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Modern science has introduced us to many strange ideas on the universe, but one of the strangest is the ultimate fate of massive stars in the Universe that reached the end of their life cycles. Having exhausted the fuel that sustained it for millions of years of shining life in the skies, the star is no longer able to hold itself up under its own weight, and it then shrinks and collapses catastrophically unders its own gravity. Modest stars like the Sun also collapse at the end of their life, but they stabilize at a smaller size. But if a star is massive enough, with tens of times the mass of the Sun, its gravity overwhelms all the forces in nature that might possibly halt the collapse. From a size of millions of kilometers across, the star then crumples to a pinprick size, smaller than even the dot on an “i”.
What would be the final fate of such massive collapsing stars? This is one of the most exciting questions in astrophysics and modern cosmology today. An amazing inter-play of the key forces of nature takes place here, including gravity and quantum forces. This phenomenon may hold the secrets to man’s search for a unified understanding of all forces of nature, with exciting implications for astronomy and high energy astrophysics. Surely, this is an outstanding unresolved mystery that excites physicists and the lay person alike.
The story of massive collapsing stars began some eight decades ago when Subrahmanyan Chandrasekhar probed the question of final fate of stars such as the Sun. He showed that such a star, on exhausting its internal nuclear fuel, would stabilize as a “White Dwarf”, about a thousand kilometers in size. Eminent scientists of the time, in particular Arthur Eddington, refused to accept this, saying how a star can ever become so small. Finally Chandrasekhar left Cambridge to settle in the United States. After many years, the prediction was verified. Later, it also became known that stars which are three to five times the Sun’s mass give rise to what are called Neutron stars, just about ten kilometers in size, after causing a supernova explosion.
But when the star has a mass more than these limits, the force of gravity is supreme and overwhelming. It overtakes all other forces that could resist the implosion, to shrink the star in a continual gravitational collapse. No stable configuration is then possible, and the star which lived millions of years would then catastrophically collapse within seconds. The outcome of this collapse, as predicted by Einstein’s theory of general relativity, is a space-time singularity: an infinitely dense and extreme physical state of matter, ordinarily not encountered in any of our usual experiences of physical world.
As the star collapses, an ‘event horizon’ of gravity can possibly develop. This is essentially ‘a one way membrane’ that allows entry, but no exits permitted. If the star entered the horizon before it collapsed to singularity, the result is a ‘Black Hole’ that hides the final singularity. It is the permanent graveyard for the collapsing star.
As per our current understanding of physics, it was one such singularity, the ‘Big Bang’, that created our expanding universe we see today. Such singularities will be again produced when massive stars die and collapse. This is the amazing place at boundary of Cosmos, a region of arbitrarily large densities billions of times the Sun’s density.
An enormous creation and destruction of particles takes place in the vicinity of singularity. One could imagine this as ‘cosmic inter-play’ of basic forces of nature coming together in a unified manner. The energies and all physical quantities reach their extreme values, and quantum gravity effects dominate this regime. Thus, the collapsing star may hold secrets vital for man’s search for a unified understanding of forces of nature.
The question then arises: Are such super-ultra-dense regions of collapse visible to faraway observers, or would they always be hidden in a black hole? A visible singularity is sometimes called a ‘Naked Singularity’ or a ‘Quantum Star’. The visibility or otherwise of such super-ultra-dense fireball the star has turned into, is one of the most exciting and important questions in astrophysics and cosmology today, because when visible, the unification of fundamental forces taking place here becomes observable in principle.
A crucial point is, while gravitation theory implies that singularities must form in collapse, we have no proof the horizon must necessarily develop. Therefore, an assumption was made that an event horizon always does form, hiding all singularities of collapse. This is called ‘Cosmic Censorship’ conjecture, which is the foundation of current theory of black holes and their modern astrophysical applications. But if the horizon did not form before the singularity, we then observe the super-dense regions that form in collapsing massive stars, and the quantum gravity effects near the naked singularity would become observable.
