̽��ֱ�� of Cambridge - theoretical physics

Five-dimensional black hole could ‘break’ general relativity

sc604 — Fri, 19 Feb 2016 06:00:00 +0000

Researchers have shown how a bizarrely shaped black hole could cause Einstein’s general theory of relativity, a foundation of modern physics, to break down. However, such an object could only exist in a universe with five or more dimensions.

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge and Queen Mary ̽��ֱ�� of London, have successfully simulated a black hole shaped like a very thin ring, which gives rise to a series of ‘bulges’ connected by strings that become thinner over time. These strings eventually become so thin that they pinch off into a series of miniature black holes, similar to how a thin stream of water from a tap breaks up into droplets.

Ring-shaped black holes were ‘discovered’ by theoretical physicists in 2002, but this is the first time that their dynamics have been successfully simulated using supercomputers. Should this type of black hole form, it would lead to the appearance of a ‘naked singularity’, which would cause the equations behind general relativity to break down. ̽��ֱ��results are published in the journal Physical Review Letters.

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General relativity underpins our current understanding of gravity: everything from the estimation of the age of the stars in the universe, to the GPS signals we rely on to help us navigate, is based on Einstein’s equations. In part, the theory tells us that matter warps its surrounding spacetime, and what we call gravity is the effect of that warp. In the 100 years since it was published, general relativity has passed every test that has been thrown at it, but one of its limitations is the existence of singularities.

A singularity is a point where gravity is so intense that space, time, and the laws of physics, break down. General relativity predicts that singularities exist at the centre of black holes, and that they are surrounded by an event horizon – the ‘point of no return’, where the gravitational pull becomes so strong that escape is impossible, meaning that they cannot be observed from the outside.

“As long as singularities stay hidden behind an event horizon, they do not cause trouble and general relativity holds - the ‘cosmic censorship conjecture’ says that this is always the case,” said study co-author Markus Kunesch, a PhD student at Cambridge’s Department of Applied Mathematics and Theoretical Physics (DAMTP). “As long as the cosmic censorship conjecture is valid, we can safely predict the future outside of black holes. Because ultimately, what we’re trying to do in physics is to predict the future given knowledge about the state of the universe now.”

But what if a singularity existed outside of an event horizon? If it did, not only would it be visible from the outside, but it would represent an object that has collapsed to an infinite density, a state which causes the laws of physics to break down. Theoretical physicists have hypothesised that such a thing, called a naked singularity, might exist in higher dimensions.

“If naked singularities exist, general relativity breaks down,” said co-author Saran Tunyasuvunakool, also a PhD student from DAMTP. “And if general relativity breaks down, it would throw everything upside down, because it would no longer have any predictive power – it could no longer be considered as a standalone theory to explain the universe.”

We think of the universe as existing in three dimensions, plus the fourth dimension of time, which together are referred to as spacetime. But, in branches of theoretical physics such as string theory, the universe could be made up of as many as 11 dimensions. Additional dimensions could be large and expansive, or they could be curled up, tiny, and hard to detect. Since humans can only directly perceive three dimensions, the existence of extra dimensions can only be inferred through very high energy experiments, such as those conducted at the Large Hadron Collider.

Einstein’s theory itself does not state how many dimensions there are in the universe, so theoretical physicists have been studying general relativity in higher dimensions to see if cosmic censorship still holds. ̽��ֱ��discovery of ring-shaped black holes in five dimensions led researchers to hypothesise that they could break up and give rise to a naked singularity.

What the Cambridge researchers, along with their co-author Pau Figueras from Queen Mary ̽��ֱ�� of London, have found is that if the ring is thin enough, it can lead to the formation of naked singularities.

Using the COSMOS supercomputer, the researchers were able to perform a full simulation of Einstein’s complete theory in higher dimensions, allowing them to not only confirm that these ‘black rings’ are unstable, but to also identify their eventual fate. Most of the time, a black ring collapses back into a sphere, so that the singularity would stay contained within the event horizon. Only a very thin black ring becomes sufficiently unstable as to form bulges connected by thinner and thinner strings, eventually breaking off and forming a naked singularity. New simulation techniques and computer code were required to handle these extreme shapes.

“ ̽��ֱ��better we get at simulating Einstein’s theory of gravity in higher dimensions, the easier it will be for us to help with advancing new computational techniques – we’re pushing the limits of what you can do on a computer when it comes to Einstein’s theory,” said Tunyasuvunakool. “But if cosmic censorship doesn’t hold in higher dimensions, then maybe we need to look at what’s so special about a four-dimensional universe that means it does hold.”

̽��ֱ��cosmic censorship conjecture is widely expected to be true in our four-dimensional universe, but should it be disproved, an alternative way of explaining the universe would then need to be identified. One possibility is quantum gravity, which approximates Einstein’s equations far away from a singularity, but also provides a description of new physics close to the singularity.

̽��ֱ��COSMOS supercomputer at the ̽��ֱ�� of Cambridge is part of the Science and Technology Facilities Council (STFC) DiRAC HPC Facility.

