Science over the last four years

It would seem that I haven't posted here in nearly four years. Life has been busy, and further, I find other social media outlets capture my attention more. But I shall try to post here now and then. So what has happened in physics in the last four years? Too much to write! But here are a few things:

• The Large Hadron Collider has pinned down many of the properties of the Higgs boson.

• Alas, there are few hints of anything unexpected, except for a "bump" in the data at six times the mass of the Higgs boson, which might be some new elementary particle. Stay tuned!

• Quantum mechanics has passed all tests thrown at it. There has been great progress in using it to improve communication security and computation--though that is still a ways off.

• There has been a lot of work in constructing materials from the small scale on up.

• Gravitational waves have been directly observed.

The last one was reported just a few months ago. Two giant black holes merged a billion light years away, and scientists were able for the first time to detect the resulting jiggling of spacetime, just as Einstein predicted. It was a stupendous achievement, and opens our ears to a whole new side of the Universe.

Read More...

Summary only...

Where does Mass Come From? Announcement July 4th, 2012!

On July 4th in Melbourne, Australia, the 5000 physicists of the ATLAS and CMS collaborations from the Large Hadron Collider in Geneva, Switzerland are going to announce their results on the search for a particle that has to do with the origin of mass—the Higgs boson.

In December of 2011, these two giant groups of scientists, engineers, computer programers and support staff, announced their first concrete results on the search for the Higgs boson.  Now, they have about twice the data, and they will be able to make a much more definitive claim.  Scientists and nonscientists around the globe are waiting excitedly for the unveiling of the results.

So what is the Higgs boson?

Imagine a world where everything is like light, able to zip around at 300,000 km/s (186,000 mi/hr).  Light is made up of just one kind a particle, called a photon.  If all the particles were like that, they would be massless.  They would not form into atoms and molecules.  Except for the frequency (color) of the various kinds of light, everything would be much the same.

It turns out our understanding of particle physics is very much like that, except that there is a mechanism which gives the particle mass so most things can't travel at the speed of light, and so they aren't all the same, and they can form atoms and molecules.  That "except" part is all due to a mechanism called "spontaneous symmetry breaking" (never mind the big term for now).  So we have a beautiful theory of lightlike massless particles which is "fixed" to explain the world as we see it by this mechanism.  The theory has been tested backwards and forwards—all of it except for this crucial mechanism.

And that's where the Higgs boson comes in.  The mechanism predicts that this particle must be there.  The trouble is, it requires an enormous amount of energy (on the scale of elementary particles) to make one, and so our biggest colliders have not been big enough to produce it.  Until now.

The Large Hadron Collider is big enough and collides enough particles per second to see it.  If the Higgs is there, as predicted in the simplest model, the LHC should see it and report evidence or even observation of it on July 4th.  If they don't see it (contrary to the rumors), then the simplest model is wrong.

So this July 4th, keep an ear and eye peeled for news about the origin of mass and the Higgs boson.


P.S.: Please never refer to the Higgs boson as the "God particle", a term made up by a PR guy, because it simultaneously insults religion and science. (It's particularly ironic because the Higgs boson doesn't do anything, not even give mass to particles—it is the smoking gun for the mechanism which does.)

Read More...

Summary only...

Sometimes It's Just Hay

Last December I wrote about rumors that an experiment called CDMS had found evidence for direct detection of the Dark Matter. I called my post "Searching for Unusual Hay in a Haystack" because the "needle" they were looking for (the dark matter) is so close in appearance to the "hay" (background events) that it is really hard to tell them apart. At the time, I said that it was quite likely that the "signal" of two events was just some background events that happened to look a lot like the signal they were looking for---that they had just found normal hay that looked a little unusual. And I concluded, "So we await future experiments with more signal and less background".

Well, that data has just been published in the journal Physical Review Letters. Here is a nice writeup of the results. In brief, an experiment called XENON100, which is much more powerful than CDMS, was able to take enough data in just its first 11 days of running to basically rule out the CDMS signal (in the plot pictured above, the solid black XENON100 line is below the dotted CDMS line on the left half of the plot, where CDMS signal events were found). Another way to put that is this: if the CDMS signal were real (not just a background fluctuation), XENON100 would easily have seen it. But XENON100 saw nothing unusual.

This is often the pattern on the frontiers of science. There is a hint of a signal, and then it is either confirmed or it is ruled out by a more powerful experiment. Alas, this time it was ruled out. So it's back to waiting for a hint of a signal from somewhere else.

[the plot is taken from the journal article, which is available here]

Read More...

