by Richard Obousy
Physicist Richard Obousy here takes a look at an intriguing new paper by Mike McCulloch, a researcher at Plymouth University. In addition to his work in theoretical physics and warp drive possibilities, Obousy is current project leader and primary propulsion design lead for Project Icarus, a joint venture between the British Interplanetary Society and the Tau Zero Foundation to re-think the original Project Daedalus starship design. In the review below, Obousy places McCulloch’s work on the Pioneer anomaly in the context of current thinking on dark matter, dark energy and the nature of mass. Does the Higgs field explain inertial mass, or are there alternatives? Read on.
Few areas of research have garnered as much attention from both the public and scientific communities as those of dark energy and dark matter – and for good reason. Both terms stem from observations of the physical universe that, simply put, don’t belong within the well-understood framework of known physics. Another phenomenon discovered in the nineties concerns an anomalous acceleration of the Pioneer probes. These ostensibly unrelated observations may, in fact, be connected to each other by an intriguing line of research currently being investigated by Mike McCulloch, a researcher at the University of Exeter. Before exploring McCulloch’s research, a brief review of dark energy, dark matter, and the anomalous Pioneer acceleration will be presented.
Dark matter is a proposal put forward to explain the observations first made by Zwicky in 1933 that galaxies were too energetic to be held together by observable matter. Zwicky originally proposed the existence of an unseen form of baryonic matter that provided the necessary gravitational force to hold the galaxies together. Due to constraints imposed by modern cosmology, the idea has evolved to assume this form of matter is non-baryonic (not made of quarks); however, the fundamental idea has remained unchanged. After decades of searching for dark matter, none has been directly detected, but a number of experiments are ongoing.
Dark energy stems from the truly astounding observation made originally by Riese and Perlmutter in the late 90’s that the rate of cosmological expansion, long thought to be either static or decelerating, is actually accelerating. For this to be happening, it is commonly believed that the universe is filled with a ubiquitous and exotic negative pressure field that drives the accelerated expansion. Although we can give this energy a name, and predict what it will do, dark energy as a ‘real’ physical field has never actually been measured in the lab, and today, dark energy remains somewhat of an enigma.
As if dark energy and dark matter haven’t dealt theoreticians enough of a blow, cracks began to appear in our understanding of gravity due to the observation made by Anderson et al in 1996 that both Pioneer 10 and 11 are experiencing an anomalous acceleration of 8.74±1.33×10-10 m/s2 directed approximately towards the sun. It is precisely this anomaly that is studied by Mike McCulloch in his recent publication in Europhysics Letters called Minimum Accelerations from Quantized Inertia (reference below). McCulloch’s work addresses the Pioneer anomaly, and within the framework of his model, one could perhaps come to a deeper understanding of dark matter and dark energy thanks to a novel idea known as MOND, or Modified Newtonian Gravity.
The basic idea that McCulloch explores is the nature of mass, and the possibility that inertial mass, in fact, changes slightly under certain conditions. It has been known since the time of Newton that all bodies attract all other bodies in the universe with a force that is proportional to their mass. This type of mass is what is known as gravitational mass. It is also known that when one applies a force to an object, it accelerates at a magnitude that is proportional to its mass. This type of mass is known as inertial mass. It is commonly assumed that gravitational and inertial mass are identical, and this has been verified by our highest precision instruments to date.
The fundamental nature of inertial mass is not precisely known and is an issue that has been pondered at least since the time of Mach. Recent efforts to codify inertial mass into the Standard Model (SM) of particle physics have resulted in the famous Higgs field, which is a ubiquitous field that bestows mass upon matter via a process known as spontaneous symmetry breaking. Although the Higgs field has not been experimentally detected, many physicists are confident that it will be found at the Large Hadron Collider.
Despite the widespread acceptance in the existence of the Higgs field, there have been alternative attempts to uncover the nature of inertial mass. One paper, Inertia as a Zero Point Lorentz Force, written in 1994 by Rueda, Puthoff and Haisch (RPH), represents a stalwart effort to model inertia as a back-reaction of matter to the quantum vacuum similar to the Unruh field. Despite not gaining widespread acceptance in the theoretical community, the paper galvanized interest in the possibility that the quantum vacuum and inertial mass may be related. The basic premise of the paper was that matter, modeled as a ‘Parton’, interacts with the quantum vacuum in such a way that any acceleration generates a Lorentz-type back-reaction to the vacuum which manifests itself macroscopically as a resistance to acceleration or, more simply, as inertial mass.
