Archive for category Physics
50 years of the PS
Last month marked the first collisions in the largest man made particle physics experiment ever conducted, the LHC. In a remarkably short time, considering the drawbacks the experiment has suffered in the past, the LHC has gotten from very rough first collisions, to prolonged, stable beams and finishing up a few days ago with the highest energy collisions in any particle physics experiment to date.
This milestone comes just in time for the 50th birthday celebrations of another important particle accelerator; the Proton Synchrotron.
This machine was the first big circular particle accelerator at CERN, and was the beam source for such experiments as the Gargamelle bubble chamber, which first discovered neutral currents in the 70’s.
On the 3rd and 4th of this month at CERN, several Nobel Laureates arrived at CERN to reminisce about the success of the PS and the development of particle physics since. Many of the talks were focussed at discussing the experiments at the PS, SPS, LEP and then the LHC, with a few talks focussing mainly on theory.
The event was a fascinating insight into the minds of some of the greatest contributors to particle physics in the last half century. It definitely left me inspired.
As testament to the Proton Synchrotron’s success, it is still used as the second stage accelerator for the protons injected into the LHC today.
COLLISIONS!
Posted by Tom in Physics, Science, Technology on November 24th, 2009
So as you may well be aware, the LHC started up again this weekend after a good 14 months off.
September 10th last year was the date penned for the long overdue startup of the Large Hadron Collider – the worlds biggest (and best) particle accelerator. Unfortunately a problem with the cooling system caused a leak of liquid helium and a saftey mechanism called a quench kicked it. It basically ruined a few magnets so the whole thing had to be shut down to be fixed.
Well, it was fixed, and this weekend saw the grand reopening. On Friday evening, the various LHC control rooms were full of physicists and excitement as the beams were reinjected into the machine. First one way around, then the other. The plan, as us postgrads understood it, was that collisions (ie, two beams in the LHC going in opposite directions and brought to a focus at the detectors) were not due until early December.
Well, we were wrong. Today (the 23rd November), the LHC injected beam 1 into the LHC, then injected beam 2 and started ‘beam synchronization’. I don’t fully understand what that means, but I know that there was one bunch per beam (about a metre in length worth of protons), and my best guess at ‘beam synchronization’ is that the two bunches were brought close together.
Obviously the beams weren’t focussed and we weren’t running at anything close to design luminosity, but a few collisions occured in all 4 detectors. An exciting moment for everyone involved with the LHC, indeed.
The CERN Press Release of the weekend’s events and a summary of the collisions explains a bit further what the goings on were this weekend, and what the plans for the LHCs immediate future are.
The coming months should be an exciting time for particle physics, and the coming years will hopefully shed some light on the darker corners of the Standard Model and beyond…
Feynman Diagrams that look like animals
In Particle Physics, there is a special tool that is used to help understand, visualise and calculate certain aspects of a particle interaction. This is the Feynman Diagram.
A lot of Feynman Diagrams are fairly dull, monotonous affairs, essentially consisting of two particles scattering off each other, like so:
However, there are some Feynman Diagrams which can, with the help of a little imagination, be made more exciting!
Recently, I discovered that, if I stare too long at Feynman Diagrams, I start to see the diagram in a different way, much like how you can often see faces while staring at a plastered ceiling, or see shapes in the clouds.
I see animals in Feynman Diagrams.
Here’s a few to get you started.
Now, any High Energy Physicist will tell you that this has already been done, in the form of the Penguin Diagram.
Which, if you squint a bit, can look a bit like a penguin. (The animal, not the chocolate bar)
The Penguin Diagram originated when a bet between Physicists was made, the loser of which had to include the word ‘Penguin’ in their next paper. So it’s all a bit artificial really…
Next, the deer diagram. It’s fairly self explanatory, the gluon emitted by the up quark represents the antlers, the rest follows from that…
The fish diagram. This one takes a little bit of thinking, but not too much. Obviously the top quark pair production vertex is the ‘nose’ of the fish, the bottom and anti-bottom quarks are the main fins of the fish and the quark antiquark/lepton antilepton pairs are the tail fins.
The lobster. This one, I’ll admit, is a bit contrived. But bear with me. The quark antiquark/lepton antilepton pairs at the very right are obviously the claws. Where the quark and anti-bottom lines form a diamond with the top anti top pair is the head, the gluon is the body (albeit a very skinny body considering the size of the head), and the protons at the left are the tail.