Summary only...

French President Sarkozy Speech in Support of Basic Research

[September 2010 Note: I am appalled but what President Sarkozy has done of late regarding the Roma People, but I am leaving up my positive impressions of him from this summer on the narrow issue of science funding]

I am attending the International Conference on High Energy Physics (ICHEP) in Paris. I'll write a subsequent post about the science. But first, in a perhaps unprecedented move, a head of state for major country has chosen to address a physics conference. Nicolas Sarkozy, President of the Republic of France, gave an impressive impassioned visionary speech to a skeptical audience of particle physicists. He won us over.

The speech lasted perhaps 1/2hr, and was constructed specifically for the assembled audience. No doubt he can use elements in other speeches, but most of it was really directed at us. He said that some of his friends asked why he would give such a speech to "those people", and the content of his speech was a ringing answer. He highlighted the need for basic research, especially in a world where fundamentalism and economic conditions threaten it. He stressed the need for politicians, such as himself, to work actively to support science. And he talked about the role basic research plays in future innovation, saying something like "you can't build a lightbulb by successive improvements to the candle". Finally, he called upon us to convey what we know to the public.

The speech was also remarkable for it humility and for its grasp of the topic of the conference, which really is, in some sense, an attempt to understand the very small in order to understand the very large.

So to President Sarkozy and his staff, all I have to say is, "Merci beaucoup".

UPDATE: Here is a page devoted to the speech with video and full text in French and English.

[photo: Mike Paterson, from the only other hint I have seen of the speech on the web thus far: http://www.guardian.co.uk/science/blog/2010/jul/27/sarkozy-high-energy-physicists-ichep]

Read More...

Summary only...

Searching for Unusual Hay in a Haystack: The Case of CDMS

Over the past two weeks, rumors have swirled around the web that the CDMS collaboration had discovered particles of "dark matter". [I have not yet written a promised post on dark matter, but there is this.] It all started with a single blog post which contained "facts", such as the statement that there was a paper in press at the journal Nature, which turned out to be false. One very connected person tweeted about the post, and it spread like wildfire. Soon the Nature editor sent the blogger a snarky letter denying the claim, which the blogger posted. Others speculated that the Nature editor was just trying to throw them off track. The next day the Nature editor posted a comment on the blog apologizing for the snarky nature of the letter, but again refuting the claims. Still rumors shot around the net about what result there might be.

So there was much anticipation Thursday when the CDMS collaboration gave two simultaneous talks announcing their results.

I watched a live stream of one of them. It proceeded in a halting fashion from the strain of the web traffic. Then, when the speaker got to the point of announcing their results, the stream froze for ten solid minutes. When it recovered, it zipped straight to her conclusions (how many of you were assuming the speaker was male--come on admit it), and I was left to guess a number of the details. But the bottom line is this: they saw 2 events with a background of 0.8. What does that mean, you ask?

The experiment looked for a very rare signal: that a particle of dark matter, which rarely interacts with anything, leaves a small ripple in the detector. The detector is located at the bottom of a mine to shield it from most cosmic rays. But there are still background events: interactions in the detector from particles which come from radioactive elements in the rock or particles which somehow survive going through hundreds of meters of rock. There are telltale signatures of dark matter particles (such as the energy and timing of the event) which help distinguish them from background particles, but occasionally a background particle mimics those signatures by chance. In the CDMS experiment, they calculate that over two years of running, that happened on average 0.8 times ( it took heroic efforts to keep it this small) . Maybe this helps: if they ran for 20 years with the same detector, i.e. 10 times longer, then they'd expect it to happen 8 times.

Now they saw 2 events. So what is the chance that those events are really signals of dark matter particles? Well, it is easier to ask "what is the chance they are background events?". If you ran for 20 years, what is the chance that 2 of the background events happened in the first two years. Using something called the Poisson distribution, they find that there is about a 1/4 chance those 2 events are both just background events. That's not a strong signal. As good as their efforts were at reducing backgrounds, it was not enough. If there were no dark matter particles and you ran the experiment for 20 years and divided them into ten two year periods, about two or three of those ten periods would happen to have 2 background events in them.

Still, if the events do turn out to be really from dark matter, it will begin to explain one of the great mysteries of science. So we await future experiments with more signal and less background.

UPDATE: Sometimes It's Just Hay

Read More...

Summary only...

