Αρχική > επιστήμη > How did I get here?

How did I get here?

Big Bang cosmology and the evolution of the universe

Posted onSaturday 8 February 2014 theguardian.com


Is the universe around us an inevitable consequence of some master equation, some theory of everything? Or was there an element of chance in how it formed, and if so how big were those chances? Most people would like a theory in which we were not ridiculously unlikely.

    Big Bang cosmology and the evolution of the universe

The development of the universe from the big bang, through inflation, electroweak symmetry breaking, the quark gluon plasma, first atoms and the cosmic microwave background, first stars, and the formation of complex structures such as life Photograph: CERN/Interactions.org image bank

This week I took my dad to Geneva, to show him CERN, and ATLAS, the experiment I work on at the Large Hadron Collider. Describing my job to people reminds me of how privileged and fortunate I am to be working on a project which would have been beyond my wildest dreams as a child, and the feeling is even stronger when describing it to my dad.

Spending a couple of days in each others company, something we rarely do, we also discussed other things, including family history. The random chances involved in how he met my mum. The fact that we recently discovered that he was severely ill when only a week or two old and may well have been very lucky to survive. The choices that ended up with me having the opportunity to go to university. Not to mention the fact that I came rather close to screwing it all up at various points. Even without being a massive egotist, it is tempting to see all that history as a tenuous chain of unlikely events which somehow led up to the picture at the end of this article. What are the chances, eh?

Well, from one point of view the chances are one. A certainty. Because if anything significant along the way had happened differently, we wouldn’t have been down there in the ATLAS cavern at Point 1 on the LHC asking the question.

This point of view can be carried over into trying to understand how the universe got to be the way it is. The image at the top of the article is an illustration of our best evidence-based picture of what happened over the past 13.8 billion years. As the universe expands and cools, there are several key stages. For example, electroweak symmetry-breaking, which we are studying at the LHC, occurred about 10⁻³⁴ seconds after the big bang (where you can see all the W and Z bosons in the picture). This is governed by various parameters in the Standard Model of particle physics, and if they hadn’t been ‘just so’ the universe would look very different and probably wouldn’t contain physicists and cosmologists. If the proton were heavier than the neutron, we wouldn’t be here.

There is a famous nuclear energy level in Carbon, the existence of which was inferred by Fred Hoyle from the observation that without it, heavy elements could never be formed in supernovae. Since heavy elements definitely exist, the probability of that energy level being there had to be 1, and experiments later verified this prediction. But taken from another point of view, the existence of that energy level is extremely unlikely. Imagine writing down a master equation, say the equation of the Standard Model:

Standard Model Lagrangian

The Standard Model Lagrangian

Even if this equation is self-consistent and mathematically sound, if I start from here, what are the chances that the particular set of parameters and symmetries it contains will lead to that Carbon energy level? Or to protons even? Or to DNA? I don’t know the answer but it seems likely they are small.

Theoretical particle physicists and cosmologists spend quite a bit of time worrying about such things. Is the universe around us an inevitable consequence of some master equation, some theory of everything (which would have to be something much more general than the equation above – the equation above would need to be one of the consequences of this bigger theory, in fact)? Or is there an element of chance, and if so how big were those chances? Most people would like a theory in which we were not ridiculously unlikely.

There is obviously some element of chance. No one much spends time trying to show that general relativity (our best theory of gravity) inevitably predicts that there will eight planets in the solar system. This is generally accepted as being a pretty random outcome of the way those underlying laws play out. Other solar systems can and do look different from ours. But before we knew that our star was just one among many, asking whether the number of planets was special, and if so how, was far from being a silly question.

Some physicists would claim we are at a similar juncture regarding the observable universe. Perhaps it is one amongst a multitude – a multiverse – of different sub-universes, in each of which the physical laws vary, and play out differently, and in some of which theoretical physicists eventually evolve and start asking questions. Perhaps asking for the universe to look “likely” or “natural” in some sense is as pointless as trying to predict the exact number of planets in the solar system using relativity. Perhaps there are other sub-universes where the strong force is a bit different, there is no Carbon energy level where it is needed, and therefore no heavy elements. Or the Higgs mass is different and so atoms and nuclei never developed at all. Or the cosmological constant was different and the sub-universe only lasted five minutes. And most of these universes couldn’t support theoretical physicists. So the probability that we are observing one of those is zero, and the probability that we are in one which can support life is one.

