Light speed

Michelson Interferometer

During the 1880's a series of experiments were performed that showed that light actually travels at a constant speed and it was this knowledge that later led Einstein to write his famous paper on Special Relativity. So how did we get from measuring the speed of light to Special Relativity?

This is a secong blog on Light, a previous post showed how Ole Rømer's had used the orbit of Io around Jupiter to work out the speed of light. While the results were out by a considerable way, they did show that the speed of light was finite and very quick. Improvements in its measurements showed that it was around 300,000 km per second! An incredible speed.


It was known that light could travel through a vacuum, but it was thought that the vacuum must contain some medium to propogate light. Much in the same way that sounds waves need air. As we know light travels really fast so coming up with experiments to try and detect the aether was going to take some smart thinking.

It was thougth that the earth must being moving through the aether during its orbit around the sun. So by measuring the speed of light in different directions at various different times, it should have been possible to measure the motion of the earth relative to the aether and this would have shown up in experiment as small differences in the speed of light. Now it was know that the speed of the earth around the sun is about 30 km/s, so the detectable effect would be this value divided by the speed of light,

30/300000 = 1/10000

about 1% of 1%. The idea is that if you are travelling in the same direction as the aether the apparent speed of light would be given by

apparent speed of light = speed of light - your speed

much in the same way that when you are travelling on the motor way, all the traffic on your side of the road seems to be almost stationary relative to you, where as on the other side of the road the traffic is belting passed.

White light interference pattern

Michelson came up with a piece of apparatus know called an interferometer (see the picture above) , sent a single source of white light through a half silvered mirror. The mirror split the beam at right angles sending half the light along path 1 and the second on path 2. The light is reflected back and meets up on the far side of the mirror and travels into an eye piece. It shows up in the eye piece as an interference pattern, similar to that shown here.

If the Earth where travelling through the aether then the small change in the speed of light would cause the pattern to drift to the left or right. The first experiments did not show the expected effect. Six years later in 1887 Michelson and Morley repeated the experiment with better equipment which would be more than capable of measuring the small shift that was expected. The result? Probably the most famous "failed" experiment in physics. They did not detect any difference in the speed of light that could not be attributed to experimental error. So what happened next? Well not a lot really, additional experiments were formed but they all came back the same. Light appeared to be a constant. It was almost 30 years later that one man realised that they had actually made one of the most important discoveries in modern physics. The discovery? The speed of light is a constant!

How can light be a constant speed? And why can't things move faster than it? This doesn't make sense. If you are travelling at 3/4 the speed of light and you see someone coming towards you at 3/4 speed of light then surely relative to each other you are travelling at 3/4+3/4 = 6/4 which is much faster than light? The answer is no, your relative speed must be less than the speed of light.  It is still thought that the speed of light is the universal maximum speed limit. Also, any object with mass cannot reach the speed of light. The only things that travel at the speed of light is light, and potentially, gravity waves (though I don't think these exist in the form we think).

(Speed of light being maximum speed in universe - at the time of writing some results have come out of CERN that cast doubt on this. If these results are true then they will change pretty much everything!)

So Einstein ponders how things look to someone moving and someone standing still and realises that the results should be the same. So he began to think about a wire  moving in a magnet field.  If you are sitting on the magnet when it is stationary there is the magnetic field and no electric field. In the conductor which is moving there is something called an electromotive force due to the magnetic field. This causes a current to flow in the wire.

A wire passing through a magnetic field

But if you are travelling on the magnet and the conductor is stationary there will be an electric field near the magnet due to the moving magnetic field.

Hold on a minute!

On the moving magnet we have an electric field, but when the magnet is stationary no electric field near the magnet? Yep.

Einstein realised that although they appeared different they must actually be the same.

Doesn't quiet make sense? Know what you mean!

Think about that situation described above for a minute. The two appear to be quiet different and yet Einstein realised that they must actually be the same and what is different is the way we are looking at things. 


This is not all however, he then goes one step further and says that light has a constant speed in a vacuum which does not matter on the motion of the emitting body!

You may be thinking, ok, he's had a couple of fairly wild ideas, and ....

From these two ideas he goes on to show how the unit of time is not fixed but is dependent on the velocity of the observers. How space contracts , how mass alters depending on how fast we are travelling, how space and time are not actually independent but are actually part of something different that is now called space-time, and on it goes...

