There is a magic rock in St. Cloud, France. It’s made not of stone, but of a metallic alloy that’s 90% platinum and 10% iridium, and it’s magic not through some supernatural force, but because scientists have declared it to have a mass of exactly 1 kilogram. Now many scientists would like to get rid of it. 

Our civilization is built upon a system of measurement standards. If two people want to trade, they have to agree on what a pound is. If you pay a contractor to build a 100 foot tall building, you have to agree on the length of a foot. Throughout history humans have had standards of measurement, often dictated by governmental decree. But since the early 1800s there has been a quest to create a truly universal standard of measurements. This became the metric system, which was further standardized to the Système international d’unités (SI) in 1960. The SI standard has become the basis for measurement across the globe. They define the physical units we use to measure things. Even in the United States, quantities like the foot and pound are defined in terms of SI units.

The most common SI units are those of the meter (length), second (time) and kilogram (mass). In the 1800s these were based upon the physical characteristics of Earth. A meter was defined by declaring the circumference of Earth to be 40,000 kilometers. A second was defined by declaring the length of an average day to be 24 hours long. A kilogram was defined as the mass of a liter (1000 cubic centimeters) of water. While these definitions initially worked well, as our measurements became more precise things became problematic. As measurement of the Earth’s circumference improved, the length of a meter would necessarily change. Since the volume of a liter is defined in terms of length, the mass of a kilogram likewise shifted. Precise measurements of Earth’s rotation showed that the length of a day varied, so even the second wasn’t entirely fixed.

There are two ways to define a set of units that don’t vary. One is by defining a particular object to be an exact standard, and the other is to define units in terms of universal physical constants. The meter and second are now defined using the later method. For example, in Einstein’s theory of relativity, the speed of light in a vacuum is always the same. No matter where you are in the universe, or how you are moving through space, the speed of light never changes. It is an absolute physical constant. This has been verified through numerous experiments, and in 1983 it was given an exact value. By definition, the speed of light is 299,792,458 meters per second. By defining this value, we also defined the length of a meter. Since the speed of light is a constant, if you know how long a second is, you know the length of a meter.

Emission spectrum of a high pressure sodium lamp. Credit: Chris Heilman

The length of a second is also defined in terms of light. By the 1960s we had developed atomic clocks based upon cesium 133. Like all elements, Cesium 133 emits light at specific frequencies. Light is emitted from an atom when an electron moves from a higher energy quantum state to a lower one, and under the right conditions they are always the same. One particular emission from cesium 133 is known as the hyperfine ground state, and it is used to regulate an atomic clock the way the swing of a pendulum regulates a grandfather clock. In 1967 the frequency of light emitted by this hyperfine transition was defined to be 9,192,631,770 Hz. By measuring the frequency, you know the length of a second.

Since the meter and second are based upon physical constants, they don’t change. They can also be measured anywhere in the universe. If an alien civilization wanted to know what units we use, we could send them a radio message with the definition for meters and seconds, and the aliens could recreate those units. But since 1889, the kilogram has been defined by a specific chunk of metal known as the International Prototype of the Kilogram (IPK). If the aliens wanted to know the mass of a kilogram, they would have to make a trip to France.

The relative mass change of kilogram copies over time. Credit: Greg L at English Wikipedia

Besides the necessary road trip for aliens, there is a big problem with using a magic rock as the standard kilogram. Since the mass of the IPK is exact by definition, it cannot change under any circumstances. If someone were to shave off a bit of metal, it would still be one kilogram by definition. Shaving the IPK down a bit wouldn’t make the kilogram lighter, it would make everything else in the world a bit heavier. Of course, that doesn’t make any sense. Shaving down a bit of metal in France doesn’t make the Statue of Liberty weigh more. The problem is with our definition of mass. And in a sense this kind of thing actually happens. In addition to the official prototype kilogram, there are official copies all over the world. By comparing the copies to the IPK, we can determine the stability of its mass. This has only been done a few times over the years, but on average the mass of the copies has increased slightly compared to the IPK. Either the official kilogram is getting lighter, or the copies are getting heavier.