Well, it’s either a lobster or Edward Scissorhands on his side with his arms upstretched…
Wow, COOL!
After the ’slight setback’ last year, that pushed the schedule back by about a year, the LHC is back to being one of the coolest places on Earth.
The magnets of the LHC, during operation, must be cooled to around 1.9 Kelvin, which is around -271.25 Celcius. COOL!
Now that the whole collider is cooled and the new quench system installed (a failsafe mechanism), the first beams can be brought back into the LHC.
At first, the energy of each proton beam will only be a fraction of their design intensity, around 450GeV.
The first collisions should happen in late November, followed by a step up in energy to a few TeV, where the next lot of collisions will happen. This is still less than half of the design intensity, but at this energy, there may still be some new physics to do. It breaks Tevatron’s record of the highest energy particle accelerator lab at least.
At the moment, it’s looking like the LHC will stay with collisions at this lower energy for quite some time, before the final push towards 7 TeV per beam.
After all, it broke once, why risk doing it again when you can get perfectly good data first!?
ANTLERS – A Note on parTicLE physics acRonymS
Since starting my PhD with the ATLAS project, I’ve learned about various acronyms used to name detectors, software, and other related projects in the particle physics community. A simple example would be LHC – Large Hadron Collider. Nothing funny going on there, it does exactly what it says on the tin. It collides hadrons (protons and lead ions in this case) and it is rather large.
Not all acronyms are quite as nice and to the point, however. I will list just a few of the more contrived ones (throughout, bold type is used to denote the letters used in the acronym).
Firstly ATLAS – A Toroidal LHC ApparatuS.
Since when did the final letters of a word count to appear in an acronym? Fair enough if you take the first two or three letters of a word, but taking the first and last letters. Come on! At least think of another word beginning with S!
Second, DEGREE – Dissemination and Exploitation of GRids in Earth sciencE.
Now we are just being stupid. Taking the last letter and not even the first letter. Of the word SCIENCE, no less.
GENIUS – Grid Enabled web eNvironment for site Independent User job Submission.
This one leaves whole words out. Important words like web and job. Which are pretty much the point of the environment. To submit jobs through the web… I can ALMOST forgive the use of ‘N for environment’, but not when you omit such crucial words!
BaBar – B-Bbar detector. Now this is just adding letters to make it sound like cartoon elephants!
Finally, ATLANTIS – ATLAS eveNT dISplay.
Do I need to say anymore?
In the world of High Energy Physics, there is definately a trend of ‘pick your acronym first’ going on. What words the represent is entirely arbitrary as long as the Acronym sounds nice. If the words have relevence to the project, it’s just a bonus.
HONOURABLE MENTIONS (for comedy value):
GIGGLE – GIGascale Global Location Engine.
LEMON – LHC Era MONitoring.
PASTA – Processors, memory, Architectures, STorage and TApes.
and for the Twin Peaks fans:
DIANE – DIstributed ANalysis Environment.
A bit of shameless scaremongering
The media loves to grab onto any new technology or experiment that has even the tiniest whiff of controversy about it and whip up some fanciful scare stories about how it will (not if, will) end the world and we all have to sign a petition and protest. Fair enough, with big experiments there is always going to be a small risk involved. The potential benefits to science and progress always far outweigh the potential risks, however, as if they didn’t the experiment just wouldn’t be feasible.
A recent example is the LHC. Everyone got into a huge panic shortly before the first beam last September when the idea came to light that a micro black hole might be formed. People immediately hear the words ‘black hole’ and ‘new experiment’ and ‘expensive’ and immediately assume a group of geurilla scientists are trying to take over the world. These people should not be allowed to leave the house.
The purpose of this post is not to address these trivial issues though. It is infact to address a much more serious and much more likely issue. That a nearby supernova will wipe out all life on Earth.
A supernova is what happens when a massive star begins to collapse under its own gravity, then, not being able to take the immense pressures and forces involved, suddenly begins rapid nuclear fusion and catastrophically explodes, expelling a vast amount of energy and leaving behind a neutron star or black hole.
