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The fine structure constant a is the fundamental physical constant characterizing the strength of the electromagnetic interaction. First considered by Arnold Sommerfeld in 1916, it can be thought of as the ratio of two energies: (i) the energy needed to bring two electrons from infinity to an arbitrary distance s against their electrostatic repulsion, and (ii) the energy of a single photon of wavenumber k = 2p/l = 1/s where l is the photon's wavelength. Richard Feynman said of this constant, "It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it. ... It's one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say the ``hand of God'' wrote that number, and ``we don't know how He pushed His pencil''. We know what kind of a dance to do experimentally to measure this number very accurately, but we don't know what kind of a dance to do on a computer to make this number come out-without putting it in secretly! ''
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Now that's interesting all by itself. Why does it have the value it does and is that number significant? But over the past few years the fine structure constant has received attention for another reason: some researchers believe it's been changing. In 2002 stories hit the press--not just the science sites, the "real" news sites, as well--carrying messages like this one:
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Black hole theory suggests light is slowing
One of Einstein's most dearly held concepts - that the speed of light is constant - is looking a little fragile. Physicists in Australia claim there is good reason to think the speed of light has slowed over time.
"Einstein would have absolutely hated this," said Paul Davies of Macquarie University in Sydney. "His entire theory of relativity was founded on the notion that the speed of light is an absolute fixed universal number."
The physicists' suggestion follows earlier measurements of a key quantity called the "fine structure constant". This quantity dictates how photons of light interact with particles such as electrons. Observations of the light from distant, superbright galaxies suggest that this "constant" was actually slightly smaller 10 billion years ago (New Scientist print edition, 11 May 2002).
Because the value of the fine structure constant depends on two quantities - the electron's charge (e) and the speed of light © - this implies that one of these two quantities has also changed. Either c has decreased over time, or e has increased.
Now Davies and his colleagues say the most likely answer is that c has decreased. They argue that if instead the charge of the electron could go up, then this would mean the event horizon of a black hole - the region from which light and matter cannot escape - would shrink over time. And that would violate one of the golden rules of physics, the second law of thermodynamics. . .
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One of Einstein's most dearly held concepts - that the speed of light is constant - is looking a little fragile. Physicists in Australia claim there is good reason to think the speed of light has slowed over time.
"Einstein would have absolutely hated this," said Paul Davies of Macquarie University in Sydney. "His entire theory of relativity was founded on the notion that the speed of light is an absolute fixed universal number."
The physicists' suggestion follows earlier measurements of a key quantity called the "fine structure constant". This quantity dictates how photons of light interact with particles such as electrons. Observations of the light from distant, superbright galaxies suggest that this "constant" was actually slightly smaller 10 billion years ago (New Scientist print edition, 11 May 2002).
Because the value of the fine structure constant depends on two quantities - the electron's charge (e) and the speed of light © - this implies that one of these two quantities has also changed. Either c has decreased over time, or e has increased.
Now Davies and his colleagues say the most likely answer is that c has decreased. They argue that if instead the charge of the electron could go up, then this would mean the event horizon of a black hole - the region from which light and matter cannot escape - would shrink over time. And that would violate one of the golden rules of physics, the second law of thermodynamics. . .
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This was based largely (I believe) on the work of an Australian team led by John Webb that was studying the spectra of quasars. In space, distance is time and the night sky is always a snapshot of the past. The furthest things we can see are thus the oldest and those happen to be the outrageously bright objects known as quasars. If the fine structure constant had been slightly different in the distant past then quasars would know about it. Webb's team studied the redshift of these objects (a sort of stretching of the light waves that occurs as they traverse an expanding universe) and found that it didn't seem to be perfectly uniform for certain dark parts in the spectrum (dark spots in a spectrum occur when something--like gas--between the quasar and us intercepts the light and absorbs certain wavelengths, leaving a gap by the time the light gets to us). This sparked their questions about whether the fine structure constant, and thus the speed of light, might've changed in the past.
But then in 2004 we had:
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A few years ago, astronomers announced that the laws of physics may have changed slightly since the big bang. Light arriving from distant corners of the Universe suggested that a fundamental constant related to the speed of light was a bit different in the past. But in the 26 March Physical Review Letters, a different team applies the same technique to newer data and concludes that if there has been a change, it is much smaller than previously claimed.