Amazing! It would have had impressed even the great Sherlock Holmes.

With Einstein's paper came the realisation that time is not a seperate entity to space. This is a mind bending idea because there is nothing in every day life to suggest that time and space cane be related in any way and yet they are. It amazes me that anyone was able to realise this, it just seems some unlikely.

So what does this tells us about the nature of times arrow? To be honest I have some ideas, but they are for another day.

Think I may have to update my post on the positron though. After all, a positron may be no more than an electron travelling backwards in spacetime.

Speed of Light

Io passing in front of Jupiter

The current value of the speed of light is around 300,000 km per second. This is a staggeringly large number, which means that the speed of light is staggeringly fast.

Think about that for a minute, 300,000 km in a single second By the time you have finished thinking for one minute, light has travelled 18 million kilometers! Moon and back just under 50 times! Light would travel round the earth 7 times in 1 second. Intellectually we know how big the earth is, it's circumferance is around 25,000 miles, but this does not give us a real sense of how big it is. Even if you have run a marathon, about 26 miles, this is about a thousandth of the distance. Can you imagine what 1000 marathons would look like end to end.

Try to imagine a train journey, that covers say, 200 miles, 300 kilometers, on a high speed train this will take around 1 hour, but Light would make the same journey in 1 milli-second. The problem here though is that 1 millisecond is too fast for us to appreciate. There are few things that happen at a thousandth of a second that we are aware of. Anything faster than 20 ms will pass undetected by the human eye.

For me the above examples are the problem I have when trying to image the speed of light. Things happen too fast or involve distances so large as to be beyond my imagination. There is actually nothing in every day experience that can really give you any sense of what the speed of light really means. You can try, but I don't think it is possible.

It is so fast that for many years it was thought that light may be instantaneous. 

Galileo had tried a couple of experiments to try and determine the speed of light but had failed. The credit for the first measurement goes to Ole Rømer, a Dutch man, way back in 1676. Sadly, most of his work was lost during a fire in 1728. How did he do it? and what number did he come up with?

At the time Rømer was observing the heavens, trying to figure out longitude in navigation was still a big problem. One potential solution was to use the times of the eclipses of the moons of Jupiter! Seriously, how clever is that? They used the Jovian system as a giant clock. This was impractical on ships but could be used on land.

Jupiter has a moon Io, and in can be eclipsed by Jupiter, and it can travel across the front of Jupiter. This can be seen using a telescope. It takes 42.5 hours to do an orbit. Jupiter is large and can get in the way of observing the point were Io enters the eclipse and  leaves an eclipse. So it is only possible to do the observation for just over half of the year.

The earth itself is moving around the sun and what this means is that for some of the observations we are actually moving towards Jupiter and for some we are moving away from Jupiter. How Ole Rømer used this knowledge, is for me, a stroke of brilliance. Here is comes.

1) He works out the length of an orbit, 42 hours 28 minutes 31¼ seconds.
2) He then works out when it should be occuring in the future.
3) He measures again 7 weeks later to discover that his calculation is out by a total of 15 minutes
4) He realises that if the orbit is constant, then the difference must be because of the change in distance between the Earth and Jupiter over the 7 weeks.
5) It was common place in those days to work out the positions of Earth and Jupiter in their orbits and with a bit of (what is now) school boy trig he could work out the change in distance over this period.
6) He then argued that the difference between calculation (step 2 above) and the observation (step 3), 15 minutes was due to the time it took light to travel the extra distance between Jupiter and Earth.

It is step 6 that matters. By having confidence in his initial results he was able to propose that the difference is due to light taking time to travel the extra distance between the Earth and Jupiter. Suffice to say that this result was controversial and many thought that her was wrong. Rømer's boss Cassini, was one of these! It may have been this objection by Cassini that lead Rømer not to formally publishing his results. It actually took until 1727, more than 50 years later, that the result was formally accepted when it was backed up by other measurements. By which time Ole Rømer was well dead.

The value he came up with was around 200,000km/s, far short of the real value of around 300,000km/s. The the error is probably due to some of the assumptions he made. While this is a considerable error, it is still a remarkable result.  It did show two things, firstly that light was not instantaneous, secondly, it did move tremendously quick.

