Has Isaac Newton ever seen the sea
Which seriously irritates the physicists
Clive Speake is at a loss. His team recently re-measured a seemingly banal basic quantity in physics - the gravitational constant. The irritating thing: The physicists' result is well above the officially recognized value up to now. But Speake is sure of his cause: “We cannot find any errors in our measurements,” says the scientist who researches the fundamentals of gravity at the University of Birmingham in the UK. And that has serious consequences: The gravitational constant is not just any physical parameter. It is a fundamental quantity that determines the strength and properties of gravity. It is responsible for ensuring that people, animals and objects stay on the ground and do not take off. At the same time, she masters the large-scale processes in space: the movements of planets, stars and galaxies happen under her direction.
A defeat for the ego
The result from Birmingham therefore brings unrest to an important chapter in physics that puzzles the researchers: For decades they have been desperately searching for the exact value of the gravitational constant - and thus the foundation of gravity. But so far her meticulous experiments have had little success. The physicists seem to be fooled by gravity. The problem: different measurements produce contradicting results. From the third digit after the decimal point, the results drift apart significantly. The current official value: 6.67384 (80) · 10–11 m3kg-1s-2. In times when researchers are used to recording their measurands to a millionth or even a billionth, this is absurd - a setback for the ego of precision physicists.
And: there may be more than a scientific annoyance behind the riddle. Some physicists suspect something spectacular as the cause of the discrepancy. It could be that something is wrong with the law of gravity. And that would have enormous consequences for the physical worldview.
In this picture, gravity is basically a mysterious unknown. Taken on its own, it can be easily packed into formulas and laws - this is ensured by the law of gravity formulated by the English naturalist Isaak Newton as early as 1686 and its extension developed by Albert Einstein 230 years later: the general theory of relativity. But the other three basic forces of nature - the electromagnetic force as well as the weak and strong nuclear force - does not want to fit the gravitation at all.
Collision with the quanta
The “standard model” - the current theoretical structure of particle physics - only deals with the three other forces. The model simply ignores gravity. Because it cannot be embedded in the abstract formalism of quantum mechanics. This theory describes very successfully structures and dynamics in the world of atoms and elementary particles.
The world of theoretical physics is thus divided into two parts: on the one hand the standard model for the microcosm, on the other hand Newton's law of gravity for everyday experience and the general theory of relativity for the universe on a large scale. For the physicists, this is an unpopular permanent temporary measure: Albert Einstein and Werner Heisenberg - a pioneer of quantum mechanics - had longingly dreamed of a unified theory. It should merge gravitation and quantum physics into a new super theory. This “quantum gravity” should also explain phenomena where today's physics fails - such as the big bang and the inner workings of black holes. So far, however, physicists have failed to develop such an overarching theory.
At the moment they are not even able to do a seemingly much more mundane undertaking - the determination of the gravitational constant. In the mathematical formulas it defines how strong the force of gravity acts between two bodies that are at a certain distance from one another. If one were to precisely measure the force between two masses that weigh exactly one kilogram and whose centers of gravity are exactly one meter apart, the value of the gravitational constant would come out.
This natural constant came into play when Isaac Newton presented his legendary law of gravitation: According to this, the attraction of two bodies depends on the product of their masses divided by the square of their distance. This means: the larger the masses are and the smaller the distance between them, the stronger the force of gravity. The gravitational constant acts as a link. And: It defines the relative strength of gravity compared to other natural forces.
However, although gravity is the only force that can be felt everywhere in everyday life, it is by far the weakest of the four basic physical forces known today. They separate 36 orders of magnitude from the electromagnetic interaction between electrical charges - a number with 35 zeros. The fact that gravity can be felt at all is due to the fact that the electrical forces usually average out: Atoms contain as many positive as negative charges and are therefore electrically neutral when viewed from the outside.
It is different with gravity: With it, the effects of all forces of attraction add up. As a result, despite its dwarfism, gravity dominates what happens on earth and in the universe: it makes us stand and walk or makes us fall, it drives streams and rivers towards the sea, it prevents earth, moon and sun from bursting, and it determines the orbit of planets around their stars and the movement of giant galaxies and galaxy clusters in the cosmos. But their weakness makes it difficult to measure gravity precisely. The British scientist Henry Cavendish noticed this when he first wanted to determine the value of the gravitational constant in 1797. He hung a dumbbell with two lead balls on a metal thread. Next to the dumbbell he placed two large weights of lead. Due to gravity, the balls attracted each other, which twisted the dumbbell and the thread a little.
However, the effect was so puny that Cavendish could only capture it with the help of a trick: he attached a mirror to the thread and followed the reflected light of a candle flame on the wall. The result was astonishing: with the rotary balance, the Briton was able to determine the value of the constant to within one percent. That is why the physicists still adhere to his measuring principle today: Heavy weights, so-called field masses, attract small test masses and move them out of their rest position. In order to increase the measurement accuracy, the scientists varied and refined the principle - for example, a US team from Washington University in Seattle who repeated the measurement in 2000.
