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Einstein's one tiny problem

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Deborah Valkenburg
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Posts: 3233

« on: May 04, 2008, 10:59:57 pm »

Einstein's one tiny problem

Otherwise solid, the theory of general relativity starts to melt at the quantum level

Nov 06, 2005 08:30 AM
Fotini Markopoulou Kalamara
Special to the Star
What did Einstein do better than Newton? He said that space-time is not a neutral stage on which other stuff — matter — moves, but it is itself made of all the processes that take place in the world and all their coincidences, or events.

Thus space-time becomes dynamic, alive in a sense; it is a collective of everything that affects every single constituent inside it. That led Einstein to general relativity, in which space-time is a geometry that tells matter where to go, and matter tells space-time how to curve.

That is all very beautiful. But now consider the other great discovery of last century, quantum physics. At the level of elementary particles, matter is subject to the laws of quantum theory. Quantum theory says that ultimately the notion of a path and thus an event — the coincidence of two paths — is not as straightforward as the geometric beauty of general relativity would have us believe. If I send an elementary particle from home to work, general relativity says it will take the shortest path. Quantum theory says that it will take all possible paths at the same time, with consequences we've tested to great experimental accuracy.

This is really a bad blow to the pretty picture of space-time geometry. If a particle ceases to have a well-defined position, then how does it tell space-time how to curve? Does every single path the particle takes curve space-time in a different way? What happens to general relativity's basic principle, that matter tells space-time how to curve and space-time tells matter where to go? Does one elementary particle on a given space-time create a subsequent superposition of space-times?

At the very least, these questions mean that general relativity is not the final theory of space-time. The quantumness of matter has to be incorporated into the theory in a consistent and coherent manner, or we have a problem.

How big is this problem? We might try to console ourselves that, for the quantum effects to be significant, one has to have very small mass, and with a small mass you can only curve space-time so little that the effect of the one particle is extremely small. You might see it near a black hole, or near the beginning of the universe.

But this argument has, in fact, left out general relativity. It only applies to one, fixed space-time with one particle in it.

Let's remember that the basic principle that Einstein used as a guide to special and general relativity is that it is physical processes and their coincidences — events — that are fundamental, and space-time is the collection of all of those.

But he considered classical processes and classical events. Our task includes repeating the exercise for quantum processes and events, since at the basic level the world is made of quantum systems — squillions of quantum particles that have to make up a space-time.

The question cannot be what happens to a given space-time when it contains a quantum particle but the other way around: How did the true constituents of the universe combine to create a space-time, a construction that makes us think that the notion of a path is natural and, with matter, an entity separate from space-time?

Ultimately, we still don't know precisely what question we should be asking. General relativity is considered a fundamental theory not least because we believe that the notion of space-time is fundamental: It is everywhere.

I believe the real lesson of quantum theory is that the space-time description of the world is good, but not fundamental. It tells us that the notion of a particle's path does not exist, and the way we deal with that is to talk of a particle taking all possible paths at the same time.

How do we go about discovering the real question to ask? We ask preliminary questions, a bunch of different ones. For example, let us say it's true that there is such a thing as a quantum superposition of space-times. What does it look like?

If you like that question, you might want to do research on Loop Quantum Gravity, an approach whose goal is a quantum version of general relativity, keeping very close to all the principles of relativity and the notion that space-time is fundamental.

The aim is to find a consistent and sensible description of such a space-time. If it exists and can be checked to be correct, then you will have learned something profound about space-time.

The first indications from this approach suggest a relational picture of space, where the geometry is replaced by a network of connectivities. It also suggests a fundamental finiteness: General relativity says that if you look inside a small region of space, you could magnify it arbitrarily and there would be more and more new things to look at, while the network picture indicates there is a limit. At some point your magnifying glass does not see any new network connections. There can only be a finite amount of information in a finite region of space.

But that is still not enough. You have to understand what quantum space-time means; in fact, what space-time means if it is quantum. It is one thing to have an elementary particle take all possible paths simultaneously, but space-time refers to what is past and what is future. What can I mean by future if something is and is not in it at the same time?

Conversely, you might think that it is peculiar to think of space-time and matter as separate things. The very interconnectivity of the two could mean that it is wrong to address questions purely on the relativity front, as Loop Quantum Gravity mostly does, and you should instead look for a unified theory.

You may want to work on String Theory. This unification suggests a picture of the world that contains extended objects — strings and membranes — instead of the ordinary point objects that electrons and protons basically are. Your relationship with space-time will be complicated. Your extended objects live in some sort of fixed space-time, usually of a higher dimension than we do, but which is not necessarily our space-time. In fact, it had better not be our space-time, because it is fixed and not dynamic.

Or you might find such approaches unsatisfactory; too baroque and intricate, not enough physics. You'd rather be the "practical" type. You might start by stating what we know: The world started as an extremely hot, high-energy soup. Planck temperature hot, compared to which the centre of the hottest star is freezing cold.

Maybe the notion of a space-time does not make any sense at all, quantum or not, at this early stage. It is plausible that space-time only emerged as the universe cooled down, and is thus not fundamental. Space-time is the frozen phase of whatever makes up the universe, just as ice is a phase of a collection of water molecules that emerges from water when we cool it down. Ice has a regularity, a solid structure that water does not have. Maybe such a regularity is all we can mean by space-time.

The optimist in this approach will say that the fact that space-time is everywhere must mean that it is natural. Its very universality must mean its reason for existence is simple and so we should be able to find that reason.

This is very attractive in its simplicity. Nonetheless, it is hard enough to show how ice emerges from water even though we have bucket loads of it to study and we know that it does, let alone understand how space-time emerges from something we do not yet perceive. It's a bit like living in a world of ice that never melts: Would you know that water is a possibility?

Einstein broke the mould and showed that space-time is not given, fixed and frozen. We still have not taken this to its natural conclusion and found out what space-time really is. To get there, we need such a fundamental rethinking of our notion of space-time that it's hard to even guess what the consequences will be. It's hard to think outside the box when the box is space-time.
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Skepticism is good, but when you reach a certain level where
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called skepticism.  It's called ignorance.

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Deborah Valkenburg
Superhero Member
Posts: 3233

« Reply #1 on: May 04, 2008, 11:07:44 pm »

Was Einstein right?

Oct 02, 2005 01:05 AM
Clifford Will
Albert Einstein's relativity has changed forever the way we think about space, time and the universe. In two papers published in 1905, he proposed a new way of looking at motion, mass and energy, and in 1915 he postulated a new conception of gravity. His 1905 papers on special relativity were just a part of his remarkable "miracle year," whose 100th anniversary physicists worldwide are celebrating with the World Year of Physics.

Some scholars have even suggested that Einstein's relativity affected such fields as art, poetry, philosophy and history. In a 1983 book on the history of the 20th century called Modern Times, British historian Paul Johnson chose to date the beginning of the modern world not at the beginning of the century, or even at the outbreak of World War I, but instead on May 29, 1919.

This was the date of a total solar eclipse in South America and Africa, during which astronomers measured the bending of starlight by the sun's gravity, thus confirming Einstein's general theory of relativity. Johnson went on to argue that the success of relativity in science was accompanied by the rise of "moral relativism" in politics and ethics, leading to many of the great catastrophes of the 20th century.

That proposition may be debatable, but what is certain is that relativity has changed physics. Special relativity underlies everything we understand about light, atoms, nuclei and quarks, and is now part of the standard toolkit of every working physicist.

General relativity has been part of a sea change in astronomy. Whereas once the universe was seen as a quiet place of steady, fixed stars and wispy nebulae, it is now a jungle out there, with its exploding supernovae, colliding galaxies, mysterious black holes and, of course, that most violent of all events, the Big Bang.

What might Einstein have thought of all this? In 1930 he wrote that he considered "the main significance of general relativity to be the simplicity of its foundation and its internal consistency." He was rather blasι about observational or experimental questions. He once famously said that, had the astronomers' 1919 data failed to verify his theory, he would have "felt sorry for the dear Lord, for the theory is correct."

But was Einstein right?

In science, beauty and elegance mean nothing in the cold, steely stare of the experimentalist's eye. The failure to pass a single experimental test can kill the most beautiful and beloved theory.

