Good morning and welcome to the Guardian's live coverage of the Large Hadron Collider's second launch

7.32am: Eighteen months after the Large Hadron Collider suffered an enormous helium leak that shut the machine down, engineers are readying the machine for its first high energy collisions.

The LHC accelerates two counter-rotating beams of protons - the subatomic constituents of atomic nuclei - to within a whisker of the speed of light, before steering them into one another. The head-on collisions release enough energy to mimic in microcosm the conditions that prevailed a fraction of a second after the big bang.

The first collisions are expected as early as 8am this morning, but at Cern things can happen faster or slower than expected. A tentative schedule of the day's events is here.

Alas I am covering the events from London. You can watch Cern's streaming coverage here. If you have work to do and can't sit around watching people chat about particle physics all morning, I'll follow it for you.

The machine has already collided particles at a combined energy (call it centre-of-mass energy) of 2.36 trillion electron volts (TeV). One electron volt is the amount of energy the particle gains when accelerated across an electric field of 1 volt.

Slamming particles together at 2.36TeV in November showed the machine, and its four huge detectors, work well. It also claimed the crown for the LHC as the most powerful collider in the world, by pipping the US Fermilab's Tevatron collider near Chicago.

Today is bigger news. Today the machine will go to half of its full energy, that is, colliding particles together with a total energy of 7TeV.

In case you are worried, the world will not end.

7.45am: We're expecting the first attempt at collisions at 8am BST. Cern has historically shut down its major colliders over Christmas for routine maintenance, but things will be different with the LHC. The machine will run from today all the way through until the end of 2011, or thereabouts.

7.49am: The machine was loaded up with particles about 40 minutes ago. The LHC has ramped up energy to 3.5TeV in each beam, but no collisions as yet.

7.54am: The beam has just been lost. Maybe a power unit tripped. It takes thousands of components to work in unison for this to work. It sounds as though there's a problem with the cryogenics, that is keeping the superconducting magnets cool.

7.59am: The Cern engineers are trying to fix a glitch in the LHC's cryogenics. You can't accelerate or steer the beams without the cryogenics working, because the superconducting magnets have to be ultra-chilled to work properly.

8.06am: Speculative (and often ludicrous) doomsday scenarios have been kicked around since the early days of particle colliders. People have wondered about making greedy black holes; "abnormal matter" that's more stable than regular matter; magnetic monopoles that might cause proton decay and so on. My favourite, or more precisely, the one that intrigues me most, is vacuum decay.

The universe seems to be in a pretty stable state. There's been time for stars and planets to form and for life to arise. But what if the stable state of the universe was an illusion? It could be in a metastable state. And a big kick of energy somewere, anywhere, could knock the universe down into a more stable state. That's vacuum decay. What's staggering about the possibility is that it doesn't just mean curtains for us and all other life on Earth. It means no more life ever, anywhere.

Here's the late, great Sidney Coleman speculating on the prospect in, I think a 1980/81 paper:

"The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in the new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated."

Sidney Coleman.

So, we'd best get on and enjoy ourselves, eh?

8.10am: It's worth bearing in mind that the LHC is the most complex machine that's ever been built. Nothing quite like it has been made before. Last year's explosion - that led to a tonne of helium bursting into the collider tunnel - was the kind of incident that people half expected. An awful lot of things have to go right for it to work at all. Losing the beams today is a minor glitch.

8.11am: It could be two hours before they are in a position to collide particles. Good thing they've started early.

8.14am: Ah judith. I hope you're joking. I've never heard anyone other than a physicist say "I'm concerned about the cryogenics." Sounds like a line from an extra on Star Trek.

8.17am: Holger Bech Nielsen must be smiling.

8.21am: Steve Myers, head of accelerators at Cern, has just been speaking. They've had two goes, both of which have failed. The first beams were lost at an energy of 2.2TeV when a power supply tripped. They've reset it and that part seems ok. The second attempt failed due to a problem with the new magnet protection system. This is interesting, because the protection system is very new. It was added during the 18 month shut down to prevent another explosion like the one in September 2008. One of the magnets has quenched, which means it warmed up. That will need to be cooled down again before they can go for collisions. Collisions at 10am earliest.

8.23am: Britain pays £80m a year in Cern subscriptions, the bulk of which goes on the LHC project.

8.27am: What the LHC might find:

The Higgs boson (thought to give mass to fundamental particles)

Supersymmetry (which doubles the number of particle species in the universe)

Extra spatial dimensions (they must be tiny if they're there)

Miniature black holes (and we're talking pin-prick sized)

Dark matter particles

And plenty more. I'll say more about the above in a mo.