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Now another group of astronomers has applied the same technique to absorption lines measured at the European Southern Observatory's Very Large Telescope in Chile. Instead of doing a complex analysis of all the data, the team selected only simple-looking absorption lines. They avoided lines that appeared to include many components and those with complete absorption at the center of the line, to make it easier to extract shifts that are smaller than one tenth of the width of an absorption line. "Our data are of uniform quality and far superior to those used in earlier studies," says team member Raghunathan Srianand, of the Inter University Center for Astronomy and Astrophysics in Pune, India. The researchers concluded that the fine-structure constant might well be constant, and that the variation seen in the prior work was highly unlikely.
The Australian team isn't convinced. Michael Murphy, who is now at the University of Cambridge, says selecting data that looks simple might give an unrealistically small estimate of the error. And if the fine-structure constant really hasn't changed, he says, "we still have to explain these data from Keck." In view of the extensive efforts to rule out systematic errors, he says the explanation "may be mundane, but it has to be bizarre." Murphy and his Australian colleagues hope to publish their own analysis of Very Large Telescope data soon.
Lennox Cowie of the University of Hawaii in Honolulu suspects that both groups are underestimating their possible errors. But Cowie, who recently coauthored an essay on related work [2], adds that "the fact that the first independent measurement is not in agreement has to make one feel the variation claim is now weak."
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Now another group of astronomers has applied the same technique to absorption lines measured at the European Southern Observatory's Very Large Telescope in Chile. Instead of doing a complex analysis of all the data, the team selected only simple-looking absorption lines. They avoided lines that appeared to include many components and those with complete absorption at the center of the line, to make it easier to extract shifts that are smaller than one tenth of the width of an absorption line. "Our data are of uniform quality and far superior to those used in earlier studies," says team member Raghunathan Srianand, of the Inter University Center for Astronomy and Astrophysics in Pune, India. The researchers concluded that the fine-structure constant might well be constant, and that the variation seen in the prior work was highly unlikely.
The Australian team isn't convinced. Michael Murphy, who is now at the University of Cambridge, says selecting data that looks simple might give an unrealistically small estimate of the error. And if the fine-structure constant really hasn't changed, he says, "we still have to explain these data from Keck." In view of the extensive efforts to rule out systematic errors, he says the explanation "may be mundane, but it has to be bizarre." Murphy and his Australian colleagues hope to publish their own analysis of Very Large Telescope data soon.
Lennox Cowie of the University of Hawaii in Honolulu suspects that both groups are underestimating their possible errors. But Cowie, who recently coauthored an essay on related work [2], adds that "the fact that the first independent measurement is not in agreement has to make one feel the variation claim is now weak."
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Two teams, similiar data, and a dash of confusion. I should point out that changing constants wouldn't be the end of the world. Such an idea has been floated at least since the great physicist Paul Dirac suggested his Large Number Hypothesis in the 1930s. Dirac was intruiged by a numerical coincidence: the ratio of the size of the visible universe to that of an atom is about the same as the ratio of the electrical to the gravitational attractive forces between a proton and electron. However, since the first number changes (the universe is getting bigger), some of the constants that give us the second ratio would have to vary with time. Cosmologies based on a variable speed of light and some quantum theories of gravity are trying out models that vary different constants.
Anyway, now they're thinking up new ways to see if the fine structure constant has changed, looking even further into the past to a time before quasars existed. The key lies in the cosmic microwave background, the photons that last bounced off the opaque, chaotic stew of the early universe before neutral hydrogen formed and the universe became transparent. These photons have crossed the entire universe, coming almost all the way from the big bang itself, in the process being stretched down into low energy microwaves.
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Probe seeks changes in fine-structure constant
23 March 2007
Cosmologists in the US have proposed a new way to measure the fine-structure constant as it was some 13 billion years ago -- and see if its value differed from that measured today. The method, which has yet to be verified using astronomical observations, involves measuring how hydrogen atoms absorbed photons from the cosmic microwave background. It could provide further evidence that this fundamental constant of nature -- which defines the strength of the electromagnetic interaction -- is not actually a constant after all (Phys. Rev. Lett. 98 11301).
Most measurements of the fundamental constants of nature have been made on Earth over the past one hundred years. However, it is possible that these quantities are different when measured elsewhere in the universe or at other times. Indeed, the ability of fundamental constants to vary over space and time plays an important role in some theories that attempt to unify gravity, electromagnetism, and the strong and weak nuclear forces.
Some physicists believe that the value of the fine-structure constant (α) has been growing since the universe was formed in the Big Bang some 13.5 billion years ago. Observations of light from distant quasars suggest that α may have been one part in 105 smaller some 11 billion years ago than it is today. Closer to home, measurements of α derived from studying the decay of radioactive isotopes on Earth suggest that the constant may have changed by one part in 107 over the past 4.6 billion years.