The RPH paper was not the first to suggest that accelerated matter is effected by the quantum vacuum. In 1976, Unruh showed that a body undergoing an acceleration in the vacuum sees a thermal radiation of temperature T that is related to its acceleration. Wien’s displacement law tells us that, for a given temperature, there will be a dominant wavelength which, via the Unruh effect, is inversely proportional to the acceleration – namely, as the acceleration gets smaller, the radiation wavelength gets bigger. As the acceleration decreases, this wavelength reaches a limiting value: the wavelength of the observable universe. Milgrom, in 1994, speculated that at this point, there would be a ‘break in the response to the vacuum’ and the Unruh radiation would be unobservable. He further speculated that this could have an effect on inertial mass. Herein lies the crux of this line of thinking – that matter’s response to the vacuum is what generates inertia.
McCulloch further develops the idea of Milgrom by allowing for a more natural development in the Unruh radiation spectrum. In the original idea by Milgrom, only the dominant wavelength was considered. McCulloch, however, develops what he calls a Hubble-Scale Casimir effect, where a range of wavelengths are allowed based on the boundary conditions of the size of the observable universe.
“The new assumption is that this Unruh radiation is subject to a Hubble-scale Casimir effect. This means that only Unruh wavelengths that fit exactly into twice the Hubble scale (harmonics with nodes at the boundaries) are allowed, so that a greater proportion of longer Unruh waves are disallowed, reducing inertia in a new, more gradual, way for low accelerations.”
Using this model, McCulloch is able to develop an equation which illustrates the modification of inertial mass for low accelerations. Put in simpler terms, as the Pioneer probes depart our solar system they experience a force due to the gravitational attraction of the sun. This force generates an acceleration which, due to its extremely small value, modifies the inertial mass of the pioneer probe. Because of this modification, the Pioneer probes, seemingly now less massive, feel a greater acceleration due to the sun than that predicted by Newtonian mechanics, creating the anomalously large acceleration.
How does this all relate to dark energy and dark matter? The answer is in the relationship between certain natural scales that occur in physics. The basic building block is the scale that characterizes the cosmological constant. We call this scale R and it is the distance scale over which the cosmological constant curves the universe. R is about 10 billion light years and is 1040 times the size of an atomic nucleus – the scale where the standard model of particle physics is applicable). R is also 1060 times the Planck scale – the scale at which we believe in GUT’s (Grand Unified Theories), where all the forces in nature behave identically. It is therefore pragmatic to wonder whether this scale R might be indicative of some new physics.
Hints at new physics at the scale R manifest themselves in the cosmic microwave background (CMB) – thermal radiation left over from the Big Bang. This radiation has been cooling as the universe expands, and is now at a fairly uniform temperature of 2.7 degrees Kelvin. Fluctuations in this temperature exist to a level of a few parts per 100,000, and the patterns of these fluctuations provide us with clues to the physics of the early universe.
Analysis of the temperature fluctuations over the last decades illustrate how much energy is contained in this radiation as a function of wavelength. It appears that the CMB is dominated by a single large peak, followed by a number of smaller peaks. It also appears that there is very little energy in the longest wavelength. This data can be interpreted as indicative of a ‘cutoff’, above which the thermal modes are less excited. What is particularly remarkable is that this cutoff occurs on a scale R which we associate with the cosmological constant.
This cutoff is somewhat puzzling from the perspective of inflation theory, which was developed by Alan Guth of MIT and, originally, by Alexei Starobinsky of the Landau Institute for Theoretical Physics in Moscow. According to the theory of inflation, the early and rapid expansion of the universe created huge regions of the cosmos with relatively uniform properties. This region is thought to be much larger than the observable universe. The cutoff indicates that, at the scale R, inflation stopped just at the point where it created a region as large as we now currently observe. If, in fact, inflation ‘switched off’ just at the point where it created the cosmos as large as we currently observe, then some physical mechanism must have been responsible for selecting this unique time to stop. This seems incredibly improbable, since nothing in the physics of inflation says anything about scales on the order of 10 billion light years.
Said another way, if inflation produced a largely uniform universe, then it likely produced uniformity on scales much larger than we observe. Thus, the patterns produced by inflation, the small fluctuations, should be visible beyond the present size of the universe. Instead – what the data indicate is that these fluctuations stop above the scale R.
Another indication that new physics may occur at scales on the order of R is an apparent asymmetry in the distribution of hot and cold spots in the CMB dubbed the ‘Axis of Evil’. This observation was first made in 2005 by Kate Land and Joao Magueijo of Imperial College London. A number of independent studies have confirmed this apparent alignment of anisotropies in the CMB.
There are additional phenomena associated with the scale R that are worth discussing. One way we can explore R is to combine it with additional constants of nature. An interesting place to start is to combine it with the speed of light, c, to give us R/c. Dimensionally, R/c gives us a time, and that time corresponds to the present age of the universe. Taking the reciprocal of this, c/R, gives a frequency, a profoundly low ‘note’ which has completed one oscillation in the entire lifetime of the universe.
Going one step further, we can explore c2/R which, dimensionally, gives us the units of acceleration. Remarkably, this number is the acceleration produced by the cosmological constant. This is the same acceleration that we currently believe dark energy is responsible for and is on the order of 10-10 m/s. This also happens to be the roughly the same anomalous acceleration that the Pioneer probes are currently experiencing!
The c2/R also crops up when we examine rotational velocity of orbiting stars in galaxies. Recall that stars are seen to rotate at a velocity that would, according to Newtonian Mechanics, be too fast for them to be held in a stable orbit. The contemporary fix for this problem is to introduce dark matter. This is not the only fix, however. For spiral galaxies, in which stars move in circular orbits, anomalous velocities (orbital velocities that, according to Newtonian Mechanics, should not be possible) are only apparent beyond a certain orbit. Within this ‘special’ orbital distance Newtonian gravity works perfectly. Because stars move in a circular orbit they experience an angular acceleration which is related to their velocity (a=v2/r). The breakdown of Newtonian gravity occurring at this ‘special’ orbital distance occurs when the stars are rotating with an angular acceleration of 1.2×10-10 m/s2, almost identical to the scale c2/R. This is thoroughly fascinating, and this string of relationships which appear to be related to the scale R represent tantalizing hints at physics beyond what is currently studied and practiced within the mainstream academic community.
Today, nobody knows for certain what this new physics is (if it really is new physics), and nobody has written down a theory codifying its behavior. Mike McCulloch, however, is arguably helping to increase momentum within this curious and remarkable area of research.
The paper is McCulloch, “Minimum accelerations from quantised inertia,” Europhysics Letters Vol. 90, No. 2 (20 May, 2010). An abstract is available, with full text here. The paper by Rueda, Haisch and Puthoff is “Inertia as a Zero Point Lorentz Force,” Physical Review A, Vol 49, No 2 (February 1994), pp.678-694.
Robert Forward’s Indistinguishable from Magic is a genial and absorbing read, a collection of essays and fiction illustrating some of the scientist’s most memorable ideas. And while gigantic lightsails driven by laser beam to other stars always come to mind when Forward’s name is mentioned, it’s fascinating to page through his thoughts on antimatter, black holes and time machines. Long a Forward admirer, I was pleased to see that another of the concepts discussed in this book recently made an appearance at this month’s solar sail conference in Brooklyn.
‘Statites’ are a Forward construct, a word he coined to describe a spacecraft that uses a solar sail to hover over a region rather than orbiting the Earth. Let Forward describe what he calls a ‘technique for hanging things in the sky’:
…I have the patent on it — U.S. Patent 5,183,225 “Statite: Spacecraft That Utilizes Light Pressure and Method of Use”… The unique concept described in the patent is to attach a television broadcast or weather surveillance spacecraft to a large highly reflective lightsail, and place the spacecraft over the polar regions of the Earth with the sail tilted so the light pressure from the sunlight reflecting off the lightsail is exactly equal and opposite to the gravity pull of the Earth.
You can see where Forward is going with this. This is a solar sail that isn’t designed for transport but for station-keeping, and it offers options that other kinds of satellite do not. But maybe satellite is the wrong word:
With the gravity pull nullified, the spacecraft will just hover over the polar region, while the Earth spins around underneath it. Since the spacecraft is not in orbit around the Earth, it is technically not a satellite, so I coined the generic term ‘statite’ or ‘-stat’ to describe any sort of non-orbiting spacecraft (such as a ‘weatherstat’ or ‘videostat’ or ‘datastat’).
Forward always noted that he had made no money from his patent, but said he didn’t want to make the mistake Arthur C. Clarke did when he failed to obtain a patent on his idea of the geosynchronous communications satellite. In a short story included in Indistinguishable from Magic called ‘Race to the Pole,’ Forward writes about a statite called the ‘Hovering Hawke’ that uses a kilometers wide square lightsail to support a powerful broadcast satellite. Such a ‘polesitter’ would, by Forward’s calculations, need to be too distant to serve as a communications satellite, but direct broadcast or weather surveillance would be robust applications.
