Posts Tagged ‘Cosmology’

Cosmic rays surprise us again

April 7, 2013
The Alpha Magnetic Spectrometer being tested at CERN by being exposed to a beam of positrons.

The Alpha Magnetic Spectrometer (AMS-02)being tested at CERN by being exposed to a beam of positrons. (Picture from Wikipedia)

[Text and figures updated on April 9th 2013 due to insight from Ryan Nichol: Thanks]

The team running the Alpha Magnetic Spectrometer (AMS-02) have produced their first set of results. And as expected, they are full of surprises.

AMS-02 is an awesomely complex device – too power-hungry, heavy and complex to be placed on its own space platform, it was attached to the International Space Station 18 months ago on the last space shuttle mission. I wrote about this here.

It has with 650 separate microprocessors, 1118 temperature sensors and 298 active thermostatically-controlled heaters. It is basically a general-purpose particle detector like those found at CERN, and represents the culmination of nearly one hundred years of ‘fishing for particles’ in the high atmosphere.

  • First we flew balloons and found that ‘radiation levels’ increased as we went higher.
  • Then we discovered a ‘zoo’ of particles not yet observed on Earth – positrons, muons, pions, and anti-protons.
  • Then we discovered that ‘cosmic rays’ were not ‘rays’ but particles. And we realised that at the Earth’s surface we only observed the debris of collisions of ‘cosmic ray’ particles with the atoms in the upper atmosphere.

Where did these primary cosmic ray particle from?  What physical process accelerated them? Why did they have the range of energies that we observed? What were they? Protons? Electrons? Positrons? We just didn’t know. The AMS-02 was sent up to answer these questions.

I have found much of the comment on the results incomprehensible (BBC Example) with the discussion being exclusively focussed on ‘dark matter’.  So I thought I would try to summarise the results as I see them based on reading the original paper.

Over the last 18 months (roughly 50 million seconds) AMS-02 has observed 25 billion ‘events’  (roughly 600 per second). However, the results they report concern only a tiny fraction of these events – around 6.8 million observations of positrons or electrons believed to be ‘primary’ – coming straight from outer space.

  • They found that – as is usual for cosmic rays – there were fewer and fewer particles with high energies (Figure 1 below)
  • Looking at just the electrons and positrons (i.e. ignoring the protons and other particles they observed) there were only about 10% the number of positrons compared with electrons, but that the exact fraction changed with energy (See Figure 2 below)
  • They found that there were no ‘special’ energies – the spectrum was smooth.
  • They observed that the particles came uniformly from all directions  – the distribution was uniform with variations of greater 4% very unlikely.
  • The electron and positron fluxes followed nearly the same ‘power law’ i.e. the number of particles observed with a given energy changes in nearly the same way – indicating that they probably have the same source.

They conclude very modestly that the detailed observation of this positron ‘spectrum’ demonstrates…

“…the existence of new physical phenomena, whether from a particle physics of astrophysical origin.”

I like this experiment because it represents a new way to observe the Universe – and our observations of the Universe have always surprised us. Observations have the power to puncture the vast bubbles of speculation and fantasy that constitute much of cosmology. I am sure that over the 20 year lifetime of this experiment, AMS-02 will surprise us again and again.

Figures

Figure 1: Graph of the number of positron events observed as a function of energy in billions of electron volts (GeV). Notice that there only roughly 100 events in teh highest energy category.

Figure 1: Graph of the number of positron events observed as a function of energy in billions of electron volts (GeV). Notice that there only roughly 100 events in teh highest energy category.

AMS Figure 6

Figure 2: Graph of the fraction of positrons compared with electrons as a function of energy in billions of electron volts (GeV). The ‘error’ bars show the uncertainty in the fraction due to the small number of events detected.

Faith in Science

September 4, 2010
The Orrery by Joseph Wright of Derby. 'Few of us know how to prove that the Earth orbits the Sun.' Photograph: The Gallery Collection/Corbis

The Orrery by Joseph Wright of Derby. 'Few of us know how to prove that the Earth orbits the Sun.' Photograph: The Gallery Collection/Corbis

Should we have faith in Science? A recent article in the Guardian by a friend of mine (Alom Shaha) suggests that ‘faith’ in science is really not appropriate.

Science has become the flavour of the month. Nowadays even comedians want to brag about their unfinished science PhDs. But is having people ‘feel’ that science is cool enough? Such a change in attitudes, though not unwelcome in itself, simply represents a shift in fashion. Alom quotes me in his article where he claims that Science is humanity’s greatest cultural achievement, and he and I both believe that as a culture we should be collectively and individually proud of Science. More importantly, Alom argues we should be collectively and individually more knowledgeable about it. In short, in addition to science ‘entertainment’ designed to make people ‘feel nice about science’, there should be an explicit media accent on science education I agree profoundly: and indeed that is the point of the Protons for Breakfast course. It’s not propaganda, it’s empowerment.

The latest instance science-flavoured stories in the media was the front page story in the Times last Thursday: “Hawking: God did not create Universe“. There is not time or space in this blog to address the issue of God: that is a big question But this newspaper story? I do have a little space and time for that: just enough to say that this is utter nonsense. Leaving aside any special knowledge Stephen Hawking might have on the subject of God, we come to the question of the origin of the Universe. Amazingly humanity has made progress on this question. Our progress has been driven through observational astronomy rather than cosmological speculation. The discovery and understanding of the microwave background radiation, for example, puts the idea of an explosion some 13 billion years ago on a sound footing. Estimates of the ratio of hydrogen to helium in the universe, tie up with our understanding of the nuclear process which must have taken place in the first moments of this explosion. We should be collectively proud of these achievements. But we should be collectively aware that many aspects of the explosion are not understood at all, most notably the supposed period of faster-than-light growth in which the Universe increased in size by a factor 10^78 i.e. a factor:

1000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000

for no reason that we understand. We can give names to the phenomena (inflation – which I think is rather understated given the colossal nature of the expansion!)  and explain that if this inflation did not occur then the universe would be observably different (the microwave background radiation would not be so uniform in different direction in the sky). We can even hypothesise about what might have caused it (an inflaton field). But in the end, we just don’t know. And we have to face the fact that we may never know. Or that there is some other aspect of this about which we are not aware.

The Hawking explanation for the origin of the Universe (and I use the word ‘explanation’ very cautiously) is that in fact there is not just one Universe which we don’t fully understand, there are apparently a stupendously large number of universes similar to ours, all unobservable from our own home universe. Forgive me, but someone who advocates that this is an ‘explanation’ of anything deserves sympathy rather than praise. The idea that this is on the front of a national newspaper should be a source of embarrassment for scientists.

Lev Landau, one of the great physicists of the twentieth century stated that ‘”Cosmologists are often in error but seldom in doubt.” I would go further. I think Cosmologists must be always in error, because I suspect there will always be things we don’t understand. But ultimately our understanding of the universe should not be a matter of faith, or fashion, and Stephen Hawking’s opinion on the matter is just irrelevant.


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