Beyond the fact that it was fast, and it was shown by Maxwell to be electro-magnetic radiation, it was not until Michelson-Morley experiment in 1887 that light showed its most peculiar property of all, and one that lead to Einstein's Special Theory of relativity. This is covered in the next post.

Nullius in verba

Short post today on one of the reasons why I started this blog. If you have been reading any of the previous posts you will realise that I tend to have a common theme running through each of them, it is this...

I have doubts about almost every current theory in Physics. Sometimes this is backed up by experiment that casts doubt on the original idea, sometimes because the idea just doesn't make sense. Other times it's just that I refuse to accept something just because everyone tells me it is so.

Take mathematics. I knew a physics lecturer once who had practically no time for mathematicians. Said they were a bit like alcohol, a good servant but a poor master. Mathematics has given us some great tools for investigating the universe, but ultimately, mathematics is just that, mathematics. This is fundementally different to physics and should be held in check when required.

Another thing I really dislike is actually the use of certain terms like "Standard", as in the "Standard Model". We know this theory has a number of .... let's call them "issues" and yet we give it a name that indicates that it is right and well accepted, which it isn't. This is also true of theories like the Big Bang, Black Holes, Dark Matter, now being described as "mature". I know what it implies, but what does that mean? Nothing of course. Take the Big Bang, ask the question, "Well, what was before the Big Bang" and you might actually get the following, totally ridiculous answer...

"It doesn't make sense to ask what was before, because time didn't exist before the Big Bang."

Translated,

"We haven't got a clue."

Another reason I have a problem with terms like "mature" is that they actually hinder scientific descovery. You think I am wrong? Aristotle came up with ideas that were thought right for almost 2000 years. They went practically unquestioned because he was a great thinker and his ideas had had time to mature, solidify, stood the test of time etc. They were wrong! It was only when people like Galileo came along and had the strength of mind to question did things move forward again.

We had the same problem with Newton's ideas, almost 200 years before Einstein put the cat in with the pigeons. Now we have the legacy of Einstein. His he correct? About some of it, yes, I think he is. About most of his work, nope, I don't think he is and people should not feel indimidated or be ignored when they stand up and provide ideas that contradict him.

Nor should people accept ideas just because it has a name that makes it sound like it must be true. Chances are it isn't. "Mature" should be reserved for use with cheese and wine, not Physics.

Take nobody's word for it.

Dark Energy For That Matter

The fantastic Vera Rubin

Dark Matter and Dark Energy, it's something you'd expect to find in a Stephen King novel, not in main stream physics. But here it is all the same.

As with many things I have a bit of a problem with dark matter, I'm not completely sure it exists, but before we get to that, let's take a little look at the state of play.

Questions: Who dreamed this one up? and why do we even need it?

The first answer is easy, it was first postulated by Fritz Zwicky, which also leads neatly onto the second question, which was also answered by Zwicky.

Zwicky was an astronomer studying the motion of galaxies in 1933. He measured the mass of the galaxies using something called the brightness method. Then he measured the mass of the same galaxy using the orbital method and found that the mass for 400 times larger than the original. This difference is now known as "the missing mass problem" and a problem it is.

Now you may be wondering if one or both of the techniques are at fault, it was a question that first crossed my mind, but there is evidence (that I will put in another post) that while complex they both have merit.

Zwicky said that there must be additional "dark" matter to explain the extra mass and no one took a blind bit of notice of his idea until the 1970s when other problems started to arise.The most famous being that discovered by Vera Rubin. Vera was measuring the predicted angular motion of galaxies and the observed motion of galaxies by galactic rotation curves.

Kepler's laws are beautiful

This is basically measuring the speeds of the stars rotating in a galaxy. The further from the centre the slower they go according to Kepler's laws. Which are amazing by the way (I will do a post on these elsewhere because the really are brilliant). Only this was not observed, the stars were all travelling at pretty much the same speed, irrespective of distance. So, how to explain it? re-enter Zwicky's idea of dark matter.

Now we have a small problem because we can't actually see this stuff and we have not been able to come up with a single experiment to find the missing 90%+ of the universe! So far all we can do is infer its existence from the behaviour of various galaxies and gravitational lensing.