Just how distant would a polesitter have to be? In this passage from the story, a scientist explains the difficulty a statite would experience maintaining a stable position as the Earth’s seasons change:
“The control problem of keeping the [statite] balanced over the pole is very tricky, especially during the summer season of that hemisphere when the polar axis is over on the sunlit side of the Earth. That’s why ‘pole-sitters’ have to be placed so far away from the Earth. If they get any closer than 250 Earth radii, they become unstable during the summer.”
Forward’s patent ran out in February of this year, but his idea is beginning to gain traction. At the Second International Symposium on Solar Sailing, which ended just last week, Matteo Ceriotti reported on work with Colin McInnes describing what Forward called ‘displaced orbits’ that would allow geosynchronous telecommunications satellites to be deployed to the north or south of the Earth’s equator. Working with graduate student Shahid Baig, McInnes (University of Strathclyde) has published a new paper that shows the viability of displaced orbits. Says McInnes:
“Satellites generally follow Keplerian Orbits, named after Johannes Kepler – the scientist who helped us understand orbital motion 400 years ago. Once it’s launched, an unpowered satellite will ‘glide’ along a natural Keplerian orbit. However, we have devised families of closed, non-Keplerian orbits, which do not obey the usual laws of orbital motion. Families of these orbits circle the Earth every 24 hours, but are displaced north or south of the Earth’s equator. The pressure from sunlight reflecting off a solar sail can push the satellite above or below geostationary orbit, while also displacing the centre of the orbit behind the Earth slightly, away from the Sun.”
Image: Analog‘s December, 1990 issue contained an article by Robert Forward describing the ‘polesitter’ concept, one of many innovative ideas the scientist introduced to a broad audience. Credit: Condé Nast.
No, we’re not in ‘polesitter’ range, not yet, anyway. But these displaced orbits would allow solar sails — McInnes is interested in hybrid sails complemented by electric propulsion systems — to be displaced between 10 and 50 kilometers from the equator. As we continue our work with solar sails, finding ways to make them robust enough to handle polar stationary orbits seems like a reasonable expectation. Another Forward concept thus moves into sharper definition. I can only imagine how much the late Dr. Forward would have enjoyed sitting in on the relevant session at the solar sail conference, and reading the McInnes paper.
The paper is Baig and McInnes, “Light-Levitated Geostationary Cylindrical Orbits are Feasible,” Journal of Guidance, Control and Dynamics, Vol. 33, No. 3 (2010), pp. 782-793 (preprint). You’ll also enjoy reading the non-fiction piece Robert Forward wrote for Analog. It’s “Polesitters,” published in Analog Science Fiction/Science Fact Vol. 110, No. 13 (December, 1990), pp. 88-94.
We’re learning a lot more about how planets interact with each other gravitationally. ‘Resonance’ is the operative term here. When planets are locked in a 2:1 orbital resonance, the outer planet orbits the host star once for every two orbits of the inner planet. A 3:2 resonance occurs when the outer planet orbits the star twice for every three orbits of the inner planet.
Resonance (technically ‘mean motion resonance’) prevents close encounters between planets and provides long-term orbital stability. And if the 2:1 resonance is the most common pattern, it’s also true that things can change when planets migrate to different parts of their system. John Johnson (Caltech) describes the result of fast inner migration:
“Planets tend to get stuck in the 2:1. It’s like a really big pothole. But if a planet is moving very fast it can pass over a 2:1. As it moves in closer, the next step is a 5:3, then a 3:2, and then a 4:3.”
Johnson’s work on resonance has born fruit in a new paper in which he and his colleagues discuss the discovery of two solar systems where gas giants in relative proximity to each other have become locked into resonance. Studying the matter helps us understand how solar systems evolve, as planets farther out in the protoplanetary disk migrate inwards, causing gravitational disturbances that can only become stable in orbital resonance.
Studying the star 24 Sextantis, some 244 light years from Earth, using radial velocity methods, the researchers have found two gas giants separated by about 0.75 AU, roughly 113 million kilometers. You can contrast this with the spacing between the largest planets in our system. Jupiter and Saturn are never closer than 531 million kilometers. The planets orbit the star with periods of 455 days and 910 days and are locked in a 2:1 orbital resonance.
A second gas giant pairing occurs around the star HD 200964, some 223 light years from Earth. Here the distance between the two gas giants can close to 0.35 AU (53 million kilometers). Johnson likens the latter pairing to that of Titan and Hyperion, two Saturnian moons, which also show a 4:3 resonance, but notes that the planets orbiting HD 200964 interact far more strongly, each being 20,000 times more massive than the combined mass of Titan and Hyperion. The planets in this system have orbital periods of 630 and 830 days respectively. Johnson adds:
“This is the tightest system that’s ever been discovered, and we’re at a loss to explain why this happened. This is the latest in a long line of strange discoveries about extrasolar planets, and it shows that exoplanets continuously have this ability to surprise us. Each time we think we can explain them, something else comes along.”