The standard kilogram hasn’t been replaced by a physical constant because we haven’t been able to measure them with enough precision. The obvious physical constant for mass would be the universal constant of gravity G. But gravity is a weak force, and measuring G is difficult. So far we’ve only measured it to about one part in 10,000, which isn’t nearly accurate enough to define mass. But there is another constant we could use, and it’s known as the Planck constant.

The Planck constant lies at the heart of quantum theory. It was first introduced by Max Planck in his study of light. When objects are heated, they emit light, and the color of that light depends upon the temperature of the object. This is known as blackbody radiation. According to classical theory, most of the light emitted should have very short wavelengths, but experimentally this wasn’t the case. Planck demonstrated that light must be quantized proportional to a small constant h, which we now call Planck’s constant. As our understanding of quantum theory grew, the Planck constant played a role not just in quantization, but quantum ideas of energy and momentum. In SI units, h has units of kg∙m²/s. If the Planck constant is defined to have an exact value, then the kilogram would be defined in terms of Planck’s constant as well as the meter and second.

In principle it’s a good idea, but it can only be done if we can measure it accurately. In 2014 the Conférence Générale des Poids et Mesures  (CGPM) decided that before such a definition could occur, three independent measurements of the Planck constant would need to be made, each with an accuracy of 50 parts per billion, and one accurate to 20 parts per billion. By June of this year three experiments have been done with uncertainties smaller than 20 parts per billion. The CGPM meets again in 2018, where it is expected they will officially define the Planck constant to be exactly 6.626069934 x 10⁻³⁴ kg∙m²/s. When that happens the prototype kilogram will no longer be a magic rock, but simply a part of scientific history.

And the aliens won’t have to make that road trip after all.

The post The Magic Rock appeared first on One Universe at a Time.

When we make measurements of things in the universe, we can express the results in some type of units. The mass of a black hole in solar masses, or the distance to Jupiter in astronomical units, for example. But to be accurate, all of these units need to be based upon well measured standards that form the basis for all other units. Defining those standards poses certain challenges. After all, how to you measure something that is used to define your measurements?

Our modern foundation of measurements is the metric system (or more formally the International System of Units). In this system, seven base units are defined by convention (meter, kilogram, second, ampere, kelvin, mole and candela), and from these all other measuring units are derived. Even most non-metric units are formally defined by metric quantities. For example, an inch is defined as 2.54 centimeters. An English pint is exactly 568.26125 cubic centimeters.

The base units and their connection to physical constants. Credit: Wikipedia

Originally, the metric base units were defined in terms of physical quantities that could be measured. A kilogram was proposed as the mass of a liter (1000 cubic centimeters) of water. The meter was proposed by declaring that the distance from the north pole to the equator along a line through Paris is 10,000 kilometers. The second was defined in terms of a standard solar day being exactly 24 hours long. While these definitions initially worked well, as our measurements became more precise things became problematic. As measurement of the Earth’s circumference improved, the length of a meter would necessarily change. Since the volume of a liter is defined in terms of length, the mass of a kilogram likewise shifted. Precise measurements of Earth’s rotation showed that the length of a day varied, so even the second wasn’t entirely fixed.

A replica of the standard kilogram. Credit: Wikipedia

To solve this problem, some of the units were redefined in terms of a specific physical object. For example, in the 1870s the meter was redefined as the distance between two marks on the prototype meter bar made of platinum and iridium. In 1889 a cylinder of platinum-iridium alloy was defined as the prototype kilogram. While these were more precise than earlier definitions, they still had problems. As physical objects they need to be handled and measured in order to calibrate units. They can also change as they interact with their environment. We know, for example, that the standard kilogram varies in mass over time relative to official copies. These are small changes, on the order of micrograms, but they limit the precision of our measurements.

