Posts Tagged ‘gravity’
“‘Space-time’ – that hideous hybrid whose very hyphen looks phoney”*…
Space and time seem about as basic as anything could be, even after Einstein’s theory of General Relativity threw (in) a curve. But as Steven Strogatz discusses with Sean Carroll, the reconciliation of Einstein’s work with quantum theory is seeming to suggest that space and time might actually be emergent properties of quantum reality, not fundamental parts of it…
… we’re going to be discussing the mysteries of space and time, and gravity, too. What’s so mysterious about them?
Well, it turns out they get really weird when we look at them at their deepest levels, at a super subatomic scale, where the quantum nature of gravity starts to kick in and become crucial. Of course, none of us have any direct experience with space and time and gravity at this unbelievably small scale. Up here, at the scale of everyday life, space and time seem perfectly smooth and continuous. And gravity is very well described by Isaac Newton’s classic theory, a theory that’s been around for over 300 years now.
But then, about 100 years ago, things started to get strange. Albert Einstein taught us that space and time could warp and bend like a piece of fabric. This warping of the space-time continuum is what we experience as gravity. But Einstein’s theory is mainly concerned with the largest scales of nature, the scale of stars, galaxies and the whole universe. It doesn’t really have much to say about space and time at the very smallest scales.
And that’s where the trouble really starts. Down there, nature is governed by quantum mechanics. This amazingly powerful theory has been shown to account for all the forces of nature, except gravity. When physicists try to apply quantum theory to gravity, they find that space and time become almost unrecognizable. They seem to start fluctuating wildly. It’s almost like space and time fall apart. Their smoothness breaks down completely, and that’s totally incompatible with the picture in Einstein’s theory.
s physicists try to make sense of all of this, some of them are coming to the conclusion that space and time may not be as fundamental as we always imagined. They’re starting to seem more like byproducts of something even deeper, something unfamiliar and quantum mechanical. But what could that something be?….
Find out at: “Where Do Space, Time and Gravity Come From?, ” from @stevenstrogatz and @seanmcarroll in @QuantaMagazine.
* Vladimir Nabokov
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As we fumble with the fundamental, we might send far-sighted birthday greetings to Jocelyn Bell Burnell; she was born on this date in 1943. An astrophysicist, she discovered the first pulsar, while working as a post-doc, in 1957. She then discovered the next three detected pulsars.
The discovery eventually earned the Nobel Prize in Physics in 1974; however, she was not one of the prize’s recipients. The paper announcing the discovery of pulsars had five authors. Bell’s thesis supervisor Antony Hewish was listed first, Bell second. Hewish was awarded the Nobel Prize, along with the astronomer Martin Ryle.
A pulsar— or pulsating radio star– a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The precise periods of pulsars make them very useful tools. Observations of a pulsar in a binary neutron star system were used to confirm (indirectly) the existence of gravitational radiation. The first extrasolar planets were discovered around a pulsar, PSR B1257+12. And certain types of pulsars rival atomic clocks in their accuracy in keeping time.
“For the moment we might very well can them DUNNOS (for Dark Unknown Nonreflective Nondetectable Objects Somewhere)”*…
When does one give up on a hypothesis?…
In 1969, the American astronomer Vera Rubin puzzled over her observations of the sprawling Andromeda Galaxy, the Milky Way’s biggest neighbour. As she mapped out the rotating spiral arms of stars through spectra carefully measured at the Kitt Peak National Observatory and the Lowell Observatory, both in Arizona, she noticed something strange: the stars in the galaxy’s outskirts seemed to be orbiting far too fast. So fast that she’d expect them to escape Andromeda and fling out into the heavens beyond. Yet the whirling stars stayed in place.
Rubin’s research, which she expanded to dozens of other spiral galaxies, led to a dramatic dilemma: either there was much more matter out there, dark and hidden from sight but holding the galaxies together with its gravitational pull, or gravity somehow works very differently on the vast scale of a galaxy than scientists previously thought.
Her influential discovery never earned Rubin a Nobel Prize, but scientists began looking for signs of dark matter everywhere, around stars and gas clouds and among the largest structures in the galaxies in the Universe…
But… over the past half century, no one has ever directly detected a single particle of dark matter. Over and over again, dark matter has resisted being pinned down, like a fleeting shadow in the woods. Every time physicists have searched for dark matter particles with powerful and sensitive experiments in abandoned mines and in Antarctica, and whenever they’ve tried to produce them in particle accelerators, they’ve come back empty-handed. For a while, physicists hoped to find a theoretical type of matter called weakly interacting massive particles (WIMPs), but searches for them have repeatedly turned up nothing.
With the WIMP candidacy all but dead, dark matter is apparently the most ubiquitous thing physicists have never found. And as long as it’s not found, it’s still possible that there is no dark matter at all. An alternative remains: instead of huge amounts of hidden matter, some mysterious aspect of gravity could be warping the cosmos instead…
Dark matter is the most ubiquitous thing physicists have never found; Ramin Skibba (@raminskibba) wonders if it isn’t time to consider alternative explanations: “Does dark matter exist?” in @aeonmag.
* Bill Bryson on dark matter, in A Short History of Nearly Everything (2003)
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As we interrogate the invisible, we might send observant birthday greetings to Val Logsdon Fitch; he was born on this date in 1923. A particle physicist, he shared the 1964 Nobel Prize in Physics with his collaborator James Cronin for their experiments proving that some subatomic reactions do not adhere to fundamental symmetry principles (and are therefore indifferent to the direction of time).
By examining the decay of K-mesons, they proved that a reaction run in reverse does not retrace the path of the original reaction, which showed that the reactions of subatomic particles are not indifferent to time. Thus the phenomenon of CP violation was discovered… and thus was demolished the faith that physicists had previously had that natural laws were universally governed by symmetry.
“Men knew better than they realized, when they placed the abode of the gods beyond the reach of gravity”*…
In search of a theory of everything…
Twenty-five particles and four forces. That description — the Standard Model of particle physics — constitutes physicists’ best current explanation for everything. It’s neat and it’s simple, but no one is entirely happy with it. What irritates physicists most is that one of the forces — gravity — sticks out like a sore thumb on a four-fingered hand. Gravity is different.
Unlike the electromagnetic force and the strong and weak nuclear forces, gravity is not a quantum theory. This isn’t only aesthetically unpleasing, it’s also a mathematical headache. We know that particles have both quantum properties and gravitational fields, so the gravitational field should have quantum properties like the particles that cause it. But a theory of quantum gravity has been hard to come by.
In the 1960s, Richard Feynman and Bryce DeWitt set out to quantize gravity using the same techniques that had successfully transformed electromagnetism into the quantum theory called quantum electrodynamics. Unfortunately, when applied to gravity, the known techniques resulted in a theory that, when extrapolated to high energies, was plagued by an infinite number of infinities. This quantization of gravity was thought incurably sick, an approximation useful only when gravity is weak.
Since then, physicists have made several other attempts at quantizing gravity in the hope of finding a theory that would also work when gravity is strong. String theory, loop quantum gravity, causal dynamical triangulation and a few others have been aimed toward that goal. So far, none of these theories has experimental evidence speaking for it. Each has mathematical pros and cons, and no convergence seems in sight. But while these approaches were competing for attention, an old rival has caught up.
The theory called asymptotically (as-em-TOT-ick-lee) safe gravity was proposed in 1978 by Steven Weinberg. Weinberg, who would only a year later share the Nobel Prize with Sheldon Lee Glashow and Abdus Salam for unifying the electromagnetic and weak nuclear force, realized that the troubles with the naive quantization of gravity are not a death knell for the theory. Even though it looks like the theory breaks down when extrapolated to high energies, this breakdown might never come to pass. But to be able to tell just what happens, researchers had to wait for new mathematical methods that have only recently become available…
For decades, physicists have struggled to create a quantum theory of gravity. Now an approach that dates to the 1970s is attracting newfound attention: “Why an Old Theory of Everything Is Gaining New Life,” from @QuantaMagazine.
* Arthur C. Clarke, 2010: Odyssey Two
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As we unify, we might pause to remember Sir Arthur Stanley Eddington, OM, FRS; he died in this date in 1944. An astrophysicist, mathematician, and philosopher of science known for his work on the motion, distribution, evolution and structure of stars, Eddington is probably best remembered for his relationship to Einstein: he was, via a series of widely-published articles, the primary “explainer” of Einstein’s Theory of General Relativity to the English-speaking world; and he was, in 1919, the leader of the experimental team that used observations of a solar eclipse to confirm the theory.
“Time is the longest distance between two places”*…
In quantum mechanics, time is universal and absolute; its steady ticks dictate the evolving entanglements between particles. But in general relativity (Albert Einstein’s theory of gravity), time is relative and dynamical, a dimension that’s inextricably interwoven with directions x, y and z into a four-dimensional “space-time” fabric. The fabric warps under the weight of matter, causing nearby stuff to fall toward it (this is gravity), and slowing the passage of time relative to clocks far away. Or hop in a rocket and use fuel rather than gravity to accelerate through space, and time dilates; you age less than someone who stayed at home.
Unifying quantum mechanics and general relativity requires reconciling their absolute and relative notions of time. Recently, a promising burst of research on quantum gravity has provided an outline of what the reconciliation might look like — as well as insights on the true nature of time…
The effort to unify quantum mechanics and general relativity means reconciling totally different notions of time; catch up on the state of play at “Quantum Gravity’s Time Problem.”
* Tennessee Williams, The Glass Menagerie
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As we set our watches, we might send carefully-calculated birthday greetings to Gabrielle-Émilie Le Tonnelier de Breteuil, Marquise du Châtelet, the French mathematician and physicist who is probably (if unfairly) better known as Voltaire’s mistress; she was born on this date in 1706. Fascinated by the work of Newton and Leibniz, she dressed as a man to frequent the cafes where the scientific discussions of the time were held. Her major work was a translation of Newton’s Principia, for which Voltaire wrote the preface; it was published a decade after her death, and was for many years the only translation of the Principia into French.
Judge me for my own merits, or lack of them, but do not look upon me as a mere appendage to this great general or that great scholar, this star that shines at the court of France or that famed author. I am in my own right a whole person, responsible to myself alone for all that I am, all that I say, all that I do. it may be that there are metaphysicians and philosophers whose learning is greater than mine, although I have not met them. Yet, they are but frail humans, too, and have their faults; so, when I add the sum total of my graces, I confess I am inferior to no one.
– Mme du Châtelet to Frederick the Great of Prussia
Obsessed with his weight…
German scale-maker Kern has been in the weighing business since 1844. To demonstrate the precision of the scientific instruments in which it now specializes, Kern has launched The Gnome Experiment.
If Earth was a perfect sphere of uniform density, then gravity would be consistent. But it’s not, which means gravity varies wherever you go. So can we chart those discrepancies using just a basic-range Kern scale?
Method
We’re shipping our Gnome Kit from scientist to scientist around the world. Join the experiment and you’ll receive:
1x Kern EWB 2.4 Scale
Pre-calibrated according to local gravity at Kern HQ, Balingen, Germany.1x Kern Gnome
The perfect test-subject for two good reasons: Gnomes are already accustomed to travelling the world. They also originate from our homeland, Germany.1x Lab gloves & 1 x Air duster
Important because dust or grease will reduce the accuracy of the results.
Readers can check up on results-to-date…
… and can sign up to participate, here.
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As we pack our bags to move south,* we might send earthy birthday greetings to civil engineer and geodesist John Fillmore Hayford; he was born on this date in 1868. Hayford, the father of the modern science of geodesy, made the first precise determination of the ellipsoidal shape and size of the earth.
* an object weighs about 0.5% more at the poles than at the Equator.
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