My name is Dr Martin Webb. I'm a junior research fellow at Oxford University, researching particle physics. But this story isn't about me. It's about my invisible pet cat Gianni.

      Gianni is a four year old male. He's Italian. When it is possible to see him he is mostly black, with four white paws, but these occasions are rare. Most of the time he is invisible.

      I was introduced to Gianni while I was in Genoa for a conference. I was staying in a pleasant seaside hotel run by an old lady with her daughter and an indefinite number of cats. I gather they ran a local animal rescue group. The proprietor introduced me to some of her feline guests and asked if I would like to adopt one. I explained that I would like it very much, but it would be impossible as there were regulations against taking animals into the UK, and as I had to travel a lot I could never take proper care of a cat.

      Undeterred, she smiled and said, "I've got just the cat for you." She disappeared, and came back carrying something invisible. "This is Gianni" she said, holding out her apparently empty hands, "you can't see him". I held out my hands and briefly felt something furry, but then it was gone. "Oh!" she said surprised, "he's not normally so nervous.... Can you give me one of your shirts?"

       This way she began a programme to introduce me to Gianni which went on for the rest of the week. She began by leaving one of my unwashed shirts in Gianni's bedding, so he would get used to my scent. Then she moved his basket to the room next to mine, where he could come and go as he pleased.

      Early one evening in the middle of the week, I was sitting at the table in my room reviewing a paper on the thermodynamics of superconducting particle detectors. I heard a soft thud and realised that Gianni had jumped onto the table. Stretching out my hand I touched the back of his head and stroked behind his ears. It was a little strange fondling an invisible cat, but I soon got used to it. After that Gianni seemed to accept me and would climb onto my lap when I was sitting in a chair, and I had to turf him off my bed before I could go to sleep.

I tried to find out more about Gianni from my host, as I was extremely curious about how she came by an invisible cat. Unfortunately this conversation stretched my Italian to the limit, and I can't be sure I understood everything she said correctly.

      Gianni, she explained, had appeared (if one can use that word) about six months ago. She didn't know where he came from. He was named after one of her daughter's boyfriends. He was invisible all the time, except when he was completely relaxed and thought nobody was looking at him. Then you could see him. But the arrival of any strangers or anything new sent him back to his usual state.

      She had known other cats who would vanish suddenly, for a short period, when they were absolutely terrified of something - presumably this had evolved as a defence mechanism - but Gianni has the only one she knew who was invisible almost all the time. Because of this, she explained, he could be quite fearless, and could get into all sorts of trouble.

      I stayed in Genoa for another week and got to know Gianni well. Once or twice I even caught sight of him, lying in the sun, or cleaning his fur. I was determined to take him back with me to Oxford. The journey gave me some sleepless nights. There were so many things that could go wrong. What if Gianni wandered off and got lost at the airport? What if a customs officer discovered him? However the day turned out to be quite uneventful.

      I left the hotel after saying goodbye to my hosts and all the other cats, carrying my suitcase in one hand, and a straw basket containing Gianni (curled up on a blanket) in the other. The apparently empty basket caused a few puzzled faces at the airport, but fortunately no suspicion. During the flight Gianni climbed onto my lap and fell asleep, apparently not the least disturbed by the experience. Usually I find flying very tedious but this time either due to the soothing influence of stroking a cat, or because I was a too concerned about other things, the time flew past easily. We got to my house by late afternoon. I left Gianni to explore this new territory and went to the supermarket to buy lots of cat food.

Over the following weeks Gianni settled down to life in Oxford and I got to know him better. He now remained visible for longer periods of time, even when he knew I was looking at him. But if I did anything he didn't like he would stay invisible. It was his way of sulking. I learnt to recognise the signs he left as he walked over carpets and brushed against furniture so I could usually guess where he was even when he was invisible. The exception to this was when he was asleep. Then it was often impossible to find him.

       Gianni liked sleeping in warm, sunny places. He liked climbing trees and furniture, and lying on top of high objects where he could watch everything happening below him. He liked regular routines and would complain if I came home late. He was also incredibly curious. I soon found my biggest problem was stopping him getting bored. He probably missed the company of other cats, which he had plenty of in Genoa. There were a few other cats in the neighbourhood, but they seemed rather suspicious of a playmate who they could smell, but not see.