Gravitational lensing happens when light from distant galaxies is bent by the large gravitational fields of black holes (not that I believe the exist, read my post on this when I finish it!) or dark matter.

But is this evidence enough to dream up this exotic matter? Personally I am not sure. I am tempted to look for other explainations. Such as? Well, MOND, Modified Newtonian Dynamics for one. The idea here is that Newton's law of gravitation has only been verified when gravitational acceleration is fairly large. We know that Newton's law breaks down when gravitation forces get really large, maybe ot also fails when gravitational accelerations are very small?

What I also find interesting is that some scientist use the missing mass problem to cast doubts on the Big Bang, which is currently the favourite for the start of everything. Can this be true? Could the Big Bang be wrong? Why not? I certainly have my doubts. But then I doubt just about everything. If dark matter does it exist then it will also raise problems for the Standard model of physics, which cannot currently explain dark matter.

The thing for me though is the idea that 90%+ of the universe may actually be made up of something that we cannot even detect. I wonder if the faster than light neutrinos have got anything to do with dark matter?

I turned on the TV one evening to here a guy describing something that was a total mystery, could not be seen or felt, and yet was believed by many to influence everything in the universe. This unseen force that would have a profound impact on mankind should it ever be shown to be true... I thought to myself, I don't remember reading that there was any programs on about God. It was actually describing dark matter. Or was it? 

Schrödinger's cat

Don't talk to me about cats!

This is a part of physics that I still don't get. 

If you take a look at physics sooner rather than later you will come across the Cat. This is not a real cat, it is a cat thought up by one of the greats of his day, Erwin Schrödinger.

Besides the cat he is also responsible for one of my favourite equations , now known as... Schrödinger's equation!

One of the concepts that comes out of quantum mechanics is something called Superposition. This is one of the real mind benders of physics, first time you see this you end up thinking... yeah right!

What it shows is that at the sub-atomic level nature gets well and truly weird. The princinple of superposition says that we just don't know what state an object is in. So it can actually be in all of them simultaneously, providing we don't look. Once we look, then the act of actually looking causes the object to be in a particular state. This is strange idea, so much so that many people have struggled with it, one being Schrödinger and his theoretical cat.

The cat is placed in a box and poison is also placed in there. We then drop in a radioactive substance. If a single atom decays then the poison is released and the cat dies. The problem is that we can not know the state of the cat, alive or dead, without opening the lid of the container. By opening the lid we are making a measurement which forces the cat into the state of being alive or ... dead. So until we open the lid, the cat is actually in this indeterminate alive/dead state.

Can the universe actually work this way? There is no single state unless we make an observation? But what does that mean? I still haven't got an idea. Einstein was not a fan. Yet there does appear to be some evidence for this strange idea. if you put two holes in a piece of card and fire photons at it one at a time it appears that the photon actually manages to interact with itself to cause a diffraction pattern to occur. In otherwords the photon of light appears to interact with itself. This is one I still struggle with. It just doesn't make sense and yet there it is.

I can only assume it is my ignorance on this one. Even if there is no one to look in the box the event will or will not have happened and the universe will move on. Events have been happening since the Big Bang (if that actually happened) and will do so long after we have gone on our way. Do these events happen differently because we are not here to see them? No, of course not. Or do they? May be we need some form of omnipotent observer who can see every event, or possibility of an event, anywhere in the universe and takes a quick look to help it determine its outcome?!

Trees fall in the forest and just because no one is around to hear them they still make a big noise. Or do they? And on that completely unsatisfactory note I will end this post. At some point I shall return. Problem is, unless you come back and take another look you won't know if it is the same post, or an updated post. Only by looking can you determine the outcome of this post.

Bending Light

This image has nothing to do with this blog!

Imagine you go to bed, but when you wake up, you are no longer in bed. You are floating inside of a square room. The room has no windows and no door, you cannot see outside the room.

Getting over the initial shock of finding yourself in such a situation, you try to figure out what is going on. You see an envelope stuck to the wall and you can just about reach out and open it. The envelope has some questions:

Are you floating in space? or are you in an elevator in free fall? How would you know the difference?

