There’s been some buzz in the news about how the Large Hadron Collider (LHC) might disprove the big bang by discovering rainbow gravity. It certainly makes for a catchy headline, but the big bang is an observational fact, regardless of whether there was a primordial singularity. No experiment the LHC is planning will disprove the big bang. The term “rainbow gravity” might seem like the most hyped aspect of these recent headlines, but actually it’s just a funny name for a somewhat interesting idea.

Rainbow gravity is one aspect of an approach to quantum gravity known as the principle of general locality. In Newton’s relativity, motion is relative, but all events are set against an absolute background space and time. In other words, all observers agree on the time and place of a particular event. In Einstein’s relativity this isn’t the case. There is no absolute spacetime framework, so different observers will disagree on things such as how long a particular event takes, or even which of two events occurred first. However, if two objects happen to interact at a specific time and place (at a specific spacetime coordinate) then all observers will agree that the event happened at the same spacetime location. In physics this means locality is absolute.

General locality is an extension of relativity where a spacetime location is not universal. If an observer in one frame sees an interaction at a specific spacetime coordinate, another observer might not. Both observers would agree the interaction was local, but might disagree about the specific location. Thus locality is relative rather than absolute. Just as relativity was used as a way to unify space and time, general locality can be used to unify spacetime and momentum into a general “phase space.” Basically, time, location and motion are combined in this model.

While it’s a pretty wild idea, it does have some testable predictions. Since particles of different energies (momenta) see different spacetimes, their paths are slightly different. This is true for light as well, so different wavelengths of light should have slightly different speeds. This isn’t something we could measure under normal conditions, but near the event horizon of a black hole the speeds could vary enough to be measurable. Just to be clear, there’s no evidence that such a thing actually occurs at this point, and “rainbow gravity” as it is called is just one idea towards quantum gravity among a great many ideas.

The LHC could also discover magical unicorns. Credit: MoongazePonies

Where this connects to the LHC is that there has been speculation that the collider might create micro black holes. It isn’t considered likely given what we know, but some very speculative models suggest that they might occur. If these speculative models are correct, and if the LHC creates micro black holes, and if general locality and rainbow gravity is correct, and if this causes the micro black holes to decay in different ways from that expected by general relativity, then the LHC might be able to detect it. Since one of the other predictions of rainbow gravity is that the universe didn’t begin with a singularity, and since the big bang is often conflated with such a singularity in the popular press, you therefore get headlines such as “LHC may disprove big bang by discovering rainbow gravity.”

Unfortunately these types of headlines are likely to become very popular. The reality is that the LHC provides an opportunity to look for new physics beyond the standard model. There are a lot of new ideas that could be put to the test when the collider becomes active at its highest energies ever, so folks are staking their claims and placing their bets. Perhaps the journalists would do well to see who actually wins this pony race before writing about the outcome.

Paper: Giovanni Amelino-Camelia, et al. Principle of relative locality. Phys. Rev. D 84, 084010 (2011)

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Later this month the Large Hadron Collider (LHC) will be up and running again, this time at nearly double the previous collision energies. It will be the highest energy at which we have actively collided particles. While this is great news for scientists eager to discover new physics, it has some people worried that the LHC could open a pandora’s box of unexpected consequences, such as the creation of a micro black hole that could engulf the planet. The standard response is to state that even if the LHC were to create micro black holes, they would pose no risk. But given that we’ve never collided particles at this energy, how do we know?

On a basic level, a black hole is simply matter packed so densely that it forms an event horizon. The key is the density, not the overall mass, so in principle extremely tiny black holes are theoretically possible. Realistically, however, the strong forces between colliding particles would prevent them from reaching a density necessary to form a black hole. Besides, on such small scales the particle/wave nature of objects can’t be ignored, and quantum theory is very clear that the quantum “size” of particles prevents the necessary density. So conventional physics says that the LHC won’t produce any black holes, even at these new energy levels.

