Large Hadron Collider | The Knowledge Dynasty

Large Hadron Collider

Tying loose ends? Gravitational waves could solve string theory, study claims

New paper suggests that the hotly contested physics thesis, which involves the existence of six extra dimensions, may be settled by cutting-edge laser detectors.

String theory makes the grand promise of weaving together all of physics into a single sublime framework. The only downside is that scientists have yet to find any experimental proof that it is right and critics question whether its predictions are even testable.

Now, a new paper has claimed that gravitational wave measurements could hold the key to whether string theory is destined to fulfil its lofty goals or be consigned to the dustbin of discarded ideas. The study suggests that the first observable evidence for the existence of extra dimensions, one of string theorys predictions, could be hidden within the ripples of gravitational waves.

“It would be amazing because general relativity and Einstein do not predict this at all,” said David Andriot, a physicist at the Max Planck Institute for Gravitational Physics in Potsdam and lead author of the study.

The crux of string theory although there are many competing versions is that all particles can be viewed as one-dimensional strings on which the fundamental forces of nature (gravity, electromagnetism and so on) act as different modes of vibration. For reasons better explained in maths than words, the framework also requires there to be at least six extra spatial dimensions, in addition to time and the three spatial ones of everyday life.

Scientists, notably those working at the Large Hadron Collider, have looked for energy vanishing into these hypothetical extra dimensions, but so far efforts have been inconclusive. One possibility is that the dimensions are coiled up so tightly that they are imperceptible; another is that they are not there at all.

Andriot is hopeful that the Laser Interferometer Gravitational-Wave Observatory (Ligo) experiment could start to answer this question.

In 2015, Ligo made the historic first observation of gravitational waves, the compression and stretching of space that Einstein predicted would occur as a mass moves through the fabric of the universe. In this case, Ligos detectors were picking up the ripples sent out across space-time following the violent collision of a pair of black holes more than a billion years ago.

A Laser Interferometer Gravitational-Wave Observatory (Ligo) technician inspects the devices twin detectors. Photograph: LIGO Laboratory/Reuters

String theory predicts that, during such cataclysmic events, ripples should also be travelling through the extra spatial dimensions and that there should be subtle interactions between the standard waves and those hidden from view.

Our study concludes that if there are extra dimensions it would lead to another mode of shrinking and stretching, said Andriot.

The latest paper, published in the Journal of Cosmology and Astroparticle Physics, concludes this would produce a breathing effect, superimposed on the main gravitational wave. The pattern might be measurable once a third detector, called Virgo, joins the twin Ligo detectors in gathering data late next year or early in 2019, although the team have not yet worked out whether the effect would be big enough to spot.

“If we have extra dimensions we can get this effect, but there are other things that could cause it. It’s not a smoking gun for extra dimensions,” said Andriot.

Christopher Berry, a scientist working on Ligo at the University of Birmingham, said it is a priority to look for the kinds of subtle modifications to gravitational waves described in the paper. “It’s one of the classic tests that we would like to do,” he said.

Such observations would be hugely significant because they are not predicted by Einstein’s general theory of relativity, meaning that our understanding of how gravity behaves would need to be revised. One option is string theory, but there are other competing theories. The absence of the breathing effect would help rule out some of these theories, or narrow the window in which they could occur.

“We expect that any deviations from general relativity would happen in the most extreme conditions; that’s where you’d expect the theory would break,” said Berry. “The best place for testing that is the collision of black holes.”

The paper also predicts that gravitational waves should ripple through each extra dimension at a characteristic frequency analogous to the way organ pipes of different lengths produce notes of different pitch. Working on the assumption that the extra dimensions are very small, a series of higher-frequency gravitational waves would be predicted. These would be at a frequency more than a billion times higher than the limit of what Ligo could detect, but which might be observable one day by a future detector.

“If this was seen, we could talk of a smoking gun,” said Andriot.

Others remain unconvinced that such observations would provide the sought-after experimental proof. Peter Woit, a theoretical physicist at Columbia University, New York, and longstanding critic of string theory, said: “The problem is that string theory says nothing at all about the sizes of these extra dimensions, they could be anything from infinitely large to infinitely small, so theres no real prediction. If we ever do see extra dimensions, there’s no particular reason to believe these have anything to do with string theory.”

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Will scientists ever prove the existence of dark matter?

Astronomers in the US are setting up an experiment which, if it fails as others have could mark the end of a 30-year-old theory.

Deep underground, in a defunct gold mine in South Dakota, scientists are assembling an array of odd devices: a chamber for holding tonnes of xenon gas; hundreds of light detectors, each capable of pinpointing a single photon; and a vast tank that will be filled with hundreds of gallons of ultra-pure water. The project, the LZ experiment, has a straightforward aim: it is designed to detect particles of an invisible form of matter called dark matter as they drift through space.