This is the anthropic principle, or one version of it. There are lots of variations on this theme, and I myself have tended to dismiss such thoughts as at best fruitless speculation and at worst a counsel of despair, a recipe for giving up on trying to understand things. The key word in the paragraph above is “observable”. We can see other solar systems now. But if we can’t observe these other sub-universes, what is the point of postulating them? A discussion at Frank Close’s Faraday lecture at the Royal Society moderated my opinion on this a little though. Frank was talking about symmetries and the Higgs, and the anthropic principle came up, as it often does in this context. Frank batted the question to the cosmologist John Barrow, who was in the audience and is one the people who has developed these ideas over the last few decades. Part of his answer was a very simple statement that for some reason I had never heard or formulated myself quite so clearly. The import to me of what he said was to remind us that there is no guarantee that the universe will be kind enough to arrange matters such that we have an unbiased theoretical and observational standpoint, and that the most scientific approach is not to ignore the possibility of anthropic bias, but to bear it in mind and as far as possible account for it. Or in his words

It is just a methodological principle. If you don’t appreciate there are selection effects that might affect your experiment, you will draw wrong conclusions from it.

To me, this removes the idea that buying a version of the anthropic principle means giving up on inquiry, and it significantly increases the chances that such speculation may in the end be fruitful.

To some people reading this, I guess I have just stated the blindingly obvious. To others not. So here is an analogy of how I am thinking about this right now.

It is two in the morning and I am with a group of three friends in a Casino. We all came in with 100 Swiss Francs each (the Casino is in Montreux, because that’s the only one I’ve ever been in). We have been playing all evening, since we entered the Casino at 10pm, and we all have money left. In fact some of us have made a significant profit.

A brief examination of this situation would lead you to believe that the Casino is likely to go bust pretty soon. Everyone is winning! Imagine a croupier (or whatever they are really called outside of James Bond) reporting the situation to the manager. This is a crisis!

The manager asks what system these crazy physicists are playing. The croupier shrugs.

“No real system. They just keep playing, it looks random. They put their money in, if they win they keep playing. If they lose all their money, they leave.”

“Ah ha!” says the manager, smiling now. “And how many of them came in at 10pm?”

The answer is that it was an ATLAS collaboration meeting, and three thousand physicists came in at the beginning. The four of us remaining at 2am are the only ones who kept winning. The Casino is ok.

From our point of view, there is no benefit in asking what was the underlying theory that led us to be the winners. It is chance. But equally, if we want to use our observations of the evening to work out the principles behind the operation of the Casino, we should not give up. What we do have to do though, is take into account the bias we have from our point of view, since only people who kept winning have managed to observe the Casino in operation for the whole evening.

Similarly, acknowledging that we may be in a self-selected corner of a multiverse is no reason to give up observing what we can and striving to understand it. In fact, acknowledging potential bias gives the best hope of eventually avoiding it. Getting the relevant data, making the observations, so we can attempt to evaluate such biases, remains a major challenge of course.

Keith and Jon Butterworth visit ATLAS at CERN

Lucky. Photograph: Abha Eli Phoboo



  1. Δεν υπάρχουν σχόλια.
  1. No trackbacks yet.


Εισάγετε τα παρακάτω στοιχεία ή επιλέξτε ένα εικονίδιο για να συνδεθείτε:

Λογότυπο WordPress.com

Σχολιάζετε χρησιμοποιώντας τον λογαριασμό WordPress.com. Αποσύνδεση /  Αλλαγή )

Φωτογραφία Twitter

Σχολιάζετε χρησιμοποιώντας τον λογαριασμό Twitter. Αποσύνδεση /  Αλλαγή )

Φωτογραφία Facebook

Σχολιάζετε χρησιμοποιώντας τον λογαριασμό Facebook. Αποσύνδεση /  Αλλαγή )

Σύνδεση με %s

Αρέσει σε %d bloggers: