From Symmetry: “Looking for strings inside inflation”

Symmetry

August 27, 2015
Troy Rummler

Theorists from the Institute for Advanced Study have proposed a way forward in the quest to test string theory.

Two theorists recently proposed a way to find evidence for an idea famous for being untestable: string theory. It involves looking for particles that were around 14 billion years ago, when a very tiny universe hit a growth spurt that used 15 billion times more energy than a collision in the Large Hadron Collider.

Scientists can’t crank the LHC up that high, not even close. But they could possibly observe evidence of these particles through cosmological studies, with the right technological advances.
Unknown particles

During inflation—the flash of hyperexpansion that happened 10-33 seconds after the big bang— particles were colliding with astronomical power. We see remnants of that time in tiny fluctuations in the haze of leftover energy called the cosmic microwave background [CMB].

CMB per Planck

ESA/Planck

Scientists might be able to find remnants of any prehistoric particles that were around during that time as well.

“If new particles existed during inflation, they can imprint a signature on the primordial fluctuations, which can be seen through specific patterns,” says theorist Juan Maldacena of the Institute for Advanced Study at Princeton University.

Maldacena and his IAS collaborator, theorist Nima Arkani-Hamed, have used quantum field theory calculations to figure out what these patterns might look like. The pair presented their findings at an annual string theory conference held this year in Bengaluru, India, in June.

The probable, impossible string

String theory is frequently summed up by its basic tenet: that the fundamental units of matter are not particles. They are one-dimensional, vibrating strings of energy.

The theory’s purpose is to bridge a mathematic conflict between quantum mechanics and [Albert] Einstein’s theory of general relativity. Inside a black hole, for example, quantum mechanics dictates that gravity is impossible. Any attempt to adjust one theory to fit the other causes the whole delicate system to collapse. Instead of trying to do this, string theory creates a new mathematical framework in which both theories are natural results. Out of this framework emerges an astonishingly elegant way to unify the forces of nature, along with a correct qualitative description of all known elementary particles.

As a system of mathematics, string theory makes a tremendous number of predictions. Testable predictions? None so far.

Strings are thought to be the smallest objects in the universe, and computing their effects on the relatively enormous scales of particle physics experiments is no easy task. String theorists predict that new particles exist, but they cannot compute their masses.

To exacerbate the problem, string theory can describe a variety of universes that differ by numbers of forces, particles or dimensions. Predictions at accessible energies depend on these unknown or very difficult details. No experiment can definitively prove a theory that offers so many alternative versions of reality.
Putting string theory to the test

But scientists are working out ways that experiments could at least begin to test parts of string theory. One prediction that string theory makes is the existence of particles with a unique property: a spin of greater than two.

Spin is a property of fundamental particles. Particles that don’t spin decay in symmetric patterns. Particles that do spin decay in asymmetric patterns, and the greater the spin, the more complex those patterns get. Highly complex decay patterns from collisions between these particles would have left signature impressions on the universe as it expanded and cooled.

Scientists could find the patterns of particles with greater than spin 2 in subtle variations in the distribution of galaxies or in the cosmic microwave background, according to Maldacena and Arkani-Hamed. Observational cosmologists would have to measure the primordial fluctuations over a wide range of length scales to be able to see these small deviations.

The IAS theorists calculated what those measurements would theoretically be if these massive, high-spin particles existed. Such a particle would be much more massive than anything scientists could find at the LHC.

A challenging proposition

Cosmologists are already studying patterns in the cosmic microwave background. Experiments such as Planck, BICEP and POLAR BEAR are searching for polarization, which would be evidence that a nonrandom force acted on it.

BICEP

PolarBear

If they rewind the effects of time and mathematically undo all other forces that have interacted with this energy, they hope that what pattern remains will match the predicted twists imbued by inflation.

The patterns proposed by Maldacena and Arkani-Hamed are much subtler and much more susceptible to interference. So any expectation of experimentally finding such signals is still a long way off.

But this research could point us toward someday finding such signatures and illuminating our understanding of particles that have perhaps left their mark on the entire universe.
The value of strings

Whether or not anyone can prove that the world is made of strings, people have proven that the mathematics of string theory can be applied to other fields.

In 2009, researchers discovered that string theory math could be applied to conventional problems in condensed matter physics. Since then researchers have been applying string theory to study superconductors.

Fellow IAS theorist Edward Witten, who received the Fields Medal in 1990 for his mathematical contributions to quantum field theory and Supersymmetry, says Maldacena and Arkani-Hamed’s presentation was among the most innovative work he saw at the Strings ‘15 conference.

Witten and others believe that such successes in other fields indicate that string theory actually underlies all other theories at some deeper level.

“Physics—like history—does not precisely repeat itself,” Witten says. However, with similar structures appearing at different scales of lengths and energies, “it does rhyme.”

See the full article here.

Please help promote STEM in your local schools.

Stem Education Coalition

Symmetry is a joint Fermilab/SLAC publication.