However, passing a test doesn't get you home free. Like participants on the game-show island, you are only allowed to survive until the next experimental test comes along.

So after 100 years, how have Einstein's theories of relativity fared?

His special theory has been so thoroughly woven into the fabric of physics that nobody except cranks and crackpots seriously questions it. Do moving clocks slow down? Ask anybody at a high-energy particle accelerator and she will tell you how she prolongs the lives of unstable particles by having them move near light speed. Without this, particle physicists would never have time to examine their properties before they decayed into other particles.

Is the speed of light independent of the speed of the emitter? Ask astronomers who have measured arrivals of X-ray bursts from orbiting neutron stars. The bursts arrive at the time expected, whether the emitting star is moving toward us or away from us.

E=mc2? Ask anyone in the business of nuclear power or weapons. Or just look around you. We exist on this tiny rock in space in part because the mass of a helium atom is a tiny bit less than the mass of the four hydrogens that combined to make it in the Sun's interior. That mass, converted to energy by Einstein's formula, is what keeps us all warm.

In the decade following Einstein's miraculous 1905, he rose steadily in the academic ranks of European physics, reaching the pinnacle of a professorship in Berlin by 1914, but to the public at large, he was unknown. Even when he published the general theory in the fall of 1915, he made hardly a dent outside the small world of physicists. News of the theory barely trickled out of wartime Germany.

But when the British astronomers announced to a war-weary world in 1919 that they had verified the general theory, the news caused a sensation and made Einstein an international celebrity. Newspapers proclaimed him the successor to Isaac Newton and the creator of a new world order where nothing was what it seemed.

At the same time, the world was told that the theory was so incomprehensible that only a select few savants could possibly understand it. One story that fuelled this notion originated with British astronomer Arthur Stanley Eddington, who headed up the teams that measured Einstein's bending of light. When told by a starry-eyed colleague that he must be one of only three people in the world who understood the general theory, Eddington is said to have replied, "Who would be the third?"

But the bending test was the second success of the general theory. Already in 1915, Einstein had solved the problem of "Mercury's perihelion." This was a tiny deviation in the orbit of the sun's closest planet that had bedevilled astronomers since the middle 1800s. Newton's gravitation theory wasn't capable of accounting for this effect, but the general theory was. When Einstein found this result in 1915, he wrote to a friend that he had palpitations of the heart from the excitement of the discovery.

But that was just about it, as far as experimental tests were concerned. Without a strong link to experiment and observation, the general theory began to decline in importance, certainly compared to other emerging branches of physics, such as quantum, atomic and nuclear physics.

Einstein himself turned most of his attention toward a quest, ultimately unfulfilled, for a unified field theory. By 1960, the general theory had been relegated to the backwaters of science. As famed general relativist Kip Thorne relates, it was during this period that a well-known Caltech astronomer advised him not to study relativity for his doctorate, because it had nothing to do with real physics and astronomy. (Thorne ignored the advice.)

But during the next decade, everything changed. The 1960s are known as a decade of social ferment, of hippies and free love, of Vietnam and pot, but it was also a decade of a revolution in astronomy.

Given that Einstein derived his theory using only principles of beauty, simplicity and symmetry, it is truly remarkable how right it has turned out to be so far

The year 1961 saw the discovery of quasars, powerful sources of radiation seen almost to the edge of the visible universe, now believed to harbour supermassive black holes.

In 1964 came the discovery of cosmic background radiation, left over from an epoch around 400,000 years after the Big Bang. In 1967 occurred the discovery of pulsars, lighthouses of radio beams made by spinning neutron stars.

Finally, in 1971 came the discovery of the first candidate for a black hole in our galaxy, in a source detected by an orbiting X-ray satellite.

These discoveries made it clear that general relativity would have an important role to play in this new astronomy. At the same time, the development of the advanced technology of precision measurement, involving atomic clocks, lasers, radar and radio, together with the emerging space program, made it possible to check whether the general theory was the right theory to use in building models for these strange phenomena.

Thus began a decades-long partnership between theory and experiment that made relativistic gravitational physics one of the most thriving branches of the subject.

Eddington's 1919 measurements of the bending of light were good to 20 per cent at best, but today, using radio telescopes around the world to measure the positions of hundreds of quasars, astronomers have verified Einstein's bending prediction to one-fiftieth of a per cent.

