On1859, the same year that Darwin published his revolutionary and controversial book, the French astronomerUrban Jean Joseph LeverrierI discovered certaindiscrepanciesbetween theobserved positions of Mercuryand those calculated with the equations of celestial mechanics. According to Newtonian gravitation, Mercury must follow an elliptical orbit around the Sun. Now, if we include the gravitational pulls of the rest of the planets, a peculiar effect appears: the orbit does not remain static in space, but also moves around the Sun This orbital rotation is known asperihelion advance(which is the point of the orbit closest to the Sun), and can be calculated with the same equations of celestial mechanics. The result, after painstaking calculations, is a continuous advance of 531 arc seconds per century. In other words,Mercury’s perihelion makes one complete revolution around the Sun every 244,000 years.
The problem was that Leverrier’s observations did not match the theoretical prediction: Mercury’s perihelion traveled 8% faster than predicted by Newtonian mechanics. Leverrier postulated that this discrepancy was due to the influence of a still undetected planet located between Mercury and the Sun, which he christened with the name ofVolcano. Years before, in 1846, he had deduced the existence of Neptune by the gravitational disturbances that it caused in the orbit of Uranus and this situation seemed to be the same. After intense and unsuccessful searches, astronomers decided that Vulcan did not exist and left the almost negligible theory-experiment discrepancy of 43 arc seconds per century parked in a dark corner of science. Atiny difference that was going to revolutionize physics.
In 1915, Albert Einstein launched hisgeneral relativity, anew theory of gravitythat completed and expanded the one formulated by Newton in 1687. And it was that insignificant little Chinese girl who had been in the shoe of astronomy for more than 70 years that gave him the accolade. Newton’s theory could not explain such a small difference; Einstein’s, yes. In the universe even the smallest differences are crucial. Since then, general relativity has been the standard theory of gravity, the foundation on which science rests.modern cosmology. Now, is it the ultimate theory? Some physicists think not. Because today we have equally tiny data that resist explanation. For example, certainmysterious movements of the moon.
The Soviet unmanned probeLuna-2-the first human device to land on another body in the Solar System- and the astronauts of the missionsApollo 11, 14 Y15 they left on the dusty surface of our satellite a series of aluminum reflectors, little mirrors measuring 50 square centimeters that allow us to calculate the distance to Earth with an accuracy of the order of one centimeter. Quite an achievement considering that the Moon is 384,000 km from us.
After analyzing 38 years of measurements, James G. Williams of theJet Propulsion LaboratoryNASA discovered thatthe lunar orbit is getting more and more elliptical. In 2006 he published his accounts: the difference between the furthest and the closest point in the orbit was unexpectedly increasing at a rate of 6 millimeters per year. After taking into account other effects, such as tidal frictions in the interior of the Earth and the Moon, Williams confirmed that the difference was still greater than expected, almost by a factor 3. At the moment, we do not have a clear explanation. to that deviation.
Another sign thatsomething may be happening with gravitywe have it in determining the unit of distance used by astronomers when referring to solar systems: theastronomical unit (AU). Defined as themean distance from the Earth to the Sun -149.6 million kilometers-It is obtained by radio wave observations of the relative positions between the inner planets, with an accuracy of between 1 and 10 meters. This is done from ground radio observatories or by telemetry from space probes. Thus, the transponders carried to Mars by the probes have been usedViking from 1976 to 1982 and those of thePathfinder in 1997.
They are then compared withplanetary positionscalculated with the equations of celestial mechanics and the value of thespeed of lightin astronomical units per day, instead of the usual meters per second. As the speed of light is a constant, it is enough to go from days to seconds to obtain the value of the astronomical unit in meters. The currently accepted value is calculated from the D405 ephemeris of the Jet Propulsion Laboratory (JPL), although in Eastern Europe the EPM2004 ephemeris of the Institute of Applied Astronomy of the Russian Academy of Sciences is used. Let’s see how little the two calculations differ, which can be attributed to experimental errors: the Americans have obtained the value of 149597870697 meters and the Russians, 149597870696.0. With an accuracy of eleven digits, it is the most accurate astronomical constant ever measured.
HoweverAU may not be as constant as astronomers assume. In 2004 E. Myles Standish of JPL stated that, using data collected between 1976 and 1997, the UA seemed to grow over time. Or what is the same, theEarth’s orbit radius is increasing. That same year, Russians Georgij Krasinsky and Victor Brumberg of the St. Petersburg Institute of Applied Astronomy analyzed more than 200,000 observations and found that it was increasing at a rate of 15 meters per century. In 2005, the astronomer Elena Pitjeva, from the same Institute, published an even more detailed analysis of 317,000 observations ranging from 1913 to 2003. The results of the astronomer, which determine the value of the AU with an accuracy of one meter, reveal that it grows about 7 meters per century. And it shouldn’t!
However, themost mysterious anomalyof all it is discovered with space probesPioneer 10Y11. The two are leaving the Solar System in different directions, but they are not where scientists expected. ThePioneer 10 was launched in March 1972 and, after studying Jupiter, is on its way toAldebaran star, where it will arrive in 2 million years. While thePioneer 11, launched the following year, passed Jupiter and Saturn and now travels toward theconstellation of the eagle; it will pass near the star Lamda Aquila in 4 million years. In the late 1990s, Michael Martin Nieto of Los Alamos National Laboratory, Philip Laing and Anthony Liu of Aerospace Corporation, and John D. Anderson, Slava Turyshev, and Eunice Lau of JPL studied data collected from thePioneer 10for 11 years andPioneer 11for 4. And they found they were slightly off course. The ships were experiencing a weak but steady deceleration: in 1998 Pioneer 10 was 36,000 miles closer than expected and the other twin craft was 6,000 miles closer. Something almost insignificant after 10.5 billion kilometers, but very significant because celestial mechanics are accurate enough to account for this anomaly. Who steps on the brakes of the ships? Many have been the hypotheses but none have been able to explain it in a satisfactory way. Is it somenon-gravitational effector is it the signal that there is something in gravity that escapes us?
THE ALLAIS EFFECT
In the middle of the last century the mathematician Maurice Allais , who would win the Nobel Prize in Economics in 1988, claimed to have found strange behavior in a pendulum. The most elementary physics tells us that the plane in which a pendulum oscillates is constant in time, it does not move, despite the fact that the Earth rotates on itself. This leaves visitors in awe of the famous Foucault pendulum located at the entrance to many science museums. But Allais claimed to have found during the solar eclipses of 1954 and 1959 that this was not the case. According to him, the plane of oscillation presented a very slight displacement – precession, in technical terms.
This same anomaly has been described by Saxl and Allen in 1970 and a team of Chinese scientists during the eclipses of 1997, 2001 and 2002. And even in 1961 the Romanian Jeverdan published this anomalous pendulum behavior in a non-English-speaking scientific journal, for which he was condemned to oblivion. Of all the alternative explanations proposed, none have explained these deviations from the theory, but it should also be noted that the different measurements made so far are not entirely convincing. More and much more precise experiments are needed to be able to establish the exact value of this anomaly. Some cheap experiments to carry out – something rare in an age of large accelerators or very expensive artificial satellites – and that if confirmed could revolutionize physics.
In 2006 Slava Turyshev was able to save from destruction part of the disks containing telemetric data, temperature and power of thePioneer until 2003. Together with Viktor Toth, a programmer from Ottawa, Canada, he devised the way to extract this data: the processing of 50 years of telemetry is no small feat. They thought that perhaps the energy released by the plutonium reactor that was lost in space as heat could explain this anomaly. However, calculations showed that it only accounted for 30%. Given these results, it is not surprising that Viktor Toth said: “If we really had a means ofmeasure deviations of Einsteinian gravity in the Solar SystemThat would be great! “
Unfortunately, what has been done withPioneer it cannot be done with the other pair of twin ships launched in the 1970s that are leaving our Solar System, theTravel: the gas that they continuously eject to stabilize themselves masks any other effects. The only opportunity we will have to verify this mysterious phenomenon will be with theNew Horizons. Launched in January 2006, it passed Jupiter in 2007 and Saturn the following year on its way toPluto, where it will arrive in 2015. Unfortunately, this ship has not been prepared for precision tracking as happened with thePioneerBut astronomers think they might be able to pull some useful data to shed some light on the mystery in 2010 and 2011. But the gravitational oddities don’t end there. In 2008, JPL scientists James K. Campbell, John E. Ekeland, James F. Jordan, and John D. Anderson advertised in the prestigious magazinePhysical Review Letters the discovery ofanomalous changes in the orbital energy of 6 spacecraftthat used the gravitational field of the Earth on their way to other planets, a classic maneuver to save fuel that is calledflyby.
The first clue of this anomaly was obtained by following theflyby fromGalileo probeon December 8, 1990. The spacecraft was approaching Earth at a speed of 8,981 meters per second. After discounting all the influences from the rest of the planets and the Moon, they found that it was going too fast. The excess was very small, on the order of 4 millimeters per second. It certainly wasn’t much, but it was there, like excess Mercury. Could it be a measurement error? It is not known. In the new step two years later the Galileo passed through the upper layers of the atmosphere, which prevented any attempt to make an accurate measurement. However, the passage of theNear Earth Asteroid Rendezvous-Shoemaker(NEAR-Shoemaker) on January 23, 1998 also showed that speed increase, almost three times greater than the Galileo measurement, 13.5 mm / s. The astrophysicists were dumbfounded. When the shipRosetta made itsflyby in 2005 the anomaly measured was 2 mm / s.
What causes that increase? Nobody knows. In the nextflyby Rosetta’s November 2007 release, nothing seemed to affect his speed. However, theCassini-Huygens, now orbiting Saturn, gained 0.11 mm / s. No wonder astronomers look forward to the next step in theRosetta before heading for its target, Comet Churyumov-Gerasimenko, on November 13 of this year. An effect that can be verified in later years, when the shipJuno launched in 2011 withdestiny to Jupiterand use the Earth’s gravitational field as a shuttle in October 2013.
Many are theexplanationsthat have been given for thesestrange phenomena. Most have to do with well-known effects, such as heat radiated fromPioneer. But if it is the thermal emission of the ship who steps on the brake, why does the ship not accelerate as the heat source cools? The rate at which they are slowing down has remained constant for 11 years. In 2006 Claus Lämmerzahl and Hansjörg Dittus of the University of Bremen and Olivier Preuss of the Max Planck Institute for Solar System Research systematically studied all the explanations given as possible causes of the anomalies of theflyby: Earth’s atmosphere, ocean tides, solid Earth tides, probe load, magnetic field, solar wind … None explains more than 10% of the total and the effect of most of the possible causes is 1,000 times smaller than necessary to affect the spacecraft.
Are we faced with anew physics? So think a few astrophysicists, who suspect that we areat the gates of a new theory of gravity. ? Astronomers have known since the 1970s that the outermost regions of galaxies appear to violate the laws of Newtonian gravitation ,? says John D. Anderson. At that time, everyone expected that the stars in those areas would behave according to the inexcusable laws of celestial mechanics: the orbital speed decreases as we move away from the galactic center. However, it was found thatall stars located in the outer areas of the galaxy move at the same speed regardless of their distance from the center. The only way to reconcile Newtonian gravity with observations was to postulate the existence of a halo of matter that envelops the galaxy. Of course, this solution raised new and uncomfortable questions. Why do we not observe any trace of its existence? If it were normal matter, it would emit a minimum of radiation, but we do not receive a single photon from that supposed halo.
Solution? A new flight forward stating that it is something totally different and strange to the one we know and about which there is not the slightest evidence of what it can be, a type of undetectable matter that can only be seen by the gravitational influence it causes. in the surrounding stars. This is thedark matter, a candy for theoretical physicists because they have a privateering license to stretch their theories and remove the most strange particles from the hat that can account for such colorful properties.