8.36am: JerryTheDog: a good point re: what will 7TeV collisions mean if they achieve them today. Bear in mind that the American Tevatron collider has been operating at nearly 2TeV for quite a while, though it is colliding protons and antiprotons. For the LHC to see something soon (and I mean within months), there would have to be a particle or an interaction that is sitting just above Tevatron's reach and well within the LHC's reach. What are the chances? Who knows. There are theories that predict a heavy Higgs boson, but the odds seem very slim. Other theories suggest there could be supersyimmetric particles (heavy twins of the particles we know and love) that are within reach of the LHC. More than likely though, it will take many months to find clear evidence of anything. The reason is that new phenomena can easily be obscured by other well-known physics. Slamming protons together is a messy business, because each proton is made of three quarks, which means you can have all kinds of subatomic detritus smothering signs of something knew. My guess is that the Higgs boson won't be seen - by the LHC at least - before it closes down for major work in 2011. Finding supersymmetric particles, or extra dimensions though, would be staggering.

8.39am: The LHC had beams running at 3.5TeV no problem last night. I think the machine's got stage fright this morning.

8.43am: Phillipe Bloch, head of physics at Cern:

"We've worked for almsot 20 years on this project...We have a lot of work in front of us for many years, so we can wait for a few more minutes."

8.46am: Right now, Cern staff are cooling part of the machine in preparation of injecting particles again. Once the particles are in, the energy will be ramped up to 3.5TeV. The ramp up usually takes about 45 mins. Then they have to make sure the beams are stable, that is circulating happily without wandering off axis or fading. Only then will they go for collisions.

8.48am: The protons hurtle around the machine in bunches. When the beams are crossed, some of these bunches smash into oncoming bunches, and some protons will hit others. You can get glancing blows, where protons simply deflect off each other. A direct collision will split the protons into their constituent quarks and gluons. The energy released on impact can condense into entirely new particles, so you see quarks, electrons and their heavier cousins, muons, all flinging out from the collision centre.

8.54am: The glitch with the cryogenics system seems to have come from electrical noise that hit a few sectors of the LHC and then vanished. Spooky, or not.

Since you ask, Cern's previous machine, the Large Electron Positron collider, had teething troubles too. Weird signals came and went. They realised, eventually, that the machine was reacting to Earth tides: the pull of the moon on the Earth's crust was making the land around the collider heave up and down, causing a 1mm change in the circumference of the 27km collider ring.

And that wasn't the end of it.

More stray signals plagued the machine that seemed to come and go with even more regularity than the moon signals. Turned out that stray currents from the French TGV train at Geneva station were passing through the ground and getting into the collider, causing slight changes in the beam orbit. Once that had been cleared up, they found yet more signals that were traced back to the water level of Lake Geneva.

These are big machines, but they are amazingly sensitive.

8.59am: Some pics from this morning's events so far...

9.08am: We've just heard from Andrzej Siemko, the man in charge of the quench protection system at Cern's LHC. The system is there to alert operators to problems with the magnets, and to shut the machine down if it looks like a magnet is in danger of being damaged. It's really sensitive. The system picked up an electrical anomaly from two sectors of the machine and flagged a warning to operators. What's interesting - and it's good news I think - is that the problem it identified did not come from one of the LHC's 8,000 superconducting magnets. It picked up a load of electrical noise that was later spotted at other accelerators at Cern (the LHC is just the biggest and shiniest).

Sounds like a glitch with the Cern power supply rather than a problem with the LHC itself. That said, if it's a generator problem, or a substation problem, it could be an annoyance.

9.09am: Friskydiscus says: "When (if) they do manage to collide any particles can you mark it with a big bold Goooooooal!!!!!!!"

But of course. It would be my pleasure.

9.13am: BristolBoy asks: "How long did it take them to discover that noise was coming from passing TGVs"

I'm afraid I can't remember quite how long it took to uncover. I don't think it was too long, because so many Cern staff pass through Geneva train station and they recognised the times when the anomalous signals were appearing. The fact that the signals came and went like clockwork Mon-Fri, and then at different times at the weekend helped. I wonder if anyone would have noticed if the collider was in the UK. Our trains seem to come and go as they please.

9.20am: Craignewzealand asks about the similarity between LHC collisions and those in the cosmos.

I think it's a fair question to ask, but it's one that gets short shrift from many particle physicists. Very quickly, the argument is that higher energy collisions have been going on for billions of years somewhere in the universe, and there's been no catastrophe yet. That we know of. In space, particles can have head on collisions, though a speeding particle slamming into stationary ion/surface is more likely. John Marburger, President Bush's former science adviser, told me these fears were like going for a swim in Scotland and worrying about being eaten by the Loch Ness monster. Theory doesn't rule out the Loch Ness monster, not absolutely anyway. No one has (really) ever seen it. And if it is there, we don't know if it likes the taste of people. I would go on, but I'm probably boring people!

