Saturday, 28 August 2010
Are Earth and Moon perpetual-motion machines?
One of the oldest and most fascinating exhibits included in Heureka’s main exhibition is the “perpetual-motion machine”. With the exception of a few breaks for maintenance, it has been spinning without interruption since 4 October 1990. The secret behind this “perpetual-motion machine” is known only by its designer, David Jones of the UK, and a select group among the maintenance staff. Wild and less wild guesses about the origin of the energy needed by the device have been presented, starting from a conventional “manget” and ending with the last explanation for every unfathomable Heureka exhibit: “It’s a sham.”
Heureka’s “perpetual-motion machine” is no sham; it has an energy source that has been hidden very cleverly. However, the device may evoke the question that, even if it is not possible to build a perpetual-motion machine on Earth, would such a thing work in space? Is Earth on its orbit round the Sun, and the Moon on its orbit round Earth, perpetual-motion machines? At least these “devices” have been working for a long time. Earth has rotated round the Sun for 4.5 billion years and the Moon has rotated round Earth for only about 50 million years less.
From the human viewpoint and time perspective, Earth and the Moon seem perpetual enough. Poets have written about the Moon’s eternal journey around Earth. It is therefore safe to predict that, at least not with our own eyes, we shall never see an end to Earth’s orbiting round the Sun, or the Moon’s orbiting round Earth.
Before studying this issue in more detail, we should determine what is meant by a perpetual-motion machine.
In the above link, item 3 would seem to describe the orbiting Earth and Moon very well. They are bodies that do not consume energy, although they are subject to forces – above all gravitation, the force keeping the entire universe together.
How, then, can Earth orbit round the Sun and the Moon round Earth forever, even though they are both affected by a strong gravitational force? Will they actually rotate forever?
The direction of gravitation to the object revolving round a central body is always at right angles to the direction of motion. Gravitation does not slow the speed of the body; instead, it turns the direction. For this reason, gravitation is a force that does not reduce the orbiting body’s kinetic energy.
In fact, gravitation is the force that keeps the body on its orbit. Without gravitation, bodies would be thrown off their orbits in the same way as a discus leaves an athlete’s hand when he releases it.
The other forces resisting the motion of an orbiting body in space are so small when compared against the body’s kinetic energy that their effect is non-existent even over a period of millions of years, let alone during a person’s lifetime. Changes are mainly caused by objects that hit Earth and the Moon − meteors and meteorites, as well as large objects that pass the orbiting bodies close enough. The former have very little impact, while the latter are fortunately very rare. Moreover, since both factors may increase or decrease the orbiting body’s speed, their impacts offset each other in the long term.
Seen from the human perspective, Earth and the Moon on their orbits are perpetual-motion machines, even though one day their motion will stop. Physical and human perpetuity are measured on different scales.
Earth has two types of energy. One is kinetic energy that gives Earth speed and prevents gravitation from pulling Earth directly into the Sun’s fiery furnace. The second type of energy is bound to Earth’s rotation. The same energies affect the Moon as well. The bodies received both types of energy when the Solar System was under formation. Fundamentally, the energy was generated in the Big Bang.
The Moon, Earth and the Sun interact with each other through gravitational force. This causes a phenomenon that can be quite significant in some areas on Earth: tides.
Tides contain energy that can even be utilised and that is released constantly into moving water when tidal levels vary. Energy is not generated from nothing. Where does the energy of tides originate?
Mostly from the rotation energy of Earth and the kinetic energy of the Moon. This also means that the rotating speed of Earth, as well as the speed of the Moon on its orbit, must gradually slow down. Can these phenomena be observed and what do they mean?
They can be observed, but not with human senses. Earth’s rotational speed decreases by some 1/1,000 of a second per year. In concrete terms, this is seen as a small element among the factors that make it necessary to add leap seconds to high-precision chronometers every few years. Each 24-hour period becomes slightly longer when the rotation speed decreases. Moreover, large-scale natural phenomena may also affect Earth’s rotation. The earthquake that caused the tsunami in the Indian Ocean at Christmas 2004 accelerated Earth’s rotation by about 3/1,000 of a second per year.
Thus, slowing of Earth’s rotation is on the order of about one millionth of a second in 24 hours. This means that the difference will become visible only after millions of years. However, some studies of fossils have shown clearly that Earth used to rotate more quickly, and that the day was correspondingly shorter in the distant past.
Because Earth and the Moon interact with each other, the Moon’s rotation is also slowing down. According to the laws governing rotation, this should be visible in the Moon’s orbit, which should spiral outward from Earth. The first astronauts on the Moon installed a mirror that reflects a laser beam sent from Earth. From the laser beam’s travel time, it is possible to calculate the distance between Earth and the Moon, and any changes in it, very accurately.
At present, the Moon is about 384,000 kilometres from Earth. According to measurements, the distance lengthens by about 3.2 centimetres per year. When the Moon was young, it was much closer to Earth than now. For instance, 4 billion years ago it was 128,000 kilometres closer than at present. Solar eclipses were more common then and extended over larger areas than today. Everything was better in the past!