It is thought there is five times more dark matter than normal matter in the universe, although it has yet to be detected directly. Finding it would solve one of sciences most baffling mysteries and explain why galaxies are not ripped apart by stars flying off into deep space.

However, many scientists believe time is running out for the hunt, which has lasted 30 years, cost millions of pounds and produced no positive results. The LZ project which is halfway through construction should be sciences last throw of the dice, they say. This generation of detectors should be the last, said astronomer Stacy McGaugh at Case Western Reserve University in Cleveland, Ohio. If we dont find anything we should accept we are stuck and need to find a different explanation, perhaps by modifying our theories of gravity, to explain the phenomena we attribute to dark matter.

Other researchers reject this view: “Theory indicates we have a really good chance of finding dark matter particles,” said Chamkaur Ghag, chair of the Dark Matter UK consortium. “This is certainly not the time to talk of giving up.”

The concept of dark matter stems from observations made in the 1970s. Astronomers expected to find that stars rotated more slowly around a galaxy the more distant they were from the galaxys centre, just as distant planets revolve slowly round the Sun. (Outermost Neptune moves round the Sun at a stately 12,000mph; innermost Mercury does so at 107,082mph.)

That prediction was spectacularly undone by observations, however. Stars at a galaxys edge orbit almost as fast as those near its centre. According to theory, they should be hurled into space. So astronomers proposed that invisible dark matter must be providing the extra gravity needed to hold galaxies together. Proposed sources of dark matter include burnt-out stars; clouds of dust and gas; and subatomic particles called Wimps weakly interacting massive particles. All have since been discounted, except Wimps. Many astronomers are now convinced they permeate space and form halos round galaxies to give them the gravitational muscle needed to hold fast-flying stars in place.

Getting close to Wimps has not been easy. Scientists have built increasingly sensitive detectors deeper and deeper underground to protect them from subatomic particles that bombard Earths surface and which would trigger spurious signals. These devices resemble huge Russian dolls: a vast metal tank containing water to provide added protection against incoming stray particles is erected and, within this, a giant sphere of an inert gas such as xenon is suspended. Wimps making it through to the final tank should occasionally strike a xenon nucleus, producing a flash of light that can be pinpointed by electronic detectors.

Despite three decades of effort, this approach has had no success, a failure that is starting to worry some researchers. We are now building detectors containing more and more xenon and which are a million times more sensitive than those we used to hunt Wimps 30 years ago, said astrophysicist Professor David Merritt, of the Rochester Institute of Technology, New York. And still we have found nothing.

Last July, scientists reported that after running their Large Underground Xenon (Lux) experiment for 20 months they had still failed to spot a Wimp. Now an upgraded version of Lux is being built the LZ detector, a US-UK collaboration while other devices in Canada and Italy are set to run searches.

The problem facing Wimp hunters is that as their detectors get ever more sensitive, they will start picking up signals from other weakly interacting particles called neutrinos. Tiny, almost massless, these constantly whizz through our planet and our bodies. Neutrinos are not nearly heavy enough to account for the gravitational abnormalities associated with dark matter but are still likely to play havoc with the next generation of Wimp detectors.

I believe the Wimp hypothesis will be truly dead when we reach that point, said McGaugh. It already has serious problems but if we get to the point where we are picking up all this background interaction, the game is up. You will not be able to spot a thing.

This point is rejected by Ghag. “Yes, occasionally a neutrino will kick a xenon nucleus and produce a result that resembles a Wimp interaction. We will, initially, be in trouble. But as we characterise the collisions we should find ways to differentiate them and concentrate only on those produced by Wimps.”

But there is no guarantee that Wimps if they exist will ever interact with atoms of normal matter. You can imagine a scenario where dark matter particles turn out to be so incredibly weak at interacting with normal matter that our detectors will never see anything, said cosmologist Andrew Pontzen, of University College London.

Indeed, it could transpire that a Wimp is completely incapable of interacting with normal matter. You would then be saying we can only make sense of the universe by proposing a hypothetical particle that we can never detect, said Pontzen. Philosophically that is a highly unsatisfactory situation. You would be saying you cannot prove or disprove a key hypothesis that underpins scientificunderstanding.

However, Pontzen also pointed out that dark matter has proved invaluable in making scientific predictions and should not be dismissed too quickly. Scientists in the late 20th century attempted to predict what the cosmic background radiation left behind by the Big Bang 13 billion years ago might look like. Those who used dark matter in their calculations were found to have got things spectacularly right when we later flew probes to study that radiation background. It shows there was dark matter right at the birth of the universe.

