# Maximum pulsar rotation?

1. Apr 8, 2010

### Plaster

What is the fastest Possible rotational velocity for a pulsar ? There must be a point where it requires an infinity of energy to go any faster.

There is also a point where it becomes a black hole. How exactly does velocity relate to a fundamental shift in terms of how matter changes spacetime in such a fundamental way ?

Relativity describes it but doesn't explain it, just like the quantum formalism. Maybe my question can be summed up thus; what is the difference between energy in black holes, energy in matter, and energy in em radiation ?

2. Apr 8, 2010

### mgb_phys

There is a point where the surface velocity is greater than the escape velocity - and so it would fly apart.

Nothing to do with velocity, a blackhole is just a question of mass, put enough extra mass onto a neutron star and it will collapse into a blackhole.
The only thing rotation would do is prevent mass sticking to it.

3. Apr 9, 2010

### Nabeshin

I think he means more that since when you spin something up you impart energy to it, you are contributing to the stress-energy-momentum tensor. So at some point, energy density should be high enough to collapse the object into a BH. Of course, this point is almost certainly far beyond the limit in which the neutron star flies apart, but it could be an interesting academic exercise.

4. Apr 10, 2010

### Ich

No, what you're also adding is tension inside the body, since it has to withstand centrifugal "force". Tension = negative pressure = "negative energy". Rotating won't collapse an object.

5. Apr 10, 2010

### Count Iblis

A trivial upper limit is obtained by assuming that it rotates at the speed of light. A neutron star with radius 10 km would then rotate about 4800 times per second. The fastest pulsars are in fact millisecond pulsars, so they are only an order of magnitude below this very crude upper limit.

6. Apr 14, 2010

### Plaster

What density would such a neutron star have to be to rotate at, say, 2000 times per sec without flying apart ?

7. Apr 14, 2010

### Ich

From http://arxiv.org/abs/0901.1268" [Broken], it seems that the maximum frequency is ~800 - 1400 Hz, depending on the mass of the neutron star.

Last edited by a moderator: May 4, 2017
8. Apr 14, 2010

### Mihael@@/&

As as 2007 March 10, XTE J1739-285 is the fastest-spinning celestial body yet known, with a period of 1,122 Hz (approximately 67,320 RPM)

9. Apr 14, 2010

### Frame Dragger

I would add that similar approaches are (trying) to be used to determine whether or not all singularities are "cloaked" by an Event Horizon. So for, yup, all are below their theorized upper limits.

10. May 17, 2011

### Cigarshaped

Let me spin off at a tangent here, for a moment. How do you know that a pulsar is rotating in the first place?
Quoting webpages (as 'authority') - I read that Tommy Gold and Franco Pacini came up with the idea of spinning neutron stars. Not well received at first. Perhaps because it was based on the primitive mechanical lighthouse lantern concept? Then suddenly 1968 the discovery of a pulsar in the Crab Nebula led to the theory being accepted!

How does that make their theory acceptable? Are we actually seeing a spinning neutron star in that x-ray time-lapse movie by NASA/CXC/J.Hester et al? I would suggest that most pulsars are more like the current, replacement lighthouse system ie a strobe light. Pulses of energy by means of some resonant phenomena, akin to the electronic circuits that oscillate within the strobe. No real speed limit to that sort of system!

Answer: No difference. It behaves like electricity in ionised gas..totally scalable (thus lab-testable).