“It turns out that the collapse of a massive star will give rise to either a black hole or naked singularity”
In recent years, a series of collapse models have been developed where it was discovered that the horizon failed to form in collapse of a massive star. The mathematical models of collapsing stars and numerical simulations show that such horizons do not always form as the star collapsed. This is an exciting scenario because the singularity being visible to external observers, they can actually see the extreme physics near such ultimate super-dense regions.
It turns out that the collapse of a massive star will give rise to either a black hole or naked singularity, depending on the internal conditions within the star, such as its densities and pressure profiles, and velocities of the collapsing shells.
When a naked singularity happens, small inhomogeneities in matter densities close to singularity could spread out and magnify enormously to create highly energetic shock waves. This, in turn, have connections to extreme high energy astrophysical phenomena, such as cosmic Gamma rays bursts, which we do not understand today.
Also, clues to constructing quantum gravity–a unified theory of forces, may emerge through observing such ultra-high density regions. In fact, the recent science fiction movie Interstellar refers to naked singularities in an exciting manner, and suggests that if they did not exist in the Universe, it would be too difficult then to construct a quantum theory of gravity, as we will have no access to experimental data on the same!
Shall we be able to see this ‘Cosmic Dance’ drama of collapsing stars in the theater of skies? Or will the ‘Black Hole’ curtain always hide and close it forever, even before the cosmic play could barely begin? Only the future observations of massive collapsing stars in the universe would tell!
A previous blog post, Patterns in Physics, discussed alternative “representations” in physics as akin to languages; an underlying quantum reality described in either a position or a momentum representation. Both are equally capable of a complete description, the underlying reality itself residing in a complex space with the very concepts of position/momentum or wave/particle only relevant in a “classical limit”. The history of physics has progressively separated such incidentals of our description from what is essential to the physics itself. We will consider this for time itself here.
Thus, consider the simple instance of the motion of a ball from being struck by a bat (A) to being caught later at a catcher’s hand (B). The specific values given for the locations of A and B or the associated time instants are immediately seen as dependent on each person in the stadium being free to choose the origin of his or her coordinate system. Even the direction of motion, whether from left to right or vice versa, is of no significance to the physics, merely dependent on which side of the stadium one is sitting.
All spectators sitting in the stands and using their own “frame of reference” will, however, agree on the distance of separation in space and time of A and B. But, after Einstein, we have come to recognize that these are themselves frame dependent. Already in Galilean and Newtonian relativity for mechanical motion, it was recognized that all frames travelling with uniform velocity, called “inertial frames”, are equivalent for physics so that besides the seated spectators, a rider in a blimp moving overhead with uniform velocity in a straight line, say along the horizontal direction of the ball, is an equally valid observer of the physics.
Einstein’s Special Theory of Relativity, in extending the equivalence of all inertial frames also to electromagnetic phenomena, recognized that the spatial separation between A and B or, even more surprisingly to classical intuition, the time interval between them are different in different inertial frames. All will agree on the basics of the motion, that ball and bat were coincident at A and ball and catcher’s hand at B. But one seated in the stands and one on the blimp will differ on the time of travel or the distance travelled.
Even on something simpler, and already in Galilean relativity, observers will differ on the shape of the trajectory of the ball between A and B, all seeing parabolas but of varying “tightness”. In particular, for an observer on the blimp travelling with the same horizontal velocity as that of the ball as seen by the seated, the parabola degenerates into a straight up and down motion, the ball moving purely vertically as the stadium itself and bat and catcher slide by underneath so that one or the other is coincident with the ball when at ground level.
There is no “trajectory of the ball’s motion” without specifying as seen by which observer/inertial frame. There is a motion, but to say that the ball simultaneously executes many parabolic trajectories would be considered as foolishly profligate when that is simply because there are many observers. Every observer does see a trajectory, but asking for “the real trajectory”, “What did the ball really do?”, is seen as an invalid, or incomplete, question without asking “as seen by whom”. Yet what seems so obvious here is the mistake behind posing as quantum mysteries and then proposing as solutions whole worlds and multiple universes(!). What is lost sight of is the distinction between the essential physics of the underlying world and our description of it.