McGaugh is unconvinced. He points to the failure of Geneva’s Large Hadron Collider, used to find the Higgs boson, to produce particles that might hint at the existence of Wimps. It was hailed as the golden test but it has produced nothing, just like the other experiments. Instead, more effort should be directed to developing new theoretical approaches to understanding gravity, he argues. One such theory is known as modified Newtonian dynamics, or Mond. It suggests that variations in the behaviour of gravity could account for the unexpected star speeds. “Such approaches should take precedence if LZ should fail to find dark matter in the next two or three years,” McGaugh said.

Ghag disagrees. “I think it is ridiculous to suggest we stop, he said. Are we just going to say OK, we have no idea what makes up 85% of the universe just because we are finding it all a bit hard? That’s not realistic.”

The uncertain nature of the problem was summed up by Pontzen. “We have been looking for dark matter for so long. Sometimes I think I should get real and admit something is up. On the other hand, the technology is getting better and we are opening up new possibilities of where to find dark matter. Which of these scenarios I feel closest to depends what sort of day I am having.”

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Could the Higgs boson have been discovered by accident?

A theorist says no. I say yes. See what you think.

We have a tendency to oversimplify complicated issues. Sometimes this gives useful clarity, but more frequently it gives a distorted impression of what I am stubborn enough to call the truth. Clarity can be seductive, but is disastrously misleading if it neglects important facts. This is true in politics, in history, and in science.

In a recent article on how we came to discover the Higgs boson, my colleague, theoretical physicist James Wells, puts it like this:

A terse and deleteriously incomplete history of the Higgs boson says that it was postulated in 1964 by the theorist Peter Higgs and then discovered in 2012 by experimentalists after a multi-decade herculean construction project at CERN to find it.

This is, he says, a distortion of how science progresses, and of what enables scientific discoveries.

The Higgs boson was the last particle of the Standard Model of physics to be discovered. It occupies a unique and essential place in that theory, and its discovery validated our understanding of how fundamental particles can have mass. You might think the history of that discovery would be thoroughly understood and agreed on, but Wells detects a false and seductive mythology: a Eureka moment for Higgs in 1964 followed by 48 years of experimental labour until the triumphant announcement by the ATLAS and CMS collaborations on 4 July 2012.

His article is a good read for anyone unafraid of an equation or two, and his main point is that elucidating the observable consequences of the idea conceived by Higgs (and Brout and Englert) in 1964 required a series of major theoretical advances over the intervening years, and an amount of hard graft at least comparable to that involved in building the experiments. The discovery papers of ATLAS and CMS cite 115 theory papers. Wells lists them in his article. They cover the original ideas, understanding their appearance in a particle collision, what other physics might fake that appearance, what the theoretical uncertainties are, and more. ATLAS and CMS could have cited many more papers, but you have to draw the line somewhere.

The point is convincingly made, and should be taken on board by anyone tempted by the simplistic lone theorist hero and herculean experimentalists mythology.

Wells makes another point, however, with which I disagree. He says that the theory input is so important that

The Higgs boson could not have been discovered experimentally by accident.

I think it could.

The Higgs boson is produced when particles are brought into collision with each other at high energies, as was done at the Large Hadron Collider at CERN. Once produced, the Higgs decays rapidly to other particles, and digging the signs of this decay out of all the other particles produced in the collisions is a large part of the experimental challenge. The Higgs can decay several different ways, and some of them would surely not have been untangled any time soon without a lot of guidance from theoretical predictions such as those described by Wells.

But one of the ways the Higgs can decay is rather striking and is not overwhelmed by fake backgrounds. This is the decay to four leptons (either electrons, muons or their antiparticles). Any experiment at a high energy collider would look for four leptons and measure their mass. In fact this is the measurement I described just last week. The bump at 125 GeV in that plot with the most physics is pretty clear – we would, I am confident, have made that plot and seen that bump, even if we hadnt been looking for the Higgs.

Now it is true that without the theory we might never have built the collider, or ATLAS and CMS. Counterfactual history is tricky. But there are good general reasons for building high-energy colliders they allow us to study the smallest constituents of matter, and we knew that was the case already without help from Higgs. So I think high energy physics would probably have got there in the end.

What is certainly true is that without the theory we would have been slower, and would not have known immediately what that bump was. Wed have been excited about it, and there would probably have been a lot of theories produced very quickly, as there were when we thought we might have another one at 750 GeV. Maybe that is what Wells means without the theory, we could not have understood the implications of the bump, or connected it to the origin of particle masses. In that case I would agree with him.

But the Higgs boson could, and I think probably would, have turned up by accident, nevertheless. And the theorists would have told us what it meant, eventually.

Jon Butterworths book Smashing Physicsis available as Most Wanted Particle in Canada & the US.

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