The researchers had hung a thin glass plate on a tungsten thread on a turntable. Then they let the table rotate slowly. Two field masses, each weighing 30 kilograms, were mounted on the outside. With each rotation, the force of gravity tugged and pulled the pendulum with the glass plate. An electronic controller controlled the rotation of the table in such a way that the tungsten thread did not twist - a trick that significantly increased the precision of the experiment.
In addition, the researchers let the field masses rotate - the other way around than the turntable. Thereby they eliminated the gravitational influence of the environment. Because since gravity cannot be shielded, the gravity field in the laboratory either has to be known exactly or it has to be cleverly averaged out. Otherwise a truck driving by or a laboratory assistant standing around could disrupt the highly sensitive measurement. The US physicists came up with the most precise value to date.
Experiments create confusion
Measurements from other groups confirmed the value. But some also deviated from it - for example an experiment at the Joint Institute for Laboratory Astrophysics in Boulder (Colorado), the results of which were published in 2010. Clive Speake and his colleagues now complete the mess. The experiment by the physicists from Birmingham is also based on the traditional principle of the Cavendish rotary balance. But the researchers have cleverly improved the concept: “We used two different methods to measure the deflection,” explains Speake. “This enabled some sources of error to be eliminated.” The British team had already carried out an experiment based on this principle in 2001 - with the unsettling result that the measured value was 0.2 per mil above the reference value from Seattle. Alarmed by this discrepancy, the researchers repeated the experiment with optimized technology. In September 2013, the team published the result: "We came up with almost the same measured value as in the first experiment," states Speake.
The result could have unpopular consequences: At the end of 2014, the next "reorganization" of the natural constants is due. In view of the new result, the responsible committee could feel compelled to widen the already astonishingly wide error bar of the gravitational constant - a major setback for the physicists.
Research is currently stuck in a dead end. "Each group thinks they have measured accurately and the fault lies with the other," says Stephan Schlamminger of the National Institute of Standards and Technology (NIST) in Boulder, Colorado. “Nevertheless, it seems to me most likely that the experimenters underestimated their mistakes or overlooked sources of error.” Therefore, experts will meet for a workshop in February to discuss ideas for even more precise experiments.
Atoms replace the dumbbells
There is already an approach for a completely new type of experiment: two research teams from Italy and the USA are trying to use something completely different as test masses instead of the usual dumbbells and pendulums - atoms. The principle: an ultra-cold cloud of rubidium atoms is gently pushed upwards with the help of a laser in a vertical metal tube that has been pumped empty of air. Pulled by gravity, the cloud then falls down again - passing close to a block made of 500 kilograms of tungsten. Once at the bottom of the tube, the particles pass an atomic interferometer. This special sensor takes advantage of the fact that the atoms have a quantum character, i.e. behave like light waves under certain circumstances. In particular, they can overlap, similar to light waves, and thus create complex light-dark patterns.
"If we push the tungsten up or down next to the tube, the gravitational field acting on the atoms changes," explains Guglielmo Tino, physicist at the University of Florence. “At the same time, the light-dark pattern in the interferometer also varies.” By precisely analyzing the change in shadow in the high-tech sensor, the Italians want to deduce the precise value of the gravitational constant. The result should be known in early 2014 - and perhaps help solve the gravity mystery.
Much is still in the dark
Should the measured value from the atomic interferometer deviate significantly from other results, this would inspire a highly interesting speculation: "Perhaps the gravitational constant is not a natural constant at all, but results from a deeper theory that we do not yet know," says Frans Klinkhamer, physicist at Karlsruhe Institute of Technology (KIT). He emphasizes: “Basically we know very little about gravity.” The longest known force also holds the most secrets.
With the general theory of relativity Albert Einstein created an ingenious conceptual structure that puts Newton's formula for gravity on a modern and fundamental basis. In this picture, masses bend and bend four-dimensional space-time, creating dents and funnels in the fabric of the universe. These curved areas have an effect on other masses, so to speak let them roll into themselves - which creates the attractive effect of gravity. The revolutionary theory has been confirmed again and again by a number of experiments, but it suffers from the drawback that it fails on small scales - with atoms and molecules. And: Einstein's formulas, which contain the gravitational constant introduced by Newton, cannot explain the underlying character of gravitation and the mechanism in which it arises: Why do masses drag on spacetime? And what are masses anyway? The physical core of the ominous force lies in the dark.
Some physicists speculate that the strength of gravity changed during cosmic evolution. To test this idea, they use an original experiment. The starting shot was given by the moon landing in July 1969. At that time, Neil Armstrong and Buzz Aldrin had placed special mirrors in the lunar dust - panels made up of 100 individual mirrors each. NASA later began sending laser flashes from Earth to the moon on a regular basis. There, around 385,000 kilometers away, the lightning strikes the mirrors. They throw the light back to earth, where telescopes pick it up. The highlight: The transit time of the lightning can be measured with high precision - and thus the exact distance between the earth and the moon. Should the gravitational constant change, this distance would develop differently than predicted by Newton's law. But so far no discrepancy has been found.