As we’ve studied the physical universe, we’ve found that the universe itself seems to have a set of absolute physical standards. Some things, such as the speed of light, the universal constant of gravity, and the charge of an electron, seem to have precise values that are absolute and unchanging throughout the universe. These physical constants could thus be used as the fundamental scale from which our metric system is defined. For some units this has already been done. In 1983 the speed of light was defined as exactly 299,792,458 m/s, and the second is defined in terms of the frequency of light from a caesium 133 atom. As a result, the meter is now defined in terms of the speed of light. But other units such as the kilogram are still based upon the 1889 prototype. There is currently a proposal to redefine the kilogram and other units in terms of physical constants. If the proposal passes, the metric system would no longer depend upon prototype objects.

But if we defined our units in terms of physical constants, what if those constants change over time? While this has been expressed as a concern, it isn’t really that much of a problem. We know from observations that the physical constants appear uniform across billions of years and billions of parsecs. All evidence so far shows them to be fixed and unchanging. And fixing our definitions in terms these physical values doesn’t prevent us from detecting the relative changes of these constants over time. So if we find they are changing we can always make new definitions for our units, just as we have in the past.

It would just be a matter of holding our measurements to a higher standard.

The post Holding Measurements To A Higher Standard appeared first on One Universe at a Time.

Gravity is the force that holds us to the Earth, and holds the planets in their orbits around the Sun. It is such a constant presence in our lives that it’s difficult to image gravity acting any other way. But what if a million years ago gravity were much weaker than it is now? What if gravity were stronger in the Andromeda Galaxy than it is in our own Milky Way? If gravity varied over space and time, how would we know?

This is an important question in astronomy, since we assume that gravity is a physical constant, meaning that it has always been the same everywhere throughout the history of the universe. Because of this we can compare observations of different regions of the sky with experiments we do on Earth to understand the cosmos. If gravity weren’t constant, a basic assumption of astronomy would have to be changed. But in science we can’t simply assume something to be true, we have to keep testing our assumptions to see that they’re accurate.

Fortunately, there are ways to study whether gravity has changed. At a basic level, it certainly appears that gravity is the same everywhere. For example, if gravity were stronger in some regions of the universe we would expect stars and galaxies in that region to be more tightly clustered together than in other regions. What we observe is than on large scales galaxy clusters are spread out fairly evenly. When we look at stars in our own galaxy, they behave just as we’d expect if gravity is constant.

While these general observations are nice, they don’t really determine whether gravity is truly a physical constant, or if it’s just “kind of” constant. To really test gravity we have to make very precise observations over space and time. One way to do this is to look at Type Ia supernovae. These massive explosions of a dying star have a maximum brightness (known as their absolute magnitude) that we can determine accurately. This brightness is determined in part by the strength of gravity, so if gravity varied across different regions of space, we would expect these supernovae to have varying absolute magnitudes as well. In 2014, a team of astronomers studied more than 500 of these supernovae, and found (to within one part in 100 million) there was no apparent difference in the strength of gravity.  There observations covered 65% of the visible universe, so across billions of light years gravity seems to be constant.

Diagram of a pulsar Source: NRAO

This year another team analyzed 21 years of data to determine whether gravity changes over time. In this case they looked at the bursts of energy coming from a pulsar. A pulsar is a dense neutron star with a strong magnetic field. Because of its magnetic field, beams of radio energy emanate from the pulsar’s magnetic poles. As the pulsar rotates, the beam can sweep across our field of view like a lighthouse. Since the rate of the pulses depends on the pulsar’s rotation, they are very regular. By measuring the pulses very precisely we can determine the pulsar’s motion and any changes in that motion with great precision.

