General relativity passes cosmic test Einsteins theory holds in extreme gravitational fields

Chalk up another win for "orthodox" scientific theories. General relativity has again passed stringent tests.

General relativity survives gruelling pulsar test — Einstein at least 99.95 percent right

An international research team led by Prof. Michael Kramer of the University of Manchesters Jodrell Bank Observatory, UK, has used three years of observations of the "double pulsar", a unique pair of natural stellar clocks which they discovered in 2003, to prove that Einsteins theory of general relativity - the theory of gravity that displaced Newtons - is correct to within a staggering 0.05%. Their results are published on the 14th September in the journal Science and are based on measurements of an effect called the Shapiro Delay.

The double pulsar system, PSR J0737-3039A and B, is 2000 light-years away in the direction of the constellation Puppis. It consists of two massive, highly compact neutron stars, each weighing more than our own Sun but only about 20 km across, orbiting each other every 2.4 hours at speeds of a million kilometres per hour. Separated by a distance of just a million kilometres, both neutron stars emit lighthouse-like beams of radio waves that are seen as radio "pulses" every time the beams sweep past the Earth. It is the only known system of two detectable radio pulsars orbiting each other. Due to the large masses of the system, they provide an ideal opportunity to test aspects of General Relativity.

The large mass of the pulsars and their proximity to each other is the key thing, resulting in a very strong gravitational field. The binary pulsar system provides an opportunity to check the validity of general relativity under conditions that are more extreme than any studied before.

Shapiro delay can be described as an apparent change in the speed of light in a strong gravitational field. It occurs because spacetime itself is warped in the field, which effectively forces light to travel a larger distance. Since the radio-frequency beam from each pulsar sweeps across Earth at a very precise frequency (22.8 milliseconds for one, 2.8 seconds for the other), like an exceptionally accurate clock, it is possible to predict exactly when the beam should be seen. Any departure from this prediction would be due to the Shapiro delay. The orbital period of the two pulsars around each other is about 2.4 hours. During this period, the distance between the pulsars varies, so the mutual gravitational fields vary correspondingly. This allows the theoretical delay time to be calculated, and the observations match the prediction very well.

This effect is distinct from the time dilation which occurs in a large gravitational field. The dilation causes time intervals to appear to lengthen. So the period of rotation of each pulsar appears to change, and the spectrum of radio waves from each object is redshifted, as the pulsars experience a change in the gravitational field. Here again, the observations match the predictions of general relativity very well.

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Additional information:

Pulsars Gyrations Confirm Einsteins Theory

News article about the research from Science. (Subscription required for full access.)

Tests of General Relativity from Timing the Double Pulsar

The actual research paper from Science. (Subscription required for full access.)

Millisecond Pulsars as Tools of Fundamental Physics

A review paper by Kramer posted to the arXiv in May 2004. It explains the underlying physics in some detail. It also describes the binary pulsar system, which had only recently been discovered.

The Confrontation between General Relativity and Experiment

An expository paper by Clifford M. Will that reviews the status of experimental tests of general relativity and of theoretical frameworks for analyzing them.

General relativity passes cosmic test - Einsteins theory holds in extreme gravitational fields.

News article at Nature.com news. (Subscription required)

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Tags: relativity, general relativity, pulsars, astrophysics

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Lockdown The coming war on general purpose computing

Cory Doctorow has an article outlining why he thinks a range of industries will be lobbying hard to make general purpose computers available to the masses - Lockdown: The coming war on general-purpose computing.

Why might other sectors come to nurse grudges against computers in the way the entertainment business already has? The world we live in today is made of computers. We dont have cars anymore; we have computers we ride in. We dont have airplanes anymore; we have flying Solaris boxes attached to bucketfuls of industrial control systems. A 3D printer is not a device, its a peripheral, and it only works connected to a computer. A radio is no longer a crystal: its a general-purpose computer, running software. The grievances that arise from unauthorized copies of Snookis Confessions of a Guidette are trivial when compared to the calls-to-action that our computer-embroidered reality will soon create.

Consider radio. Radio regulation until today was based on the idea that the properties of a radio are fixed at the time of manufacture, and cant be easily altered. You cant just flip a switch on your baby monitor and interfere with other signals. But powerful software-defined radios (SDRs) can change from baby monitor to emergency services dispatcher or air traffic controller, just by loading and executing different software. This is why the Federal Communications Commission (FCC) considered what would happen when we put SDRs in the field, and asked for comment on whether it should mandate that all software-defined radios should be embedded in “trusted computing" machines. Ultimately, the question is whether every PC should be locked, so that their programs could be strictly regulated by central authorities.