2008 Nobel Prize in Physics

The 2008 Nobel Prize in Physics goes to two different achieve- ments.  Both relate to symmetry breaking, but in very different ways.  All three recipients, Yoichiro Nambu, Makoto Kobayashi and Toshihide Maskawa, certainly deserved the prize, but Nambu should have gotten the prize years ago, and they should given the prize to Nicola Cabibbo as well—after all it is called the Cabibbo-Kobayashi-Maskawa mechanism!

In brief, this is what they did.

Nambu explained how protons and neutrons could get mass in the same way that superconductivity happens.  If that doesn't sound ground-breaking, I don't know what does!  He showed that a symmetry in something called a quantum field theory can be "spontaneously broken".   

Think of looking down at a pencil from above.  What is the most symmetric way of placing the pencil?  Why on its point!  That way, if you rotate your view, it looks the same.  But of course that is not a stable situation.  The pencil immediately falls over.  Which way?  Well, it goes spontaneously in a random direction.  And after it falls, the situation isn't invariant under rotations—the symmetry is broken.  That is in essence what Nambu showed worked in a quantum field theory—that the most stable state need not be the most symmetric one.

Now on to a completely different topic.  There are three families of quarks: up-down, charm-strange, and top-bottom (yes, these are silly names).  Back when we knew about only two of them, Nicola Cabibbo realized that they could mix together—that if you started a process with a strange quark, there was a chance you could end it with a down quark.  Such mixing is controlled by just one parameter, the Cabibbo angle.  (It's an angle of rotation in quark mixing space.)

A puzzle at the time was how to explain CP violation—the observation that the symmetries of charge conjugation (C, switching + and -) and parity (P, switching left and right) were not actually good symmetries of nature.   In other words, if you took some processes and switched both + and - charges and left and right, you didn't get the same result (trust me, that's odd).

Kobayashi and Maskawa realized in 1973 that if there were a third family of quarks (there is, but no one knew it then), one would get not only other mixing angles, but something called a phase.  This phase was just what one needed to explain CP violation.

In brief, they came up with a mechanism which explained the existing observation of CP violation by proposing that there was a third family of quarks—and lo and behold a third family of quarks was found a few years later!  Come to think of it, Cabibbo, Kobayashi, and Maskawa should have won years ago.

Read More...

Summary only...

Is the Large Hadron Collider safe?

The Large Hadron Collider, usually referred to by scientists as the LHC, had its first preliminary test today.  All went well.  But what does the LHC do, and is it safe?

What is the LHC?

The LHC is a "particle collider".  It has two main parts: beams and detectors.  Two beams of protons will be channeled at near the speed of light around a tunnel 27 km in circumference, one clockwise, one counterclockwise, by a pair of rings made of 9000 superconducting magnets.  The beams will cross in several places, allowing the particles within them to collide (hence the term "collider").  The by-products of those collisions will be observed by two enormous detectors (as well as two somewhat smaller ones).  It short, it collides beams of particles and detects what happens.

What is the LHC for?

Physicists have learned a lot about the fundamental constituents of matter by bashing particles together.  The higher the energy scale of the collisions, the deeper, in a sense, one can probe.  We now understand what particles make up all the matter we can see, and what particles are responsible for forces.  For example, as I said in a previous post, we understand about electrons and their siblings (yes, I know I haven't gotten around to doing the followup posts yet), and we understand that the electromagnetic force comes from the particle of light, the photon.  In fact, we have understood how these particles and forces behave in terms of some rather beautiful symmetries.  A symmetry is an invariance, as in "looks the same in a mirror", or "runs the same if you switch all the red and black cables for one another".   A key point is that a symmetry can be broken.  For example, you don't look the same in a mirror.  Even if you part your hair down the middle, there is always some freckle to give away that it is a mirror image.

Our theory of particle physics using symmetries works great, except for understanding why most of the particles have mass.  If the symmetries of the theory were not broken,  these particles would have to be massless.  We need to understand how the symmetries are broken—we have to find the freckles.   The main freckle is called the Higgs boson (please, can we stop using the awful term "God particle"!).  It has never been seen.  We think that is how the electron gets its mass, but we don't know for sure.  And we don't understand how the Higgs boson might fit into a more complete theory.

The LHC is designed to find the Higgs boson, and we hope it will point us to a more complete theory of matter and energy.  It may also shed light on the dark matter, but that is a post for another day.

Is the LHC Safe?

Sometimes the LHC is described as "recreating the Big Bang".  This sort of language is colorful, and conveys the grand nature of the endeavor, but it also makes it sound scary, and, more to the point, is completely inaccurate.  The LHC will probe a new frontier for humans, but the kinds of collisions that will take place in it happen in and around the Earth all the time.  Cosmic ray protons hit protons in the atmosphere and create sprays of particles just like in the LHC.  If you were to wait in one location, it would be quite rare that you would see a collision at the same energies as the LHC, but across the whole atmosphere they happen all the time.  If these collisions were dangerous, they would have done their damage long ago.  

