Tag: particle accelerators

All the science in ‘The Cloverfield Paradox’

I watched The Cloverfield Paradox last night, the horror film that Paramount pictures had dumped with Netflix and which was then released by Netflix on February 4. It’s a dumb production: unlike H.R. Giger’s existential, visceral horrors that I so admire, The Cloverfield Paradox is all about things going bump in the dark. But what sets these things off in the film is quite interesting: a particle accelerator. However, given how bad the film was, the screenwriter seems to have used this device simply as a plot device, nothing else.

The particle accelerator is called Shepard. We don’t know what particles it’s accelerating or up to what centre-of-mass collision energy. However, the film’s premise rests on the possibility that a particle accelerator can open up windows into other dimensions. The Cloverfield Paradox needs this because, according to its story, Earth has run out of energy sources in 2028 and countries are threatening ground invasions for the last of the oil, so scientists assemble a giant particle accelerator in space to tap into energy sources in other dimensions.

Considering 2028 is only a decade from now – when the Sun will still be shining bright as ever in the sky – and renewable sources of energy aren’t even being discussed, the movie segues from sci-fi into fantasy right there.

Anyway, the idea that a particle accelerator can open up ‘portals’ into other dimensions isn’t new nor entirely silly. Broadly, an accelerator’s purpose is founded on three concepts: the special theory of relativity (SR), particle decay and the wavefunction of quantum mechanics.

According to SR, mass and energy can transform into each other as well as that objects moving closer to the speed of light will become more massive, thus more energetic. Particle decay is what happens when a heavier subatomic particle decomposes into groups of lighter particles because it’s unstable. Put these two ideas together and you have a part of the answer: accelerators accelerate particles to extremely high velocities, the particles become more massive, ergo more energetic, and the excess energy condenses out at some point as other particles.

Next, in quantum mechanics, the wavefunction is a mathematical function: when you solve it based on what information you have available, the answer spit out by one kind of the function gives the probability that a particular particle exists at some point in the spacetime continuum. It’s called a wavefunction because the function describes a wave, and like all waves, this one also has a wavelength and an amplitude. However, the wavelength here describes the distance across which the particle will manifest. Because energy is directly proportional to frequency (E = h × ν; h is Planck’s constant) and frequency is inversely proportional to the wavelength, energy is inversely proportional to wavelength. So the more the energy a particle accelerator achieves, the smaller the part of spacetime the particles will have a chance of probing.

Spoilers ahead

SR, particle decay and the properties of the wavefunction together imply that if the Shepard is able to achieve a suitably high energy of acceleration, it will be able to touch upon an exceedingly small part of spacetime. But why, as it happens in The Cloverfield Paradox, would this open a window into another universe?

Spoilers end

Instead of directly offering a peek into alternate universes, a very-high-energy particle accelerator could offer a peek into higher dimensions. According to some theories of physics, there are many higher dimensions even though humankind may have access only to four (three of space and one of time). The reason they should even exist is to be able to solve some conundrums that have evaded explanation. For example, according to Kaluza-Klein theory (one of the precursors of string theory), the force of gravity is so much weaker than the other three fundamental forces (strong nuclear, weak nuclear and electromagnetic) because it exists in five dimensions. So when you experience it in just four dimensions, its effects are subdued.

Where are these dimensions? Per string theory, for example, they are extremely compactified, i.e. accessible only over incredibly short distances, because they are thought to be curled up on themselves. According to Oskar Klein (one half of ‘Kaluza-Klein’, the other half being Theodore Kaluza), this region of space could be a circle of radius 10-32 m. That’s 0.00000000000000000000000000000001 m – over five quadrillion times smaller than a proton. According to CERN, which hosts the Large Hadron Collider (LHC), a particle accelerated to 10 TeV can probe a distance of 10-19 m. That’s still one trillion times larger than where the Kaluza-Klein fifth dimension is supposed to be curled up. The LHC has been able to accelerate particles to 8 TeV.

The likelihood of a particle accelerator tossing us into an alternate universe entirely is a different kind of problem. For one, we have no clue where the connections between alternate universes are nor how they can be accessed. In Nolan’s Interstellar (2014), a wormhole is discovered by the protagonist to exist inside a blackhole – a hypothesis we currently don’t have any way of verifying. Moreover, though the LHC is supposed to be able to create microscopic blackholes, they have a 0% chance of growing to possess the size or potential of Interstellar‘s Gargantua.