Gravitational interactions in this environment are quite powerful. This Caltech news release notes that the gravitational tug between HD 200964’s two planets is 3 million times greater than the gravitational force between Earth and Mars, 700 times larger than that between the Earth and the Moon, and 4 times larger than the pull of the Sun on the Earth.
As to the history of these worlds, the paper on this work notes their current positions and their likely changes over time:
In both the 24 Sex and HD 200964 systems, the planets reside well within the so-called snow line, beyond which volatiles in the protoplanetary disk can condense to provide the raw materials for protoplanetary core growth. For a pre-main-sequence, 1.5 M [solar mass] star the snow line is located beyond 2-3 AU… It is therefore likely that the planets around 24 Sex and HD 200964 formed at larger semimajor axes and subsequently experienced inward orbital migration.
Both of these stars are massive and dying, subgiants that have evolved off the main sequence and have run out of hydrogen for nuclear fusion. The eventual fate of such stars is to become a red giant, but neither of the stars has progressed that far. While red giants are problematic for radial velocity methods because their pulsations mask the spectral shifts that would reveal orbiting planets, subgiants have not expanded to that point and planet hunting remains possible. In fact, using the Keck Subgiants Planet Survey, Johnson and team are learning a great deal about such systems:
“Right now, we’re monitoring 450 of these massive stars, and we are finding swarms of planets. Around these stars, we are seeing three to four times more planets out to a distance of about 3 AU — the distance of our asteroid belt — than we see around main-sequence stars. Stellar mass has a huge influence on frequency of planet occurrence, because the amount of raw material available to build planets scales with the mass of the star.”
The paper is Johnson et al., “Retired A Stars and Their Companions VI. A Pair of Interacting Exoplanet Pairs Around the Subgiants 24 Sextan[t]is and HD 200964,” accepted for publication in The Astronomical Journal (abstract).
by Larry Klaes
The Faces from Earth project, run so energetically by Tibor Pacher, is planning its next ‘E.T. Are You Out There?’ campaign, following a successful campaign in May that introduced interstellar concepts to school children in five countries. In this piece, journalist Larry Klaes looks back at the Voyager spacecraft, which will be the subject of the new Faces from Earth campaign. The Voyagers electrified all of us with the discovery of volcanoes on Io and a possible ocean beneath Europa’s ice, and the ensuing stream of images from planets and moons never before seen up close. They also carried golden discs bearing information about their builders. As of this morning (EST), Voyager 1 is 15 hours, 44 minutes, 56 seconds in light-travel time from home, at the edge of the Solar System but, as Larry makes clear, hardly forgotten.
In the first decade of the Space Age, humanity succeeded in sending a handful of robotic space probes to Earth’s two nearest planetary neighbors, Venus and Mars. The voyages of these mechanical vessels, which only took a matter of months, were brief in their visits to these alien worlds. Nevertheless, these new kinds of explorers gave scientists their first knowledge of the true natures of these places after centuries of speculation.
Much farther beyond, where the Sun is eventually reduced in appearance to just a very bright star, is the realm of the outer gas giant worlds. These planets are many times larger than all of the inner terrestrial globes put together and lack solid surfaces in the same sense as our Earth and its celestial brethren. The Jovian planets also keep in their mighty gravitational grips collections of moons and rings of debris that would qualify them as whole solar systems in their own right.
But for humanity in the early days of space exploration, these alien places were very far away and full of unknowns, including whether a fast-moving spacecraft could navigate the natural boundary between the terrestrial and Jovian realms known popularly as the Asteroid Belt without being smashed to pieces by potentially deadly dust and meteoroids. In addition, a spacecraft of that era would take decades to reach all the outer worlds; such vessels were still on the proverbial drawing boards, while most of the actual probes which did reach the nearest worlds in functioning order often did so with a lot of luck and engineering skill.
Beginnings of a Grand Tour
In that same time, when the two main players of the Space Age were preparing to see who could place the first humans on the lunar surface, it was determined that the outer planets would align in such a way in their solar orbits in the late 1970s that they could be reconnoitered by a quartet of nuclear-powered space probes flying past each world in just one decade. The plan and the mission were appropriately named the Grand Tour.