The same simple problem illustrates another feature, that physics works equally well in a local time-dependent or a global, time-independent description. This is already true in classical physics in what is called the Lagrangian formulation. Focusing on the essential aspects of the motion, namely the end points A and B, a single quantity called the action in which time is integrated over (later, in quantum field theory, a Lagrangian density with both space and time integrated over) is considered over all possible paths between A and B. Among all these, the classical motion is the one for which the action takes an extreme (technically, stationary) value. This stationary principle, a global statement over all space and time and paths, turns out to be exactly equivalent to the local Newtonian description from one instant to another at all times in between A and B.
There are many sophisticated aspects and advantages of the Lagrangian picture, including its natural accommodation of basic conservation laws of energy, momentum and angular momentum. But, for our purpose here, it is enough to note that such stationary formulations are possible elsewhere and throughout physics. Quantum scattering phenomena, where it seems natural to think in terms of elapsed time during the collisional process, can be described instead in a “stationary state” picture (fixed energy and standing waves), with phase shifts (of the wave function) that depend on energy, all experimental observables such as scattering cross-sections expressed in terms of them.
“The concept of time has vexed humans for centuries, whether layman, physicist or philosopher”
No explicit invocation of time is necessary although if desired so-called time delays can be calculated as derivatives of the phase shifts with respect to energy. This is because energy and time are quantum-mechanical conjugates, their product having dimensions of action, and Planck’s quantum constant with these same dimensions exists as a fundamental constant of our Universe. Indeed, had physicists encountered quantum physics first, time and energy need never have been invoked as distinct entities, one regarded as just Planck’s constant times the derivative (“gradient” in physics and mathematics parlance) of the other. Equally, position and momentum would have been regarded as Planck’s constant times the gradient in the other.
The concept of time has vexed humans for centuries, whether layman, physicist or philosopher. But, making a distinction between representations and an underlying essence suggests that space and time are not necessary for physics. Together with all the other concepts and words we perforce have to use, including particle, wave, and position, they are all from a classical limit with which we try to describe and understand what is actually a quantum world. As long as that is kept clearly in mind, many mysteries and paradoxes are dispelled, seen as artifacts of our pushing our models and language too far and “identifying” them with the underlying reality that is in principle out of reach.
This is the fourth in our series of podcasts. Dawkins has talked about a wide range of scientists before, and now he introduces us to Fred Hoyle, one of the astronomers who originally proposed the steady state theory of the universe. The steady state theory may have been disproved, but Hoyle’s contributions to science–and science fiction–still remain.
Transcript after the jump. DORIAN DEVINS: Outside of the realm of biology, you have a lot of physicists and mathematicians as well, and it struck me that you have Fred Hoyle in here—a lot of people may not be familiar with Fred Hoyle.
RICHARD DAWKINS: Yes, Fred Hoyle was an English astronomer, astrophysicist. He was one of the three physicists who proposed the steady state theory of the universe, which is now out of fashion. Indeed, it’s almost certainly wrong, disproved by the evidence. But it was a very, very interesting theory. According to the steady state theory, there never was a beginning to the universe. The universe has always been in existence; and the expanding universe, the galaxies pulling apart, that is true, but the gaps between the galaxies get filled with spontaneously created new matter, so there are new galaxies being created in the gaps that are left as the other, older galaxies pull apart. Now, that theory is wrong, but it was never obviously silly. You might think “Well how on Earth can matter just spontaneously be created?” And Hoyle’s point was well that’s no more odd than the idea that it should be spontaneously created in the first place, at the time of the Big Bang. So it was an interesting theory; its now been disproved. He had another great claim to fame, which was that he worked out how the elements, the chemical elements, are formed in the interior of stars. We now know that in this case, Hoyle was absolutely right, that all the elements apart for hydrogen and helium I think, are made in the interior of stars. And we’re all made of star stuff, that was the poetic phrase that Carl Sagan used to quote. I think maybe he got it from Joni Mitchell or the other way around. But anyway, that all comes from Fred Hoyle. He was also a science fiction writer. His first science fiction book, The Black Cloud, is a wonderful story. I mean it’s just a feast, it’s just a riveting science fiction story marred by the fact that its hero is such a deeply unpleasant character. And all the heroes of Fred Hoyle’s science fiction books are the same deeply unpleasant character, you can’t help wondering where that unpleasant character came from.
I realize I’m privileged to have access to some of the world’s cutting edge science, but last week was particularly special with a visit to University College London to hear a mixture of astrophysicists and astrobiologists talk to journalists about their cutting edge work,organized by the ABSW, the Association of British Science Writers, of which I’m a member.
Now we all know scientists can sometimes waffle, but this brave half-dozen weren’t allowed that luxury. The format for the talks was a pecha kucha – born in Japan, you have 20 slides, each lasting for exactly 20 seconds, to get your point across. That’s 6 minutes, 40 seconds (and not a second more) to say who you are, what you do and pitch for a place in the science columns of Britain’s newspapers.
First up, Giovanna Tinetti asked what exoplanets are actually made of. For those out of the loop, exoplanets are those orbiting other stars, far beyond out own solar system. We weren’t sure such things even existed until the 1990s, but nowadays there are more than 700 confirmed cases, with hundreds more candidates awaiting confirmation. recently some astronomers have gone so far as to sayy that every star in our galaxy must have planets orbiting.The most productive way to search for these faraway worlds is by using the Kepler Space Telescope. Looking back along a populous spiral arm of the Milky Way, this other Hubble is a study in concentration, staring fixedly at a single window on the stars, watching for the most minute variation in their light. And by analying this light – the chemical clues hidden within the spectra, scientists like Giovanna can tell what planets hundreds of light years away are made from. She’s looking for those that are habitable. Soon, New Earth need not be a thing of science fiction stories, especially if Giovanna’s plans for ECHO, the Exoplanet Characterisation Observatory, are approved by ESA (the European Space Agency).
Ofer Lahav, Professor of Astronomy at UCL, chose to talk about dark energy, the mysterious entity that apparently makes up three quarters of out universe, but which we didn’t even know was there until 1998. For me the most incredible, unexpected discovery of the last fifty years has been that the rate of expansion of the universe is increasing. No one expected this. Everyone wants to know why, but Ofer was impressively agnostic in his views. Either an entity we call dark energy permeates space itself, acting as Einsteins cosmological constant, or the best theories we have are very wrong. Once upon a time our best theory was Newton’s, but it couldn’t explain why Mercury orbited the Sun the way it did. Along came Einstein, General Relativity and a revolution in science. With the dark energy anomaly, are we on the cusp of another such paradigm shift?
On 11 September 2013, an unusually long and bright impact flash was observed on the Moon. Its peak luminosity was equivalent to a stellar magnitude of around 2.9.
What happened? A meteorite with a mass of around 400 kg hit the lunar surface at a speed of over 61,000 kilometres per hour.
Rocks often collide with the lunar surface at high speed (tens of thousands of kilometres per hour) and are instantaneously vaporised at the impact site. This gives rise to a thermal glow that can be detected by telescopes from Earth as short duration flashes. These flashes, in general, last just a fraction of a second.
The extraordinary flash in September was recorded from Spain by two telescopes operating in the framework of the Moon Impacts Detection and Analysis System (MIDAS). These devices were aimed to the same area in the night side of the Moon. With a duration of over eight seconds, this is the brightest and longest confirmed impact flash ever recorded on the Moon.
Our calculations show that the impact, which took place at 20:07 GMT, created a new crater with a diameter of around 40 meters in Mare Nubium. This rock had a size raging between 0.6 and 1.4 metres. The impact energy was equivalent to over 15 tons of TNT under the assumption of a luminous efficiency of 0.002 (the fraction of kinetic energy converted into visible radiation as a consequence of the hypervelocity impact).
The detection of impact flashes is one of the techniques suitable to analyze the flux of incoming bodies to the Earth. One of the characteristics of the lunar impacts monitoring technique is that it is not possible to unambiguously associate an impact flash with a given meteoroid stream. Nevertheless, our analysis shows that the most likely scenario is that the impactor had a sporadic origin (i.e., was not associated to any known meteoroid stream). From the analysis of this event we have learnt that that one metre-sized objects may strike our planet about ten times as often as previously thought.
Monthly Notices of the Royal Astronomical Society is one of the world’s leading primary research journals in astronomy and astrophysics, as well as one of the longest established. It publishes the results of original research in astronomy and astrophysics, both observational and theoretical.
