black Hole has no Hair" theorem includes angular momentum as one of the physical properties that is preserved by a Black Hole,in which case the stuff inside it, as it approaches the singularity at the center, probably approaches light-like speeds, as required by the Law of Conservation of Angular Momentum.
The article says: "What stops them [from disintegrating at faster than 760 rps] is the phenomenon predicted by Einstein's theory of relativity -- the rippling of the fabric of space and time. Known to scientists as gravitational radiation ... " Could someone explain, for a humble layman, what that has to do with the speed of rotation?
The article is poorly written in this reagard; I had the same question you have after reading it. But as best I can figure, the "centrifugal" forces generated by rotation of the pulsar in excess of the predicted 760 rps limit results in the outer layers of the pulsar's surface bulging out asymmetrically, resulting in displacement of the center of mass (and hence center of gravity) of the star away from the axis of rotation. This would cause the pulsar's gravitational field to appear to "wobble" as it's center moves slighty closer and then father away from an observer at approximately right angles to the axis of rotation. This would manifest itself as gravity waves, which would carry away energy from the pulsar. I'm guessing that the magnitude of the gravity waves (and the energy in them) is probably related to the moment of inertia of the asymmetry, which is proportional to r2(I think...); thus as the star rotates faster, the rate of energy loss through gravitational radiation would increase at an exponential rate (as the asymmetry begins to move further and further away from the axis of rotation, increasing it's moment of inertia), which would tend to slow it down. Furthermore, I'm guessing that above the 760 rps limit, the the gravitation of the pulsar is insufficient to "hang on" to the asymmetry, which means it tends to keep moving ever further outward, moving the center of mass and gravity further away from the axis of rotation, vastly increasing the loss of energy (read angular momentum) thru gravity waves.
The analogy might be an automobile tire; there exists some speed above which the tire cannot hold iteself together angainst the centrifugal forces that are trying to fling it apart; when that point is reached, the tire starts to distort not in a nice symmetric fashion, but rather the weakest point starts to deform the most, which unbalances the tire while further increasing the centrifugal forces on the overly deformed part -- which in turn deforms it even more, resulting in imminent catastrophic failure of the tire as a result of the positive feedback loop of deformation => increasing displacement from the axis of rotation => more centrifugal force on the displaced portion of the tire => more deformation => and so on until it fails.
But I'm about as much an expert on this as you are (IOW, the above is my WAG).... so I'll wait for the big dogs to weigh in on this one.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.