Extended Reading Material

Are the laws of nature changing with time?
Feature in Physics World: April 2003

Precise measurements on the light from distant quasars suggest that the value of the fine-structure constant may have changed over the history of the universe. If confirmed, the results will be of enormous significance for the foundations of physics

WHAT do we mean by "the laws of nature"? The phrase evokes a set of divine and unchanging rules that transcend the "here and now" to apply everywhere and at all times in the universe. The reality is not so grand. When we refer to the laws of nature, what we are really talking about is a particular set of ideas that are striking in their simplicity, that appear to be universal and have been verified by experiment. It is thus human beings who declare that a scientific theory is a law of nature and human beings are quite often wrong.

The development of a scientific theory has always followed the need to understand an observation for which no satisfactory explanation previously existed. When developing new theories, physicists tend to assume that fundamental quantities such as the strength of gravity, the speed of light in a vacuum or the charge on the electron are all constant. And when these theories are found to predict the results of new observations, our belief that these quantities are actually fundamental constants becomes even stronger.

Moreover, despite the rapid changes in technology in recent decades, the timescale on which fundamental new discoveries in physics are made is typically comparable to a human lifespan. This means that theories developed decades ago can appear as if they have been carved in granite.

Fundamental constants

The end result is a natural reluctance to change our understanding of the world. But it is vital to remember the limitations that have been involved in testing these assumptions. Many of the experiments we carry out to test theories are restricted to the here and now to Earth-bound research labs or to the small part of the universe that we can observe with telescopes. If we could somehow do our experiments in a different place or at a different time, we might well find that the results are different. Indeed, that is what appears to happen when we measure something called the fine-structure constant in the very distant past.

What is the fine-structure constant?

Have the laws of nature remained the same since the Big Bang some 13.5 billion years ago? Paul Dirac first posed this question in 1937, and he was still interested in this idea when he visited the University of New South Wales (UNSW) in Sydney in 1975 where I am now based. Dirac attempted to link the strength of gravity, which describes the large-scale properties of the universe, with the various constants and numbers that characterize the small-scale properties of the universe. In doing so, he claimed that one of the constants of nature, the strength of gravity, should change with time.

Although observations subsequently ruled out Dirac's ideas, advances in many areas of physics and astronomy have resulted in a whole new set of opportunities for us to search for any hint that the constants of nature might vary. The particular question that I have been vigorously pursuing with colleagues at UNSW and elsewhere can be stated as follows: is the fine-structure constant really constant, or has its value changed over the history of the universe?

The fine-structure constant, a, is a measure of the strength of the electromagnetic interaction, and it quantifies the strength with which electrons bind within atoms and molecules. It is defined as a = e2/h bar c ˜ 1/137, where e is the charge on the electron, h is Planck's constant divided by 2p, and c is the speed of light in a vacuum. The fine-structure constant is of particular interest because it is a dimensionless number. This makes it even more fundamental than other constants such as the strength of gravity, the speed of light or the charge on the electron (see Constants with and without dimensions).

There are theoretical reasons why a and other dimensionless constants might vary with time. The holy grail of theoretical physics is to find a single unified theory that describes the four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. Although the strengths of these four forces differ, as do the distances over which they operate, most physicists believe that a unified theory will be discovered. If such a theory is not found, a great deal of the elegance and beauty of fundamental physics will be lost.

Simulated quasar absorption spectrum

Einstein's theory of gravity the general theory of relativity only requires three spatial dimensions. However, the leading contender for a unified theory requires extra dimensions beyond our familiar three. We do not know if these unified theories are correct, but if extra dimensions do exist, they must be tiny compared with our ordinary spatial dimensions.

The concept of attributing a "size" to a dimension may seem strange but it is important. The current size of the universe is determined by the distance that light has travelled since the Big Bang (i.e. about 13.5 billion light-years) and by the amount by which it has expanded since then. This means that the actual size of the universe is about 40 billion light-years and rising.

Are the extra dimensions predicted by unified theories also expanding at the same rate as the universe? The answer to this question is no. If the tiny extra dimensions were expanding at this rate, then the strength of gravity would also be changing very rapidly, and there is no evidence for this. However, it may be possible to infer the presence of these extra dimensions if they exist by detecting small changes in the strength of gravity or the other three forces.

It has been predicted, for instance, that "large" extra dimensions might cause small deviations in the inverse-square law of gravity over distances of less than 1 mm. However, recent measurements by John Price and co-workers at the University of Colorado at Boulder have failed to find any evidence for this over distances of about 100 µm (J C Long et al. 2003 Nature 421 922). This is just one of many experiments that have been set up to perform high-precision tests on constants, forces and fundamental symmetries in recent years.

There are several ways to measure possible changes in a with time. We can measure the absorption spectra of quasars at different redshifts, as we have done at UNSW. We can compare the "ticking rates" of atomic clocks made of different elements (see Searching for changes in the fine-structure constant using atomic clocks). We can also study the cosmic microwave background or the creation of the elements in the early universe. However, one of the first methods used to probe how a might have changed over the past two billion years relies on what must be one of the most unusual processes ever studied by physicists the so-called natural nuclear reactor at Oklo in Central Africa.

