We know that outside the earth’s atmosphere, there is a vacuum. We can create a vacuum here on earth too. But have we ever wondered what might still be there in a vacuum? Of course in outer space, there is light or all kinds of electromagnetic radiation. But does there exist any matter that we can’t see? Apparently, yes! Quantum fluctuations continuously create fundamental particles in a vacuum.
These quantum particles are very small and are insignificant at scales that concern our daily life, or at least we thought so until now. But recently a team of researchers at MIT LIGO laboratory has measured a tiny displacement of the 40kg mirrors at the ends of the LIGO detector, caused by these fluctuations. The LIGO detector is basically an L shaped interferometer, consisting of two 4km long tunnels, at the end of which the mirrors are suspended.
The measurements match the theoretically predicted value. The displacement is very small, about 10^-20 meters. “A hydrogen atom is 10^-10 meters, so this displacement of the mirrors is to a hydrogen atom what a hydrogen atom is to us — and we measured that,” says Lee McCuller, a research scientist at MIT’s Kavli Institute for Astrophysics and Space Research.
The measurements were done while testing the equipment that’s designed to reduce the quantum noise generated specifically by the photons of the laser that is used in the LIGO detector. This equipment called ‘noise squeezer’ helps the researchers to further increase the sensitivity of the detector towards gravitational waves by reducing the effects of quantum fluctuations on the position of mirrors.
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What changes happens in the measurement of the gravitational waves if the mirror in the LIGO detecter moved less than the diameter of a hydrogen atom ? How can we measure such small movements in the mirror ?