Early on the project was threatened with termination, not by some hazard in space but by budgetary problems on Earth. To stay alive in NASA, the Grand Tour was scaled back to explore just the two nearest gas giants, Jupiter and Saturn. The vessel numbers were reduced from four to two: The remaining probes were christened Mariner 11 and 12, following in the line of American space probes that had opened the way to understanding the inner Solar System. By the time the vessels that remained from the initial outer worlds exploration plan were ready to be launched into the heavens in the late summer of 1977, there were a number of further significant changes to the mission.
Image: The Voyager spacecraft. Voyager’s ‘Golden Record’ can be seen attached to a panel of the spacecraft’s instrument housing. Credit: NASA.
Up front, the twin spacecraft had their names changed from Mariner 11 and 12 to Voyager 1 and 2. This was done both to reflect their expanded designs and goals beyond what the earlier Mariners had accomplished and to make the probes and their missions more exciting to the public. The Voyager team also hoped that, though the craft were still officially meant to explore just Jupiter and Saturn, they would be strong and adaptable enough to complete most of the original Grand Tour plan by reaching Uranus and Neptune just over one decade hence.
Finally, just months before the two Voyagers would leave Cape Canaveral in Florida aboard separate powerful Titan 3E/Centaur rockets, a small group of far-seeing individuals convinced NASA to place a sampling of sights and sounds of our world and our species engraved onto two golden records which were subsequently attached to the sides of the Voyagers. These discs would accompany the probes past the outer worlds into the wider realm of the Milky Way galaxy. These artifacts would serve as a long-lasting record and tribute to the beings who built and launched these early interstellar wanderers and as a greeting for either their distant children or other intelligences that may move among the stars.
Exploring the Gas Giants
With their missions spanning the second decade of the Space Age, the two Voyagers truly revolutionized our understanding of the outer Solar System, in spite of the fact that they were not the first vessels from Earth to explore that region of our celestial neighborhood. That honor went to Pioneer 10 and 11, which flew past Jupiter in 1973 and 1974, respectively, with Pioneer 11 going on to flyby Saturn in 1979. The Pioneer probes then headed off into interstellar space carrying golden plaques engraved with basic information about humans, our Solar System, and our place in the galaxy. Nevertheless, the improved technologies aboard the Voyagers allowed scientists to surpass what was seen and found at and about those enormous globes by either the Pioneers or Earth-bound astronomers of the era.
At their first destination, Jupiter, the Voyagers revealed the incredibly complex patterns of the planet’s cloud patterns, including the Great Red Spot, which was confirmed to be a hurricane system three times the size of Earth that has been churning in the Jovian atmosphere for at least four centuries. Amazing as this was, what captured even more attention from the scientists, media, and public alike were the four large Galilean moons that circled Jupiter, collectively named after the Italian astronomer who discovered them in 1610. These moons were truly worlds in their own right and not the relatively sedate places initially thought to be.
Image: This dramatic view of Jupiter’s Great Red Spot and its surroundings was obtained by Voyager 1 on Feb. 25, 1979, when the spacecraft was 9.2 million kilometers from Jupiter. Cloud details as small as 160 kilometers across can be seen here. Credit: NASA Planetary Photojournal.
The innermost of the Galilean moons, named Io, turned out to have highly active volcanoes spewing molten sulfur all over its surface and far into space. Alien volcanoes had been seen before, on the planet Mars, but Io’s were anything but extinct, to say nothing of being almost completely unexpected before the Voyagers came on the scene in 1979. The next moon nearest to Io, called Europa, was a contrast: The moon’s surface was icy and smooth, populated by long dark lines across its face, with only a few impact craters large enough to be visible to Voyagers’ cameras. But underneath Europa’s covering of ice appeared to be a different story: A global ocean of briny liquid water perhaps sixty miles deep with twice the volume of all the water on Earth! Though certainly not visible to the instruments of its mechanical discoverers, serious speculations on the possibility for living creatures and what forms they might take in the distant waters of Europa wasted little time in appearing.
Thanks to the Voyagers, worlds that were once hardly even considered as abodes of geological activity and life were now seen as even better prospects for living organisms than the traditional worlds in those categories. Voyagers’ discoveries at Jupiter, perhaps more than any other place the probes would fly past on their journeys out of the Solar System, truly changed humanity’s perspectives on the alien realms inhabiting the outer reaches of our celestial neighborhood. Witnessing the truly dynamic nature of our Solar System through the Voyagers also enriched and expanded our thinking about worlds and beings around other suns, made all the more plausible by the discoveries of extrasolar planets in the years since the primary Voyager missions, of which most thus far appear to be similar to Jovian worlds.