<The strange story of the Oklo reactor

Natural uranium contains two isotopes. Uranium-235, the isotope that is useful for nuclear energy, is relatively rare and accounts for just 0.7% of all natural uranium. Its less-radioactive sibling, uranium-238, makes up the other 99.3%. In 1972 scientists from the French atomic energy commission noticed something mysterious in soil samples taken from a uranium mine in Gabon in Central Africa: the relative abundance of uranium-235 was a factor of two lower than expected.

One possibility was that a band of hi-tech terrorists had been stealing and stockpiling the missing uranium for purposes even more evil than blowing up innocent atolls. However, isotopes of various other elements also appeared to be depleted in a pattern that was strikingly similar to that observed among the waste products from modern nuclear reactors. The most plausible explanation is that there must once have been a "natural" nuclear reactor at Oklo. Although natural nuclear reactors were predicted by Paul Kuroda of the University of Arkansas as long ago as 1956, Oklo is the only known example (see image).

Natural wonder

What appears to have happened is that oxygenated water slowly dissolved the uranium-235 that was stored in surface rock about two billion years ago. Back then the natural concentration of uranium-235 would have been about 3% it is much lower now because uranium-235 decays about six times faster than uranium-238. Over time the uranium-235 would have become concentrated in nearby algae mats, which acted as filters, and eventually enough of it would have collected to reach criticality and form a natural nuclear reactor. This reactor would have "burned" the uranium-235, thus explaining the low levels of the isotope found at Oklo.

But what has this got to do with a? In 1976, four years after the Oklo reactor was discovered, Alexander Shlyakhter of the Leningrad Nuclear Physics Institute made the connection. Samples from Oklo revealed a relative abundance of samarium-149 that was a factor of 45 lower than other terrestrial samples and Shlyakhter showed that ambient neutrons could convert samarium-149 into samarium-150 if they had exactly the right energy.

This resonance was due to a delicate balance between the strong nuclear force and the repulsive electromagnetic force in samarium. Moreover, the resonance energy depended on a, so if the value of a was different two billion years ago, then the depletion of samarium-149 would also have been different. The details of the calculation are complicated, but they show that any fractional change in the value of a since the time that Oklo was active cannot be greater than 10-7 (see Olive et al. in further reading).

Very recently a new geological measurement technique known as "rhenium dating" has produced potentially even more stringent results. The ages of iron meteorites obtained using rhenium dating are consistent with those found by other methods. From this we can show that the beta-decay lifetime of rhenium cannot have changed by more than 0.5% over the age of the solar system. This translates to an upper limit on any fractional change in the value of a of the order 107 over about 4.6 billion years.

While 4.6 billion years is a long time, the universe itself is about 13.5 billion years old. Is it possible to test for changes in the value of a even earlier in the history of the universe? The answer is yes with the help of quasars.

Using quasars to look at the fine-structure constant

Quasars are compact but highly luminous objects. Indeed, they are so luminous that they can be studied in intricate detail using ground-based telescopes despite being vast distances away from us. We think that quasars contain black holes at their centres and that the immense gravitational force exerted by the black hole is extremely efficient at converting matter in its vicinity into light.

Nature kindly co-operates by scattering quasars throughout the universe. Since quasars are seen in all directions in the sky, they provide a powerful way of charting almost the entire universe. And, like any astronomical object, whenever we look at a quasar we see it as it was in the past. We see the Sun as it was just over eight minutes ago because that is how long it takes the light from the Sun to reach the Earth. Similarly, some quasars are so far away that we see them as they were billions of years ago. Indeed, by observing quasars we can build up a continuous "universal history" that starts when the universe was only about one billion years old and continues up to the present day.

However, we cannot study a with any reasonable precision using the quasars themselves. Rather, we must examine what happens when the radiation from a quasar passes through a galaxy that lies between the Earth and the quasar. The quasar emits light over a broad range of wavelengths (figure 1). However, when this light passes through the gas around the galaxy, a characteristic pattern of absorption lines will be superimposed on it.

The presence of an absorption line at a particular wavelength reveals that a specific element is present in the gas cloud, and the width of each line shows the quantity of the element that is present. In addition to hydrogen, which is ubiquitous in the universe, these "bar codes" reveal that the gas clouds contain a range of other elements, including magnesium, iron, zinc, silicon, aluminium and chromium.

Moreover, the bar code reveals what was happening when the light passed through the cloud, which could have happened as long ago as just one billion years after the Big Bang. Although the gas cloud would have evolved into something quite different by today, its bar code provides us with a permanent imprint of its state in the distant past including information about the value of a at that time.

Therefore, if we compare the bar codes that we find in quasar absorption spectra with the bar codes we measure for the same atoms and ions in the laboratory, we can find out if the physics that is responsible for the absorption of radiation by atoms has changed over the history of the universe. In other words, we can find out if a has changed.

Author: John Webb