9.23am: Fresh beams expected in just under one hour.

9.27am: WendyZu, Apolloman: Not much to add re: the picture except that the yellow curved tracks are charged particles that are kicked out of the collisions and bent by the detector's magnetic field. Some of the other markings are due to the calorimeter in the detector being triggered - they give a measure of the amount of energy carried by the particles produced.

This shot is taken looking down the beampipe or collision axis.

9.29am: In case you've ever wondered what the Cern scientists have been up to since the LHC switched on at low energy in November...

9.33am: I've been trying to think what this all reminds me of and I've got it. Sitting at Kennedy Space Centre waiting for the shuttle to launch. I've been three times and seen zero launches. Each time, the weather closed in or there was a technical glitch of some sort. The whole process is so complex and the price of beign careless and going ahead too early is so high, that everything moves at a (sensibly) cautious pace. Cern can't afford another huge accident like the one that shut this machine down for 18 months in September 2008. These guys have the patience of saints.

9.41am: So. The machine is still being cooled down so that it can take another filling of protons. These are injected as beams with an energy of 450 billion electron volts. Once those are whizzing around in the machine, the accelerator fields will ramp up, steadily spinning the particles up to 3.5 trillion electron volts per beam.

In case you find this frustrating: In the final year of operation of Cern's last collider, the Large Electron Positron Collider, the machine was being pushed so hard, it tripped out roughly every 15 minutes. That is, it would run for 15 mins, then break down. The beams then had to be filled again, stabilised, and then steered into collisions again. No one said particle physics was easy...

9.48am: Lemon3: I refer you to the priceless exchange between Robert Wilson, former director of Fermilab, and Senator John Pastore, during a Congressional hearing over the value of building a new particle accelerator. Pastore is sure there must be something about the machine that can be steered towards a defence app:

Pastore: Is there anything connected with the hopes of this accelerator that in any way involves the security of this country?

Robert Wilson: No sir, I don't belive so.

Pastore: Nothing at all?

Wilson: Nothing at all.

Pastore: It has no value in that respect?

Wilson: It has only to do with the respect with which we regard one another, the dignity of men, our love of culture. It has to do with, are we good painters, good sculptors, great poets? I mean, all the things we really venerate and honor in our country and are patriotic about. It has nothing to do directly with defending our country except perhaps to make it worth defending.

9.53am: Wilson is a tad pretentious in that exchange, I'd say. The real question: why do particle physics is a fair one. To learn about the universe and its make-up is enough for me to chip in my tax quids. bear in mind that 25% of the mass of the universe is dark, that we don't know what it is. Plenty more is not understood at all. The history of physics is littered with experts claiming the end of physics is nigh; that we are on the brink of knowing everything. It's no more true today than it was in Lord Kelvin's day.

Beams back in 20 mins.

SirTachyon: I do hope so!

9.55am: ieclark: Forgive me, I'm an enthusiast. This thing costs the UK more than a million quid a week, I like to know how it's getting on!

9.59am: harmonyfuture: It depends if Hawking radiation works as advertised. Bear in mind you need an awful lot of matter to make a dangerous blackhole, and bunches of protons just aren't enough to do that. If you get teensy ones, and it's a huge if, they'll be fabulously hot and are expected to radiate themselves out of existence in the blink of an eye.

10.02am: Jacktatum: There's a proper science journalist covering this? Where?!

The machine uses magnetic fields to steer the beams into head-on collisions at four points around the accelerator ring. Well, it's not a ring as such, but it's almost circular. The beams are crossed inside the four detectors: Atlas, CMS, LHCb and Alice.

10.08am: SirTachyon: I'm not at all sure how you'd go about looking for a tachyon. The buggers move faster than the speed of light, so I for one won't be able to catch one. You might get some joy here.

10.10am: New analysis shows that the glitch that's holding things up was due to magnetic coupling of the main circuits in super proton synchrotron (the injecting accelerator) and the LHC, not electrical noise. Or is it the Higgs particle wooing its way back in time and jinxing the machine. Again.

10.17am: New bunch of protons has just been injected. Where do they get all the protons from? Tis easy. Get a big can of hydrogen, put an electrical voltage across it, strip off the electrons and ditch them. Eeach hydrogen atom has one proton for a nucleus, and one electron orbiting it. Once you've got rid of the electrons, you have positively charged protons, which you can steer and accelerate with fields.

