View Full Version : How does relativity affect a rotating pulsar?
2009-Dec-09, 11:29 AM
When a "solid" object such as a pulsar spins at significant fractions of C, such that two opposite points on the equator, or the equator versus the pole, experience the effects of relativity, are there any interesting consequences for the object itself? What about for objects in orbit around the pulsar, or "on the surface" and "running" against the spin such that the runner remains stationary from the point of view of an observer outside the system?
"Solid objects" aren't allowed in relativity theory.
This issue has come up before, perhaps a couple of years ago. If you search for posts by me going back ages you should find the thread, if you are willing to trawl through the list.
2009-Dec-09, 09:16 PM
It seems like the concept was touched on in this thread http://www.bautforum.com/space-astronomy-questions-answers/54587-non-rotating-stars.html but was not actively addressed. I have not been able to find other posts by you that seem to be on the topic. :x
Although your original reply "'solid objects' aren't allowed in relativity theory' is probably as good a place to start as any. I'm interested in whether the relativistic effects would disrupt the normal processes of a star but a mathematically perfect sphere might formed of a single indivisible particle might make just as interesting a topic.
2009-Dec-10, 12:20 AM
A forum or web search on "Ehrenfest paradox" will show what kzb means about "solid" objects being disallowed in relativistic rotation. Something has to give!
2009-Dec-10, 05:16 AM
Since such massive spinning objects accreted that way, rather than having been spun up from a standstill, I don't imagine there's any need to "give."
2009-Dec-10, 05:26 AM
When a "solid" object such as a pulsar spins at significant fractions of C, such that two opposite points on the equator, or the equator versus the pole, experience the effects of relativity, are there any interesting consequences for the object itself? ?
Yes: gravitational wave emission.
A fast rotating pulsar can take the shape of an oblate spheroid and have significant GW emisson characteristics.
See for example: http://adsabs.harvard.edu/abs/1983Natur.303..308C
2009-Dec-10, 09:15 AM
You will find that pi is no longer pi for a spinning disk, as the circumference is length contracted, while the radius is not.
Well, rather the circumference is not 2.pi.r.
2009-Dec-10, 08:21 PM
Since such massive spinning objects accreted that way, rather than having been spun up from a standstill, I don't imagine there's any need to "give."Millisecond pulsars are indeed "spun up" from a state of slower rotation. Pulsars are also observed to "spin down" from high rates of rotation. Either way, as they make these transitions they're dealing with the sort of relativistic problem that underlies the Ehrenfest "paradox".
2009-Dec-11, 03:31 PM
On the Wikipedia page
they talk about an observer on the train(in a thought experiment) not noticing a contraction, but surely they would see the train coaches on the opposite side traveling at twice the speed, and so contracted even more?
2009-Dec-11, 11:16 PM
Thank you. :) This is a bit mind melting and trying to wrap my head around it will keep me occupied for a while....
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