Sunday, 27 April 2014

There is no electromagnetic field - get over it!

A quick Sunday post on something I've written a little about before. But first, a quote
"Give me a child until he is seven, and I will give you the man"
This, of course, has a religious origin, but it applies in many human endeavours.  Including science.

I've taught at a university level for more than a decade, and I've noticed something slightly odd. Namely students coming into university seemed to have some particular fixed notions which you have to work hard at to shift.

There are a number of examples of this, including things like the conservation of energy, which was drilled into them at school but when you tell students that energy is generally not conserved in Einstein's theory of relativity, they initially stare in disbelief.

And when it comes to quantum mechanics, they seem to think that electrons are really little hard balls that sometimes behave as a wave, whereas photons are really waves that sometimes act as particles. Really they need to think of electrons and photons as the same kind of quantum wavy thingies.

The other is electric and magnetic fields, blue and red lines that snake through space. Uncovering the existence and influence of the electromagnetic field has taken centuries, and disparate lines of evidence were pulled together by James Clark Maxwell who gave us a famous set of equations (strangely known as Maxwell's equations). They look like this;

Now, I know a few people at maths-o-phobic, but all this says is "tell me where the charges are and how they are moving, and then you can calculate the electric and magnetic fields" (it's actually a little more complicated than that, but it's a good approximation).
Back to students, who learn about electric and magnetic fields at school. They may have seen Maxwell's equations. These red and blue lines really snake through space. They are there. They are real.

But they are not.

When quantum mechanics was developed through the twentieth century, how particles interacted through the electromagnetic force was one of the big questions. And it was the  great Richard Feynman who is the person most associated with working this out.

I strongly urge you to read his own words that led him to find the solution, but what he had to do is get rid of the electromagnetic field as we understand it from Maxwell's equations! Instead, charged particles interact through the exchange of individual photons, and we get the famous Feynman diagrams.
But these photons are only emitted and received when particles interact, and so if we have a lone electron sitting there, it doesn't fire out photons all over the place, hoping another electron comes along and it can interact. A lone electron is not surrounded by a Maxwell electric field. It's not there. No electric field. No magnetic field either.

The actual picture is (of course) a little more complex than this, but it takes students a bit of time to banish the picture that space is filled with red and blue field lines.

And, in truth, that's one of the greatest things that happens in science, when you have to throw out a picture that you thought was the "reality" of the universe, and realise that you were wrong. And on that note, here's a Sunday thought: think about what you currently think about the actual workings of the universe - there's a good chance your probably wrong.

Monday, 21 April 2014

The Greatest Experiment you've never heard of!

This Easter weekend is almost over, and has been quiet as the kids are off at camps. So, I'm going to write about something I think is very important, but many don't know about.

Let's start with a question - what was the greatest year of the last century in terms of scientific discovery?

Many will cite 1905, Einstein's miracle year, which I admit is a pretty good one. Then there is 1915, when Einstein sorted out general relativity. Again, a good year.

But I'm going to say 1956.  You might be scratching your head over this. 1956 was a good year - Elvis recorded Heartbreak Hotel and the Eurovision Song Contest was held for the first time - but in terms of science, what happened?

Well, there was a prediction in a paper and an experiment which changed the way we really understand the Universe. But what's this all about?

The key thing is concept called parity. Basically, all parity asks the question of what the Universe looks like when viewed in a mirror. Again, you might be scratching your head a little, but let's take a look at a simple example.

We know that the particles of light, the photon, carries a spin. Here's a picture from wikipedia
Now, it might seem that a mass-less photon, travelling at the speed of light, "spins", but it is one of those quantum-mechanical things.

The key thing here is that a photon can spin either clockwise or anticlockwise to the direction of motion of the photon. Normally these are called left-hand or right-hand photons. So, if I have a left-hand spinning photon (the |L> up there) and look at it in a mirror, then its spin would flip and it would look like a right-hand photon (|R>), which is something perfectly acceptable. The mirror representation of the photon could quite happily occur in the actual universe.

Photons are not the only particle that spins, electrons and neutrinos do, as do quarks, and as quarks can spin, so to do the composite particles they make up, like the proton and neutron. So, entire atoms can be spinning.

One of the important laws of the Universe is that spin (well, more correctly, angular momentum) is conserved. So when an atom emits or absorbs a photon, then the total amount of spin doesn't change.

Below is an example of what happens when hydrogen emits a 21cm radio photon. In this case, we care about the spin of the proton and the spin of the electron, which are aligned before the emission. But after the emission, the photon carries off some spin, and the electron's spin has flipped so the total angular momentum remains the same.
If we hold up a mirror and flip the spins of the proton and the electron before the emission, the emission can proceed as the electron can still flip and a photon is emitted, but now spinning in the opposite direction. This can happen in the real Universe as well as the mirror Universe.

Hopefully, by now, you are going "Well, duh!". Isn't this obvious. And, yes, it is. In fact, holding a mirror up to either of (or combinations of) the electromagneticstrong or gravitational interactions, this seems to be the case.

But something happened in 1956 that changed everything. Two researcher, Yang and Lee reviewed the evidence of whether these parity rules hold for the weak force, the force responsible for radioactive decay. And they concluded that the evidence didn't say that the mirror universe must resemble the one we live in if we look at weak interactions. 

OK, if you are lost. Read on, it will make sense. Yang and Lee proposed an experiment, an experiment undertaken by Wu. The experiment was to a heavy spinning, nucleus (Wu used cobalt) and cool it down in a magnetic field, which gets all of the cobalt nuclei spinning in the same direction. The cobalt nucleus undergoes a radioactive decay and spits out an electron. And what was noticed is that the nucleus spits out more electrons in one direction than the other (see left most picture below).

If we hold a mirror up to left-most picture you get the next one across. All that happens is that the spin of the cobalt nucleus reverses, but still more electrons get spat out of the bottom in both our universe and the mirror universe.

But do we see the mirror image actually occurring in our universe? The answer is no! The situation is actually as seen in the two right-most images - reverse the spin by flipping over the cobalt, and you flip the direction that most of the electrons come out of! The mirror does not occur in the Universe.

Why? Well, electrons can spin, and happily spin this way and that. But the problem is not the electron, but the other particle emitted during radioactive decay, the neutrino. Neutrinos carry spin, just like the photon, but unlike the photon, neutrinos can only spin one way! Neutrinos are always left-handed (anti-neutrinos are always right handed) and as angular momentum must be conserved, it dictates the direction the electrons are emitted.


(Image from the excellent HyperPhysics)

For our mirror image to occur within our actual universe, we would need right-handed neutrinos, and they don't exist!

This violation of parity was a big shock to the physical world as it shows that, in terms of the weak interaction, the universe is inherently asymmetrical. I think this is extremely cool.

Two final comments before I go an enjoy the sunshine.

Firstly, Yang and Lee got the 1957 Nobel Prize in Physics for their work, a year after they published their paper! The speed tells you how important and amazing the result was, although Wu did not get the Nobel for the experiment. As I've said before, I'm no historian, but one has to wonder if the fact she was a woman had anything to do with it!

And secondly, why does the neutrino have only one spin direction? Wouldn't the universe be neater if everything obeyed the rules of parity conservation? Why does the Universe behave like this? We don't know, but maybe if we asked, the Universe would simply respond by singing some Lady Gaga and point out that it was simply born this way.

Anyway. 1956 - what a year.

Sunday, 13 April 2014

Gravitational lensing in WDM cosmologies: The cross section for giant arcs

We've had a pretty cool paper accepted for publication in the Monthly Notices of the Royal Astronomical Society  which tackles a big question in astronomy, namely what is the temperature of dark matter. Huh, you might say "temperature", what do you mean by "temperature"? I will explain.

The paper is by Hareth Mahdi, a PhD student at the Sydney Institute for Astronomy. Hareth's expertise is in gravitational lensing, using the huge amounts of mass in galaxy clusters to magnify the view of the distant Universe. Gravitational lenses are amongst the most beautiful things in all of astronomy. For example:
Working out how strong the lensing effect is reveals the amount of mass in the cluster, showing that there is a lot of dark matter present.

Hareth's focus is not "real" clusters, but clusters in "synthetic" universes, universes we generate inside supercomputers. The synthetic universes look as nice as the real ones; here's one someone made earlier (than you Blue Peter).

 Of course, in a synthetic universe, we control everything, such as the laws of physics and the nature of dark matter.

Dark matter is typically treated as being cold, meaning that the particles that make up dark matter move at speeds much lower than the speed to light. But we can also consider hot dark matter, which travels at speeds close to the speed of light, or warm dark matter, which moves at speeds somewhere in between.

What's the effect of changing the temperature of dark matter? Here's an illustration
With cold at the top, warmer in the middle, and hottest at the bottom. And what you can see is that as we wind up the temperature, the small scale structure in the cluster gets washed out. Some think that warm dark matter might be the solution to missing satellite problem.

Hareth's had two samples of clusters, some from cold dark matter universes and some from warm, and he calculated the strength of gravitational lensing in both. The goal is to see if changing to warm dark matter can help fix another problem in astronomy, namely that the clusters we observe seem to be more efficient at producing lensed images than the ones we have in our simulated universes.

We can get some pictures of the lensing strengths of these clusters, which looks like this
This shows the mass distributions in cold dark matter universes, with a corresponding cluster in the warm dark matter universe. Because the simulations were set up with similar initial conditions, these are the same clusters seen in the two universe.

You can already see that there are some differences, but what about lensing efficiency? There are a few ways to characterise this, but one way is the cross-section to lensing. When we compare the two cosmologies, we get the following:

There is a rough one-to-one relationship, but notice that the warm dark matter clusters sit mainly above the black line. This means that the warm dark matter clusters are more efficient at lensing than their cold dark matter colleagues.

This is actually an unexpected result. Naively, we would expect warm dark matter to remove structure and make clusters puffy, and hence less efficient at lensing. So what is happening?

It took a bit of detective work, but we tracked it down. Yes, in warm dark matter clusters, the small scale structure is wiped out, but where does the mass go? It actually goes in to the larger mass halo, making them more efficient at lensing. Slightly bizarre, but it does mean that we have a way, if we can measure enough real clusters, it could give us a test of the temperature of dark matter!

But alas, even though the efficiency is stronger with warm dark matter, it is not strong enough to fix the lensing efficiency problem. As ever, there is more work to do, and I'll report it here.

Until then, well done Hareth!

Gravitational lensing in WDM cosmologies: The cross section for giant arcs

The nature of the dark sector of the Universe remains one of the outstanding problems in modern cosmology, with the search for new observational probes guiding the development of the next generation of observational facilities. Clues come from tension between the predictions from {\Lambda}CDM and observations of gravitationally lensed galaxies. Previous studies showed that galaxy clusters in the {\Lambda}CDM are not strong enough to reproduce the observed number of lensed arcs. This work aims to constrain the warm dark matter cosmologies by means of the lensing efficiency of galaxy clusters drawn from these alternative models. The lensing characteristics of two samples of simulated clusters in the warm dark matter ({\Lambda}WDM) and cold dark matter ({\Lambda}CDM) cosmologies have been studied. The results show that even though the CDM clusters are more centrally concentrated and contain more substructures, the WDM clusters have slightly higher lensing efficiency than their CDM counterparts. The key difference is that WDM clusters have more extended and more massive subhaloes than CDM analogues. These massive substructures significantly stretch the critical lines and caustics and hence they boost the lensing efficiency of the host halo. Despite the increase in the lensing efficiency due to the contribution of massive substructures in the WDM clusters, this is not enough to resolve the arc statistics problem.

Sunday, 6 April 2014

The Multiverse is not not the answer!

With the recent BICEP2 results, the question of the Multiverse has raised it's less than pretty head. The Multiverse is a topic which polarizes people, some who embrace the idea that our Universe is just one of countless trillions and trillions and trillions of other universes out there. Others get a little cross with the popularity of the topic. And then there are those (looking at you slashdot commenters) who generally talk nonsense.
Over at Starts with a Bang Ethan Siegal tells us that The Multiverse is not the answer. The essential crux of Ethan's argument is that, in calling on the Multiverse, cosmologists are giving up on science. What does he mean?

The Multiverse is not a single concept, there is more than one kind, but the basic idea is that, as well as our Universe, which we find quite cosy for life, there are a myriad of other universes. These universes are not identical to our own, but possess different physics, different masses of the fundamental particles, different strengths of the fundamental forces, different forces even. 

I won't go into detail here (as I am going to go into detail elsewhere - watch this space!) but most of these universes would be sterile, completely devoid of life. But aren't we lucky to find ourselves in a universe in which we can live? Of course, we have to find ourselves in such a universe, otherwise we would not be here to ask the question. This is the Anthropic Principle (a concept more wildly and crazily discussed than the Multiverse).
So, what's Ethan's problem? Well, if are going to think that we are just one of this myriad of universes, then there is no point asking any questions about why the electron has the mass it does, or why gravity is weaker than the weak force. It's just the roll of the cosmological dice that gave these values, and these values are just right for us to live comfortably. Calling on the Multiverse is akin to saying "God did it" and so it not science. We can pack-up our cosmological bag, and go home.

But I don't think Ethan is correct. As he points out in his article, we still have mysteries to solve. One of the most pressing is to get gravity and the other forces to play nicely together in extreme conditions of the early Universe. This could be the long sought after theory of quantum gravity which will allow us to see back into the creation of the Universe. Some think that the constants of nature will fall out of this ultimate theory, as we will know why an electron has the mass it does.

But there are other things we need to understand, such as the details of inflation, minor things like how it started, how it stopped, and, well, most other things about it. We could end up with several competing theories that describe our inflating universe, and producing multiple universes as it goes. The differing models may imprint themselves on our observations, and we can tell them apart, but we may never see the other universes out there as they are beyond our horizon.
Your multiverse generating model does, however, have a big hurdle to overcome, and that is that it must produce at least one universe, one out of the potential bazzion universes it can produce, that is the Universe, the one we find ourselves living in. If your model cannot account for our cosmic home, then off to the rather large and overflowing dustbin of science. 

So, I don't think anyone is giving up. I don't think cosmology is moving from science into pseudo or non-science. And while we may never be able to see the sibling universes all around us, our mathematical picture of the evolution of the multiverse is still testable, and is still science. Freaky science, but science all the same. 

There's a lot more to come on this topic, so watch this space.