Oddbean

Neutrons in isolation are unstable. They will energetically decay, becoming a proton, electron, and an anti-electron-neutrino with a half-life of about 10 minutes. This is called ß-decay. This reaction is reversible through a process known as electron capture. If a proton and an electron are forced into near proximity, they can combine to form a neutron, emitting a normal electron-neutrino. In the collapsing iron core of a high mass star, in the last microsecond before it blows itself to kingdom come as a type 2 supernova, the electrons and protons within the iron are forced together and undergo this inverse ß-decay becoming neutrons and emitting an overwhelming barrage of neutrinos. The newly formed neutrons rapidly sort themselves out into energy shells. Neutrons are fermions. Identical fermions cannot co-exist together in the same system. Thus the neutrons in the collapsed core separate into a vast number of different energy shells. Low energy neutrons are deeper in, higher energy neutrons are farther out. There is no way for the higher energy neutrons to lose energy since all the lower shells are full. All those neutrons are in a constant state of vibration and collision. Higher energy neutrons vibrate and collide faster than lower energy neutrons. All that vibration and collision creates an outward pressure that holds back the inward crush of gravity. So the object, now only a few miles in diameter, settles into a stable state called degenerate matter. This object is often called a neutron star; and the degenerate matter within it is sometimes referred to as neutronium. A teaspoon of neutronium would weigh several billion tons. Much of the angular momentum of the original star remains within the neutron star. However, since the radius has shrunk by three or four orders of magnitude, the angular velocity has increased by a corresponding factor. These objects can spin at thousands of RPM. Moreover, much of the original star's magnetic field is trapped within the neutron star, and has likewise been compressed by several orders of magnitude. Thus, the neutron star is a very powerful spinning magnet. When you spin a magnet you create an electric field. The electric field across a newly formed neutron star is enormous. That field accelerates vast numbers of charged particles to stream outwards from the magnetic poles. However, the powerful magnetic field lines force those particles to move in a spirals as they stream outwards. When charged particles move in spirals they induce electromagnetic waves -- light. And those light waves travel in straight beams emitted from the magnetic poles of the neutron star. Those beams of light carry away some the rotational energy of the Neutron star. This creates a drag on its rotation. But given that the star contains a solar mass of neutronium spinning at thousands of RPM, it has plenty of kinetic energy to spare and so the reduction in rotational velocity is very gradual. If the magnetic poles of the neutron star are not perfectly in line with its spin axis, then those beams of light swing around through space like a lighthouse beacon. If they happen to be in line with the Earth, we see the object emitting pulses of light as the beams pass by. We call these pulsating stars Pulsars.

This was a fascinating read start to finish! Inspiring to read more. The part where the rotating magnet induces a current that emits EM waves in the direction of the poles is not clear. Is that because those particles are trapped to the magnetic field lines that themselves wobble around the axis of the rotation?

All that comes from Maxwell's laws (which eventually derive from Special Relativity). A moving magnetic field creates a stationary electric field. A moving electricc field creates a stationary magnetic field, etc. If we start with the premise that the neutron star is a spinning magnet then the moving magnetic field creates a stationary electric field aligned with the spin axis of the star. Any charged particle within that electric field will be accelerated along the electric field lines. Charged particles that move through a magnetic field are acclerated at right angles to the that field. That translates to circular motion around the magnetic field lines. So the combination of the electric field accellerating those particles linearly outward, and the magnetic field accellerating them circularly around the field lines, casues the particles to move in a tight spiral outwards. Each such charged particle has it's own electric field which is being accelerated both linearly and circularly. This creates a circularly rotating magnetic field at right angles, which ceates a circularly rotation electric field, which creates a circularly rotation magnetic field -- or, to say this differently, an electromagnetic wave. The propogation of an electromagnetic wave is perpendicular to the axis of rotation of the wave, and is therefore outward along the magnetic field lines of the star. I think I got that all right. ;-) From: Giszmo at 08/22 13:57 > This was a fascinating read start to finish! Inspiring to read more. > > The part where the rotating magnet induces a current that emits EM waves in the direction of the poles is not clear. Is that because those particles are trapped to the magnetic field lines that themselves wobble around the axis of the rotation? CC: unclebobmartin

Each of these details makes sense. I studied physics after all 😅 But it's still not intuitively clear. I guess I need to get an intuition for synchrotron radiation. I asked Chat-GPT about the beam's focus and tend to doubt it's correct with this example: https://i.nostr.build/9MKHMV2m0a2moa5h.png

I think it's just two right angle rotations that cancel each other out. The first is the ciculation of the charged particles at right angles to the magnetic field. The second is the right angle propogation of the electromagnetic waves induced by those circulating particles. From: Giszmo at 08/23 11:36 > Each of these details makes sense. I studied physics after all 😅 But it's still not intuitively clear. I guess I need to get an intuition for synchrotron radiation. > > I asked Chat-GPT about the beam's focus and tend to doubt it's correct with this example: > > https://i.nostr.build/9MKHMV2m0a2moa5h.png CC: unclebobmartin