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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.

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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.”

Read more: https://www.theguardian.com/science/2017/jul/05/gravitational-waves-string-theory

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.”

Read more: https://www.theguardian.com/science/2016/dec/31/dark-matter-existence-space-astronomers-us-experiment

First edition of Isaac Newton’s Principia set to fetch $1m at auction

Rare European copy of key mathematics text is going under hammer at Christies in New York with record guide price.

A first edition of Sir Isaac Newton’s Principia Mathematica could become the most expensive print sold of the revolutionary text when it goes under the hammer with a guide price of at least $1m (790,000) this month.

The extremely rare continental copy being sold by auction house Christies in New York is one of a handful of texts thought to have been destined for Europe and has minor differences from those distributed in England by Newton and the book’s editor, Edmond Halley.

The list price of between $1m and $1.5m is thought to be a record for the book. An English version also bound in red morocco leather, which was said to have been presented to King James II, sold for more than $2.5m in 2013. Its list price was $600,000.

About 400 copies of Principia’s first edition were printed, of which the continental versions accounted for about 20%. Halley, the astronomer best known for the comet named after him, encouraged Newton to organise his theories into a text and paid for the printing because the Royal Society of which he and Newton were members had run out of funds.

The society retains two copies of the book, including the original manuscript on which the first print run in 1687 was based, which is described as its greatest treasure.

Written in Latin, the books full title is Philosophi Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). It laid out Newton’s groundbreaking theories in areas such as gravity and the forces of motion, and introduced a more rigorous mathematical method to physical science.

Keith Moore, the head of the Royal Society library, described it as a benchmark in human thought.

“It’s not just the history and development of science; it’s one of the greatest books ever published,” he said. “It was hugely influential in terms of applying mathematics to basic physical problems.”

Moore said the large sum set to be attracted by the book could be in part due to the growing influence of science within culture, as well as the huge earnings of some technology entrepreneurs.

“People who have big books these days maybe are the kinds of people who have made their money on the internet or the web … If you have a few million quid to spend, why wouldnt you buy a copy of Principia Mathematica?

“If you’ve made your money from a really cool algorithm, you will probably appreciate Newtonian physics.”

Despite its wide-ranging impact, and the books use as a foundational physics text being unsurpassed until Einstein’s general theory of relativity, Principia did not make a list last year of the top 20 most important academic books of all time. The list was topped by Charles Darwins On the Origin of Species.

But because it was published almost two centuries earlier, first editions of Principia are rarer and likely to continue selling for far larger amounts. One of the highest prices paid for a first edition of Darwins book laying out the theory of evolution was 103,000 in 2009, and subsequent sales have been lower.

While the prices differ, the impact of the two texts was comparable, Moore said. What Newton does in the 1680’s is revolutionise the physical sciences. The fundamental laws of physics.

Darwin’s great work published in 1859 revolutionised the biological sciences in the same way. They are similar books in the impact they had.

  • The picture caption on this article was amended on 5 December 2016 to clarify that the copy of Principia Mathematica up for sale is not the one held by Cambridge University.

Read more: https://www.theguardian.com/science/2016/dec/05/principia-sir-isaac-newton-first-edition-auction-christies-new-york

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