Science Item: The weight of a pound of feathers     [1986]

[This item, which was a response to  recent scientific speculations about anti-matter, was carried in the February 1986 issue of the “Irish Democrat”, under the editor’s regular pseudonym“Feicreanach”,whichbis  Irish for “insightful”. Desmond Greaves was a regular reader of the “New Scientist” for much of his life.]

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Scientists are always cooking up something new, and it’s no wonder the Pope sometimes looks askance at them, as in his recent warning against atheism.

Do you remember the old school joke, “Which weighs more, a pound of lead or a pound of feathers?”  You were supposed to say a pound of feathers and so be the April fool.

Well don’t be so sure. The heavier may be the feathers if some work carried out in Seattle has any foundation.

It’s all about gravity. An experiment done a few centuries ago was to drop two heavy objects, one a few times the weight of the other, over the side of the leaning tower of Pisa. They both hit the ground at the same moment.

Of course they went faster and faster as they went down and their acceleration under the influence of gravity was basically due to the vast mass of the Earth attracting what were after all very small objects in comparison.

This attraction affects the swing of a pendulum and made possible the old-fashioned grandfather clock which keeps constant time because the acceleration due to gravity never changes.

The Seattle scientists thought they’d like to make quite sure and checked the work of a Hungarian experimenter of the early part of this century, who experimented (I oversimplify here) with pendulums with knobs made of different substances.

They did not swing at the same rate, but he thought this was due to experimental errors. Now his results have been looked at again and the Americans have spotted a remarkable coincidence. A pendulum made of lead will beat slower time than one made of feathers. And it will be slower in proportion to its chemical composition. Every atom of lead, uranium, osmium and other heavy metals is packed with protons and neutrons. And the deviation from standard time is in proportion to this property of the atoms.

Well, why is that? Have we discovered the science-fiction writer’s dream of anti-gravity that drives space-ships helter-skeler  through the universe?  Or is there here the discovery of an entirely new force, a new repulsive property of matter?

Gravity itself is a very weak force.   You can see that from the flight of birds and how little children can jump up and down. They get their energy from electrical forces which are strong. But if this is a new force it is a  hundred times weaker than gravity. So no wonder it was not noticed.

It’s enough to set us all speculating. Relativity theory can, one supposes, be regarded as starting in Trinity College, Dublin, when Fitzgerald showed that a perfectly round bullet fired in space will contract in the direction it is fired in and thus become slightly egg-shaped [George Francis Fitzgerald, 1851-1901, renowned physicist].

But Einstein, who accepted that the Hungarian’s deviations were errors, transmuted Newton’s theory of universal gravitation into a vision of the whole universe, which science is constantly taking further.

Halley’s comet is proving a poor show. But there’s a grand show you can see any clear night once you get out of the sodium lighting, though springtime is best. It is the enormous circle of the Milky Way bridging the whole sky. It is composed of millions of stars, the bright ones mostly near, the faint ones mostly far away. Actually the sun is part of it and takes thirty million years to go round it. But it’s not the only galaxy. There are thousands of the things, those visible with the naked eye looking like little smudges.

Now all these galaxies seem to be flying apart.  So at one time they must have been all bunched together. Hence the theory that the whole thing began with a great explosion. Then comes the question of where will it end? For years it was thought they would fly apart forever. But more recently the view has gained ground that they will get to a maximum distance and then tumble in on us. The universe is so constructed that wherever you are you think you’re at the centre of it, but we’ll not go into that. Actually this is true of the surface of the earth, and when people thought it was flat it was all very puzzling to them.

The question is of course what decides whether the galaxies go on flying apart, or tumble in on us?  Obviously it is the strength of gravitation. This depends on the total amount of matter in the universe, and even with the strongest telescopes it is not possible to see more than a tenth of what would be needed for the great rebound.

But what about matter we can’t see?  Evidence for this is mounting and gradually it is being winkled out of its hidey-holes and measured up. It looks as if the amount of matter in the universe is just about the right amount to bring it all back to where it started and begin the whole process again, a cycle involving perhaps millions of millions of years.

(At this point I throw in the parenthesis, What for?  There is only one answer, however  you take it. . . . God knows!)

But people have argued that to explain the explosion that is supposed to have started everything off it is necessary to assume anti-gravitation, a mighty repulsive force. And if  it all started from a great black hole where protons and neutrons (nucleons  as they are called)  were tightly packed together in almost infinite numbers, then this new weak repulsive force might be very strong indeed.

This is a course sheer speculation, and there are those with the expertise necessary to speculate professionally. But if there is anything in the studies that have been made, it shows that nothing in physics can be taken for granted and what a vital starting point is careful experimentation.

So now which is heavier, a pound of feathers or a pound of lead?