Relativity on the human scale


Updating Einstein’s Universe and Magic Universe

Relativity on the human scale

The most gratifying physics I’ve seen for a while comes in today’s Science magazine, from James Chin-Wen Chou and his colleagues in the Time and Frequency Division at the National Institute of Standards and Technology in Boulder, Colorado. They detect well-known effects of relativity on the rate of time passing, but now on the scale of ordinary human activities.

Standard atomic clocks employ microwaves to ensure their regularity, but Chou’s team used laser light in a pair of aluminium-27 optical clocks (invented in 2005), which gives about 100 times better accuracy. In one experiment, they used an electric field to jiggle the aluminium ion at the heart of a clock and showed that time passed more slowly in accordance Einstein’s Special Relativity theory, about the effect of motion on time. The effect of atomic motion as slow as 8 metres per second (about 30 km/h) was detectable.

Raising a clock makes it run a little faster. Credit: Chou et al., Science, 24 September 2010 – see reference.

Especially pleasing for me was another experiment, in which one clock was jacked up just 33 cm relative to the other. The clock gaining height ran faster because it was further from the Earth’s centre of gravity, and the gravitational field was slightly weaker, in accordance with General Relativity. As the change in clock rate was only about 40 parts in a billion billion (1018), its detection was a tour de force for the NIST team.

This effect of altitude on time was the key to the efforts by Martin Freeth of BBC-TV and me to make Einstein’s theory of gravity, General Relativity, comprehensible to the public, in our film “Einstein’s Universe” (1979).

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Star positions matter


Updating The Chilling Stars

Why star positions matter for climate physics

The Making of History’s Greatest Star Map is an excellent account of the European Space Agency’s Hipparcos mission by the project scientist, Michael Perryman. It brings back vivid recollections:

  • of dismay after the launch in 1989, when the satellite failed to go into the right orbit and frantic steps were needed to improvise a survivable orbit and re-configure the observing programme.
  • of satisfaction when operations continued despite unplanned exposure to the Earth’s radiation belts, as well as some nasty solar flares, until the radiation damage became fatal in 1993.
  • of the appetizer in 1994, when early results of the Hipparcos star mapping helped in accurate prediction of the impacts of the fragmented Comet Shoemaker-Levy 9 on the planet Jupiter.
  • of joy on Isola di San Giorgo, Venice, in 1997 when the Hipparcos science team announced their first large-scale results, after a huge computational effort.

Hipparcos in an ESA impression

Astrometry took that great leap forward 30 years after Pierre Lacroute of the Strasbourg Observatory first proposed a space mission to measure the positions of stars, 20 years after Erik Høg of the Copenhagen Observatory refined the concept, and 17 years after ESA earmarked it as something to do. Ground-based astrometry had stalled, because of imprecisions due the turbulence of the atmosphere, and its remaining aficionados had little lobbying power. As a result, Hipparcos remained a distinctly European space project – the first in which there was no competition with the US or Soviet space science programmes.

Applications of the Hipparcos Catalogue of 100,000 plus stars and the Tycho 2 Catalogue with 2.5 million stars (to a lesser but still unprecedented accuracy) have ranged from detecting a bend in the Milky Way Galaxy to checking Einstein’s theory of gravity, General Relativity. But wanting to pursue here the relevance of Hipparcos to climate physics, I’m pleased to see that Michael Perryman points the way.

Michael Perryman. Photo by Richard Perryman

In The Making of History’s Greatest Star Map, pp. 236-243, Perryman notes the role of Hipparcos in refining observations the wobbles of the Earth’s axis, which are involved in the pacing of ice ages (the Milankovitch theory). Then he points to the link between solar activity and climate change, as evidenced by the Little Ice Age, the Medieval Warm Period and other variations. As to the mechanism for the solar connection, Perryman singles out the suggestion that cosmic rays, modulated by solar activity, influence cloud cover.

He continues the story with the Sun’s journey through the Galaxy and the icy intervals on Earth that correspond to exposure to intense cosmic rays when passing through spiral arms. That’s a major topic in The Chilling Stars and, as Perryman says, the Hipparcos data have improved our knowledge of motions in the Galaxy.

It’s reassuring when a professor of astronomy with no scientific or political axe to grind gives serious attention to the cosmic-ray/climate link (the Svensmark hypothesis). Let me reciprocate by reviewing what’s said about the climate-related significance of Hipparcos and its successor Gaia in The Chilling Stars and see if it needs updating or extending.

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What did Hawking discover?


Updating Einstein’s Universe and Magic Universe

What did Stephen Hawking discover?

Stephen Hawking. Photo: Channel 4

We’re so used to it, we’re not surprised to see an elderly gentleman immobile in a wheelchair, his lips hardly moving, performing for most of this week as the host presenter of Channel 4 TV’s five-part history-of-science series “Genius of Britain”. Like the footballer David Beckham or the actress Joanna Lumley, the theoretical physicist Stephen Hawking is now a national treasure. And with his voice synthesiser he can seem like a visitor from outer space, setting us earthlings right about this and that.

But I’m irritated by the implication in the last programme, which I’ve just watched, that Hawking himself was a primary advocate of the Big Bang, in opposition to the great Fred Hoyle’s Steady State Theory. No mention of the l’atome primitif of the Belgian cosmologist Georges Lemaître in 1927, but he was a Catholic priest and perhaps inadmissible to Hawking’s co-presenter Richard Dawkins. No mention either of Martin Ryle, the Cambridge radio astronomer who first showed observational evidence that confounded Hoyle’s expectations.

Three decades ago, the BBC’s Alec Nisbett and I were the first to put Hawking on international television, in a two-hour blockbuster on particle physics, “The Key to the Universe”. At that time, his dreadful disease of nerve and muscle left his mumbles intelligible only those most familiar with him. We added a voice over, as if he were speaking a foreign language.

For Nisbett and me, Hawking was not only an up-and-coming physicist but an image of the frailty of Homo sapiens confronted by a confusing and often violent cosmos. After describing his then-recent suggestion that small black holes could explode, producing new particles, we incited Hawking to use stirring words to climax our show.

The Big Bang is like a black hole but on a much larger scale. By finding out how a black hole creates matter we may understand how the Big Bang created all the matter in the Universe. The singularity in the Big Bang seems to be a frontier beyond which we cannot go. Yet we can’t help asking what lies beyond the Big Bang. Why does the Universe exist at all?

My son Robert is always asking questions. Why this? Why that? Every child does. It is what raises us from being cavemen.

On one view, we are just weak, feeble creatures at the mercy of the forces of Nature. When we discover the laws that govern those forces we rise above them and become masters of the Universe.

Hawking then rose to stardom, taking up Newton’s chair as Lucasian Professor of Mathematics in Cambridge in 1979, and publishing the phenomenal best-seller A Brief History of Time in 1988. He has appeared frequently in “The Simpsons” as well as in scientific documentaries and docu-dramas. He made a sub-orbital spaceflight in 2007 at the age of 65 and during the weightless period he could move freely in his chair for the first time in four decades.

His courage, doggedness and success in the face of a crippling disease has been inspirational to paraplegics the world over. Concern for his predicament has also encouraged many people who might otherwise not have been interested in science to pay some attention to it.

In those respects I’m second to none in my admiration of Hawking. But we’re entitled to ask, as about any other scientist, what his enduring contribution to progress in research has been.

To try to get the tone right, I’ll begin by quoting an eminent but more reclusive theorist, Peter Higgs of Edinburgh, who saw the need for a particle that would give mass to matter, and how it might work. The race to find the Higgs particle, a.k.a the God particle, is now on, using the most powerful accelerators of our time.

But Higgs found himself in hot water during the Edinburgh Festival of 2002. He attended a dinner party celebrating a play based on the work of Paul Dirac, who predicted the existence of antimatter. It was not surprising that the conversation turned to why the public, who knew about Hawking, had never heard of the more important Dirac.

Thinking the dinner was private, Higgs commented about Hawking: “It is very difficult to engage him in discussion, and so he has got away with pronouncements that other people would not. His celebrity status gives him instant credibility that others do not have.”

Next morning Higgs found his remarks quoted in The Scotsman and repeated widely elsewhere. The shocked reaction confirmed Higgs’s point, that someone considered superlatively brilliant as well as handicapped was supposed to be exempt from the normal give and take with his fellow scientists.

Let’s check what I say about Hawking in Einstein’s Universe and Magic Universe and ask

  • Is it fair?
  • Does it need updating?