There are some exotic theoretical ideas that predict the creation of black holes at the new LHC levels. These models aren’t taken very seriously, but we are looking for new physics after all. On the off chance that micro black holes are created, does that mean we’re doomed? No, because quantum theory also says that black holes evaporate due to Hawking radiation. The rate of evaporation depends upon the size of the black hole, so that the smaller the black hole, the faster it evaporates. If a micro black hole on the order of LHC energies were to form, it would evaporate before anything else could be captured by it. So there’s no danger of creating a black hole that starts devouring the Earth.

But that’s all theory, you might say, how can we be sure our understanding of the risks are correct? At about 14 TeV, the new collision energies of the LHC are higher than anything we’ve ever created, but they aren’t nearly the upper limit of what we’ve observed. Cosmic rays strike the Earth with much greater energies all the time. In fact the highest energy cosmic ray ever detected was about 400 million TeV, which is well beyond any energy level we could attain for the foreseeable future. None of these cosmic rays have created a black hole that consumes Earth, and cosmic rays have been striking our planet for billions of years.

So we can look forward to new discoveries from the LHC, with no worries of a doomsday scenario.

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When the Large Hadron Collider was frantically searching for the Higgs boson, you could hear murmuring speculation about the possibility of the LHC creating black holes that could destroy the Earth. There was a lawsuit filed against Cern in an effort to prevent such a catastrophe.  Part of this was fueled by our love of a good doomsday scenario, but part of it was driven by the idea that the LHC was producing the highest energy collisions ever known. While the LHC is humanity’s highest energy particle collider, it doesn’t produce the highest energy collisions known. Much higher collisions are produced by cosmic rays.

Cosmic rays are high energy particles, typically protons, seen to impact the Earth’s atmosphere.  The energy of the LHC can accelerate a proton to an energy of about 7 TeV, or 7 trillion electron volts. An electron volt is the amount of energy a proton would get moving across an electric potential of one volt.  Seven trillion is a lot of energy for a proton, and means it it traveling at about 99.9999991% of the speed of light. That’s just 3 m/s shy of the ultimate speed limit.  Cosmic rays at TeV energies strike the Earth all the time. In a given square kilometer, one will strike every few seconds.  These cosmic rays aren’t nearly as efficient at producing exotic particles as the LHC, but they do produce a particle cascade as they strike our atmosphere.

The most powerful cosmic ray was observed in 1991. It was a proton with an energy of 400 million TeV. That’s such an astounding amount of energy that it has been nicknamed the Oh-My-God! particle.  At that energy, the proton was moving so close to the speed of light that if it trailed a photon for a year it would only be about 50 nanometers behind.  Because of time dilation, its experience of time was extraordinarily slowed relative to us. In traveling a million light years, it would only experience a couple minutes of time.

We still aren’t sure what produces these ultra-high energy cosmic rays. The leading candidates are active galactic nuclei and supernova remnants.  The Oh-My-God particle had so much energy that it was likely undeflected by things like galactic magnetic fields, so it was hoped that it could provide a clue as to its source. However in the direction of the OMG particle there isn’t anything that could produce such a high energy particle.

In 2015 the LHC plans to double particle energies to 14 TeV. That is still far less than cosmic particle accelerators, so there’s no need to worry about creating black holes or destroying the Earth through particle research.

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Now that the CERN results have been announced, what do we know?

1. We’ve discovered a particle at a level of about 5 sigma.
2. It has an energy (mass) of about 125 GeV (about 125 times heavier than a proton), which is in the range expected for the Higgs.
3. It appears to be a Higgs-type particle, but more work needs to be done to confirm it is indeed the Higgs.

Now that some of the graphs are being released you can get an idea of how hard it is to pin down the Higgs.  Below is a graph plotting the number of events per unit energy as a function of expected mass.  The dotted line is what we would expect if there  were no Higgs.  The small bump at around 126.5 GeV is the signature of the Higgs.  That small bump is (part of) what tells us the Higgs exists.