In all, The Cloverfield Paradox is a waste of time. In the 2016 film Spectral – also released by Netflix – the science is overwrought, stretched beyond its possibilities, but still stays close to the basic principles. For example, the antagonists in Spectral are creatures made entirely as Bose-Einstein condensates. How this was even achieved boggles the mind, but the creatures have the same physical properties that the condensates do. In The Cloverfield Paradox, however, the accelerator is a convenient insertion into a bland story, an abuse of the opportunities that physics of this complexity offers. The writers might as well have said all the characters blinked and found themselves in a different universe.

A new dawn for particle accelerators in the wake

During a lecture in 2012, G. Rajasekaran, professor emeritus at the Institute for Mathematical Sciences, Chennai, said that the future of high-energy physics lay with engineers being able to design smaller particle accelerators. The theories of particle physics have for long been exploring energy levels that we might never be able to reach with accelerators built on Earth. At the same time, it will still be on physicists to reach the energies that we can reach but in ways that are cheaper, more efficient, and smaller – because reach them we will have to if our theories must be tested. According to Rajasekaran, the answer is, or will soon be, the tabletop particle accelerator.

In the last decade, tabletop accelerators have inched closer to commercial viability because of a method called plasma wakefield acceleration. Recently, a peer-reviewed experiment detailing the effects of this method was performed at the University of Maryland (UMD) and the results published in the journal Physical Review Letters. A team-member said in a statement: “We have accelerated high-charge electron beams to more than 10 million electron volts using only millijoules of laser pulse energy. This is the energy consumed by a typical household lightbulb in one-thousandth of a second.” Ten MeV pales in comparison to what the world’s most powerful particle accelerator, the Large Hadron Collider (LHC), achieves – a dozen million MeV – but what the UMD researchers have built doesn’t intend to compete against the LHC but against the room-sized accelerators typically used for medical imaging.

In particle accelerator like the LHC or the Stanford linac, a string of radiofrequency (RF) cavities are used to accelerate charged particles around a ring. Energy is delivered to the particles using powerful electromagnetic fields via the cavities, which switch polarity at 400 MHz – that’s switching at 400 million times a second. The particles’ arrival at the cavities are timed accordingly. Over the course of 15 minutes, the particle bunches are accelerated from 450 GeV to 4 TeV (the beam energy before the LHC was upgraded over 2014), with the bunches going 11,000 times around the ring per second. As the RF cavities switch faster and are ramped up in energy, the particles swing faster and faster around – until computers bring two such beams into each other’s paths at a designated point inside the ring and BANG.

A wakefield accelerator also has an electromagnetic field that delivers the energy, but instead of ramping and switching over time, it delivers the energy in one big tug.

First, scientists create a plasma, a fluidic state of matter consisting of free-floating ions (positively charged) and electrons (negatively charged). Then, the scientists shoot two bunches of electrons separated by 15-20 micrometers (millionths of a metre). As the leading bunch moves into the plasma, it pushes away the plasma’s electrons and so creates a distinct electric field around itself called the wakefield. The wakefield envelopes the trailing bunch of electrons as well, and exerts two forces on them: one along the direction of the leading bunch, which accelerates the trailing bunch, and one in the transverse direction, which either makes the bunch more or less focused. And as the two bunches shoot through the plasma, the leading bunch transfers its energy to the trailing bunch via the linear component of the wakefield, and the trailing bunch accelerates.

A plasma wakefield accelerator scores over a bigger machine in two key ways:

  • The wakefield is a very efficient energy transfer medium (but not as much as natural media), i.e. transformer. Experiments at the Stanford Linear Accelerator Centre (SLAC) have recorded 30% efficiency, which is considered high.
  • Wakefield accelerators have been able to push the energy gained per unit distance travelled by the particle to 100 GV/m (an electric potential of 1 GV/m corresponds to an energy gain of 1 GeV/c2 for one electron over 1 metre). Assuming a realistic peak accelerating gradient of 100 MV/m, a similar gain (of 100 GeV) at the SLAC would have taken over a kilometre.

There are many ways to push these limits – but it is historically almost imperative that we do. Could the leap in accelerating gradient by a factor of 100 to 1,000 break the slope of the Livingston plot?

Could the leap in accelerating gradient from RF cavities to plasma wakefields break the Livingston plot? Source: AIP
Could the leap in accelerating gradient from RF cavities to plasma wakefield accelerators break the Livingston plot? Source: AIP

In the UMD experiment, scientists shot a laser pulse into a hydrogen plasma. The photons in the laser then induced the wakefield that trailing electrons surfed and were accelerated through. To reduce the amount of energy transferred by the laser to generate the same wakefield, they made the plasma denser instead to capitalise on an effect called self-focusing.