You start to ponder this and realise that this is a tricky one. If you are in free fall, just like an astronaut in orbit you will not feel gravity. But if you were in deep space far from any object you could be stationary and not feel any gravity. So you wonder, am I moving or am I stationary.

Then a thought enters your head.

"Hold on one minute. If I had a laser that could fire a beam from one side of the room to the other then I would know if I was in free fall."

Your thinking goes like this.

"if I am in space and not moving and I fire a laser from one side of the room to the other then it will follow a straight line, but, if I am in free fall then by the time the laser reaches the other side of the room the lift will be accelerating due to gravity and will have dropped ever so slightly, so it will actually seem as if the light as moved up the wall."

So you look around and bingo, you spot a laser, a spirit level and an extra sensitive detector capable of detecting movements in the Angstrom range (1 Angstrom is 0.1 nm which is 1/10 billionth of a meter - very small) next to the envelop.

You get the laser horizonal and fix the detector to the wall opposite. You fire the laser and discover that the beam travels straight and true. So you repeat the experiment on each of the other two pair of walls (if you are in a room there are 6 walls - floor and ceiling count as two). Each time the beam goes straight across.

So you smile to yourself safe in the knowledge that you must be floating in space either stationary, or at a constant velocity. Clever eh?

Just then you find yourself falling to the floor as gravity returns. Ah, you think, I must be accelerating I am on the move. But then one wall opens and you find that you are at the bottom of the elevator and you are stationary.

This can't be right you think. If the light travelled to a point directly opposite then it must actually have been bent down slightly to compensate for the downward drop of the elevator. It is at that point you realise that you were actually accelerating all the time! You also realise that you can't tell the difference between floating out in space and being in free fall in a gravitational field, without looking out the window!

That doesn't make sense you think light doesn't bend it goes in a straight line! ... and then you remember reading that light is actually bent in a gravitational field!

This is known as the Equivalence principle and was used by Einstein to help develop his theory of General Relativity. Cool eh?

Theories and Higgs

Can you see it?

Does the Higgs boson exist? is it really as important as people make out? or is it just hype? Is it really God's particle? If it is God's particle then does its existence prove the existence of God?

Physics is all about trying to explain how the universe works. We come up with an idea, try to find an experiment to see if the idea stands up to testing and if it does it gives us some clue to how the universe might work. Sometimes its the other way round. We find a result that we weren't expecting and then we try and come up with an idea to fit.

This is what is called scientific process and, by its nature, means that we are going to be wrong most of the time. Why? because we don't have all the evidence, and just like any detective story, a single piece of evidence can turn the entire story/theory on its head.

Some of the greatest discoveries about gravity were made by Newton back in the 1600's. He managed to explain how gravity behaves. Was he correct? No. But his observations and ideas were so good that it took ages to realise that they were wrong and that they are very good approximations. Does that make them invalid and useless? Absolutely not. They are an amazingly accurate description of gravity and only fail at the extremes of gravity. Newtonian mechanics were more than enough to send man to the moon.

Could Newton have done better? I don't think he could. He was already pushing the boundaries of what we knew and his insights were, and still are, amazing.

The early models of the atom were also completely off the mark. At the turn of the last century there were still some scientists arguing that atoms didn't actually exist. They were just an abstract mathematical model. It was Einstein who realised that the evidence of atoms had actually been discovered almost 80 years earlier. All of the scientist at that time had missed the clue except him.


Our understanding of magnetism, electricity and light was initially nonsense. For many it is with hindsight that we see the truth. Then we look back and are amazed that they could have taken so long to realise this or that, it seems so obvious now! Yet, at the time, given the evidence, many theories are justified. They are also completely wrong!

Making best guesses using insufficient evidence is bound to the guess being wrong. Does this mean we shouldn't bother? Of course not. But it does mean that we should always bare in mind that theories are just that. Chances are they are built on insufficient data and probably miss that vital bit of information that will give some bright spark that Eureka moment.

Some theories prove to be better than others and some have greater longevity. Relativity is still going strong. Quantum electro dynamics, the theory of the interaction of light and matter as been shown, by experiment, to be fantastically accurate. The standard model gives us the theory of the universe so far and this is were Higgs comes in. There are others, many, but are they right? Are any of them right? Personally I expect not, but they will do until we find something better. Owt is better than nowt as they say in the stranger parts of England.

The discovery of sub atomic particles opened up a new branch of physics which promises to give us a deeper insight into the workings of the universe than ever before. Particle accelerators have got bigger and bigger giving us a mass of "new particles" to wonder about.

Out of this work theories were developed to try and explain all this, one of them being the Standard model.  This was not the work of a single man, although there have been significant contributions by individuals, it is more of a team effort, much as the work to discover the particles is a massive team effort at places like CERN and the Tevatron accelerator out in the US (Before it closed down in Sept 2011).

The Standard Model has made a number of predictions about what should exist. This is not new to science. In fact many great theories not only explain what is being seen but also what may be seen. Dirac proposed that there must be antiparticles way before they were discovered. Mendeleev, of the periodic table, was able to predict the existence of elements before they were found. Others used Newton's work on the motion of planets to predict the existence of Neptune before it was discovered. These are great moments in science.

So the Standard Model predicts the existance of this fairly large particle (thought to have the mass of about 130 protons) that has yet to be found. If it is found then it will add new strength to the validity of the theory. If it is not then it will raise doubts. Even if it is not found the theory still as value. Just in the same way that Newtons. Everything that it has predicited that was found to be true is still true. The moon didn't suddenly start misbehaving as soon as Einstein point out that Newton may be wrong.

Any new theory that comes along to replace the Standard Model needs to explain all the things the standard model got right and then give new answers to the areas were the Standard Model went wrong.

So Higgs particle, what about that then? Is it the God particle? Nah, that was just a name dreamed up by marketing folk to help flog books, nothing more. People, not just physicists, love to find things out, to try and discover truth and God is a big subject. Stick the word God on the front of a science book and it's sales improve dramatically.

Find or no find, it will give us a better understanding of the universe. But will it prove or disprove God? Of course not. There is NO scientist of science book that can prove or disprove the existance of God.

So, after a year or more of particle smashing, why haven't we found it yet?

One of the challanges with the Higgs particle is that it even if it does exist. It does not do so for very long. It's life time is so short as to be impossible for the human mind to comprehend and also to short to measure directly, and probably always will be. So what we have to do, is not look for the Higgs directly, but evidence of its having been. So we are looking through the aftermath trying to figure out the nature and cause of the devistation and hoping that what we find will lead us to the conclusion that only 1 type of bomb could have caused what we see and that bomb was a Higgs boson.

It is more complex than this though, because it is not a single explosion. We are not looking at the result of two single protons colliding. The results are of billions of collisions, billions of little bombs going off and it is a question of trying to sort through all of these to find the elusive Higgs.

Are they going to find it over at CERN. The real answer is that we don't know. Does it matter if they do, or they don't? Well, if they do then it strengthens the Standard model, if they don't, it definitely puts a dent in the model. In a way, from the Physics point of view it does not really matter. If it isn't found then new theories will evolve. If it is found then Higgs must be up for a Nobel prize.

My real concern is that the Standard Model does not explain things like dark energy (but this could easily be because dark energy may not exist!). It does not give a full theory of gravitation. It cannot explain dark matter, nor something called a neutrino oscillations. May be our interpretation of these things is wrong and it is the Standard Model that is correct. Who's right? who's wrong? time will tell of course, which is a strange saying, given what we know about time!

Lets face it, at the time I am writing this we are still not sure if a neutrino can travel faster than the speed of light which would totally blow the lid on just about everything! How exciting would that be?

What do I think about Higgs boson? In a way I would like there to be a particle so that Prof Higgs gets the Nobel prize. Whether he wanted it or not, the search for the Higgs boson has now, publically, defined his career. If it is not found then it would appear that his "lifes" work was a failure. Which is not the case because he wrote the paper over 50 years ago, but such is history and public perception.

From a physics point of view, I don't think it exists and I have a small concern that a "find" will be misinterpretation of the data rather than a geniune find. This may result in many wasted years of theoretical development because  people do not do research in areas now considered closed or unimportant, much in the same way that General Relativity was ignored.

My predictions for the year 2012 at CERN... no Higgs and faster than light Neutrinos.

UPDATE: looks like there may be a Higgs and neutrinos don't travel faster than light! Prof Higgs got the Nobel Prize, well done that man.