Inset image: A video of a very thin black ring starting to break up into droplets. In this process a naked singularity is created and weak cosmic censorship is violated. Credit: Pau Figueras, Markus Kunesch, and Saran Tunyasuvunakool

Reference:
Pau Figueras, Markus Kunesch, and Saran Tunyasuvunakool ‘End Point of Black Ring Instabilities and the Weak Cosmic Censorship Conjecture.’ Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.116.071102

Researchers have successfully simulated how a ring-shaped black hole could cause general relativity to break down: assuming the universe contains at least five dimensions, that is.

As long as singularities stay hidden behind an event horizon, they do not cause trouble and general relativity holds

Markus Kunesch

Marc Van Norden

Star field - 4

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Large Hadron ‘insider’

fpjl2 — Fri, 14 Jun 2013 10:33:57 +0000

In October 2000, I arrived in Geneva from Stansted airport ready to start a two year research job at the European centre for particle physics (CERN) with my heart in my throat. This was before all of the recent excitement about the Large Hadron Collider, but for a particle physicist, CERN is the ultimate temple of physics and for me, it was my lifetime ambition to work there. I had recently daringly, and many said stupidly, turned down a life-time lectureship at another university so that I might have a chance of working at CERN (the offer came with an inflexible initial start date).

CERN really is an amazing place. There are some 10,000 physicists and engineers working on the site, which straddles the Franco-Swiss border, just outside Geneva, underneath the Jura Mountains. Everyone is working for ̽��ֱ��Cause - our common goal is to find out what the stuff that makes up our universe is like, and how it behaves.

When I arrived, the Large Hadron Collider had not been built and the previous one was toward the end of its operation. While I was there, there was a potential hint of a higgs signal in the data. All of a sudden, CERN was completely abuzz. Everyone was trying to find out the latest rumours. There were four independent experiments back then, all competing with each other. ̽��ֱ��idea behind this was that they can check each other's results, and keep each other unbiased. But because of this competition, the experiments were rushing to make a big discovery before the others, but they also wanted to keep their data secret so as not to give the others a clue.

I am a theoretical physicist: I do the mathematics and interpret experimental data, rather than actually run the experiments. My theoretical colleagues had spies on the experiments, which they were trying to push for information to find out the latest on the Higgs boson. Then they would tell their friends, and all sorts of wild rumours would start to fly about. Mostly this wasn't done for any personal advantage, it was just that we were fascinated, and really wanted to know what was going on at the cutting edge as early as possible. Once every month or two, the experiments would hold seminars and do official releases of data. Sometimes we already knew what they were going to say but sometimes it was a surprise.

In the end, it turned out that the hint of a signal that we were all getting so excited about was just a random fluctuation of the data. We couldn't really know this for sure though until we had seen the LHC data, and that didn't arrive for another eight years or so.

At the time though, since there was the hint of a higgs boson signal in the data, and since the collider only had a year or so left before shutdown to make way for the LHC, the accelerator engineers started to ramp up the energy as much as possible. This was a risky strategy, because parts started to break down, being under a lot of strain (I imagine the accelerator engineers, like Scottie from Star Trek, shouting "she cannae take any more captain!") My friend worked on one of the experiments, and many times he was paged from the pub and had to taxi up to the experiment to try and get it working once it had all broken down.

Since the beam was still on, the experiment was losing valuable data that all of the other experiments were taking, and could lose out on a discovery as a result. One time, his boss had to be called in at 1am from a birthday party to coordinate everyone. ̽��ֱ��first thing to do was pour coffee down him to try to sober him up.

People often think that the biggest man made experiment on earth will be ultra high-tech and efficiently squeaky clean. In some ways this is true, but there was also a sort of Heath-Robinson aspect too. For instance, the accelerator is a complicated beast with thousands of different magnets and sub-pieces, all with complex and nervy feedback across them. As a result, driving it is something of a black art: apparently you get the "feel" of how it is behaving that day and some of the operators were particularly good at this knack. During 1998, the best operators by far were the French accelerator engineers: they just had the most experience, and an uncanny sense of how it would behave in the following five minutes. That year, the football world cup was in France, and we were praying that France would be knocked out early because the rate of good beam was really low: all the good French operators were taking days off to watch their team's matches.

It was hard getting research jobs in the subject back then: the competition was ultra-tough, and I was far from sure I would be able to get the next job. Whenever I thought about leaving the subject, I think how sad I would be to read about a big discovery (like the recent Higgs boson discovery) and to know that I could have been involved, at least in some small way. I really feel that I am super lucky to be still researching in the field. I work from the ̽��ֱ�� of Cambridge, but visit CERN several times a year for research. If you ever go to Geneva, I thoroughly recommend you to get on the CERN website several weeks beforehand, and book yourself in to a guided tour. I guarantee it will be an incredible scientific journey.

Inset image: Ben Allanach working it out during his time at CERN

In a recent talk for TEDx, theoretical physicist Professor Ben Allanach explored the research he undertook during the two years he spent working on the Large Hadron Collider at CERN in Switzerland. Here, he takes us back to his time as one of the scientists working on the biggest scientific experiment in human history.

We were praying that France would be knocked out of the World Cup because the rate of good beam was really low

Ben Allanach

̽��ֱ��micro frontier: Benjamin Allanach at TEDxDanubia 2013

Sir Cam

̽��ֱ��Large Hadron Collider projected onto the Old Schools, the ̽��ֱ��'s administrative centre, during the 800th Anniversary celebration year

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