A laser’s electromagnetic field, as it travels through the plasma, makes electrons near it wiggle back and forth as the field’s waves pass through. The more intense waves near the pulse’s centre make the electrons around it wiggle harder. Since Einstein’s theory of relativity requires objects moving faster to weigh more, the harder-wiggling electrons become heavier, slow down and then settle down, creating a focused beam of electrons along the laser pulse. The denser the plasma, the stronger the self-focusing – a principle that can compensate for weaker laser pulses to sustain a wakefield of the same strength if the pulses were stronger but the plasma less dense.

The UMD team increased the hydrogen gas density, of which the plasma is made, by some 20x and found that electrons could be accelerated by 2-12 MeV using 10-50 millijoule laser pulses. Additionally, the scientists also found that at high densities, the amplitude of the plasma wave propagated by the laser pulse increases to the point where it traps some electrons from the plasma and continuously accelerates them to relativistic energies. This obviates the need for trailing electrons to be injected separately and increases the efficiency of acceleration.

But as with all accelerators, there are limitations. Two specific to the UMD experiment are:

  • If the plasma density goes beyond a critical threshold (1.19 x 1020 electrons/cm3) and if the laser pulse is too powerful (>50 mJ), the electrons are accelerated more by the direct shot than by the plasma wakefield. These numbers define an upper limit to the advantage of relativistic self-focusing.
  • The accelerated electrons slowly drift apart (in the UMD case, to at most 250 milliradians) and so require separate structures to keep their beam focused – especially if they will be used for biomedical purposes. (In 2014, physicists from the Lawrence Berkeley National Lab resolved this problem by using a 9-cm long capillary waveguide through which the plasma was channelled.)

There is another way lasers can be used to build an accelerator. In 2013, physicists from Stanford University devised a small glass channel 0.075-0.1 micrometers wide, and etched with nanoscale ridges on the floor. When they shined infrared light with wavelength of twice the channel’s height across it, the eM field of the light wiggled the electrons back and forth – but the ridges on the floor were cut such that electrons passing over the crests would accelerate more than they would decelerate when passing over the troughs. Like this, they achieved an energy gain gradient of 300 MeV/m. This way, the accelerator is only a few millimetres long and devoid of any plasma, which is difficult to handle.

At the same time, this method shares a shortcoming with the (non-laser driven) plasma wakefield accelerator: both require the electrons to be pre-accelerated before injection, which means room-sized pre-accelerators are still in the picture.

Physical size is an important aspect of particle accelerators because, the way we’re building them, the higher-energy ones are massive. The LHC currently accelerates particles to 13 TeV (1 TeV = 1 million MeV) in a 27-km long underground tunnel running beneath the shared borders of France and Switzerland. The planned Circular Electron-Positron Collider in China envisages a 100-TeV accelerator around a 54.7-km long ring (Both the LHC and the CEPC involve pre-accelerators that are quite big – but not as much as the final-stage ring). The International Linear Collider will comprise a straight tube, instead of a ring, over 30 km long to achieve accelerations of 500 GeV to 1 TeV. In contrast, Georg Korn suggested in APS Physics in December 2014 that a hundred 10-GeV electron acceleration modules could be lined up facing against a hundred 10-GeV positron acceleration modules to have a collider that can compete with the ILC but from atop a table.

In all these cases, the net energy gain per distance travelled (by the accelerated particle) was low compared to the gain in wakefield accelerators: 250 MV/m versus 10-100 GV/m. This is the physical difference that translates to a great reduction in cost (from billions of dollars to thousands), which in turn stands to make particle accelerators accessible to a wider range of people. As of 2014, there were at least 30,000 particle accelerators around the world – up from 26,000 in 2010 according to a Physics Today census. More importantly, the latter estimated that almost half the accelerators were being used for medical imaging and research, such as in radiotherapy, while the really high-energy devices (>1 GeV) used for physics research numbered a little over 100.

These are encouraging numbers for India, which imports 75% of its medical imaging equipment for more than Rs.30,000 crore a year (2015). These are also encouraging numbers for developing nations in general that want to get in on experimental high-energy physics, innovations in which power a variety of applications, ranging from cleaning coal to detecting WMDs, not to mention expand their medical imaging capabilities as well.

Featured image credit: digital cat/Flickr, CC BY 2.0.

The Large Hadron Collider is back online, ready to shift from the “what” of reality to “why”

The world’s single largest science experiment will restart on March 23 after a two-year break. Scientists and administrators at the European Organization for Nuclear Research – known by its French acronym CERN – have announced the status of the agency’s upgrades on its Large Hadron Collider (LHC) and its readiness for a new phase of experiments running from now until 2018.

