artikel keren satu lagi, sorry masih pake bahasa enggris 
--dan itu artinya, nickname saya gak usah diganti 
Professor Einstein, you can relax. E still equals mc² . Probably …
Renowned physicist Frank Close urges caution before we abandon the theory of relativity and prepare for time travel
Partial view of a graphic explaining Cern's neutrino experiment.
Full version
The barman said: "Sorry, we don't serve neutrinos." A neutrino enters a bar.
This is but one of many tweets inspired by the news that neutrinos – ghostly subatomic particles – may travel faster than light. If so, science fiction could become science fact, with wonderful paradoxes such as effects preceding their causes. One example would be the punchline preceding the story (in case, like me, it took a while for you to decode the joke).
As a scientist I have grown up to believe this law of nature: the only thing that travels faster than light is a rumour. The story that scientists at Cern, Europe's giant particle physics laboratory near Geneva, had apparently created neutrinos that travelled faster than light, hit the news on Friday morning while I was half asleep and seemed to be the latest example of this law.
But as I awoke, and the story refused to go away, I began to panic that I would have to rewrite my book Neutrino – which it seemed was rapidly being overtaken by events. My only consolation was that this revision would be but a small tremor in the unimaginable change to our understanding of life, the universe and, indeed, everything, if this claim turned out to be true. The physics textbooks in the libraries of the world would be wrong; the foundations of science would crumble. Particles travelling faster than light, capable of carrying information, would alter everything. So, what's going on and why does it matter?
Einstein's theory of relativity was one of the great revolutions of 20th-century thought, and arguably the greatest theoretical construct of the human mind. When Isaac Newton built his laws of motion in the 17th century, he imagined space and time as some invisible matrix through which we pass without changing them. The metronome ticks steadily on as we move through a permanent static three-dimensional space. Einstein's vision was that space and time are fluid, intertwined, affected by our motion: the faster you move, the slower you age. This has many wonderful implications, such as the puzzle of the twins – Tweedledum who stays at home while Tweedledee takes a high-speed gap year and returns home wiser but, surprisingly, younger than his sibling.
The fact that space and time are elastic, stretching and warping in synchrony with our passage, is weird, but inescapably true. The beams of particles at Cern, travelling within a mere fraction of light speed, arrive at their destination on time only when the subtleties of relativity are included in the accounting. GPS satellites locate you precisely, but have to include Einstein's arithmetic in the calculations. Some experiments at Cern agree with the predictions of relativity to better than one part in a trillion – that is like measuring the distance across the Atlantic Ocean to better than the width of a human hair – but only when relativity is taken into account.
For scientists certainly, and for many of us, perhaps surprisingly, Einstein's theory of relativity is needed to keep track of our daily affairs.
Albert Einstein believed that nothing can exceed the speed of light. Photograph: Philippe Halsman/AFP
What has any of this to do with the speed of light?
Einstein's edifice is constructed on an experimental fact: that the velocity of light is independent of your own motion. Whether you are moving towards the source, or away from it, or are stationary, doesn't matter: speed of light is universal. This is counterintuitive. A fast racing car overtakes a slower one more gradually than it does the static spectators at trackside; however, a light beam passes everyone the same – spectators or Lewis Hamilton would measure the same speed. Counterintutitive certainly, but true, and it led to Einstein's world-view. And one of the basic consequences of Einstein's theory is that the speed of light – in a vacuum – is nature's speed limit. Nothing can travel through a vacuum faster than light.
Has Cern overthrown this paradigm? I doubt it. Light travels slower through water, glass, even air, than through a vacuum. Radio waves do, too. So light can be slowed down, but not sped up: the vacuum is nature's open road where light travels at the speed limit. We need to be careful when asking what exactly has the Cern experiment done, or, more pertinently, how did it do it?
Cern produces beams of neutrinos, ghostly particles that can travel through the earth as easily as a bullet through a bank of fog. A beam travels down through the surface of the Earth in a straight line, the Earth's surface curving upwards away from it initially, eventually bending downwards until, 730km later, at Gran Sasso, a laboratory near Rome, the neutrino beam re-emerges. This journey has taken about 1/500th of a second.
If you could send a light beam through the Earth, it should arrive at the same instant as the neutrino – if the neutrino travels at light speed – or slightly before it (if the neutrino travels slower than light) but not later, as that would require the neutrino to travel faster than light. If we could do that experiment, it would be clear cut. The problem is, we cannot. The Earth is transparent to neutrinos, but opaque to light.
If we know the distance from Cern to Rome precisely enough, and the time that the neutrino took to get there, then the ratio of distance to time – kilometres per second – gives the speed. In effect this is what the experiment does, but even this is not straightforward.
Measuring the time to accuracies of nanoseconds involves accounting for the time that electronic signals take to pass through circuits, into readouts and onwards to further parts of the complex of counters, computer chips and the myriad pathways of the nanoworld. If you have all of these measured, and if they are indeed everything you need to know, then you can determine the time elapsed – with some uncertainty. This they have done. However, if there is some unexpected bottleneck, unrecognised and hence unaccounted for, the timing might be a few nanoseconds amiss.
Then there is the measurement of the distance. Determining this to an accuracy of about 10 centimetres in 730km is required – and, apparently, is possible by geodesy. But precisely how this is done is, to me at least, still one of the many mysteries in this experiment. You certainly don't do it with a tape measure, even if you had one that was accurate to atomic sizes. Sending a radio signal up to a satellite, at the instant the neutrino leaves Cern, which then passes it on down to the receiver in Rome, and comparing which arrives first, and by how much, has its own difficulties. The speed of radio waves through the atmosphere is affected by magnetic fields, and by other phenomena; it is far from simply a radio beam passing through a vacuum at "the speed of light".
I would bet that a subtle error in the measured distance or time is more likely than that their ratio – the inferred speed – exceeds Einstein's speed limit.
Ultimately nature knows the answers and we have to find them by experiment. If it is possible to travel faster than light – in a vacuum – then it doesn't matter how many physicists say nay: the truth will out. And if it is true? I shall rewrite Neutrino and replace email with numail (neutrino-mail) – it's faster.
Frank Close is professor of theoretical physics at Oxford University and emeritus fellow at Exeter College, Oxford, and the author of Neutrino (OUP)
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Originally Posted by E = mc²
