Date: October 17, 2010

Title: Can the Laws of Physics Change?

Podcaster: Stuart Clark

Description: Can the laws of physics change, and if they do what does this mean for our understanding of the Universe?

Bio: Dr Stuart Clark is an award-winning astronomy author and journalist. His books include The Sun Kings, and the highly illustrated Deep Space, and Galaxy. His next book is Big Questions: Universe, from which this podcast is adapted. Stuart is a Fellow of the Royal Astronomical Society, a Visiting Fellow of the University of Hertfordshire, UK, and senior editor for space science at the European Space Agency. He is also a frequent contributor to newspapers, magazines, radio and television programmes. His website is www.stuartclark.com and his Twitter account is @DrStuClark.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by AAVSO. The American Association of Variable Star Observers (AAVSO) is a worldwide, non-profit scientific and educational organization of amateur and professional astronomers interested in stars that change in brightness–variable stars.

Founded in October 1911 to coordinate variable star observations made largely by amateur astronomers for Harvard College Observatory, the AAVSO has grown to become the world leader in variable star astronomy, with members in 45 countries and an archive of over 17 million variable star observations.

As we begin our 99th year, the AAVSO is proud to support excellent education and outreach initiatives like the 365 Days of Astronomy podcast.

Transcript:

CAN THE LAWS OF PHYSICS CHANGE?

Hello I’m Dr Stuart Clark, astronomy author and journalist. Today I’d like to explore the question: Can the laws of physics change?

Science has enjoyed unprecedented success in describing nature with mathematics. The equations derived have become our way of understanding the laws of physics and of predicting the behaviour of physical systems. It seems unlikely that the laws themselves can change, but what about the so-called constants of nature?

There are many constants. They are the values that cannot be derived from theory, and so can only be determined by measurement. They are used in the laws of physics as conversion factors to create exact mathematical relationships between quantities.

Some of the constants are self explanatory, such as the speed of light. Others seem more abstruse, such as the Planck constant, which governs the way nature breaks energy up into small ‘packets’. Despite calling these quantities constants, there has been a creeping suspicion over the last 15 years or so that some of them – particularly the speed of light – may be changing slowly with time.

The universe is bathed in microwaves. Traditional physics explains the almost uniform temperature of this background as a result of a sudden period of exponential expansion early in the Universe’s history – but what drove this inflation is still a mystery. In 1993 physicist John Moffat pointed out that if the speed of light had been higher in the past, photons of light could have travelled much further and so could have equalized the temperature across a much wider expanse of space without the need to invoke inflation.

Astronomers now study distant quasars – early galaxies powered by matter falling into black holes − in the hopes of catching the last vestiges of any change in the speed of light. But we have to be careful when drawing conclusions from the measurement of constants that have units attached to them. The speed of light is measured in units of length and time. If a variation is detected, the researchers cannot be sure whether it is the speed of light that has varied, or the rate at which the clock has ticked, or the length of the ruler. So they concentrate on examining dimensionless constants. Say you measure the ratio of a proton’s mass to an electron’s mass, then the units – kilograms – will cancel out and the resulting constant will simply be a number.

The so-called fine-structure constant is dimensionless. It is obtained by combining the speed of light with the Planck constant and the charge on an electron. It affects the outer structure of each atom, which controls the way an atom’s electrons react with passing light beams. If the speed of light were to change with the passage of time, the fine-structure constant would change also, as would the characteristic pattern of lines produced by atoms.

In 1999, John Webb of the University of New South Wales led a team observing 128 quasars out to 10 billion light years. They collected the quasar light, split it into spectra, looking for the fingerprints of intervening atoms. The spectral lines changed in a way that was consistent with the fine-structure constant having increased slightly during the course of cosmic history, by around 1 part in 100,000 during those 10 billion years.

Numerous groups are trying to verify or disprove this idea because the discovery of changing constants has enormous consequences for our understanding of the Universe. It points to physics beyond Einstein, perhaps even to the elusive ‘theory of everything’.

Most physicists believe that the best candidate for a theory of everything is string theory. This complex mathematical theory replaces particles with strings wiggling in higher dimensions than the three we are directly familiar with. According to string theory, only if all the higher dimensions are taken into account will the value of physical constants remain truly constant.

In the case of gravity, mass in kilograms and distance in metres are equated to a force in newtons by Newton’s ‘gravitational constant’, Big G. This too has been another target for physicists searching for variations in the constants but Big G is difficult to measure accurately.

By 1987, physicists thought Big G was known to an accuracy of 0.013 percent. Improved experiments in 1998 forced this to be re-assessed to a lesser accuracy of just 0.15 percent. The value of Big G is extraordinarily imprecise when compared with the force of electromagnetism, which is known to 2.5 million times greater accuracy. This lack of precision has led to speculation about whether the constant might be changing slowly over time, in effect changing the strength of gravity. Such a variation would gradually change the orbits of stars and planets, affect the sizes of celestial objects, and determine how brightly stars shine.

Measuring the distance of the Moon using lasers from Earth has shown that the value of Big G cannot be changing by more than one part in a million per year. Other physicists search for temporary changes in the strength of gravity brought on by the movement of Earth around its orbit.

This is because Einstein’s theories of relativity rest upon the central tenet that the laws of physics are the same, no matter where or when you are located in the Universe or how you are moving. How to transform what one observer can see into the viewpoint of another is known as the Lorentz transformation but if the constants change, the Lorentz transformation no longer works precisely, and a Lorentz violation is said to have taken place.

String theory allows small Lorentz violations to have taken place in the big bang, imprinting themselves on the fabric of space-time and these could make Big G display a different value over the course of a single year as Earth orbits the Sun and so travels in different directions through space. The obvious way to test for this is to drop objects throughout the year and measure how fast they fall. Comparing measurements taken six months apart should yield the greatest difference because then the Earth is travelling in opposite directions through space. The best place to conduct the experiment is in space, because when an object is in free fall, small gravitational variations can be measured very precisely. A number of missions hoping to pursue this research are currently on the drawing board.

Physicists will continue to search for changes in the constants of nature – both long-term and short-term effects – for as long as they believe that string theory is the way to unite gravity with the other forces. By measuring the amount of change, they will be able to home in on the correct version of string theory and understand better its picture of a multi-dimensional Universe.

Newton’s theory of gravity is said to have been inspired by watching an apple fall to the ground; earlier, Galileo is said to have dropped objects from tall buildings to discover that all objects fall at the same speed, regardless of their composition or mass. It would therefore be entirely fitting if our next breakthrough in understanding the Universe could come from measuring falling objects in orbit.

End of podcast:

365 Days of Astronomy
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