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galaxies cluster


Updating Einstein’s Universe

Galaxies cluster in Einstein’s way

To prove Albert Einstein wrong and so share a little of his glory has been a goal for generations of physicists and astronomers. But the findings of X-ray astronomy help to show that his theory of gravity, called General Relativity or GR for short, remains stubbornly resistant to detectable error 95 years after Einstein promulgated it. In a report soon to be published in Monthly Notices of the Royal Astronomical Society, David Rapetti and colleagues at Stanford and Honolulu say, “Our results represent the most robust consistency test of General Relativity on cosmological scales to date.”

X-rays from galaxy cluster Abell 3376, 600 million light-years away in the Columba constellation. Chandra (NASA/CXC/SAO/A.Vikhlinin) and ROSAT

For seeing whether Einstein’s writ runs reliably throughout known space and time, clusters of galaxies can serve like the standard weights and measures used to check a shopkeeper’s scales. Bound together by gravity as the largest objects in the Universe, galaxy clusters fill the chasm between local distances (Earth, Solar System, Milky Way Galaxy and its neighbours) and the microwave background radiation from the edge of the observable cosmos. By visible light a galaxy cluster resembles a swarm of flies, but to X-ray telescopes in space it looks like a big balloon. That’s because a very hot gas cloud, more massive than all the galaxies put together, fills the space between them.

Theorists can reckon how big the clusters ought to be, and how they should grow over time, according to various theories of gravity, and check the expectations against the X-ray observations. Rapetti and Co. used results from NASA’s Chandra X-ray Observatory to enhance the data on 238 clusters of galaxies seen by Germany’s Rosat X-ray satellite, which ceased operating in 1999. The aim was to gauge how quickly the galaxy clusters grew over cosmic time. If a rival theory called DGP were right, gravity should leak away into some other cosmic dimension and the growth of the clusters would be slowed. It wasn’t.

Another recent “X-ray test” for General Relativity also uses observations of galaxy clusters by Chandra and Rosat – in this case 49 relatively close ones. Fabian Schmidt of the California Institute of Technology and his colleagues found that the cluster masses were too low to fit another theory, called f(R), but appropriate to Einstein’s theory.

Prompting these tests is one of the biggest issues in cosmology. Since 1998, when astronomers unexpectedly found that the expansion of the Universe is accelerating, theorists have been divided about whether or not to accept the Einsteinian view of the matter. The possibility of acceleration always lurked in a “cosmological constant” that Einstein introduced into his 1917 equation describing the Universe as a whole. But its implementation requires a huge invisible driver called Dark Energy. The other theories mentioned here, DGP and f(R), are among the attempts to do without Dark Energy by revising the theory of gravity – by inventing, in effect, a new kind of force in Nature.

Having seen off one of the possibilities, Schmidt and his colleagues write that “The abundance of galaxy clusters promises to be a good probe of other modified gravity scenarios as well.” Meanwhile, Uncle Albert scores twice. The way the clustering galaxies behave fits both his theory of gravity and his cosmological constant very nicely, thank you.


D. Rapetti et al., Mon. Not. R. Astron. Soc. in press, released online 13 April 2010. Text available at:

F. Schmidt et al., Phys Rev D, 80, 083505, 2009

About Einstein’s Universe


Occasional postings will comment on news relating to Einstein’s ideas. This introduction explains my interest.

About Einstein’s Universe

Let’s make relativity plain,” was my answer when BBC-TV in London and WGBH in Boston asked, in 1978, how we should celebrate the centenary of Albert Einstein’s birth in the following year. Not just Special Relativity, high-speed travel and E=mc2. Those are fairly easy to talk about. General Relativity (GR), Einstein’s theory of gravity, was the challenging task.

What spurred me were memories of struggling as a student with GR, taught at Cambridge as a branch of higher mathematics. Also irritation at being told again and again that GR was beyond the grasp of ordinary mortals, six decades after our Albert dreamed it up. He was an intuitive physicist and not a brilliant mathematician. Those fancy equations were supplied by other people. So what were the pictures in his head?

Filming for “Einstein's Universe”: Sidney Drell, John Archibald Wheeler and Peter Ustinov, with a younger Nigel Calder. Photo: Joan Williams, BBC/WGBH

Months of fun followed, in the course of which Martin Freeth, producer-director for the BBC, borrowed the McDonald Observatory in Texas and brought to it the eminent physicists and astronomers who were to explain Einstein’s ideas to a genuine layman. This was the actor Peter Ustinov, who also spoke Einstein’s own words and twice acted the part of a time traveller. Specially made for the programme and flown out from London was a billiard table configured so that a ball representing a planet could orbit in the warped space around the Sun, or fall into a black hole. Read the rest of this entry »