Introducing Ourselves to the Cosmos
Thirty-three years after leaving Earth and twenty-one years after Voyager 2 had flown past the last of the gas giant planets, Neptune, both Voyagers continue to function and return priceless data on regions of the outer Solar System where no human-made spacecraft has ever been before. This area, known as the heliosphere, is considered part of the cosmic boundary between our Solar System and where true interstellar space lies. Perhaps before they expire around 2025, one or maybe both of the Voyagers will last long enough to perform one more scientific mark by revealing the constituents of deep space beyond the influence of our Sun.
The very fact that the Voyagers would be propelled into the Milky Way galaxy by their interactions with the giant planets of the outer Solar System is what inspired the late Cornell astronomer and science popularizer Carl Sagan and others to create what has become known as the Voyager Interstellar Records. While he and others knew the odds of the Voyagers ever being found by other intelligences were small, the fact that the probes would be only our third and fourth artifacts sent to the stars compelled Sagan and his companions to utilize this opportunity to preserve something of ourselves where it could last far longer than anywhere on Earth, perhaps one billion years or more.
Image: The Voyager ‘Golden Record,’ containing the thoughts, images and sounds of Earth. Credit: NASA.
Most importantly, while the Voyagers were built and launched by the United States of America, the golden records were designed to represent as much of our whole human species and the rest of life on Earth as possible in images, words, sounds, and music. Scanning through the contents of the records (which can be done from this Web site: http://goldenrecord.org/) one gets a definite sense of our being one species on just one world among hundreds of billions of stars in a vast Universe composed of billions of galaxies. To quote Carl Sagan in the Epilogue of the 1978 book on the Voyager Interstellar Record titled Murmurs of Earth: “But one thing would be clear about us: No one sends a message on such a journey, to other worlds and beings, without a positive passion for the future. For all the possible vagaries of the message, they could be sure that we were a species endowed with hope and perseverance, at least a little intelligence, substantial generosity and a palpable zest to make contact with the cosmos.”
Carrying on Voyager’s Work
Faces from Earth is paying tribute to the Voyager missions and the grand concepts embodied by them and all those who made it possible. For in a very real sense the space probes became reality by a collective effort of our civilization, both directly and indirectly. The Voyagers gave us our first real taste, both through their journeys past immense and amazing alien worlds as never witnessed before and the messages and information they carry in those small golden discs on their sides, of what the Cosmos is really like and how we in turn appear in relation to all that vastness of space and time.
Faces from Earth is keeping alive those representations and messages for the current and future generations of humanity, for we are even now becoming Citizens of the Galaxy, no longer confined to the savannahs or villages or even this single planet. While we are still very much the biological creatures of our distant ancestors, we now have an awareness and abilities they never dreamed of. We must mature into the species we are moving towards, one which embodies and embraces new worlds and new ways of life scarcely imagined in the past or even now. Faces from Earth will work together with all those who share the dream of being part of a humanity achieving its full potential among the stars.
For more on the ongoing Voyager missions, try these sites:
NASA/JPL Voyager Web site:
Voyager Interstellar Record information and links (see External Links at end of main page):
Where are the Voyagers right now?
Time travel holds such perennial fascination that even though its relationship with interstellar issues is slim, I can’t resist reporting on new ideas about it. John Cramer’s time experiments seem stuck in limbo, but now we have new work from Seth Lloyd (MIT) and colleagues about one way out of the paradoxes time travel seemingly creates. The ‘grandfather paradox,’ returning to the past to kill your own grandfather and thus causing your future self not to exist, seems inevitable if we grant the existence of what are called ‘closed timelike curves’ (CTCs), the paths through spacetime that would let a time traveler interact with his or her self in the past.
Ways Around Paradox
Lloyd’s team gets past that problem by describing a particular version of closed timelike curves formed with what is called ‘post-selection.’ The idea is to describe these CTCs in terms of quantum mechanics, starting with the assumption that time travel is a communications channel from the future to the past. Is there, then, a quantum communication channel to the past? The researchers consider quantum teleportation, in which a quantum measurement combined with classical communication lets quantum states be transported between sender and receiver.
The paper then applies quantum teleportation to timelike curves with postselection (P-CTCs):
We show that if quantum teleportation is combined with post-selection, then the result is a quantum channel to the past. The entanglement occurs between the forward- and backward-going parts of the curve, and post-selection replaces the quantum measurement and obviates the need for classical communication, allowing time travel to take place. The resulting theory allows a description both of the quantum mechanics of general relativistic closed timelike curves, and of Wheeler-like quantum time travel in ordinary spacetime.