This is what modern “big science” looks like folks, and it’s pretty awesome.

So what now?

Now that we know the mass range of this particle, experiments can be tuned  specifically to that range to study the overall properties.  Efforts will focus on measuring those properties to confirm it is the Higgs as theory predicts.  Work will also focus on determining if there is only one type of Higgs (as the standard model predicts) or if there are multiple types of Higgs (as predicted by things such as string theory).

Note: This was written 4 July 2012 on Google+.  I’m posting it here just to have a copy.

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Tomorrow CERN is expected to announce the results of the search for something called the Higgs boson.  So just what is the Higgs boson, and why would finding (or not finding it) be such a big deal?  For that we have to cover a bit of background on particle physics.

You are likely familiar with the basic concept of atoms.   Atoms were once thought to be the building blocks of everything, and it was assumed that atoms were indivisible.  We now know that atoms are made up of a nucleus of protons and neutrons, surrounded by a cloud of electrons.  Often the electrons are pictured as orbiting the nucleus like planets, but that’s not really how it works.

As far as we know, electrons aren’t made up of anything smaller.  They are a type of elementary particle.  Protons and neutrons however do have parts, known as quarks.  Protons and neutrons are made of 3 quarks each, so they are not elementary particles.  The quarks are the elementary particles.

It turns out the electron and the quarks that make up protons and neutrons (known as up and down quarks) are not the only elementary particles there are.  There is also the neutrino, which has similarities to the electron but doesn’t have electric charge.  Electrons and neutrinos are part of a group of elementary particles known as leptons, and it turns out there are six known leptons.  It also turns out that there are six types of quarks (up, down, charm, strange, top and bottom).

So it seems that everything in the universe is built out of the 6 types of quarks and the 6 types of leptons.  Twelve elementary particles that are the building blocks of the universe.  Of course this raises the question of how these particles interact.  In Newton’s gravity, masses interact through the gravitational field, however in particle physics a field is produced by a type of particle known as a gauge boson.  There are four types of fields in the universe:  gravity, electromagnetism, weak nuclear and strong nuclear, which means there are four types of gauge bosons:  the graviton (which has never been observed directly), the photon, the W boson, and the gluon.  The basic idea is that particles move because of boson interactions.  So, for example, if an elementary particle has charge, then the gauge photons interact with it, which is why two charges attract or repel each other.

Given all that, what about mass?  You might say that since gravity is due to the graviton boson, gravitons must produce the effect of mass just like photons produce the effect of charge, and you’d be partly right.  However there are actually two properties of mass.  One is that masses attract each other gravitationally.  The other is that a mass resists changing its motion (known as inertia).  It is this second property that makes larger masses harder to move than smaller ones.  Gravitons determine how masses interact gravitationally, but gravitons don’t determine a particle’s inertia.  So there must be a fifth type of gauge boson that determines a particles inertial mass.  This new type of particle is known as the Higgs boson.

So why is it so important to find the Higgs boson?  It turns out that all the quarks, leptons and bosons are part of a theory known as the Standard Model.  If the standard model is correct, then the 12 types elementary particles are the only ones that exist, the four gauge bosons tell them how to move, and the Higgs boson determines their inertial masses.  The Higgs must exist, otherwise the standard model is wrong.  The theory also predicts that if the Higgs boson exists, the Large Hadron Collider at CERN must be able to observe its effects.  That means that either we find the Higgs boson (and the standard model is right) or we don’t find it (and the standard model is wrong).  Since the standard model is the foundation of all particle physics, proving it or disproving it is a huge deal.  As in Nobel prize big deal.  That is why everyone is so keyed up about the announcement tomorrow.

Of course there is a third possibility for tomorrow.  It could be that CERN announces the discovery of something, but something that isn’t the Higgs.  We’ll just have to wait and see.  Either way, once they announce things, I’ll try to write a post talking about what it means.

Note: This post was written on 3 July 2012.  It is posted here just to have a copy.

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