      Taking him out was no problem at all. I went on several walks around Oxford with my basket with Gianni, standing with paws on the rim, looking at everything we passed. Soon he learnt to climb on my shoulder, which he preferred being higher up. If I walked parallel to a wall, such as the one in front of the natural history museum, he would leap off and walk along the top, then leap back on my shoulder once the wall finished. Occasionally he would lose his balance and dig his claws into my back until he had climbed back into place, but with practice he managed very well. He didn't like walking at my feet and would only do so in large open spaces without any other people.

As these short excursions went so well, I had the idea of taking him with me to my lab in the physics department. Thus one Friday morning, shortly before the start of Michaelmas term I took Gianni with me to work. It was a calm, warm day. I left early before there were many people about. Gianni vanished as soon as we were out of the house but I could feel his claws digging into my shoulder and the tip of his tail on my neck.

      I wasn't aware of any rules forbidding University staff from taking their invisible pet cats into the department, but just in case I decided not to mention it to anyone. Anyway, if I introduced Gianni to anyone it would surely lead to all sorts of questions, which I didn't have the time to answer.

      There was quite a crowd of people at the entrance to the department, which made Gianni at bit nervous, but once we reached my office he calmed down. I sat down at my desk and started to check my email. I heard Gianni jump onto the desk, then onto the floor, and then I lost track of him.

I work on an experiment searching for the electric dipole moment of the neutron. Neutrons are subatomic particles, which together which protons and electrons make up atoms, from which all ordinary matter is made. The neutron is an electrically neutral particle. But its electric charge may not be zero everywhere. If the neutron has slightly more positive charge on one side, and slightly more negative charge on the other side, it would possess what we call an electric dipole moment.

       So far no experiment has found the electric dipole moment of the neutron. But that doesn't mean it doesn't have one. It may just be very small. Therefore our team is developing an instrument of unprecedented sensitivity. If the neutron does have an electric dipole moment, we hope to be able to measure it.

       Measuring the precise shape of the neutron may seem like a very obscure academic activity. But it could explain one of the biggest mysteries in contemporary physics - why the Universe exists at all?

      The fundamental laws of particle physics are, to a first approximation symmetric - they look the same whether time runs backwards or forwards. But we know there must be some subtle asymmetry hidden somewhere, as a universe described by totally symmetric physical laws would be very boring. It would contain equal amounts of matter and it's mathematical opposite, antimatter. These would simple annihilate together, leaving the universe full of energy, but empty of stars, planets, scientists and cats. An asymmetry in the laws of physics, would mean a non-zero neutron electric dipole moment. So if we can find it, we can explain why our universe exists at all.

Our team includes scientists from the UK, France and Japan. The experiment itself is located in Grenoble, France. There a large number of neutrons are generated by a nuclear reactor. They are then cooled so they are moving very slowly, and stored in a cell with large electric and magnetic fields. The neutrons then spin around like spinning tops. By measuring a shift in their spin precession frequency when we change the electric field, we can determine the electric dipole moment of the neutron.

      Unfortunately the experiment is easily disrupted by any change in the magnetic field which can make it look as if we've found an electric dipole moment even when we haven't. Therefore the experiment has to be surrounded by lots of magnetic shields, and we need to measure the magnetic field inside it very precisely. Our group at Oxford was invited to join the experiment because we have considerable expertise in measuring magnetic fields using SQUID magnetometers (I'll explain what SQUIDs are later). For the last few months, we have been developing a system to monitor the magnetic field in the neutron experiment.

That morning, among all my other emails from students, lecturers, administrators and Viagra salespeople, there was a message telling me I had a delivery in stores. I went downstairs to see what it was, and found a new SQUID readout system had arrived from the manufacturers in America. I carried it up to the laboratory adjoining the office, eager to try it out.

       I unpacked an electronic control box and a preamplifier and plugged it all together. The SQUID chips themselves had already been supplied by another manufacturer. However as SQUIDs only operate at very low temperatures, before I could try it out, I had to cool it down. This is done using liquid helium.