As best I can untangle this (and we’ll deal with Wheeler in a moment), the post-selection idea means that time travel paradoxes are ruled out. Try to perform the event causing the paradox and something will happen to make the action fail. Moreover, although this theory of post-selection in timelike curves was created to deal with quantum mechanics in CTCs following the principles of general relativity, the authors think it extends to other contexts. Quantum theory that allows entanglement, in other words, seems to allow time travel even when no spacetime closed timelike curve exists.
Tunneling Through Time
Lloyd’s team says this quantum time travel can be thought of as ‘a kind of quantum tunneling backwards in time, which can take place even in the absence of a classical path from future to past.’ That’s a helpful thought, given that the extreme distortions of spacetime required by more traditional time travel thinking in a relativistic context are all but impossible to create.
Interestingly, there already exists a growing literature on entanglement and projection in the development of timelike curves, all described briefly in this paper. But the authors are particularly careful to note John Wheeler’s ideas impinging on quantum time travel, ideas that Richard Feynman described in his Nobel Prize lecture. This is worth repeating:
‘I received a telephone call one day at the graduate college at Princeton from Professor Wheeler, in which he said, “Feynman, I know why all electrons have the same charge and the same mass.”
“Because, they are all the same electron!”
And, then he explained on the telephone, “Suppose that the world lines which we were ordinarily considering before in time and space – instead of only going up in time were a tremendous knot, and then, when we cut through the knot, by the plane corresponding to a fixed time, we would see many, many world lines and that would represent many electrons, except for one thing. If in one section this is an ordinary electron world line, in the section in which it reversed itself and is coming back from the future we have the wrong sign to the proper time – to the proper four velocities – and that’s equivalent to changing the sign of the charge, and, therefore, that part of a path would act like a positron.”’
And now we’re really in Wonderland. Post-selection accepts only particular results, meaning that the only states that can be teleported via quantum entanglement are those that are consistent with the world we know. Time travel in this guise is necessarily consistent with our reality and forbids any actions that would create paradoxes. The authors put it this way: “…although any quantum theory of time travel quantum mechanics is likely to yield strange and counter-intuitive results, P-CTCs appear to be less pathological. They are based on a different self-consistent condition that states that self-contradictory events do not happen…”
Ratcheting Up Improbabilities
In an article on this work in Science News, Laura Sanders takes note of the fact that ruling out paradoxes means that unlikely events may happen with greater frequency:
“If you make a slight change in the initial conditions, the paradoxical situation won’t happen. That looks like a good thing, but what it means is that if you’re very near the paradoxical condition, then slight differences will be extremely amplified,” says Charles Bennett of IBM’s Watson Research Center in Yorktown Heights, N.Y.
For instance, a bullet-maker would be inordinately more likely to produce a defective bullet if that very bullet was going to be used later to kill a time traveler’s grandfather, or the gun would misfire, or “some little quantum fluctuation has to whisk the bullet away at the last moment,” Lloyd says. In this version of time travel, the grandfather, he says, is “a tough guy to kill.”
So we have no paradoxes but we seem to be distorting probability, a very strange result but maybe a bit less strange than the paradoxes we’ve avoided. Time travel makes for eerily seductive fiction — who would not wonder about traveling into the past to see loved ones again, or to remedy some unintentional wrong — and judging from the number of emails I received pointing me to this paper, the idea is as compelling now as it has ever been. I hadn’t realized how far back time travel has resonated in history, but the paper notes an account in the Hindu epic called the Mahabarata in which King Revaita visits the Brahma’s palace, stays for only a few days, and returns to Earth only to find that many eons have passed in his absence.
This is more or less the idea behind the creaky science fiction story “Out Around Rigel” (Astounding Stories, December 1931), in which Robert H. Wilson imagines the first journey to another star and uses the event as a way to teach Einsteinian special relativity (the first time this was done in science fiction, to my knowledge). The crew returns to find a thousand years have passed during their six-month journey. But this is a time travel account from the standpoint of relativistic spacetime. Quantum mechanics, in this paper’s estimate, might give us options other than that one-way ticket to the future.
How post-selection would work in quantum mechanics has yet to be determined, but the authors discuss the possibility of testing their theory experimentally by using quantum teleportation. Can people ever hope to take a journey into their own past with a self-consistent, non-paradoxical outcome? Science fiction writers will want to mull over the findings of this thorny, mind-bending paper and especially note the extensive literature treating entanglement and projection in the creation of closed timelike curves.
The paper is Lloyd et al., “The quantum mechanics of time travel through post-selected teleportation,” available as a preprint. Be aware as well of Lloyd et al., “Closed timelike curves via post-selection: theory and experimental demonstration” (preprint). This story on Physorg.com also discusses Lloyd’s work and ponders non-linearity in quantum mechanics.