Astronomers have long believed that if a star with a mass between 1.4 and 3 times that of the sun were to expend all its fuel, the end result would be a collapse to a neutron star—an object with a diameter of perhaps 12 miles, but still retaining its original mass. Such a small object, even if it continued to emit radiation, was thought to be almost impossible to detect. In 1967, however, the first such an object was discovered, by pure chance.
Jocelyn Bell, an astronomy student at Cambridge University, was assigned the task of investigating fluctuations in the strength of radio waves from distant galaxies. She found unexpectedly that certain places in the heavens were emitting short, rapid bursts of radio waves at regular intervals. Each burst lasted no more than one hundredth of a second. The interval between successive bursts was extraordinarily constant—it did not change by more than one part in 10 million. A clock with this precision would lose or gain no more than a second a year.
No star or galaxy had ever been observed to emit signals as bizarre as these. At first some astronomers thought that intelligent beings on other stars might be beaming a message to the earth, and referred to the Morse-code stars as LGM’s, or Little Green Men. But it soon became apparent that the radio pulses had a natural and not an artificial origin. One of the main reasons for this conclusion was the fact that the signals were spread over a broad band of frequencies. If an extraterrestrial society were trying to signal other solar systems, its interstellar transmitters would require enormous power in order for the signals to carry across the trillions of miles that separate every star from its neighbors. The only feasible way to do this would be to concentrate all available power at one frequency, as we do when we broadcast radio and TV programs. It would be wasteful, purposeless and unintelligent to diffuse the power of the transmitter over a broad band of frequencies.
Realizing that the cause for the strange pulsing must be natural, scientists began a search for the explanation for these peculiar signals. The first clue to the answer was the sharpness of the pulses. From the fact that each pulse lasted for one hundredth of a second or less, astronomers concluded that a pulsar is an incredibly small object—far smaller than a white dwarf. This conclusion was based on the fact that when an object emits a burst of radio waves, the waves from different parts of the object arrive at the earth at different times, blurring the sharpness of the original pulse. The smaller the object, the sharper the pulse. From this line of reasoning astronomers calculated that the objects with such sharp radio bursts must be no more than 12 miles in radius!
This extremely small size fits very well the predictions for a neutron star. Such a star can be created as the aftermath of supernova explosion, where the remaining mass of an exploding star can be imploded to form a pure ball of neutrons, but with most of the mass of the original star packed it.
Knowing that a neutron star could be the natural result of a supernova explosion, astronomers had been investigating the center of the Crab Nebula, where a supernova explosion had been recorded by Chinese astronomers in the year AD. 1054. (It was not observed in Europe, which was in the height of the dark ages). The Crab Nebula, the nearest supernova remnant known, thus seemed to be the logical place to find a neutron star remnant of the supernova.
In 1968, a pulsar was discovered at the center of the Crab Nebula, at precisely the place where a neutron star had been expected. Suddenly, many items of evidence fit together like the pieces of a jigsaw puzzle. Clearly the neutron star and pulsar were two names for the same thing—a fantastically compressed, super-dense ball of matter, created when a massive star collapses at the end of its life.
But what then causes the sharp, regularly repeated bursts of radiation from which pulsars derive their name? Astronomers believe that a pulsar, like the sun and most other stars, is subject to violent storms which may last for years, spraying particles and radiation out into space. Each storm occurs in a localized area on the surface of the pulsar and sprays its radiation into space in a narrowly defined direction. When the earth lies in the path of one of these streams of radiation, our radio telescopes pick up the signals, which indicate to us the presence of the pulsar. The explanation for the periodic bursts of radiation is then that the pulsar is spinning about its axis, and as it spins the stream of radiation from its surface sweeps through space like the light from a revolving lighthouse beacon. If the earth happens to lie in the path of the rotating beam, it will receive sharp bursts of radiation once in every rotation of the pulsar. And because a pulsar (or neutron star) is very small, it may rotate at a high rate of speed. For the pulsar PSR 1937+21, found in the Crab Nebula, this is a rotation of one hundred times per second.
Currently more than 1820 pulsars have been identified in the heavens around us—very small objects with extremely sharp, regular radio pulses. This in itself is rather a puzzle. The radio signals from pulsars are quite weak, and we are only able to detect those which are relatively close to the earth. Thus there must be a very large number out there which we cannot detect. Even more astonishingly, if we only see those quasars whose spin axes are roughly perpendicular to the earth/pulsar line, as the lighthouse analogy for the pulsing would require, then there must be a very large number of pulsars in our immediate vicinity which we do not detect because we do not fall in the discharge area of their rotating radio beam. If the rotating radio beam model is correct, then pulsars are some of the most familiar objects in our surroundings. It is estimated that there are as many as a million pulsars in our galaxy.
An additional question arises from the large number of pulsars observed. If neutron stars are only caused as the result of a nova or supernova, there must have been very many such events in the past. And yet such events are relatively rare. This raises the question of how so many neutron stars could have formed, and whether they are really the cause of pulsars.
The pulse rate of the first pulsar discovered, in the Crab Nebula, was 0.033 seconds, or about 100 pulses per second, which translates into a rotational rate of 100 rotations per second. That is, the neutron star thought to be the source of pulses is believed to rotate 100 times a second, or 60,000 revolutions per minute (RPM). Remember the old time records at 33 rpm and 45 rpm. The neutron star at the center of the Crab Nebula, 10 miles in diameter, is thought to be spinning 2000 times faster than those old-timey records, which were just a few inches in diameter. But that’s nothing!
Since the first pulsar was discovered, a large number of what are known as millisecond pulsars have been found—object with extremely short periods. Thus while many pulsars have a pulse rate of one half to several seconds, the millisecond pulsars pulse at far higher rates—up to 716 times per second.
There appears to be two different types of pulsars. The slower pulsing pulsars have a relatively high velocity through space—around 400 km per second, while the millisecond pulsars move at a more leisurely and normal, 90 km per second. No reason for this difference is known.
Let’s think about this for a moment. The fastest pulsing pulsar found so far has a period of about 0.0014 seconds per pulse, or 716 pulses per second. If we attribute the pulses to rotation of the neutron star, then it is rotating at a rate of 43,000 rpm. Taken alone this rotation rate is very high, but not exceptional. Laboratory centrifuges are available which spin at over 100,000 rpm, and flywheel generators are being designed for hybrid automobiles for rotation rates about this figure. But these devices are a few feet in diameter at most, and automobile rotors have a limiting circumferential velocity of about 6000 feet per second before they begin to disintegrate. But a millisecond pulsar is supposedly several miles in radius, and at 716 revolutions per second the circumferential velocity would be nearly one half the speed of light!
To accelerate an object with a mass larger than the sun to such fantastic radial velocity (called spin-up) would take enormous energy, the source of which is a complete mystery to astronomers. To make matters worse, their spin-down rate—the rate the millisecond pulsar spin is slowing, is extremely small (almost zero!), suggesting that these objects are perhaps two to three times as old as the currently estimated age of the universe. And if these objects are not slowing down appreciably, where does the energy to create the energetic emitted radio beam come from? Emitted energy would necessarily result in a loss of mass and a resultant change in spin rate. But this is not observed!
Some of the characteristics of millisecond pulsars in particular, and all pulsars in general, raise questions, at least to me, about rotation and the lighthouse effect as the cause of the pulsing observed. Therefore I would like to raise the possibility of a different means for producing the observed effect from neutron stars—a new concept stemming from our discussions on the nature of gravity.
In another segment we described how the gravitational forces acting on an atom may be related to the relative freedom of the atomic nucleus to move about uninhibited by the swarm of electrons normally surrounding it (the Nuclear Binding Theory of Gravity presented elsewhere). That is, unconstrained atomic nuclei have large gravitational forces, while those constrained from moving have lower gravitational forces. With this in mind we might question what happens gravitationally when all the molecules of a star are squeezed together until there is virtually no possibility of movement on the part of an atomic nucleus (i.e. become a neutron star).
We have always assumed that atoms retain their same gravitational forces no matter under what conditions the atoms find themselves, but this assumption is not founded in experimental evidence. It has just been taken for granted since Newton first put forth his theory of gravity. Suppose, instead, that during the collapse of a star, in the brief period before total collapse, the molecules are highly ionized, and therefore gravitationally very active. This intense gravitational force then accelerates the star’s collapse into a super-dense neutron star.
But let us suppose now that when the star has collapsed to become a neutron star, it essentially loses its gravitational power because the atomic nuclei that hold the key to its gravitational forces become immobile. Its gravitational field suddenly becomes much weaker than it was just before collapse. Now, with insufficient gravitational force to hold the atoms together into their extremely dense form, the star rapidly expands—literally explodes—from the strong repulsive forces within its core that are no longer held in check by gravity.
Immediately after the neutron star explodes, it once again becomes a light-emitting star again, and because hordes of free atomic nuclei are released during the explosion, it becomes gravitationally active again as well. With intense gravitational forces again present, the star once more collapses to become a neutron star, and the cycle repeats.
In other words, the wax and wane of the gravitational forces near the core of a neutron star cause it to pulsate, much like a human heart, but much faster, and it is this regular pulsation we observe, instead of rotation! The sharp pulse of radio energy observed at regular intervals would then probably be generated during the periodic explosion from compact star to normal star.
It may be a totally unrelated fact, but in 1976 a Soviet research team and a group from Birmingham, England independently announced that the sun oscillates with a period of 2 hours, 40 minutes! Wouldn’t it be interesting if experiments found that the sun’s gravitational field varied as well. In another segment we will find just such an effect!
The concept of a neutron star pulsation also gives rise to something new—a pulsating gravitational force. The gravitational attraction of such an object would vary according to the part of the pulsation cycle it was measured in. Measured from a distance over a period of time, the gravitational force would be the average forces present—somewhere between the lowest force, when all the atoms are packed together—to the largest forces when the neutron star had exploded and the atomic nuclei were again highly ionized.
The concept of a pulsating neutron star as the cause for pulsar observations is a highly attractive alternative to the rotating beacon effect. It eliminates the need for excessive spin rates, energy production which is not supported by any apparent loss in mass, the missing spin-up energy, and the extremely low spin-down rate.
Interestingly, we have painted ourselves into a box—a little black box, if you will. For if a neutron star loses its gravitational force when atoms are squeezed so tightly they cannot move, then what would happen in the core of a black hole?
The very same thing!
In other words, there could never be such a thing as an immutable black hole—it too would necessarily pulsate, for when it reached its greatest density, it would lose the gravitational attraction needed to maintain its minimum size, and would then expand. In fact, we would be hard pressed to know if a pulsar were a pulsating neutron star or a pulsating black hole—the effect would be essentially the same. And since astronomers have maybe identified only one object they are pretty sure is a black hole, perhaps this is the reason. Just maybe there is no such thing as a stable black hole!
· The source of pulsed radio energy observed in Pulsars, thought to be rotating neutron stars, may be really from a gravitationally induced pulsation of otherwise normal neutron stars or black holes.
· It is possible that black holes and neutron stars cannot exist as stable objects, since they may lose their gravitational attraction when they are compacted to their greatest density.