       Helium has the lowest boiling point of any liquid, at 4.2K or -269°C. We keep it in large sealed vacuum flasks called dewars. The SQUID chips were mounted at the end of a dipstick, which I fastened to the top of the dewar with an o-ring seal, and very slowly, lowered it into the liquid.

       Liquid helium is very expensive and it evaporates very easily. Putting a sample at room temperature into liquid helium has the same effect as sticking a red-hot poker into a tub of water. The trick is to lower it very slowly so the evaporating gas cools the stick before you push it into the liquid. This way you don't loose so much helium

       At this point Mike arrived. Mike is a new first year graduate student working in our group. He's enthusiastic, quick to learn, and he's not afraid to ask stupid questions. But having only been working with us for a few weeks he had an awful lot to learn. I tried to explain what I was doing, but didn't get very far before he interrupted.

       "What's a SQUID?" he asked.

       "A Superconducting Quantum Interference Device", I answered.

       "What's that?"

       "It's an instrument for measuring magnetic fields," I explained, "the most sensitive there is. Do you remember Faraday's law?"

       "A changing magnetic field through a loop of wire... induces a voltage across the ends of the wire..." he replied correctly.

       "Exactly." I replied. "So what happens in a loop of superconducting wire?"

       He started to think about it, but I was impatient so I told him the answer, "a superconducting wire," I explained, "when cooled below its transition temperature, has zero resistance. Therefore the voltage is the same throughout the loop, so the magnetic field through the loop cannot change."

       He looked puzzled, "so if it can't change, what happens if you put a magnet through it?" he asked.

       "It induces a current in the superconductor, which circulates around the loop, which generates its own field, which cancels the applied field, so the total magnetic flux through the loop doesn't change." I explained with enthusiasm.

       He thought about this, before nodding, then repeating, "so what's a SQUID".

I realised this conversation could go on for some time. "In a SQUID," I explained quickly, "you have a superconducting loop, with two tiny breaks. If the breaks are small, superconducting electrons can tunnel across the gap. Apply a bias current across the loop, and due to the quantum physics of the set-up, the voltage across the loop varies with the magnetic flux through it. Add a preamplifier, feedback coil and control electronics, and you have the world's most sensitive magnetometer."

       There was a pause while he took this all in.

       "Just think of it as a black box," I said, "which measures the magnetic field strength. Could you hold this for a moment, I have to check something."

      I indicated the dipstick I was holding halfway into the helium dewar with my right hand. Mike took it from me, and before he could ask any more questions I walked off to check what Gianni was up to.

He wasn't in the office so I went back to the lab to look for him. Like most working laboratories it's a complete mess. We ran out of cupboard space some years ago and since then everything just gets piled on top of desks and the floor. In addition to the usual computers, workbenches, and liquid helium dewars we have two refrigerators, which we use to cool our detectors down to milli-kelvin temperatures. These are made up of a series of cylindrical thermal shields which stack inside one another. This is connected to a mass of pipes, pumps and tanks used to cool it down.

      This is all supported by an array of scaffolding. We access the top from a raised platform. As this was the highest point you could get to in the room, I guessed Gianni would be somewhere up there. Sure enough, I found him exploring a network of pipes right at the top. I looked around for anything that might be dangerous to cats, and after convincing myself he was safe I left him and went back to work.

With Mike's help I connected the cables between a computer, the control electronics, the preamplifier and the dipstick with the SQUIDs. I had written some software to let us adjust the bias current and other parameters and we could view the output signal on the screen. We turned it on and tuned all the controls. Everything seemed to work fine. But I wanted to check that it was really measuring the magnetic field. I looked around the lab for a magnet, but couldn't fine anything. So I went to the lab next door and borrowed an old transformer that contained a strong magnet. Then I then waved it up and down in front of the dewar - and we saw the signal on the computer screen move up and down accordingly.

      For a first test I was amazed how well that had gone. Usually nothing works the first time you try it out. However there was still a lot of work to do before the final system would be ready. We would have to improve the resolution, add some more magnetic shielding, see how it would all fit into the rest of the experiment, and develop some more software.

      Therefore I asked Mike to take some detailed measurements with the system - to determine the noise level and resolution. Then I went back to my office to work on some programming.