Tailwind for string theory
It would suit some theorists if something was wrong with Newton's law of gravity - for example, the proponents of string theory. It is considered a promising candidate for a unified theory of physics - that formula that is supposed to finally reconcile gravity with the other natural forces. The string hypothesis assumes that matter does not consist of almost point-like elementary particles, as previously assumed, but rather of vibrating, unimaginably tiny threads or strings: the "strings".
The theory plays out in ten to eleven dimensions. They could be "rolled up" within the familiar spatial directions and influence the strength of gravity at very small distances. The result: At distances smaller than a few millionths of a meter, Newton's venerable formula would lose its validity. Researchers all over the world are looking for such deviations.
Another mystery: at the end of the 1990s it became surprisingly clear that the universe was not expanding more and more slowly, as expected. Instead, the cosmic expansion is accelerating steadily - a mysterious something seems to be driving the universe apart like a yeast dough. The cosmologists gave the incredible cosmic villain the name "dark energy".
Evidence of a bizarre field?
What is behind this remains unclear to this day. A speculative attempt at an explanation is the “quintessence”. The experts use it to describe an energy field that pervades the entire universe and changes over time. So far, however, there has been no evidence that it actually exists. If one were to find deviations in the law of gravitation, this could be a first indication that the bizarre universal field is more than a physical fiction.
The law of gravity may also fail on large scales: in the distant depths of the universe - especially in those outer regions of galaxies and nebulae where matter experiences little acceleration due to gravity. A theory called "MOON", the modified Newtonian dynamics, could apply there. “This could explain why galaxies are stable and don't fly apart,” says KIT physicist Frans Klinkhamer. Unknown hypothetical particles that hold the star systems together as part of "dark matter" are not used in this theory.
If such speculations prove to be true, it would be no wonder that different experiments arrive at completely different values for the gravitational constant: Since each experiment takes place under different conditions, it could only ever measure the value that applies to these conditions. The large discrepancy between the measured values for the basic quantity of gravity would then not be a nuisance, but a spectacular pointer to new physics. ■
When cycling, FRANK GROTELÜSCHEN struggles with gravity. Uphill his pulse is racing, downhill he is afraid of flying out of the curve.
by Frank Grotelüschen
Ever since the English naturalist Isaac Newton sat dozing under a tree and an apple allegedly fell on his head, gravity has been causing confusion. After Newton's lightning-fast discovery, which he translated into a formula, almost 330 years have now passed and many new insights into gravity have matured. But the force that makes the masses attractive and pulls everything earthly to the ground poses more puzzles to scientists than ever. In sophisticated experiments they try to decipher the physical nature of gravity.
The search for the swirling space
In the picture of general relativity, gravity arises from deformations of space-time. When objects such as stars, planets or even people move at an accelerated rate, they generate defects that propagate in waves in space and time - this is what the formulas of Einstein’s theory demand.They are similar to the annular waves that form on the surface of a lake after a stone is thrown into it. Such a “gravitational wave” could be proven by a characteristic pattern of periodic compressions and expansions of the space that it leaves behind during its passage. Although scientists at several large research institutions worldwide - including the "geo600" heavy listening post at the University of Hanover - have been looking for traces of gravitational waves for many years, they have so far not been able to directly detect these fluctuations in the space-time structure. This is mainly due to the fact that the rhythmic variations that cause the hypothetical gravity waves are tiny: even gravitational waves that emanate from a supernova explosion - one of the most energetic events in space - would only increase the distance between the earth and the sun change the diameter of a hydrogen atom. The distortions that the detectors on earth, which are just a few kilometers in size, would have to detect are around a hundred millionth smaller. Other objects also emit significantly weaker gravitational waves. If it were possible to detect gravitational waves, that would not only enable new insights into the cosmos. It would also be a brilliant confirmation of Albert Einstein's general theory of relativity. If the proof fails, however, it would feed doubts about the conventional conception of physicists of the nature of gravity.
· Physicists despair of measuring gravity.
· Perhaps the fundamental constant of gravity is not constant at all.
· Tricky experiments with atoms should now shed light on the darkness.
· Some evidence gives the string theory a tailwind.
more on the subject
Clive Speake's homepage at the University of Birmingham: www.birmingham.ac.uk/staff/profiles/ physics / speake-clive.aspx
Institute for Astronomical and Physical Geodesy of the Technical University of Munich: www.iapg.bv.tum.de/iapg.html
Information about the satellite missions GOCE and GRACE: www.goce-projektbuero.de/goce.html www.csr.utexas.edu/grace
Good to know: gravity
Gravitation or gravity is one of the four known basic forces of nature. It starts from all bodies or particles that have a mass. Thanks to the action of gravity, they attract each other. How strong this attraction is is described by Newton's law of gravity and - in a generalized form - the general theory of relativity. Unlike the electromagnetic or Coulomb force, for example, gravity cannot be shielded. It permeates every known form of matter. According to the current state of knowledge of physicists, there is also no antigravity that would cause repulsion instead of mass attraction. The gravitational constant is of fundamental importance for understanding gravity - a numerical value that can be found in all formulas for the effect of gravity. It determines the strength of this force in comparison to the other physical forces of nature.
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