10.21am: ieclark: It's a fair point. I was at Cern for "big bang day" in 2008. It was overdone by the media I think. Especially for poor listeners of the R4 Today programme who were treated to an audio explanation of a dot appearing on a screen. I haven't been asked to live blog this for the Guardian. I just wanted to. It's the biggest experiment in physics in a generation. It might well go badly wrong. I won't live blog every minute, but since I'm going to be watching the coverage anyway, I thought I might write about it while I'm at it. I do take your point about media overkill though. What does everyone else think?

10.26am: Gregmcdougall asks: "What would the effect on living tissue (other than the assumed cold) of these high-energy beams be?"

A good question.

In the mid-70s, a Russian physicist called Anatoli Bugorski was checking a faulty accelerator when the proton beam came on and hit his head. He says he saw a bright flash "brighter than a thousand suns".

His face swelled up and skin started falling off. He lost some hearing but otherwise his brain remained in quite decent shape. He could still function, but got tired quickly. It was a crazy accident to have happened.

10.27am: Proton beams are circulating. Now they have to be ramped up to high energy. You go slow and steady at this stage. If you go too fast and a power unit trips out, you basically slide back down that ladder and have to start all over again.

10.35am: SirTachyon: Bear in mind that around a billion protons will be colliding every second when the LHC is up and running. The detectors will look for different things, but there is plenty of overlap too. You can hunt for known particle decays-so the Higgs might decay into quarks of certain energies. But, so many other things can do that too, that you have to record a huge number of collisions before you can be sure you're seeing something on top of what you'd expect without a Higgs existing. This isn't the kind of experiment that gives you an answer within hours or days or even weeks. You collect data and watch the statistics of different collisions. If a certain decay process happens more than is predicted, then it could be something new being created. The particles that are created are all unstable - otherwise we'd be finding them down the back of our sofas all the time. So unstable particles decay soon after being created, into other more stable particles that are picked up by the detectors. You then collect these debris particles, measure their masses and energies, and work backwards to see what could have decayed into them. Apologies for rushing this - I'll do a proper explainer some time if there's interest.

10.45am: Lyn Evans, head of the LHC, speaking. They're going for ramping up in 1 minute.

More to come from the DG Rolf Heuer.

10.50am: jacktatum: A good point - there is indeed stable dark matter. There could well be a whole family of dark particles, some stable, others not so. All of the particles we know of are the ones that either make us up, or interact with the ones that make us up. A guy called James Wells at Cern speaks well on this - and talks about particle physics being to anthropocentric. There is no reason why we should have a privileged view of the universe; that we should be able to see all, or even the majority of particles and forces at work in the universe. We might be privvy to a minority reality. It's all fun stuff.

10.58am: Someone asked: What if the LHC doesn't find anything.

Frankly, it'd be a disaster. But. The LHC absolutely should find something. Somewhere in the energy range it covers (up to 14TeV) there must be something that explains something called electroweak symmetry breaking. This is the process that separated the electromagnetic force (which allows us to see) from the weak force (that plays a role in sunshine production) a picosecond after the big bang. The process is thought to have been caused by the Higgs field, which in breaking the electroweak symmetry, gives mass to certain bosons (force carrying particles) and also the quarks and electrons that make up normal matter. Finding the Higgs boson (which could weigh the equivalent of 115 - 140 billion electron volts) will suggest that Higgs's ideas about the origin of mass are correct.

But still. What if it finds nothing?

That really would be a disaster for particle physics. Because I can't see governments readily funding another collider (for example, a giant linear collider) if existing theories have not been pruned back to reveal some promising avenues of discovery.

Someone else asked about the giant size of these machines. I couldn't agree more. They are the most high-tech things we have, yet still seem a bit Space 1999. This machine, if you pulled it up from the ground and stood it up, would reach 5 miles into the sky. Whatever happened to miniaturisation"?

Progress is at hand. Physicists are working on ways of accelerating particles to huge energies over short distances. One technique, uses "plasma wake fields" to get particles "surfing" along a plasma wave at incredible speeds. It'll be a while before a collider built on this technology is built though.

11.02am: CuriousJohn: How would we recognise evil incarnate against LHC backgrounds?

An aside.: when I was a student I asked Martin Bell (he of the white suit) if he believed in true evil. Turns out he does (or did back then). Like it's an entity. One for the philosophers I suspect.

Back to reality:

Ramping up of the beams is going on now. They expect to be at top energy, 3.5TeV, in 25 mins. The energy right now is 950GeV (billion electron volts). They began from 450 GeV.

