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17.25 The Forces of Nature

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Location of galaxies as a function of red shift from the 2df galaxy survey. Click here for original source URL


For a deeper knowledge of the very early universe, we must consider the fundamental forces of nature. Like the ancient Greek thinkers before them, modern scientists hope to discover the simplest possible description of the physical world. The scientific method is based on the belief that diverse physical phenomena have a simple underlying basis. Think of the rich diversity of structures in the cosmos, from the swirl of spiral galaxies and the mystery of black holes to the filigree of arcs and voids in the distribution of galaxies. The single force of gravity has shaped all of these structures. Think also of the variety of the material world, from lustrous precious metals and inert and colorless gases to the radioactive fizzing of heavy elements. Atomic forces yield this variety.

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An overview of the various families of elementary and composite particles, and the theories describing their interactions. Fermions are on the left, and Bosons are on the right. Click here for original source URL.

The quest for symmetry continues with the fundamental forces of nature. There are four forces in the physical world. The two with which we are most familiar — gravity and electromagnetism — operate over an infinite range and contribute to the structure of the macroscopic world. The electromagnetic force is 1028 times stronger than gravity. If you doubt this, take a balloon and notice how the slight static charge of the balloon rubbed against your head will hold it on the ceiling against the full attractive force of the entire Earth. Similarly, a modest magnet holds a nail against the Earth's gravity. In normal matter, electromagnetic forces are literally neutralized by the equal numbers of negatively charged electrons and positively charged protons.

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The Rutherford model of an atom. Click here for original source URL.

The other two forces of nature operate over very short distances within an atom. The weak nuclear force is actually far stronger than gravity, but its range is 100 times smaller than an atomic nucleus. It is responsible for the radioactive decay of massive nuclei and for the decay of neutrons when they are not locked in atomic nuclei. The strong nuclear force has a range about the size of an atomic nucleus, and it acts to bind neutrons and protons inside a nucleus. To do so, it must overpower the electromagnetic force, which would otherwise cause the positively charged protons in a nucleus to repel each other. Within an atomic nucleus, physicists have shown that protons and neutrons are made of even smaller particles called quarks.

The properties of the four forces of nature are summarized below:

• Strong nuclear force: relative strength = 1, operates on quarks, holds atomic nuclei together

• Electromagnetic force: relative strength = 10-12, operates on all charged particles, holds atoms together, controls EM waves

• Weak nuclear force: relative strength = 10-14, operates on quarks and electrons, involved in radioactive decay

• Gravity: relative strength = 10-40, operates on all particles, governs motion and structure of planets, stars, and galaxies

The four forces of nature could not appear more different. Two forces have infinite range and two have short range. The forces are transmitted by particles with different masses and they differ by a factor of nearly 1040 in strength! The ultimate expression of physicists' belief in symmetry is the search for unification a theory demonstrating that the four forces of nature are manifestations of a single unified force.

Our low-energy world may represent a situation of "broken" symmetry, in which the underlying similarities between forces and particles are concealed. Imagine spinning a roulette wheel. At first, the ball has high energy and moves in circles. At the end, when the wheel has stopped, the ball rests in one of 37 numbered slots. We might watch the results of many spins and conclude that the ball has 37 different states. However, at high energy the ball always has the same motion; the symmetry is broken only at low energy. For another example, imagine a large number of pencils balanced on their flat ends. Viewed from above or the side, they present a picture of uniformity or symmetry. However, if they all topple (a state of lower potential energy), the result would be a tangle of skewed and overlapping pencils with no symmetry at all. We find ourselves in the low-energy worlds, staring at the jumble of forces and subatomic particles, trying to imagine the simplicity of the high-energy universe.

Physicists took the first step toward unification in the 1970s with a theory that united the electromagnetic and weak nuclear forces. This electroweak theory was convincingly confirmed by experiments using particle accelerators in 1983. In the early universe, the electromagnetic and weak nuclear forces were unified at temperatures above 1015 K, or a time within 10-12 seconds after the big bang. In the present-day universe, the symmetry is broken and the two forces appear quite distinct. Emboldened by this success, theorists have attempted to bring the strong nuclear force under the umbrella of unification in grand unified theories. (We use the plural "theories" because physicists have come up with several versions of unification.) There is a theoretical expectation of the energy or temperature at which the forces of nature become unified. The weak, strong, and electromagnetic forces are predicted to be equal at a phenomenal temperature of 1028 K, corresponding to a time only 10-35 second after the big bang! This temperature is far beyond the range of any existing or planned particle accelerator. The only possible laboratory for grand unified theories is the very early universe.

Grand unified theories might clear up several of the mysteries of the big bang. Experimental physicists discovered a slight asymmetry between the behavior of matter and antimatter in 1964. Grand unified theories predict an excess of matter over antimatter at temperatures near 1028 K, but those theories give no concrete prediction of the amount of the excess. Nevertheless, it is exciting when theories invented to explain a laboratory phenomenon might explain why the universe contains far more matter than antimatter.

Grand unified theories also contain an insight into why inflation might have happened. As the universe passed through the temperature at which the strong force separated out from the weak and electromagnetic forces, it changed its state — much in the way that water changes its state when it freezes to become ice. When water freezes to become ice, energy is released. (The analogy even extends to the loss of symmetry at lower temperatures. Water molecules move freely and point in all directions, while ice crystals are aligned in particular directions.) The very early universe cooled so quickly that it borrowed energy from the vacuum of space. The law of conservation of energy can be broken for tiny periods of time — these momentary violations of the physics of the everyday world are called quantum fluctuations. Borrowed energy drove the sudden inflation of the universe 10-35 seconds after the big bang. When the strong force became distinct, the energy from that transition flooded into the universe as 1089 photons, and the expansion continued more leisurely. This scenario sounds truly fantastic, but one of its predictions has already been confirmed. Inflation takes quantum fluctuation and boosts them into the seeds of large-scale structure. Theory predicts that there is no preferred size of a quantum fluctuation; they should occur equally on all scales. Observations with the WMAP satellite showed that the tiny fluctuations in the microwave background have exactly this property.


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Temperature map of the universe, as measured by WMAP. Click here for original source URL.