I continued programming for most of the day, with a short break for lunch. In the mid-afternoon I was working my way through a list of compilation errors when Mike tapped lightly on my door.

      "It's stopped working", he said.

      I followed him back to the lab and looked at the computer screen, sure enough something was wrong. Instead of a steady signal, the output was jumping all over the place.

      "Has it run out of helium?" I asked Mike.

      "No, it's still half full", he replied.

      "Are all the connections okay?"

      "Yes, I checked that"

      "And what happens if you turn it off and on again?"

      "It works for a minute or so then the problem reappears" he replied glibly.

      It seemed he had tried everything obvious. But just in case I turned it off myself, then restarted it and retuned the SQUIDs. Just as he had said it worked briefly then the signal went crazy again. I turned it off again, unplugged the cables on the top of the dipstick and check the connections with a multimeter. Everything seemed fine.

      I sat down and tried to imagine what could be wrong. Mike fidgeted nervously. Whatever was causing the problem seemed to be in the control electronics. Maybe we would have to send it back to America. But the manufacturers had assured me over the phone that they had thoroughly tested it. I touched the side of the electronics box. It felt quite hot. Maybe they hadn't tested it with a 230V power supply, and that was causing it to overheat. Unless...

       I reached on top of the box and picked up an invisible cat, which had been lying, fast asleep on the warm surface, blocking the air vents. I put Gianni over my shoulder, but he woke up straight away and jumped off with a soft mew.

      "Maybe we should install another fan" I said to Mike, "it seems rather hot. Leave it turned off until it cools down then try again."

At just before seven o'clock I walked in the main gate of my college, with Gianni balanced on my shoulder and made my way across the main quad to the senior common room. After the events of that afternoon I was a little apprehensive about leaving him on his own, but I decided it was probably less risky than taking him into the dining hall, where there would be a lot of people, and a lot of food. I left him on a cushion in a quiet corner, then walked over to the dining hall. I arrived just as they were sounding the bell for dinner.

      In fact, as it was still the summer vacation there were only a small number of fellows and guests eating in college that night, so dinner was served in a small room to the side of the main hall. There were two elderly philosophers, who had both retired some years ago, but kept coming into the college; a linguist and his two guests, who sat discussing University politics, and a young mathematician who had joined the college two months ago. She worked in some area of topology. Her name was Julia. She was originally from New Zealand, but had also worked in America before coming to Oxford.

      The food was rather good that evening. An avocado salad followed by trout with a cheesecake for desert. After a couple of glasses of wine I was chatting happily enough with everyone else, and I had all but forgotten Gianni.

      After dinner the linguists and philosophers wandered off their own way, and Julia and I went back to the common room. As soon as I opened the door, Gianni jumped down from a chair and rubbed himself against my legs. I bent down and touched the top of his head briefly.

      I got a cup of coffee from the urn by the door and sat down in an armchair opposite Julia. I poured some milk in my coffee, then I got up again to fetch another saucer and poured some milk for Gianni, which I left on the coffee table in front of me. If Julia asked me about this I would have to invent some story - that I was doing some experiment to see how quickly milk evaporated to test a theory I'd thought up over dinner, but she didn't say anything.

      I talked with Julia about research funding applications, while watching the ripples in the milk as Gianni lapped it up. He drank about half of it and then jumped off the table onto a longhaired rug. I watched the carpet hairs move as he landed. Then I noticed there were still ripples in the milk saucer. For a moment a thought he had jumped back on the table but then the milk was calm again and I assumed I had just imagined it. I blinked and looked back the rug.  Now he was lying down in the centre - I could see the outline of his body where the hairs were pressed down. He rolled from side to side at bit. Then I noticed the hairs on the other side of the rug were also depressed. For a moment I was completely confused, then it all became clear - there was a second invisible cat there.

      Once I had realised this it was obvious what was happening. The two cats were rolling over on the rug, prodding each other with their paws and play fighting. I leaned back in my chair relieved to have solved this enigma, and I caught Julia's eye. She was smiling. The other cat must be her companion.

       I had always assumed that mine was probably not the only invisible cat in the world. But it seemed a sufficiently improbably thing to meet one of them, that I assumed the chance I would meet two was essentially zero. But I guess they're more common than I thought. Maybe the city is full of them.