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Although we rarely stop to think about the origin of the elements of our bodies, we are directly connected to the greater universe. In fact, we are literally made of stardust that was liberated from the interiors of dying stars in gigantic explosions, and then collected to form our Earth as the solar system took shape some 4.5 billion years ago. Until about two decades ago, however, we knew only of our own planetary system so that it was hard to know for certain how planets formed, and what the history of the matter in our bodies was.
Then, in 1995, the first planet to orbit a distant Sun-like star was discovered. In the 20 years since then, thousands of others have been found. Most planets cannot be detected with our present-day technologies, but estimates based on those that we have observed suggest that almost every star in the sky has at least one extrasolar planet (or exoplanet) orbiting it. That means that there are more than 100 billion planetary systems in our Milky Way Galaxy alone! Imagine that: astronomers have gone from knowing of 1 planetary system to some 100 billion, in the same decades in which human genome scientists sequenced the 6 billion base-pairs that lie at the foundation of our bodies. How many of these planetary systems could potentially support life, and would that life use a similar code?
Exoplanets are much too far away to be actually imaged, and they are way too faint to be directly observed next to the bright glow of the stars they orbit. Therefore, the first exoplanet discoveries were made through the gravitational tug on their central star during their orbits. This pull moves the star slightly back and forth. Only relatively heavy, close-in planets can be detected that way, using the repeating Doppler shifts of their central star’s light from red to blue and back. Another way to find planets is to measure how they block the light of their central star if they happen to cross in front of it as seen from Earth. If they are seen to do this twice or more, the temporary dimmings of their star’s light can disclose the planet’s size and distance to its star (basically using the local “year” – the time needed to orbit its star – for these calculations). If both the gravitational tug and the dimming profile can be measured, then even the mass of the planet can be estimated. Size and mass together give an average density from which, in turn, knowledge of the chemical composition of that planet comes within reach.
With the discoveries of so many planets, we have realized that an astonishing diversity exists: hot Jupiter-sized planets that orbit closer to their star than Mercury orbits the Sun, quasi-Earth-sized planets that may have rain showers of molten iron or glass, frozen planets around faintly-glowing red dwarf stars, and possibly some billions of Earth-sized planets at distances from their host stars where liquid water could exist on the surface, possibly supporting life in a form that we might recognize if we saw it.
Guided by these recent observations, mega-computers programmed with the laws of physics give us insight into how these exo-worlds are formed, from their initial dusty disks to the eventual complement of star-orbiting planets. We can image the disks directly by focusing on the faint infrared glow of their gas and dust that is warmed by their proximity to their star. We cannot, however, directly see these far-away planets, at least not yet. But now, for the first time, we can at least see what forming planets do to the gas and dust around them in the process of becoming a mature heavenly body.
A new observatory, called ALMA, working with microwaves that lie even beyond the infrared color range, has been built in the dry Atacama desert in Chili. ALMA was pointed at a young star, hundreds of light years away. Its image of that target star, LH Tauri, not only shows the star itself and the disk around it, but also a series of dark rings that are most likely created as the newly forming planets pull in the gas and dust around them. The image is of stunning quality: it shows details down to a resolution equivalent to the width of a finger seen at a distance of 50 km (30 miles).
At the distance of LH Tauri, even that stunning imaging capability means that we can see structures only if these are larger than about the distance of the Sun out to Jupiter, so there is a long way yet to go before we see anything like the planet directly. But we will observe more of these juvenile planetary systems just past the phase of their birth. And images like that give us a glimpse of what happened in our own planetary system over 4.5 billion years ago, before the planets were fully formed, pulling in the gases and dust that we now live on, and that ultimately made their way to the cycles of our own planet, to constitute all living beings on Earth.
What a stunning revolution: from being part of the only planetary system we knew of, we have been put among billions and billions of neighbors. We remember Galileo Galilei for showing us that the Sun and not the Earth was the center of the solar system. Will our society remember the names of those who proved that billions of planets exist all over the Galaxy?
Headline image credit: Star shower, by c@rljones. CC-BY-NC-2.0 via Flickr.