The general theory's first big success (explaining the Mercury anomaly) has gotten even better through the techniques of interplanetary radar to improve measurement of all the planetary orbits. Theory matches observation to a tenth of a per cent.

One of the most sensitive tests of the general theory wasn't even foreseen by Einstein. It was radio astronomer Irwin Shapiro who pointed out in 1964 that, when a radar signal passes through the warped space-time near a body like the sun, it would suffer an additional tiny delay in its passage. This "Shapiro time-delay" has now been measured many times, most recently during the flight of the Cassini spacecraft on its way to Saturn. That measurement confirmed the effect to one part in 100,000.

A remarkable test of the theory was provided by the serendipitous discovery in 1974 of a "binary pulsar," a pulsar in orbit around another neutron star. Because the pulsar emits radio beeps with amazing regularity, astronomers could act like cops with a speed-trap gun and measure the pulsar's comings and goings about its companion by measuring the variations in the pulse frequencies.

They discovered that the orbit was actually shrinking slowly, and the pulsar was speeding up in its orbit, causing the orbital period to decrease. The effect was minuscule, only 75 millionths of a second per year, out of an eight-hour orbit. But this is what the general theory predicts, because binary star systems should emit gravitational waves and thereby lose energy and shrink. The predicted rate agrees with the observed rate to a fifth of a per cent. The 1993 Nobel Prize in Physics went to Joseph Taylor and Russell Hulse, the discoverers of this extraordinary system.

Einstein made a third crucial prediction in addition to the bending of light and Mercury's perihelion, called the gravitational frequency shift. This is a shift to higher frequencies of a beam of light that moves downward in a gravitational field, or to lower frequencies for an upward moving beam.

An equivalent way of stating this is that time appears to move faster for clocks high in a gravitational field compared to those below. Einstein actually had this idea in 1907, when he realized the equivalence between gravity and an accelerating elevator, yet he regarded it as a crucial test of general relativity. But it wasn't tested until 1960, when scientists studied the behaviour of gamma rays going up or down a tower on the campus of Harvard University.

Later experiments using atomic clocks on planes and rockets verified the effect with high precision. Today, this effect leads to a remarkable link between relativity and daily life. The Global Positioning System (GPS) uses atomic clocks on a constellation of 24 satellites to provide precise navigation and time-transfer for users on the ground and in space.

The system is so precise that the effects of both of Einstein's relativities must be taken into account: special, because the satellites are moving fast compared to Earth clocks; and general, because they are at high altitudes. Without proper handling of these relativistic corrections, GPS would fail to function at its stated accuracy.

So it seems that Einstein was right. Not because the theory is elegant, not because Einstein was a great scientist, but because experiment says so.

But will he stay "right?" The theory predicts black holes, and while there is strong evidence that they exist, astronomers would like to see if their properties match the predictions.

The theory predicts gravitational waves, and while the binary pulsar gives strong indirect support of this, scientists would rather detect them directly, and have recently begun operating large gravitational-wave observatories on three continents with this in mind.

Detecting gravitational waves would open a new window on the universe, making possible the exploration of warped space-time near black holes and exploding stars.

Still, a number of physicists believe there may be physics beyond Einstein. Attempts to unify the interactions of physics in an overarching quantum model, such as string theory, all involve making small modifications of standard general relativity, not the least of which is moving from the four dimensions of space-time to 10 dimensions.

The recent astronomical evidence that our universe is expanding faster with time rather than slower may require a return to what Einstein called his greatest blunder, the addition of a "cosmological term" to his equations.

And laboratory scientists are making delicate measurements of the force of gravity between tiny objects separated by fractions of a millimetre to see if gravity at short distances behaves the same as gravity at solar-system distances.

How well the general theory will fare in the face of this experimental onslaught is anybody's guess. But given that Einstein derived his theory using only principles of beauty, simplicity and symmetry, it is truly remarkable how right it has turned out to be so far.
Report Spam   Logged

Skepticism is good, but when you reach a certain level where
you're grasping at straws and making little sense... it's not
called skepticism.  It's called ignorance.
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