One worry that has been stated in the press is that the LHC might produce mini black holes.  Well, that is a possibility if there are extra dimensions of space that become visible just at the LHC energy scale, but that is unlikely (not quite as crazy as it sounds though).  But such mini black holes would not be like the monsters you may have seen in Sci Fi.  They would be tiny (way smaller than protons) and would decay in a fraction of a second.

Could these mini black holes be stable?  First of all, even if they were, a mini black hole would take hundreds of millions of years to grow appreciably in size in the Earth, so it could not be the doomsday machine some have feared.  But everything we know about the theory says that such mini black holes must decay very rapidly due to quantum processes.   Mini black holes are essentially  just another kind of particle that decays. 

If all of that is not enough to convince you that the LHC is safe, here is a final comfort:  we have seen pulsars.  Comforting eh?  You see, pulsars are like canaries in the coal mine.  They are spinning neutron stars.  Neutron stars are dense cinders of dying stars that just barely avoided collapsing on themselves into black holes (the large kind).  They would feel the effects of a mini black hole much much faster than the Earth would.  They too are bombarded by cosmic rays all the time.  They recreate the LHC experiments each second.  If particle collisions created mini black holes that somehow were stable, all neutron stars would quickly be triggered into collapsing.  We see pulsars, so that can't have happened.

So the LHC is not a threat.  It is just a tool to look for freckles.

An engineer leans on a magnet in the 27km-long tunnel that houses the Large Hadron Collider (BBC News; Image: Cern/Maximilien Brice)

Read More...

Summary only...

Electrons and Their Siblings

Electrons are the little points of charge that are racing right now through the logic board of your computer, just so you can read this blog. They are tiny. In fact, they are too small to measure. We know that they are smaller than 1/1000th the size of a proton†. They weigh 1/1800th as much as the proton. They have one unit of electric charge.

The photo is of J. J. Thompson, who used the pictured Cathode Ray Tube to discover the electron in 1897.

There are two siblings of the electron which are just like electrons, except heavier. They are called the muon and the tau.

[first of three posts designed to explain what this giant thing is.]

Muons (μ) weigh about 200 times as much as the electron. They are not stable, and decay in about 2 millionths of a second, on average. That's actually very long-lived for an unstable particle. They are produced in the upper atmosphere by cosmic rays, and because they are going near the speed of light, they live until they reach the ground (I'll explain that later). There are hundreds of them going through you every second. Don't worry, we have spent our entire time as a species bathed in a background of particles going through us. Most of them pass straight through, and our bodies are well equipped to deal with a small amount of radiation.

Taus (τ) weigh about 3500 times as much as the electron (they are almost twice as heavy as a proton). They are even less stable than muons, and decay in about a trillionth of a second. Since they are shorter-lived and produced less frequently in cosmic rays, there are far fewer taus going through you than muons.

[next Neutrinos and Their Siblings]

† 'Size' becomes less well defined for elementary particles. What do use to measure it? Here I mean that electrons don't seem to be made of other particles and so are pointlike, as far as we can tell. (Protons and neutrons are made of quarks, and they do have a size, a quadrillionth of a meter, better known as a femtometer.)

[Note: The numbers for mass in the table are actually in units of Gev/c², and the proton weighs 0.938 Gev/c², not 1, but one shouldn't worry about such small differences when one is getting a sense of scale.]

[confidence: established, my qualifications: trained]

Read More...

Summary only...

House-Sized Particle Device Circles Europe

The KATRIN spectrometer is part of an experiment to try to measure the tiny mass of the electron neutrino.  I'll discuss what that means in three subsequent posts, Electrons and Their Siblings, Neutrinos and Their Siblings, and How to Measure Electron Neutrino Mass.  

 

In this post, let's just consider the trip the KATRIN spectrometer took in January 2007.  

They had to get it from where it was made, near Munich, Germany, to Karlsruhe, Germany, a distance of 400 km.  But there was no way to do this directly, so they had to take the 9000 km route depicted on the map!  

 

Along the way, it had to navigate some tight quarters!

Here's a video of the trip through that town, Eggenstein-Leopoldshafen, just north of the destination of Karlsruhe.  The best part is from about 1:00 to 4:00.

Be sure to tune in for future physics posts which will explain what this thing does. 

Read More...

Summary only...