| The Dominance of Gravity.
Why is Gravity Dominant? The Story of Four Forces.
Although no one would do so very seriously, one could define astronomy as the science of objects which are big enough to be dominated by self-gravity. This is clearly inadequate! Among other things, we need to study the innermost parts of atoms, the nuclei, so that we can understand how nuclear reactions provide the energy of the stars; and the clouds of electrons around atoms explain much of what we will learn about the light emitted by hot objects. So extremely small things, as well as large, must be part of our study. Nevertheless, it is not much of an exaggeration to say that in astronomy, gravity drives everything.
At first glance, this is surprising. There are four known forces in physics, and of these gravity is by far the weakest. Let us consider them (briefly) in turn.
1 The electric force. This is the force which makes two particles repel each other if they have the same charge (like a pair of electrons, each of which carries a negative charge) or attract each other if they have different charges (like an electron and a positively-charged proton). This is the force which controls our day-to-day lives: the electrical attraction and repulsion between the electrons and protons in the atoms which make up our bodies hold all those atoms, and those of the materials around us, in a careful balance. It is the force which maintains the crystal rigidity of a mineral or rock, and which explains the structural strength of a piece of bone or a tree branch. In chemical reactions, such as the burning of coal or the much slower `burning' (metabolism) of food to provide us with energy, electrical bonds are broken and reattached in new ways as the atoms regroup to form different compounds: for instance, the carbon in coal combines with oxygen atoms to form carbon monoxide and carbon dioxide exhaust, releasing energy in the process.
2 The `strong' force. Given the existence of an electric force, you may be wondering how an atom can hang together at all. After all, in the very centre or nucleus of the atom, one usually finds many positively-charged protons. (The exception is the element hydrogen, which has only one proton in its nucleus; all other elements have two or more protons. Oxygen, for instance, has eight protons.) Yet the nucleus is tiny, and the protons must be in very close proximity. Why doesn't the electric repulsion between all these positively-charged particles blow the whole nucleus apart? The answer is that at these very tiny distances, there is another force, the so-called strong force, which provides a mutual attraction which allows the nucleus to hang together.
3 The `weak' force. This is a force which determines some aspects of the radioactive decay of sub-atomic particles, in ways I will describe briefly a little bit later in the course and spend a fair bit of time on in the companion Phys 016 ('Stars and Galaxies') course. It is a topic of particular interest here at Queen's since the weak force is involved in the production of special sub-atomic particles called neutrinos, which are thought to be emitted from the core of the sun in prodigious numbers as a result of the thermonuclear reactions taking place there. The Physics Department at Queen's is the headquarters of an international experiment called the Sudbury Neutrino Observatory (the SNO project), an experiment in which, believe it or not, a special kind of `telescope' has been built deep in a mine in Sudbury to detect the neutrinos which pass essentially unimpeded through the miles of rock overhead. I will describe this in more detail in Phys 016 at the time that I describe the structure of the sun and stars and their sources of energy.
4 Gravity. Most of you have a pretty good idea, intuitively at least, of the nature of gravity. As the course progresses I will discuss Isaac Newton's introduction of the notion of universal gravitation and the law which governs its behaviour, and also tell you about how diffferent our modern understanding of gravity is, thanks to the brilliant insights provided by Einstein in the twentieth century. For the moment, let me merely note that gravity is by far the weakest of the four forces I have been describing. It is weaker than the electric force, for example, by a factor of ten-to-the-fortieth-power.
What exactly does this mean? The easiest way to understand this is to imagine, say, a pair of electrons sitting near each other in empty space. Since each of them has some small mass, they attract each other by gravity, and might be expected to slowly approach each other under the influence of this force. But they are also charged particles, each one bearing a negative charge: thus they also repel each other. The important point is that the repulsive force which the electrons feel is roughly 10**(40) times as strong as the attractive gravitational force - an absolutely enormous factor of about
"10 000 000 000 000 000 000 000 000 000 000 000 000 000!"
In other words, the effects of gravity are utterly negligible in such circumstances. But on very large scales, as we have noted, gravity dominates the behaviour of matter. How?
The answer is that gravity is cumulative and unscreened. By cumulative, I mean that if you gather bits of matter together, their gravitational effects always add up. This is not true for the electric forces of large lumps of ordinary matter. Think of the moon and the Earth, for instance. Each of them contains trillions upon trillions of electrons, and you might expect these two large collections of electrons, even though separated by a quarter of a million miles, to feel an enormous mutual repulsion. But the atoms which make up the Earth and moon also contain many positively-charged protons (indeed, most objects are electrically neutral, by and large, containing equal numbers of electrons and protons). Since the protons in the moon attract the electrons in the Earth with as strong a force as the electrons repel them, and vice versa for the protons in the Earth, the net electrical effects are negligible. But the enormous amount of matter in each of the moon and Earth leads to a large net attractive gravitational force between these two bodies. Indeed, as we noted in an earlier lecture, the self-gravity of each of them (the gravitational effect of the total lump of matter on its own bits and pieces) explains their spherical shape, among other things.
By unscreened, I mean that nothing you can put between the Earth and the Moon, or between any two bodies at all, can block the influence of gravity in the way that an opaque screen can stop light from passing from one body to the other. Some of you may know of an early science fiction novel called `The First Men in the Moon,' by H.G. Wells. In that novel, a scientist named Cavor invents a material, Cavorite, which acts a gravitational screen: if you sit on a slab of it, for instance, you no longer feel the Earth's gravity and can float up into space. This is how Wells's intrepid explorers reach the moon. But the modern understanding of gravity absolutely precludes such a device, especially in the way in which gravity is understood following Einstein -- something we will explore later.
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0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.
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(Monday, 17 May, 2021.)