11.07am: jacktatum: there are Higgsless theories, for sure. Some break electroweak symmetry using extra dimensions. Quick thing on electroweak symmetry breaking. Before it's broken, there are four bosons that are all massless and carry the electroweak force. They are the photon, W+, W- and Z bosons. After symmetry breaking, the Ws and Z become massive but the photon remains massless. That separates the two, because the photon, being massless, can still act over infinite distance. Because the W & Z get so heavy, they make sure the weak force can only act over a tiny distance. It's so short range, it basically acts only on contact.

11.10am: tinylittlebear: Indeed. But on Friday I was speaking to woman who recently left Cern to work on particle beams for cancer treatment in Oxford. Charged particles dump their energy in tissue at a defined point, so by being tricksy, you can fire particles at a tumour and get them to dump their energy (heat) into the tumour and frazzle it. So what nearly did for Bugorski is a life extender (I'm not sure life saving is possible) for loads of people. Another example of how pushing tech and science with projects like the LHC lead to full on benefits for broader society.

11.15am: Emptycan: For the larger part, the British scientists at Cern seem to have faired quite well, but others, such as the nuclear physicists, are in very real trouble. Across the board cuts are probably coming to science. Lord Drayson still talks of a budget ringfence for science, but universities are cutting PhD studentships already. The Chancellor announced £600m of cuts to higher education and research budgets last year. We have to wait for the CSR to know for sure, but it does not look good, and probably won't be good whoever gets in. The Tory science spokesman has talked of science cuts being inevitable.

I suspect the US and Singapore are looking very attractive to young, mobile British scientists right now.

11.16am: Beam at 2TeV. Cern DG just calling in to the lab.

11.20am: Cern DG, Rolf Heuer:

"I'm physically in Japan, but mentally in Geneva. I'm more nervous here than I would be in Geneva."

Oh dear. Journalist asking Cern's DG where his superfast Dan Brown jet is.

What a numpty.

11.22am: CuriousJohn: Thanks for that - be great if you can shed more light on the situation over there.

11.28am: Hilarious. A Cern physicist has just been talking about creating black holes. He worked out that to make a black hole, you need to compress 1,000,000,000,000,000,000,000,000,000,000,000,000 protons into the space of a billionth of a millionth of a millimetre. That number of protons would make up three times the population of Geneva. Apparently.

This from Brian Cox on Twitter:

"If anyone else says "black hole" today I'm going to come round and chin em".

11.32am: Proton beams are now circulating at 3TeV. The goal is 3.5TeV. Edging ever closer.

11.37am: EurObeideZakani: manipulating the Higgs field isn't going to happen any time soon. You'd need to heat your part of the universe above 10,000,000,000,000,000 degrees to influence it. Which isn't really on. A bit dangerous too. If you change the Higgs field, you'll alter the size of atoms (though not their mass very much), and destablise normal matter. That's not a good thing to do.

We have 3.5TeV beams now.

Collisions coming when they're stable.

11.40am: SirTachyon: The LHC has a lot of different Twitter feeds. Try searching under LHC or Cern hashtags. Fabulously, my keyboard doesn't have a hash key.

11.42am: I do hope there's a big red button marked: "Collide".

Expecting collisions in 10-15 mins.

11.43am: Watch the first collisions here for max applauding potential...

11.44am: After everything the Ghostbusters told us about not crossing beams...

11.45am: The beams are being adjusted in preparation for colliding them.

11.47am: In case you fancy going down the bookies - some odds on the creation of anything new and exciting.

11.55am: Operators are stabilising the beams in preparation for collisions. All looking good. But I've said that before.

11.58am: Beams are separated for now by magnetic fields. You take those away gently until they're not there, then....collision time.

11.59am: Beams aligned and ready for collision. Nearly.

12.05pm: Gooooaaalllll!

We have collisions. The detectors are not yet fully on and taking data, but particles are colliding.

12.07pm: Fabiola Gianotti, head of the Atlas team:

"We got something like 40 events per second, which is the expected rate.

"It's the beginning of a new era of physica exploration."

12.08pm: We'll have pics of the highest energy manmade collisions as soon as we can get them.

12.12pm: I'll be hanging around for a little while to grab some pics and make sure everyone gets home safely, or whatever the we-just-started-up-the-world's-largest-collider equivalent is. Thanks to everyone who joined me this morning, it was great to have you along for the ride.

Collision event at 7 TeV Photograph: CERN Geneva

12.56pm: The Atlas detector has just released its first images. Don't ask me to interpret them just yet!

2-Jet event at 2.36 TeV Photograph: CERN Geneva

And a second picture from Atlas:

Cern Atlas Photograph: CERN Geneva

And finally: