As Scott suggests -
New Scientist 13th July 2007
It's the theory everyone loves to hate.
Depending on who you ask these days, string theory is either untestable, disconnected from reality or not even science. Right?
Not so fast. While critics have been chipping away at its claim to be a "theory of everything", string theorists themselves have realised they must find ways to put their models to the test. They may still be far from being able to observe a string in a laboratory, but experiments planned for the near future - and even one currently under way - could provide tantalising evidence either for or against string theory.
Ever since the development of the two main pillars of modern physics - Einstein's general theory of relativity, which describes how gravity sculpts the universe at large, and quantum mechanics, which describes how subatomic particles behave - their incompatibility has had researchers hunting for a deeper theory to unify them. Many have thought that string theory, which emerged in the 1970s, is the most promising candidate because it is a quantum explanation of gravity, albeit one with flaws.
According to string theory, all particles and forces arise from vibrations of tiny string-like objects. What appear to us as electrons, protons or even gravity are in fact the same thing, but wiggling about in different ways. In order to produce such variety, strings have to wiggle in more complex patterns than are possible in three dimensions, so string theory requires at least six additional ones. To have gone unnoticed all this time, these dimensions must be curled up like submicroscopic origami. What's more, the number of forms that this origami can take is truly vast, with each one corresponding to a different universe with different particles and different fundamental constants.
How can experiments possibly test this? A string is smaller than a trillionth of a trillionth of an atom, rendering direct tests impossible even with the most powerful microscopes. Little wonder that scathing criticism of string theory as untestable and unscientific has mounted in the past few years. Now the string community is fighting back by devising creative, if indirect, ways to look for signs of strings - from hidden dimensions to ripples in space-time and other potential signatures of a stringy universe. The time has come to put string theory to the test.
Enter Joe Polchinski, a string theorist at the University of California, Santa Barbara, who is leading a search for strings in an unlikely place: outer space. Polchinski and others think strings that formed just after the big bang may have been stretched out by the expansion of the universe, reaching astronomical sizes and therefore becoming detectable. "Strings formed at the right moment in the early universe can stretch as the universe expands, and so today be as long as galaxies or even span the visible universe," he says.
Brane annihilation
The idea was proposed in 2002 by Saswat Sarangi and Henry Tye of Cornell University in Ithaca, New York. It came from string theory's version of inflation - the generally accepted notion that the universe underwent a brief period of exponential expansion just a fraction of a second after the big bang (New Scientist, 3 March, p 33). String theorists postulate that our three visible dimensions of space are actually confined to the surface of a membrane, or brane, floating in a higher-dimensional space; when a brane and a so-called anti-brane meet, they annihilate each other in a burst of energy, just as matter and antimatter do.
Such a collision would result in a tremendous expansion of space: if this happened near our brane, it and the surrounding space would inflate, matching standard cosmological models of inflation. Strings would be forged in the crash, and though most would remain tiny, some would expand along with the universe (New Scientist, 18 December 2004, p 30). Polchinski claims that space may therefore be strewn with a filamentary network of vibrating cosmic "superstrings". "Many of the most promising models of inflation in string theory do produce them," he says.
If these strings exist, they will be impossible to see directly. We could, however, observe their gravitational effects, since they pack a huge amount of mass into each unit length. A string could act as a gravitational lens as it passes in front of a star relative to Earth; the string's gravity will deflect passing starlight, producing a double image of the star from our point of view. The two images would have telltale features only a cosmic string could produce, such as equal brightness.
As strings vibrate, they should also produce ripples in the fabric of space-time known as gravitational waves. What's more, oscillating strings could be the most powerful source of gravitational waves in the universe, producing a distinctive pattern of strong bursts that would differentiate them from other sources. Although such waves have yet to be observed from any source, they are a key prediction of general relativity. Astronomers are already looking for them using the Laser Interferometer Gravitational-Wave Observatory (LIGO), while NASA and the European Space Agency (ESA) are planning to launch the far bigger Laser Interferometer Space Antenna (LISA) in 2015.
According to Polchinski, though, our best bet for observing gravitational waves emanating from strings is to use pulsars. A pulsar is a rapidly spinning neutron star that fires out a beam of electromagnetic radiation as it rotates, like a lighthouse. These flashing beacons act as some of the most accurate clocks in the universe, and a gravitational wave rippling between a pulsar and Earth would disturb the otherwise precise timing of the pulses arriving here. The most likely cause of such fluctuations would be black holes colliding, but waves from strings would yield a unique timing pattern that would make them stand out. "Over the next five to 10 years," Polchinski says, "these [pulsar observations] will probe the most interesting models."