Even this is a shadow of what is to come. After all, this was the year in which we saw the debut of open source shape files for converting AR-15 rifles to full-automatic. This was the year of crowd-funded open-sourced hardware for genetic sequencing. And while 3D printing will give rise to plenty of trivial complaints, there will be judges in the American South and mullahs in Iran who will lose their minds over people in their jurisdictions printing out sex toys. The trajectory of 3D printing will raise real grievances, from solid-state meth labs to ceramic knives.

It doesnt take a science fiction writer to understand why regulators might be nervous about the user-modifiable firmware on self-driving cars, or limiting interoperability for aviation controllers, or the kind of thing you could do with bio-scale assemblers and sequencers. Imagine what will happen the day that Monsanto determines that its really important to make sure that computers cant execute programs which cause specialized peripherals to output custom organisms which literally eat their lunch.

Regardless of whether you think these are real problems or hysterical fears, they are, nevertheless, the political currency of lobbies and interest groups far more influential than Hollywood and big content. Every one of them will arrive at the same place: “Cant you just make us a general-purpose computer that runs all the programs, except the ones that scare and anger us? Cant you just make us an Internet that transmits any message over any protocol between any two points, unless it upsets us?"

There will be programs that run on general-purpose computers, and peripherals, that will freak even me out. So I can believe that people who advocate for limiting general-purpose computers will find a receptive audience. But just as we saw with the copyright wars, banning certain instructions, protocols or messages will be wholly ineffective as a means of prevention and remedy. As we saw in the copyright wars, all attempts at controlling PCs will converge on rootkits, and all attempts at controlling the Internet will converge on surveillance and censorship. This stuff matters because weve spent the last decade sending our best players out to fight what we thought was the final boss at the end of the game, but it turns out its just been an end-level guardian. The stakes are only going to get higher.

As a member of the Walkman generation, I have made peace with the fact that I will require a hearing aid long before I die. It wont be a hearing aid, though; it will really be a computer. So when I get into a car—a computer that I put my body into—with my hearing aid—a computer I put inside my body—I want to know that these technologies are not designed to keep secrets from me, or to prevent me from terminating processes on them that work against my interests.

Last year, the Lower Merion School District, in a middle-class, affluent suburb of Philadelphia, found itself in a great deal of trouble. It was caught distributing, to its students, rootkitted laptops that allowed remote covert surveillance through the computers camera and network connection. They photographed students thousands of times, at home and at school, awake and asleep, dressed and naked. Meanwhile, the latest generation of lawful intercept technology can covertly operate cameras, microphones, and GPS tranceivers on PCs, tablets, and mobile devices.

We havent lost yet, but we have to win the copyright war first if we want to keep the Internet and the PC free and open. Freedom in the future will require us to have the capacity to monitor our devices and set meaningful policies for them; to examine and terminate the software processes that runs on them; and to maintain them as honest servants to our will, not as traitors and spies working for criminals, thugs, and control freaks.

Cory is giving a talk at the Long Now next week - Cory Doctorow Seminar Primer.

If geek culture had a class president, Cory Doctorow would be frontrunner for the position. He writes for BoingBoing, uses Ubuntu, played a hero in XKCD, published several rebellious young-adult sci-fi novels (under CC licenses, no less), and has worked on two continents fighting for the rights of internet users. He’s spent the better part of the last couple decades encouraging content-producers to embrace the new models of distribution made possible by the internet and fought them tooth and nail when they seek to hold it back.

His outspokenness doesn’t come from a single statement like “information wants to be free.” Doctorow argues in a recent essay called Lockdown that enforcing Copyright law in the digital era is about more than protecting the rights of intellectual property holders; it has rather become a kind of trojan horse for the surveillance industrial complex and threatens to severely curtail the individual autonomy of the world’s citizens. Computers are infusing everything, he explains. They increasingly extend our embodiment and cognition and can thus be enabling and liberating. Computing’s inherent flexibility therefore offers a form of freedom; commercial or governmental interests that seek to control computing for their own needs or simply out of a fear of the new way must be resisted in order to protect that freedom.

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