One of the most famous supernovae is SN1987A, which is the first supernova to occur in 1987. It was highly luminous and even visible to the naked eye for a while. But even then, 1987A was a distant supernova.
A supernova closer to home, say, within a kiloparsec, has a high chance of ending a lot of life.
Some calculations:
Assumptions:
1. The stars in the Milky Way are spherically distributed.
2. The mass of the Milky Way is entirely composed of stellar objects.
3. All stars in the Milky Way are roughly as massive as the Sun.
Taking these assumptions into consideration, we will most likely come out with an underestimate of the stellar number density of the Milky Way, purely based on assumption number 1.
Mass of the Milky Way: ~ 6×10^11 Solar Masses (1.42×10^42 kg)
Radius of the Milky Way: ~ 50,000 Light Years (4.7×10^20 m)
Average Density of the Milky Way: ~ 3.2×10^-21 kg/m^3
Now assumption number 4, the stars are uniformly distributed throughout the Milky Way.
Density within a radius of 1kpc: ~4×10^38 kg
Stars within a radius of 1kpc (using assumptions 2 and 3): ~200,000,000
So if we make an underestimate that 1% of the stars in the Milky Way are large enough to produce enough energy in a supernova to kill us all, that makes close to 2 million high mass stars that are close enough to wipe out all life on Earth.
In fact, it’s been proposed that the reason for one of the mass extinctions in the Earth’s history was caused by a nearby supernova.
The reason we aren’t all kicking up a fuss about it? Well, for one, there’s absolutely nothing we can do about it. There’s no way to predict when a supernova will happen (aside from observe the stars for evidence of their evolutionary stage) and there’s definately no way to stop one. The second reason is that a supernova is a relatively rare event. The Milky Way has around 100 billion stars. The average rate of supernovae in our galaxy is around 5 per century. So the probability of a single star in the Milky Way going supernova is around 3×10^-14. This means that we can expect a nearby (within 1kpc) star to go supernova around once every 1,000,000 years.
A mass extinction caused by a nearby supernova has, literally, a one in a million chance of happening.
The Physicists Favourite Thought Experiment
A recent post on a forum I visit got me thinking about this idea.
First, imagine you are on board a high speed train and you have a gun. This isn’t Murder on the Orient express though, this is an experiment.
Say the train is moving at the same speed as the gun can fire a bullet (assuming this speed is the initial velocity of the bullet and the train is moving in a vaccuum, so air resistance is negligible), call this 500 m/s.
Now, fire the bullet out of the window directly ahead. What happens?
Well, without air resistance the bullet isn’t limited by terminal velocity, so the bullet is moving at 500 m/s before it is fired, then gains an extra 500 m/s once it is fired. The bullet is moving at 1000 m/s. Or, with respect to the passenger on the train, the bullet is moving at 500 m/s.
Now, point the gun directly backwards, away from the direction of motion and fire. Again, relative to the passenger the bullet travels at 500 m/s, but relative to an observer standing by the tracks, the bullet appears to be falling straight down.
Then that got me thinking about a similar problem that is all too common in Physics classes.
Imagine you are on board a spacecraft travelling at the speed of light.
You have a torch, you point it in the direction you are moving and turn it on.
What happens?
Well, the light travels away from you at the speed of light.
Now, what about a stationary observer? (that is, the spacecraft is moving relative to the observer)
Nothing can travel faster than light, right?
Right.
Well, the observer also sees the light travel at the speed of light in the direction the craft is travelling.
But then the observer would see the light moving at the same speed as the craft, yet the craft sees the light moving away at the speed of light.
A contradiction?
That’s where Einstein comes in with the Special Theory of Relativity.
This states that light moves at a constant velocity relative to all observers, irrelevant of their velocity or direction.
This is allowed to happen by a phenomena known as time dilation. That is, time slows down the faster you get.
This then leads to the famous ‘twin’ thought experiment.
Take a set of twins, give them identical clocks, leave one on Earth and set one off on a journey round the Universe at very, very high velocity.
When the travelling twin comes back, his clock will be lagging behind the stationary twin on Earth. Thus, he has aged less.
Time dilation is a very interesting concept of both Special and General Relativity, and has had a profound impact on modern Physics, even modern life. In fact, GPS satellites orbitting Earth must take into account relativistic effects, something called gravitational time dilation, when sending positioning data.
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