Positron

A genius

So this evening I find myself reading a physics paper written back in 1928 by Paul Dirac. The reason I am reading this paper is because I've been watching the 1981 Horizon program about Richard Feynman.

Initially I'd thought about doing a blog on Feynman, but there has been so much written about him that you can find it elsewhere. I'm sure he'd say talking about physics is more important.

Feynman talked about was his work on Quantum Electro Dynamics (for which he was awarded the Nobel Prize) that came out of him doing work on the Dirac Equation.

Now I vaguely remember the Dirac equation from days gone by and while I didn't remember exactly what it was I did remember that it was a biggie. Dirac had written a paper on the relativistic wave equation of the electron. Which is the paper that I have just read prior to writing this.

Two things have come out of me reading this paper. The first is that I am now aware just how rusty I have become at Physics! The second is on page 612 of the paper. This is one of those great moments in Physics. Dirac doesn't say that a new particle exists as such but he does point out that electrons can have either positive or negative charge. This is the start of a thought process that leads to him predicting the existence of an anti electron in a paper he published three years later in 1931. This later became known as the positron.

Now here is one of those many instances where we see just how difficult it can be matching theory and experiment. In 1929 Dmitri Skobeltsyn actually saw a positron using a Wilson cloud chamber (the particle detector of its day) as did Chung-Yao Chao, but his results were inconclusive and he did not follow it up. Had they been fully aware of Dirac's work, they may have realised what they were seeing, but no. So the prize for discovery went to Carl Anderson who discovered the positron on August 2 1932. He was also the person who came up with the name positron.

First image of a positron

How do we know they saw a positron and not something else? Take a look at the image on the right. This is the picture Anderson took of the positron. Can you see it? It's a bit like one of those scans you see at the hospital where you can't figure out what your looking at. Ok, here we go...

The thick line running from left to right is a piece of lead that seperates the top of the cloud chamber from the bottom. In the middle, looking a bit like a thin curved scratch is a line starting at the bottom (about 7 o clock) and curving off to the left at the top (about 11 o clock). Can you see it now? That is the IT! That is the track left by the positron.

The thing that looks a little bit like a hair on a lense is the first recognised proof of anti-matter. It won Carl Anderson the 1936 Nobel prize for physics.

How do we know we are moving from the bottom to the top and not the other way? the positron curves more at the top than the bottom. Which means it has lost energy by the time it reaches the top.

How do we know it as a positive charge and not a negative charge like the electron? The cloud chamber is  placed in a magnetic field. These fields cause positively charge particles like positrons and protons to curve in the opposite direction to negatively charged particles like electrons. In this case positively charged particles bend to the left, negatively charged to the right. So from the curve we know that is positively charged.

Taking a look at the amount of curve and the speed of the particle in the picture tells us that we have a particle that has the same mass to charge ratio as an electron.

This combination of evidence, the positive charge and the charge to mass ratio lead to the conclusion that the positron had been sighted.

This is the end of the post. I started with Feynman and ended with the positron, which is probably the way it should be. Especially as it was Feynman who later suggested that a positron may just be an electron travelling backwards in time! Blimey Richard, you know how to make my head spin.

What follows is further information on how the charge to mass ratio was found.

Charge to Mass Ratio

How do we know that it is a positron and not some other charged particle like a proton? Just over 30 years earlier. In 1897 J J Thompson had calculated the charge to mass ratio of the electron. Using Netwon's second law of motion, F=ma, and something called the Lorentz force law it is possible to get the charge to mass ratio, but how? The Lorentz force is giveb by the following equation

F = q[ E + (v x B) ]

now this may look a bit scarey and when we combine it with  F = ma, it can look even worse because we end up with

m a = q [ E + ( v x B) ] ,

where m is the mass of a particle and a is its acceleration. Divide both sides by q gives us

 (m / q) a = E + (v x B)

now here is the cool part. If we only apply a magnetic field, E =0 and B we know because we apply it our selves and have measured it. Then the equation becomes

( m / q) a = v x B .

( m / q) is the mass to charge ratio.  We get v and a from watching just how fast the particle is moving. So now we have the mass to charge ratio. When we compare this value with that J J Thompson got for the electron we find it is the same!