From physicsworld.com: “Have alien civilizations built cosmic accelerators from black holes?”

physicsworld.com

Mar 19, 2015
Hamish Johnston

Cosmic collider: could an advanced civilization harness a black hole

Has an advanced alien civilization built a black-hole-powered particle accelerator to study physics at “Planck-scale” energies? And if such a cosmic collider is lurking in a corner of the universe, could we detect it here on Earth?

Brian Lacki of the Institute for Advanced Studies in Princeton, New Jersey, has done calculations that suggest that if such an accelerator exists, it would produce yotta electron-volt (YeV or 1024 eV) neutrinos that could be detected here on Earth. As a result, Lacki is calling on astronomers involved in the search for extraterrestrial intelligence (SETI) to look for these ultra-high-energy particles. This is supported by SETI expert Paul Davies of Arizona State University, who believes that the search should be expanded beyond the traditional telescope searches.

Like humanity, it seems reasonable to assume that an advanced alien civilization would have a keen interest in physics, and would build particle accelerators that reach increasingly higher energies. This energy escalation could be the result of the “nightmare scenario” of particle physics in which there is no new physics at energies between the TeV energies of the Standard Model and the 1028 eV Planck energy (10 XeV) – where the quantum effects of gravity become strong. “The nightmare of particle physics is the dream of astronomers searching for extraterrestrials,” says Lacki.

An important problem facing alien physicists would be that the density of electromagnetic energy needed to reach the Planck scale is so great that the device would be in danger of collapsing into a black hole of its own making. However, Lacki points out that a clever designer could, in principle, get round this problem and “reaching [the] Planck energy is technically allowed, if extremely difficult”.

Not surprisingly, such an accelerator would have to be rather large. Lacki believes that if electric fields are used for acceleration, the device would have to be at least 10 times the radius of the Sun. However, a magnetic synchrotron-type accelerator could be somewhat smaller. As for what materials could be used to make the accelerator, Lacki says that normal materials could not withstand the strong electromagnetic fields. Indeed, one of the few places where such a high energy density could exist is in the vicinity of a black hole, which he argues could be harnessed to create a Planck-scale accelerator.

“Vast amounts of pollution”

Colliding particles at tens of XeVs is only half the battle, however. Lacki calculates that the vast majority of collisions in such a cosmic collider would be of no interest to alien researchers. To get useful information about Planck-scale physics, he reckons that the total collision rate in the accelerator would have to be about 1024 times that of the Large Hadron Collider. “As such, accelerators built to detect Planck events are extremely wasteful and produce vast amounts of ‘pollution’,” explains Lacki.

While much of this pollution would be extremely high-energy particles, that in principle could reach Earth, it is unclear whether they could escape the intense electromagnetic fields within the collider. Furthermore, like colliders here on Earth, the builders of a cosmic machine would probably try to shield the surrounding region from damaging radiation. Indeed, Lacki’s analysis suggests that neutrinos are the only particles that are likely to reach Earth.

These neutrinos would have energies that are a billion or more times greater than the highest energy neutrinos ever detected here on Earth. However, unlike their lower-energy counterparts, these accelerator neutrinos would be much easier to detect because they interact much more strongly with matter. Lacki calculates that the majority of such neutrinos passing through the Earth’s oceans will deposit their energy in the form of a shower of secondary particles. While the oceans are far too murky for physicists to detect the light given off by the showers, Lacki reckons that the sound of a shower could be detected by a network of hydrophones in the water. However, because these neutrinos are expected to be extremely rare, he calculates that about 100,000 hydrophones would be needed to have a chance of detecting the neutrinos.

Whole of the Moon

Another possibility, albeit less sensitive, is to use the Moon as a neutrino detector. Indeed, the NuMoon experiment is currently using a ground-based radio telescope to try to detect showers created when 1020 eV neutrinos smash into the lunar surface.

While the detection of YeV neutrinos would not be proof that an alien accelerator exists – some theories suggest that they could be produced naturally by the decay of a cosmic strings – Lacki says that spotting such high-energy particles would be an important breakthrough in physics.

While Davies is keen to expand SETI, he does identify one important drawback of looking for cosmic colliders. “My main problem is that once the [alien] experiments are done, there would be no need to keep the thing running, so unless there are mega-machines like this popping up all over the place, there would be only transient pulses,” he told physicsworld.com.

Davies believes that it is very difficult for humans today to understand why an advanced civilization would want to build a Planck-scale collider. “Why do it? Perhaps to create a baby universe or some other exotic space–time sculpture,” he speculates. “Why do that? Perhaps because this hypothetical civilization feels it faces a threat of cosmic dimensions. What might that threat be? I have no idea! However, a civilization that knows a million times more than humanity might perceive all sorts of threats of which we are blissfully unaware.”

Lacki’s calculations are described in a preprint on arXiv.

See the full article here.

Please help promote STEM in your local schools.

Stem Education Coalition

PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.