Dark matter faces a peculiar challenge: it has been almost 80 years since Fritz Ziwcky first postulated its existence and nobody knows what it is; There is not even a direct observation to confirm that it is there. There are currently a dozen experiments underway in underground laboratories – such as the one in Canfranc in the Aragonese Pyrenees – waiting to see a dark matter particle colliding with an atomic nucleus. Twelve others try to detect the annihilation of a pair of dark matter particles while waiting for the LHC in Geneva, the famousthrottlethat broke down shortly after opening, can produce it. Faced with this drought of evidence, what physics is asking for is an act of faith, very similar to what it asked in the 19th century with the also undetectable ether, infinitely rigid and elastic at the same time.
THE FOUNDATION FOR RESEARCH IN GRAVITY
In 1948 the businessman Roger Babson created this foundation destined to find a way to build a gravity shield. As in science fiction movies, or the famous cavorite stone with which HG Wells took his protagonists to the Moon, its purpose is to find a way to prevent gravity from developing its work. It even created an annual award for those scientists who made progress in this field. The foundation disappeared shortly after Babson’s death in 1967, but his award continues, which Stephen Hawking has won several times.
Such a situation has sown well-founded skepticism among some cosmologists. Chicago Fermilab scientists Scott Dodelson and Michele Liguori are looking for evidence to justify the existence of anew theory of gravityable toexplain the large-scale structure of the universe. “We have to be damned sure that we can’t explain the universe other than by accepting dark matter,” says Dodelson. The way they point is to modify Einstein’s theory, make it more precise, in the same way that the German genius did when completing the universal gravitation of Isaac Newton.
One possibility is the callcovariant relativistic theory of gravity, formulated in 2004 by Jacob Bekenstein. It is based on a modified version of Newtonian theory published in 1983 by Mordehai Milgrom. According to this Israeli physicist, Newton’s theory does not apply well to regions of extremely low density of matter. According to him, gravity is more intense in diffuse systems and this can explain the movement of stars in the outer regions without having to make use of the invisible and mysterious dark matter. Thus, in the same way that the Newtonian theory does not apply in strong gravitational fields and must be replaced by the Einsteinian, a new theory may be necessary for regions with very weak fields. This initial Milgrom proposal has been taken up, completed and corrected by Bekenstein. But there is still a long way to go. “We have to check all the predictions of the new theory and compare it with the dark matter paradigm to see if it is a viable alternative,” adds Liguori.
Another physicist, John Moffat, has proposed anon-symmetric gravitational theorywhich also tries to explain the mystery of dark matter. In essence it follows the same path that Einstein traveled in the last years of his life and failed: to find a unified field theory. The matter is quite technical and the key lies in cCompare the equations of the electromagnetic field with those of the gravitational field. There is a peculiar difference between them that, if it were solved, would point to “the existence of anew fundamental force in the universe“, dice Moffat.
The mathematical complications of this scientific adventure are Homeric and so far only approximations have been found to what could be a new theory, but the few physicists who work on it think that they will be able to expel that inexplicable dark matter from the universe.
And if we did not have enough of it, in 1997 cosmologists added an even greater complication: that year it was discovered that theuniversewasspeeding up. General relativity says that if there is no matter other than burning, it is impossible then “something” must be pushing. The fact that? Nothing easier. We already had dark matter to explain the motion of stars, then let’s introduce something even stranger to explain this acceleration:dark energy. However, a small number of scientists argue that it is only a new indication that we must modify our conception of gravity. Among them are Martin Kunz and Domenico Sapone. The fundamental problem, as these two physicists state, is that “we cannot directly observe dark energy or modified gravitation.” There is no way to experimentally distinguish between the two: “they both seem the same,” Kunz adds. His hope is to study certain aspects related to the phenomenon known asgravitational lens, that is, the effect that the presence of nearby gravitational fields has on the path traveled by a ray of light. Time will tell if Einstein was right after all.
Miguel Angel Sabadell