Sure, catching sight of a cosmic string would be a boon for string theory, but is there any observation that would serve as a death knell? For many sceptics, it's not that string theory is so hard to prove correct that puts them off, but rather that you can't falsify it. "I'm not aware of any test that if it fails will prove string theory wrong," says physicist Brian Greene of Columbia University in New York. "That's a real headache. You'd like to have a situation where you have a prediction, and if it's right the theory is right, and if it's wrong the theory is wrong."
However, there is one test that could at least send string theory back to the drawing board. It involves another source of gravitational waves: inflation itself. The rapid expansion is thought to have sent ripples through space-time, and cosmologists are trying to uncover the imprint of these "primordial" gravity waves in the cosmic microwave background (CMB), the faint radiation from the big bang that pervades the universe. If they succeed, say physicists Andrei Linde and Renata Kallosh of Stanford University in California, string theorists will have some explaining to do.
That's because the strength of primordial gravity waves should be directly proportional to the energy of inflation itself. Earlier this year, Linde and Kallosh discovered that string theory sets a limit on that energy: they calculated that had inflation been too energetic, the six curled-up dimensions posited by string theory would have unfurled and grown just as large as the three we see around us. In order to preserve the six-dimensional origami, inflation must have occurred at relatively low energies, producing only weak gravity waves. In fact, according to Linde and Kallosh, no string-based inflation model can possibly create detectable gravity waves (www.arxiv.org/abs/0704.0647).
So any sign of gravitational waves in the CMB would be a serious blow to string theory. "It would be a very important observation to make us rethink our models of string theory, or rethink our models of string inflation," Linde says. Such an observation would mean that string theory is incompatible with inflation and also with the related "dark energy" that recent studies suggest makes up 70 per cent of our universe and is causing the expansion of space to speed up (New Scientist, 17 February, p 28).
The moment of truth could arrive with a pair of upcoming experiments. Clover, a trio of telescopes in the Atacama desert in Chile, is specifically designed to detect the imprint of gravitational waves on the CMB. This collaboration between the University of Oxford, the University of Cambridge and Cardiff University, UK, is expected to become operational next year. And ESA's Planck Surveyor spacecraft, scheduled for launch next year as well, will make the most sensitive measurements of the CMB to date, which should enable it to test for gravity waves.
Not everyone thinks these tests will be useful, however. "Not seeing something is hardly evidence for string theory," says Nobel laureate Sheldon Glashow of Boston University, Massachusetts, an outspoken critic of string theory. He feels that such a result would mean very little. "String theorists are very wise. They can come up with a way to explain anything." String theory is simply not testable, he says. "There are an enormous number of string theories and they describe zillions and zillions of universes, none of them observable in any way. It sounds to me like angels dancing on the head of a pin."
?String theories describe zillions of universes, none of them observable. It sounds like angels dancing on the head of a pin?
In the meantime some theorists, including string pioneer Leonard Susskind at Stanford, have suggested that string theory is in fact already being tested at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York. It's a controversial claim, and it highlights one of the most unexpected aspects of string theory.
?Observations could make us rethink string theory. Some theorists suggest that it is in fact already being tested?
Back in 1997, Juan Maldacena of the Institute for Advanced Study in Princeton, New Jersey, made a startling finding: that a version of string theory in a bizarrely shaped universe with five large dimensions and five curled-up ones is mathematically equivalent to an ordinary quantum theory of particles living on the four-dimensional surface of that universe. So the string world of the higher-dimensional space can be thought of as a holographic projection of the ordinary particles milling about on its surface (New Scientist, 27 April 2002, p 22). The particles in Maldacena's model are similar to the quarks and gluons that comprise every atom in our own universe, so could we likewise show that some similar version of string theory corresponds exactly to our own system of particle physics?
If so, say the string supporters, experiments like that at RHIC offer a way to test the theory (New Scientist, 16 October 2004, p 35). There, physicists slammed gold nuclei together at enormous speeds to produce a mini fireball 300 million times hotter than the surface of the sun. Inside the fireball, thousands of quarks and gluons formed a kind of plasma, just as they did in the first fraction of a second after the big bang. The plasma's existence had been predicted by quantum chromodynamics (QCD), the conventional theory that describes protons and neutrons as bundles of quarks held together by gluons. Quarks are perpetually bound together except at extreme temperatures, like those of the big bang or the RHIC fireballs.
When they first created the fireball, physicists were shocked: the quark-gluon plasma behaved nothing like the gas-like substance predicted by QCD. Instead, it behaved like a liquid. Ordinary QCD can't explain this result - but string theory might. According to Maldacena's holographic conjecture, every object or group of objects on a four-dimensional surface has a counterpart in the 10-dimensional interior. So what is the string-theory counterpart of the quark-gluon plasma? Surprise: a black hole.
Quarks and strings
Prior to RHIC's creation of the plasma, string theorists had predicted that a similar plasma on the four-dimensional surface of Maldacena's model universe should correspond exactly to a black hole in the higher-dimensional space. What's more, the event horizon of that black hole has a measurable viscosity, which describes how it responds to objects falling in, or to gravitational waves rippling across it. Calculating the viscosity of the black hole, string theorists were able to predict the viscosity of the experimental plasma. After the RHIC results, physicist Dam Thanh Son of the University of Washington, Seattle, and his colleagues performed the calculations and found that the viscosity of the observed quark-gluon plasma closely matched the value predicted by string theory. It seems that the quark-gluon plasma is indeed described by the same equations as a higher-dimensional black hole.
So what? Some physicists believe that the plasma and its black hole counterpart are simply analogous - they just happen to be described by the same mathematics. "But the connection may be deeper," says Susskind. He and some others think that the plasma and the black hole are not merely analogous, but are actually the same thing viewed at different energy scales and in different numbers of dimensions, so that experiments on quarks and gluons are in fact experiments on strings. "Is that testing string theory?" asks Susskind. "I think so."
The relationship between quarks and strings received further support in March when Igor Klebanov of Princeton University and his colleagues solidified the mathematical relationship between QCD and a particular string model (Physical Review Letters, vol 98, 131603). They proved that when gravity is strong in the string model, the force between quarks in QCD is weak, and vice versa. Their equations allow physicists to translate the QCD description into the string description and back again. In the quark-gluon plasma, for instance, the force between quarks is incredibly weak; the quarks, which are usually stuck together, are set free to wander about. This corresponds to incredibly strong gravity in the string world - a black hole.
"We're still very far from being able to say, here is the exact string theory that describes QCD," Susskind says. "But the connections that show up between nuclear physics and string theory are fascinating. Sceptics won't consider this evidence for string theory, but nuclear physicists will use string theory and in time discover how accurately it describes these experiments."
Critics should take heed. Experiments now show that string theory may be testable after all. One study at a time, string theorists seem to be homing in on models that will make specific, falsifiable predictions. "I don't think we will have a 100 per cent confirmation of string theory for a very long time," says Susskind. "But we will gather little pieces of evidence here and there - mathematics, heavy ion collisions, the discovery of supersymmetry [see "Brian Greene on string theory"]. All of these things will add up to something that can't be ignored."
From issue 2612 of New Scientist magazine, 11 July 2007, page 30-34
Brian Greene on string theory
String theorists are on the edge of their seats waiting for the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland, to power up and begin slamming protons together at extreme energies. The accelerator will begin collecting data next year, and may provide evidence for two key aspects of string theory: extra dimensions and supersymmetry. "If these tests turn out to be positive, they won't be a smoking gun for string theory itself, but will be for certain features of the theory," says physicist Brian Greene of Columbia University in New York.
If extra dimensions exist, they could give themselves away by swallowing debris coming out of the LHC's particle collisions. Because energy is always conserved in collisions, if a final tally reveals that some energy is missing, extra dimensions could be the culprit. "Evidence for extra dimensions won't prove string theory," Greene says, "but it will be a wonderful piece of circumstantial evidence."
Another piece of evidence would be supersymmetry, a central feature of string theory. It posits a relationship between matter particles such as protons and electrons, and force-carrying particles such as photons and gravitons. In a supersymmetric world, every matter particle has a superpartner that the LHC may have enough energy to finally produce. "If supersymmetry is found, that won't prove string theory correct, but it will establish a critical feature," says Greene.
Unfortunately, if supersymmetry is not observed, that won't rule out string theory. "One of the main criticisms I hear over and over again is that string theory is a moving target that can explain anything and therefore nothing," Greene says. "The thing that often gets lost in the discourse is that string theorists follow the mathematics - they don't dream up this or that wizardry to explain some particular thing, they follow where the mathematics takes them."
New Scientist 13th July 2007
It's the theory everyone loves to hate.
Depending on who you ask these days, string theory is either untestable, disconnected from reality or not even science. Right?
Not so fast. While critics have been chipping away at its claim to be a "theory of everything", string theorists themselves have realised they must find ways to put their models to the test. They may still be far from being able to observe a string in a laboratory, but experiments planned for the near future - and even one currently under way - could provide tantalising evidence either for or against string theory.
Ever since the development of the two main pillars of modern physics - Einstein's general theory of relativity, which describes how gravity sculpts the universe at large, and quantum mechanics, which describes how subatomic particles behave - their incompatibility has had researchers hunting for a deeper theory to unify them. Many have thought that string theory, which emerged in the 1970s, is the most promising candidate because it is a quantum explanation of gravity, albeit one with flaws.
According to string theory, all particles and forces arise from vibrations of tiny string-like objects. What appear to us as electrons, protons or even gravity are in fact the same thing, but wiggling about in different ways. In order to produce such variety, strings have to wiggle in more complex patterns than are possible in three dimensions, so string theory requires at least six additional ones. To have gone unnoticed all this time, these dimensions must be curled up like submicroscopic origami. What's more, the number of forms that this origami can take is truly vast, with each one corresponding to a different universe with different particles and different fundamental constants.
How can experiments possibly test this? A string is smaller than a trillionth of a trillionth of an atom, rendering direct tests impossible even with the most powerful microscopes. Little wonder that scathing criticism of string theory as untestable and unscientific has mounted in the past few years. Now the string community is fighting back by devising creative, if indirect, ways to look for signs of strings - from hidden dimensions to ripples in space-time and other potential signatures of a stringy universe. The time has come to put string theory to the test.
Enter Joe Polchinski, a string theorist at the University of California, Santa Barbara, who is leading a search for strings in an unlikely place: outer space. Polchinski and others think strings that formed just after the big bang may have been stretched out by the expansion of the universe, reaching astronomical sizes and therefore becoming detectable. "Strings formed at the right moment in the early universe can stretch as the universe expands, and so today be as long as galaxies or even span the visible universe," he says.
Brane annihilation
The idea was proposed in 2002 by Saswat Sarangi and Henry Tye of Cornell University in Ithaca, New York. It came from string theory's version of inflation - the generally accepted notion that the universe underwent a brief period of exponential expansion just a fraction of a second after the big bang (New Scientist, 3 March, p 33). String theorists postulate that our three visible dimensions of space are actually confined to the surface of a membrane, or brane, floating in a higher-dimensional space; when a brane and a so-called anti-brane meet, they annihilate each other in a burst of energy, just as matter and antimatter do.
Such a collision would result in a tremendous expansion of space: if this happened near our brane, it and the surrounding space would inflate, matching standard cosmological models of inflation. Strings would be forged in the crash, and though most would remain tiny, some would expand along with the universe (New Scientist, 18 December 2004, p 30). Polchinski claims that space may therefore be strewn with a filamentary network of vibrating cosmic "superstrings". "Many of the most promising models of inflation in string theory do produce them," he says.
If these strings exist, they will be impossible to see directly. We could, however, observe their gravitational effects, since they pack a huge amount of mass into each unit length. A string could act as a gravitational lens as it passes in front of a star relative to Earth; the string's gravity will deflect passing starlight, producing a double image of the star from our point of view. The two images would have telltale features only a cosmic string could produce, such as equal brightness.
As strings vibrate, they should also produce ripples in the fabric of space-time known as gravitational waves. What's more, oscillating strings could be the most powerful source of gravitational waves in the universe, producing a distinctive pattern of strong bursts that would differentiate them from other sources. Although such waves have yet to be observed from any source, they are a key prediction of general relativity. Astronomers are already looking for them using the Laser Interferometer Gravitational-Wave Observatory (LIGO), while NASA and the European Space Agency (ESA) are planning to launch the far bigger Laser Interferometer Space Antenna (LISA) in 2015.
According to Polchinski, though, our best bet for observing gravitational waves emanating from strings is to use pulsars. A pulsar is a rapidly spinning neutron star that fires out a beam of electromagnetic radiation as it rotates, like a lighthouse. These flashing beacons act as some of the most accurate clocks in the universe, and a gravitational wave rippling between a pulsar and Earth would disturb the otherwise precise timing of the pulses arriving here. The most likely cause of such fluctuations would be black holes colliding, but waves from strings would yield a unique timing pattern that would make them stand out. "Over the next five to 10 years," Polchinski says, "these [pulsar observations] will probe the most interesting models."
Sure, catching sight of a cosmic string would be a boon for string theory, but is there any observation that would serve as a death knell? For many sceptics, it's not that string theory is so hard to prove correct that puts them off, but rather that you can't falsify it. "I'm not aware of any test that if it fails will prove string theory wrong," says physicist Brian Greene of Columbia University in New York. "That's a real headache. You'd like to have a situation where you have a prediction, and if it's right the theory is right, and if it's wrong the theory is wrong."
However, there is one test that could at least send string theory back to the drawing board. It involves another source of gravitational waves: inflation itself. The rapid expansion is thought to have sent ripples through space-time, and cosmologists are trying to uncover the imprint of these "primordial" gravity waves in the cosmic microwave background (CMB), the faint radiation from the big bang that pervades the universe. If they succeed, say physicists Andrei Linde and Renata Kallosh of Stanford University in California, string theorists will have some explaining to do.
That's because the strength of primordial gravity waves should be directly proportional to the energy of inflation itself. Earlier this year, Linde and Kallosh discovered that string theory sets a limit on that energy: they calculated that had inflation been too energetic, the six curled-up dimensions posited by string theory would have unfurled and grown just as large as the three we see around us. In order to preserve the six-dimensional origami, inflation must have occurred at relatively low energies, producing only weak gravity waves. In fact, according to Linde and Kallosh, no string-based inflation model can possibly create detectable gravity waves (www.arxiv.org/abs/0704.0647).
So any sign of gravitational waves in the CMB would be a serious blow to string theory. "It would be a very important observation to make us rethink our models of string theory, or rethink our models of string inflation," Linde says. Such an observation would mean that string theory is incompatible with inflation and also with the related "dark energy" that recent studies suggest makes up 70 per cent of our universe and is causing the expansion of space to speed up (New Scientist, 17 February, p 28).
The moment of truth could arrive with a pair of upcoming experiments. Clover, a trio of telescopes in the Atacama desert in Chile, is specifically designed to detect the imprint of gravitational waves on the CMB. This collaboration between the University of Oxford, the University of Cambridge and Cardiff University, UK, is expected to become operational next year. And ESA's Planck Surveyor spacecraft, scheduled for launch next year as well, will make the most sensitive measurements of the CMB to date, which should enable it to test for gravity waves.
Not everyone thinks these tests will be useful, however. "Not seeing something is hardly evidence for string theory," says Nobel laureate Sheldon Glashow of Boston University, Massachusetts, an outspoken critic of string theory. He feels that such a result would mean very little. "String theorists are very wise. They can come up with a way to explain anything." String theory is simply not testable, he says. "There are an enormous number of string theories and they describe zillions and zillions of universes, none of them observable in any way. It sounds to me like angels dancing on the head of a pin."
?String theories describe zillions of universes, none of them observable. It sounds like angels dancing on the head of a pin?
In the meantime some theorists, including string pioneer Leonard Susskind at Stanford, have suggested that string theory is in fact already being tested at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, New York. It's a controversial claim, and it highlights one of the most unexpected aspects of string theory.
?Observations could make us rethink string theory. Some theorists suggest that it is in fact already being tested?
Back in 1997, Juan Maldacena of the Institute for Advanced Study in Princeton, New Jersey, made a startling finding: that a version of string theory in a bizarrely shaped universe with five large dimensions and five curled-up ones is mathematically equivalent to an ordinary quantum theory of particles living on the four-dimensional surface of that universe. So the string world of the higher-dimensional space can be thought of as a holographic projection of the ordinary particles milling about on its surface (New Scientist, 27 April 2002, p 22). The particles in Maldacena's model are similar to the quarks and gluons that comprise every atom in our own universe, so could we likewise show that some similar version of string theory corresponds exactly to our own system of particle physics?
If so, say the string supporters, experiments like that at RHIC offer a way to test the theory (New Scientist, 16 October 2004, p 35). There, physicists slammed gold nuclei together at enormous speeds to produce a mini fireball 300 million times hotter than the surface of the sun. Inside the fireball, thousands of quarks and gluons formed a kind of plasma, just as they did in the first fraction of a second after the big bang. The plasma's existence had been predicted by quantum chromodynamics (QCD), the conventional theory that describes protons and neutrons as bundles of quarks held together by gluons. Quarks are perpetually bound together except at extreme temperatures, like those of the big bang or the RHIC fireballs.
When they first created the fireball, physicists were shocked: the quark-gluon plasma behaved nothing like the gas-like substance predicted by QCD. Instead, it behaved like a liquid. Ordinary QCD can't explain this result - but string theory might. According to Maldacena's holographic conjecture, every object or group of objects on a four-dimensional surface has a counterpart in the 10-dimensional interior. So what is the string-theory counterpart of the quark-gluon plasma? Surprise: a black hole.
Quarks and strings
Prior to RHIC's creation of the plasma, string theorists had predicted that a similar plasma on the four-dimensional surface of Maldacena's model universe should correspond exactly to a black hole in the higher-dimensional space. What's more, the event horizon of that black hole has a measurable viscosity, which describes how it responds to objects falling in, or to gravitational waves rippling across it. Calculating the viscosity of the black hole, string theorists were able to predict the viscosity of the experimental plasma. After the RHIC results, physicist Dam Thanh Son of the University of Washington, Seattle, and his colleagues performed the calculations and found that the viscosity of the observed quark-gluon plasma closely matched the value predicted by string theory. It seems that the quark-gluon plasma is indeed described by the same equations as a higher-dimensional black hole.
So what? Some physicists believe that the plasma and its black hole counterpart are simply analogous - they just happen to be described by the same mathematics. "But the connection may be deeper," says Susskind. He and some others think that the plasma and the black hole are not merely analogous, but are actually the same thing viewed at different energy scales and in different numbers of dimensions, so that experiments on quarks and gluons are in fact experiments on strings. "Is that testing string theory?" asks Susskind. "I think so."
The relationship between quarks and strings received further support in March when Igor Klebanov of Princeton University and his colleagues solidified the mathematical relationship between QCD and a particular string model (Physical Review Letters, vol 98, 131603). They proved that when gravity is strong in the string model, the force between quarks in QCD is weak, and vice versa. Their equations allow physicists to translate the QCD description into the string description and back again. In the quark-gluon plasma, for instance, the force between quarks is incredibly weak; the quarks, which are usually stuck together, are set free to wander about. This corresponds to incredibly strong gravity in the string world - a black hole.
"We're still very far from being able to say, here is the exact string theory that describes QCD," Susskind says. "But the connections that show up between nuclear physics and string theory are fascinating. Sceptics won't consider this evidence for string theory, but nuclear physicists will use string theory and in time discover how accurately it describes these experiments."
Critics should take heed. Experiments now show that string theory may be testable after all. One study at a time, string theorists seem to be homing in on models that will make specific, falsifiable predictions. "I don't think we will have a 100 per cent confirmation of string theory for a very long time," says Susskind. "But we will gather little pieces of evidence here and there - mathematics, heavy ion collisions, the discovery of supersymmetry [see "Brian Greene on string theory"]. All of these things will add up to something that can't be ignored."
From issue 2612 of New Scientist magazine, 11 July 2007, page 30-34
Brian Greene on string theory
String theorists are on the edge of their seats waiting for the Large Hadron Collider (LHC) at CERN near Geneva, Switzerland, to power up and begin slamming protons together at extreme energies. The accelerator will begin collecting data next year, and may provide evidence for two key aspects of string theory: extra dimensions and supersymmetry. "If these tests turn out to be positive, they won't be a smoking gun for string theory itself, but will be for certain features of the theory," says physicist Brian Greene of Columbia University in New York.
If extra dimensions exist, they could give themselves away by swallowing debris coming out of the LHC's particle collisions. Because energy is always conserved in collisions, if a final tally reveals that some energy is missing, extra dimensions could be the culprit. "Evidence for extra dimensions won't prove string theory," Greene says, "but it will be a wonderful piece of circumstantial evidence."
Another piece of evidence would be supersymmetry, a central feature of string theory. It posits a relationship between matter particles such as protons and electrons, and force-carrying particles such as photons and gravitons. In a supersymmetric world, every matter particle has a superpartner that the LHC may have enough energy to finally produce. "If supersymmetry is found, that won't prove string theory correct, but it will establish a critical feature," says Greene.
Unfortunately, if supersymmetry is not observed, that won't rule out string theory. "One of the main criticisms I hear over and over again is that string theory is a moving target that can explain anything and therefore nothing," Greene says. "The thing that often gets lost in the discourse is that string theorists follow the mathematics - they don't dream up this or that wizardry to explain some particular thing, they follow where the mathematics takes them."
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