In this case the team looked at a pulsar that rotates hundreds of times a second (known as a millisecond pulsar because the pulses are only milliseconds apart). This particular pulsar also has a wide orbit around a companion star, so the orbit of the pulsar could be measure. If gravity were to change the orbit of the pulsar would correspondingly change. But the team found that over a 21-year period there was no observed change of the strength of gravity. Specifically, they found that the strength of gravity could have changed no more than 1 part in a trillion.

We’ll never be able to prove with 100% certainty that gravity doesn’t change, but every experiment we have done so has found no evidence of any change over the entire history of the universe. As best we can tell, gravity truly is a constant of nature.

Paper: W. W. Zhu et al. Testing Theories of Gravitation Using 21-Year Timing of Pulsar Binary J1713+0747. ApJ 809 41 doi:10.1088/0004-637X/809/1/41 (2015)

The post How Do We Know Gravity Is Constant? appeared first on One Universe at a Time.

One of the great strengths of astronomy is it’s ability to observe the past. Since light takes time to travel across the universe, more distant objects are seen at an earlier time than closer objects. This means we can do things such as study the evolution of galaxies, and observe the universe in its youth. It also means we can test whether physical constants change over time. But geology also gives us the ability to observe the past, and sometimes it tells us things about astronomy.

The geology of Oklo. Credit: Mossman et al., 2008

Take, for example, the natural reactor at Oklo in Gabon, Africa. It was discovered in a uranium mine in 1972, when it was noticed that some samples from the mine had a noticeably lower concentration of uranium 235. What’s more, the region also had elevated concentrations of fission byproducts such as ruthenium 99 and neodymium 142. This is the type of thing you’d expect to see in a nuclear reactor, but it was found in a region of rock about 1.7 billion years old. It is the first and only known location of a naturally occurring fission reactor.

What happened was that groundwater seeped into the uranium deposits, which acted to dampen the speed of neutrons emitted by the U235 (similar to the way we now use heavy water in modern reactors). The slower neutrons were then able to strike other U235 elements at a speed capable of inducing fission, thus creating a steady chain reaction.

The thing about fission reactions is that their rates and byproducts are critically dependent upon various constants of nature, so the Oklo reactor can be used as a way to test whether these constants have changed over time. In particular, there have been several papers looking at one particular constant known as the fine structure constant. What we’ve found is that at the time of the Oklo reactor, roughly 2 billion years ago, this constant appeared to have the same value it has today. Specifically, it could have changed by no more than one part in 10 million. It agrees with other astronomical studies that also show physical constants haven’t changed over billions of years, but it’s important because it reaffirms the fact that what we observe in the universe is consistent with what we measure here.

Astrophysics works, and we know that thanks in part to a bit of geology.

Paper: Thibault Damour & Freeman Dyson. The Oklo bound on the time variation of the fine-structure constant revisited. Nuclear Physics B, 480, 25, pp 37–54. (1996)

The post Rock of Ages appeared first on One Universe at a Time.

Yesterday I wrote about how we test whether unitless constants such as alpha (α) change over the history of the universe. You might also have noticed that I said if such constants did change, then it would mean either fundamental physical constants change or there is some exotic physics going on. We looked at the physical constants yesterday, so today let’s look for exotic physics.

As I mentioned yesterday, the value of alpha can affect the line spectra of atoms and molecules, which makes it easy to observe astronomically. In testing for changes in alpha over space and time, astronomers looked at distant molecular clouds. These are low-gravity environments, so they are similar to the type of environment we have here on Earth. But if you want to look for exotic physics, you probably want to look for situations that are either very high energy, or very high gravity.

Credit: UNSW

Since alpha depends on three constants (electron charge, light speed, and Planck’s constant), it shouldn’t be affected by strong gravitational fields such as the ones near a white dwarf or neutron star. But there are theoretical models that connect Planck’s constant to the universal gravitational constant (G). These models attempt to develop a theory of quantum gravity which would connect quantum theory to general relativity. Some of these models introduce things that would allow the gravitational field to change the value of alpha. Of course this change would only be seen with very strong gravity cases.