Before the experiment was shut down in late 2013, the LHC became famous for helping discover the elusive Higgs boson, a fundamental (that is, indivisible) particle that gives other fundamental particles their mass through a complicated mechanism. The find earned two of the physicists who thought up the mechanism in 1964, Peter Higgs and Francois Englert, a Nobel Prize in that year.

Though the LHC had fulfilled one of its more significant goals by finding the Higgs boson, its purpose is far from complete. In its new avatar, the machine boasts of the energy and technical agility necessary to answer questions that current theories of physics are struggling to make sense of.

As Alice Bean, a particle physicist who has worked with the LHC, said, “A whole new energy region will be waiting for us to discover something.”

The finding of the Higgs boson laid to rest speculations of whether such a particle existed and what its properties could be, and validated the currently reigning set of theories that describe how various fundamental particles interact. This is called the Standard Model, and it has been successful in predicting the dynamics of those interactions.

From the what to the why

But having assimilated all this knowledge, what physicists don’t know, but desperately want to, is why those particles’ properties have the values they do. They have realized the implications are numerous and profound: ranging from the possible existence of more fundamental particles we are yet to encounter to the nature of the substance known as dark matter, which makes up a great proportion of matter in the universe while we know next to nothing about it. These mysteries were first conceived to plug gaps in the Standard Model but they have only been widening since.

With an experiment now able to better test theories, physicists have started investigating these gaps. For the LHC, the implication is that in its second edition it will not be looking for something as much as helping scientists decide where to look to start with.

As Tara Shears, a particle physicist at the University of Liverpool, told Nature, “In the first run we had a very strong theoretical steer to look for the Higgs boson. This time we don’t have any signposts that are quite so clear.”

Higher energy, luminosity

The upgrades to the LHC that would unlock new experimental possibilities were evident in early 2012.

The machine works by using powerful electric currents and magnetic fields to accelerate two trains, or beams, of protons in opposite directions, within a ring 27 km long, to almost the speed of light and then colliding them head-on. The result is a particulate fireworks of such high energy that the most rare, short-lived particles are brought into existence before they promptly devolve into lighter, more common particles. Particle detectors straddling the LHC at four points on the ring record these collisions and their effects for study.

So, to boost its performance, upgrades to the LHC were of two kinds: increasing the collision energy inside the ring and increasing the detectors’ abilities to track more numerous and more powerful collisions.

The collision energy has been nearly doubled in its second life, from 7-8 TeV to 13-14 TeV. The frequency of collisions has also been doubled from one set every 50 nanoseconds (billionth of a second) to one every 25 nanoseconds. Steve Myers, CERN’s director for accelerators and technology, had said in December 2012, “More intense beams mean more collisions and a better chance of observing rare phenomena.”

The detectors have received new sensors, neutron shields to protect from radiation damage, cooling systems and superconducting cables. An improved fail-safe system has also been installed to forestall accidents like the one in 2008, when failing to cool a magnet led to a shut-down for eight months.

In all, the upgrades cost approximately $149 million, and will increase CERN’s electricity bill by 20% to $65 million. A “massive debugging exercise” was conducted last week to ensure all of it clicked together.

Going ahead, these new specifications will be leveraged to tackle some of the more outstanding issues in fundamental physics.

CERN listed a few–presumably primary–focus areas. They include investigating if the Higgs boson could betray the existence of undiscovered particles, the particles dark matter could be made of, why the universe today has much more matter than antimatter, and if gravity is so much weaker than other forces because it is leaking into other dimensions.

Stride forward in three frontiers

Physicists are also hopeful for the prospects of discovering a class of particles called supersymmetric partners. The theory that predicts their existence is called supersymmetry. It builds on some of the conclusions of the Standard Model, and offers predictions that plug its holes as well with such mathematical elegance that it has many of the world’s leading physicists enamored. These predictions involve the existence of new particles called partners.

In a neat infographic by Elizabeth Gibney in Nature, she explains that the partner that will be easiest to detect will be the ‘stop squark’ as it is the lightest and can show itself in lower energy collisions.

In all, the LHC’s new avatar marks a big stride forward not just in the energy frontier but also in the intensity and cosmic frontiers. With its ability to produce and track more collisions per second as well as chart the least explored territories of the ancient cosmos, it’d be foolish to think this gigantic machine’s domain is confined to particle physics and couldn’t extend to fuel cells, medical diagnostics or achieving systems-reliability in IT.

Here’s a fitting video released by CERN to mark this momentous occasion in the history of high-energy physics.

Featured image: A view of the LHC. Credit: CERN

Update: After engineers spotted a short-circuit glitch in a cooled part of the LHC on March 21, its restart was postponed from March 23 by a few weeks. However, CERN has assured that its a fully understood problem and that it won’t detract from the experiment’s goals for the year.