Now Benjamin Wandelt and Rishi Khatri of the University of Illinois at Urbana-Champaign have proposed a way of measuring α as it was 10-100 million years after the Big Bang – during the so-called “dark ages”, when the universe was cool enough for neutral hydrogen atoms to exist, but before stars and galaxies formed. During this period, the hydrogen atoms absorbed cosmic microwave background (CMB) radiation at a wavelength of about 21 cm, which corresponds to a transition between two atomic energy states. The result is an absorption line in the CMB that endures to this day.
Wandelt and Khatri have shown that the precise wavelength of the transition is very sensitive to changes in α. Since microwave radiation from the dark ages can be detected today, they reckon that the precise location of the 21-cm line and the relative strength of the absorption can be used to determine α as a function of time -- after correcting for the redshift caused by the expansion of the universe.
As hydrogen atoms absorbed photons throughout the dark ages, Wandelt and Khatri believe that it should be possible to track changes in α over a period of about 100 million years. Indeed, because hydrogen was ubiquitous during the dark ages, the researchers are confident that their technique could be used to create spatial maps of α, which could prove useful in the search for dark energy.
Unfortunately, the scheme cannot take advantage of the current generation of microwave telescopes -- such as the Wilkinson Microwave Anisotropy Probe (WMAP) satellite – which do not focus on the region of the microwave spectrum containing the 21 cm absorption line from the dark ages. However, Wandelt told Physics Web that the measurements could be made using the Long Wavelength Array (LWA) telescope that is currently being built in the US state of New Mexico.
Another complication is that the signal is buried beneath an intense background of radiation that originates from within our own galaxy. However, Wandelt is confident that this background can be subtracted to yield a measurement of α to an accuracy of about 0.1% within a decade.
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23 March 2007
Cosmologists in the US have proposed a new way to measure the fine-structure constant as it was some 13 billion years ago -- and see if its value differed from that measured today. The method, which has yet to be verified using astronomical observations, involves measuring how hydrogen atoms absorbed photons from the cosmic microwave background. It could provide further evidence that this fundamental constant of nature -- which defines the strength of the electromagnetic interaction -- is not actually a constant after all (Phys. Rev. Lett. 98 11301).
Most measurements of the fundamental constants of nature have been made on Earth over the past one hundred years. However, it is possible that these quantities are different when measured elsewhere in the universe or at other times. Indeed, the ability of fundamental constants to vary over space and time plays an important role in some theories that attempt to unify gravity, electromagnetism, and the strong and weak nuclear forces.
Some physicists believe that the value of the fine-structure constant (α) has been growing since the universe was formed in the Big Bang some 13.5 billion years ago. Observations of light from distant quasars suggest that α may have been one part in 105 smaller some 11 billion years ago than it is today. Closer to home, measurements of α derived from studying the decay of radioactive isotopes on Earth suggest that the constant may have changed by one part in 107 over the past 4.6 billion years.
Now Benjamin Wandelt and Rishi Khatri of the University of Illinois at Urbana-Champaign have proposed a way of measuring α as it was 10-100 million years after the Big Bang – during the so-called “dark ages”, when the universe was cool enough for neutral hydrogen atoms to exist, but before stars and galaxies formed. During this period, the hydrogen atoms absorbed cosmic microwave background (CMB) radiation at a wavelength of about 21 cm, which corresponds to a transition between two atomic energy states. The result is an absorption line in the CMB that endures to this day.
Wandelt and Khatri have shown that the precise wavelength of the transition is very sensitive to changes in α. Since microwave radiation from the dark ages can be detected today, they reckon that the precise location of the 21-cm line and the relative strength of the absorption can be used to determine α as a function of time -- after correcting for the redshift caused by the expansion of the universe.
As hydrogen atoms absorbed photons throughout the dark ages, Wandelt and Khatri believe that it should be possible to track changes in α over a period of about 100 million years. Indeed, because hydrogen was ubiquitous during the dark ages, the researchers are confident that their technique could be used to create spatial maps of α, which could prove useful in the search for dark energy.
Unfortunately, the scheme cannot take advantage of the current generation of microwave telescopes -- such as the Wilkinson Microwave Anisotropy Probe (WMAP) satellite – which do not focus on the region of the microwave spectrum containing the 21 cm absorption line from the dark ages. However, Wandelt told Physics Web that the measurements could be made using the Long Wavelength Array (LWA) telescope that is currently being built in the US state of New Mexico.
Another complication is that the signal is buried beneath an intense background of radiation that originates from within our own galaxy. However, Wandelt is confident that this background can be subtracted to yield a measurement of α to an accuracy of about 0.1% within a decade.
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