Recently, a paper was published in Physical Review Letters that looked at line spectra from a white dwarf known as G191-B2B. Since the surface gravity of a white dwarf is about 100,000 times that of Earth’s, this provided a good test of whether alpha is affected by gravity. Comparing the line spectra with those observed on Earth, they found no discernable difference, which means any variation in alpha can be no larger than one part in 10,000.

These results put a damper on any quantum gravity models where alpha could vary significantly (such as those with strong interactions between scalar and electromagnetic fields). However 1 in 10,000 is not too strong a constraint, so the results still leave the door open on more subtle variations that might give us hints about quantum gravity.

For now, however, alpha remains constant.

The post Gravity Check appeared first on One Universe at a Time.

Within physics there are certain physical quantities that play a central role. These are things such as the mass of an electron, or the speed of light, or the universal constant of gravity. We aren’t sure why these constants have the values they do, but their values uniquely determine the way our universe works. For example, if the mass of electrons were smaller, atoms would be smaller. If the gravitational constant were larger, you’d need less mass to create a black hole, and neutron stars might not exist.

There are some who like to play “what if” scenarios about what the universe would be like if various fundamental constants had different values. There are others who make anthropic arguments that the constants must have the values they have in order for us to exist. But those are discussions for another time. Regardless of any speculation, we do know the values of these fundamental quantities very well. The mass and charge of an electron are known to about one part in a billion. The gravitational constant, perhaps the least well measured, is known to about one part in ten thousand.

These quantities are often referred to as fundamental constants, because it is generally thought that these quantities never change. Whatever the mechanism (or random chance) that gives them their values, once the universe began they became “locked in” as it were. There are several theoretical arguments as to why this is, but broadly it stems from the fact that the laws of physics appear to be the same everywhere in the universe. Gravity works the same way in distant galaxies as they do in our own. So if the laws of physics are the same everywhere in space, it’s reasonable to assume they are the same everywhen in time.

But how could we tell if they weren’t constant. After all, we do all of our observations in our corner of the universe, and in the present day. How can we test fundamental quantities in the past? One way it to look at what are known as unitless quantities. Fundamental quantities like electron mass have units (kilograms and the like), but you can multiply and divide certain fundamental quantities to create a number where the units cancel out. One of these quantities is known as alpha (α), which is a product of the charge of an electron, the speed of light, and a constant from quantum theory known as Planck’s constant.

Diagram illustrating quasar observations.
Credit: J. C. Berengut

Since alpha has no units, its value is always the same, regardless of what units you use. The only way its value could change is if electron charge, light speed, or Planck’s constant changed in relation to each other (or if there is some exotic physics we don’t yet understand). The nice thing about alpha is that its value is central to the line spectra of atoms and molecules. If alpha had a different value, the wavelengths of light emitted or absorbed would shift slightly. So we can observe light from distant objects to test the constancy of alpha.

In 2010, a research team looked at light from distant quasars that had passed through large intergalactic clouds of gas. They found evidence of some slight variation of alpha depending on the direction we looked in the sky, which would imply a spatial variation of the physical constants. This made lots of news in the press, but the findings were not strong enough to be conclusive.

Then in 2012, a different team looked at the spectra of distant alcohol clouds (yes, there are clouds of alcohol in space), and measured another unitless constant, this time the ratio of the electron mass to the proton mass. What they found was that the mass ratio has changed no more than one part in a billion over the course of seven billion years. In other words, they are constant to the limits of our observation.

So it seems some things really do never change.

The post Variables of Nature appeared first on One Universe at a Time.

One of the big criticisms of astrophysics is how it claims to actually know what has occurred billions of light years away, and thus billions of years ago. After all, no human was there, so who’s to say that the physics we have on Earth is the same as the physics of the distant cosmos?  It’s a valid criticism. Astrophysics works with the assumption that the laws of physics are the same everywhere.  That’s a huge assumption given that our most distant space probe has barely left the solar system.  So how do we know our assumption is valid?

It turns out there is a way to test this assumption, and that is by looking at what are known as physical constants.  Within physics there are certain things such as the speed of light and the charge of an electron that always have the same value.  They are known as physical constants, and if the laws of physics are the same everywhere then these values should never, ever change.

Spectra from different quasars compared. The spectra redshifts, but the pattern remains the same. Credit: NOAO

If any of these constants did change, then it would affect what we observe.  For example, if the speed of light changed, then the pattern of line spectra from different elements would also change.  Not just redshift or blueshift, but the spacing between lines would shift.  So when we look at distant objects in the Universe, we can measure whether these types of relations change over time.

Several of these types of experiments have been done, and so far all of them have found absolutely no change to within the limits of the experiment.  We know, for example, that the ratio of proton mass to electron mass is has changed by no more than 1 part in a billion over the past 7 billion years.  We know the speed of light has remained constant for at least a similar period.

Now a new paper in the Publications of the Astronomical Society of Australia adds a new constant to this list.  The universal constant of gravity, known as G.  The gravitational constant G is what determines the strength of gravitational attraction between two masses.  If G were larger, then masses would be attracted more strongly.

What the authors of this paper did was to look at 581 type Ia supernovae.  This particular type of supernova has a consistent absolute magnitude (brightness), which is why they are used as standard candles to measure galactic distances.  They can be identified by the way they dim over time, which is due to the radioactive decay of elements such as Nickel-56.  If the gravitational constant changed over time, the absolute magnitude of these supernova would change relative to the decay of Nickel-56.  What they found was that there was no observable difference in these supernovae.  This means that G has changed by no more than one part in a hundred million over the last 9 billion years.

Once again, the physics of the Universe appears to be universal.  To the limits of our observations, physics is the same everywhere and everywhen. So we really can have confidence that what we observe “out there” can be interpreted by our understanding of physics here on Earth.

Paper: Jeremy Mould, Syed A. Uddin. Constraining a possible variation of G with Type Ia supernovae. Publications of the Astronomical Society of Australia, 2014.

The post Gravitational Constant appeared first on One Universe at a Time.

When Isaac Newton proposed his universal law of gravity, he was actually making a rather bold claim, specifically that the distant stars and planets are governed by the same physical laws that govern the Earth.  This was a radical split from the traditional Aristotelian view that the heavens were fundamentally different from terrestrial physics.

Newton’s law of gravity was the first step toward the broader idea of universality, the idea that the laws of physics (whatever they may be) are valid throughout the universe.  It is a necessary foundation of astrophysics, because it allows us to compare phenomena we observe in distant space with experiments we do in the lab.  Of course, that raises the question of how we could possibly know this. What if physics depended on where and when you are?  What if it changes gradually, so that our galaxy follows basically the same physics, but a galaxy billions of light years away follows a different set of rules?

One of the ways you can test this assumption is by looking at what are known as dimensionless constants.  These are constants determined by combining certain physical values so that the units cancel out.  For example, the ratio of the electron mass to the proton mass or the fine structure constant for electromagnetism.  By measuring different physical quantities, we can determine the value of these constants in the lab.  We can also make observations of distant objects to determine the constants.  If the laws of physics are the same everywhere, then these constants can’t change and our two determinations should give the same result.

Last year, astronomers observed how methanol absorbed light in the distant galaxy PKS 1830-211, pictured below.

From that, they determined the ratio of electron to proton mass.  They found it agreed with the lab result to within one part in ten million.   The light they observed left PKS 1830-211 seven billion years ago, so this shows that the laws of physics seven billion years away and seven billion years in the past were the same as they are today.

It seems that science is valid everywhere, which is pretty awesome when you think about it.

The post Here and There appeared first on One Universe at a Time.