An Astronomy Park Around Virgo! C O N T E N T S
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An Astronomy Park around Virgo! C O N T E N T S NEWS FROM THE COLLABORATION 3 h - The Gravitational Voice is an internal Squeezing light to catch gravitational waves publication of the European Gravitational Observatory (EGO) and the Virgo Collaboration. FROM THE WORLD 6 The content of this newsletter does not The Japanese Large-scale GW Telescope: KAGRA necessarily represent the opinion of the management. LISA Pathfinder An Astronomy Park Editor: Carlo Bradaschia Editorial Staff: NEWS FROM THE SITE 13 Henrich Heitmann Gary Hemming The 2012 VESF School Martin Mohan The new EGO optics laboratory Séverine Perus The Biathlon 2012 Frédéric Richard Production: Séverine Perus OUT & ABOUT 16 Copenhagen, a drama Published in electronic format on the EGO Web: www.ego-gw.it LETTERS TO THE EDITOR 20 E D I T O R I A L While reading the following two articles on wind turbines in the April issue of Europhysics News: C. le Pair, F. Udo and K. de Groot: Wind turbines as yet unsuitable as electricity providers. http://dx.doi.org/10.1051/epn/2012204 and P.J. Eecen, H.A. Bijleveld and B. Sanderse: Wind energy research - development of advanced design tools. http://dx.doi.org/10.1051/epn/2012203 I was surprised to read such sharp opinions in such a widely diffused and normally soft magazine. Hence, I thought it would be interesting to hear the reactions of the EGO and Virgo folk and, as such, I advertised the two articles by e-mail. We publish in this issue of h, in the column Letters to the Editor, a subsequent exchange of letters and we mention the fact, upon the suggestion of Andrea Vicere, that EPS and SIF are organising a school dedicated to the theme: http://en.sif.it/activities/energy_school/2012 with energy storage being one of the central topics. We will be happy to receive and publish further comments on this hot subject. C. BRADASCHIA Editor-in-Chief THE GRAVITATIONAL VOICE - FEBRUARY 2012 2 N E W S F R O M T H E C O L L A B O R A T I O N one. It does not take into account the fact that guitar strings (or the Squeezing light to catch electromagnetic field constituting a laser light wave) follow the rules of gravitational waves quantum mechanics. A quantum oscillator behaves in a quite peculiar way. We cannot That the expected amplitude of a for the VIRGO sensitivity is thermal determine exactly its position and gravitational wave is quite small is motion. But let us suppose that in a its velocity at the same time: a fact familiar to readers of h. So it not too far future thermal noise will accordingly with the so-called should not come as a surprise that be reduced by a large factor. Then Heisenberg uncertainty relation the catching a gravitational wave is a we will find that there is a kind of product of the indeterminacy of these difficult task. It is mainly a struggle noise which is linked in a very two quantities cannot be smaller against various noises which do their intimate way with the measurement than a value proportional to a best to mask the expected effect, a procedure used. We can call it optical fundamental physical constant called distance change between two noise or, for reasons that will be h. This has interesting consequences suspended mirrors. discussed in the following, quantum that I will try to illustrate with some noise. examples. Due to the smallness of the quantities The surprising point about quantum involved, measuring the position of noise is that it could seem that it a mirror with a ruler is out of gives a limit to the sensitivity that Figure 2 The behavior of a question and the VIRGO way to do cannot be evaded for fundamental quantum ball oscillating at the that is by using interference. In a reasons, no matter how smart we bottom of a valley. The oscillation nutshell, a light wave (the laser are. This is called Standard Quantum is along the blue axis, while time beam) is sent to a far away mirror, Limit (SQL). increases along the red one. Each and the reflected wave is observed. color corresponds to a possible The information about the mirrors When we think about a wave, we classical motion which starts at the position is contained in how many imagine something which is similar same position with a different initial times the wave oscillates before to the picture sketched in Figure 1. velocity. coming back. There is something which can oscillate, such as a guitar string, and Obviously the mirror moves for a the position and the velocity of lot of reasons unrelated to the a given piece of the string is well presence of gravitational waves, and defined at any time. The trouble with currently the fundamental limitation this picture is that it is a classical Figure 1. The VIRGO way to measure the position of a mirror. The light sent to the mirror (in red) is reflected back (in green) to the observer. In Figure 2 I represented the possible positions of a small quantum ball oscillating inside a valley as a function of time. In drawing the picture I supposed that initially the ball was found with pretty good accuracy displaced in the positive direction from the bottom of the valley. The initial ball velocity is thus uncertain, this uncertainty becoming larger and larger with the precision of our knowledge of the initial position. Several initial velocities will be possible, and for THE GRAVITATIONAL VOICE - JULY 2012 3 N E W S F R O M T H E C O L L A B O R A T I O N each of them the position of the ball the possibilities. As we are interested works. However increasing the laser will change in the following in a in measuring the displacement of power is not a painless option, as it different way. The uncertainty about the wave (that is, its phase), a leads to several issues which are not the position will grow reaching a measurement will select one of the easy to deal with. But let us suppose maximum after a quarter of the classical oscillations compatible with that these could be solved: still we oscillation period, and then it will the given indeterminacy. By looking find that quantum noise cannot be start to decrease and so on, at Figure 2 we see that there are reduced arbitrarily in this way. continuously oscillating between a several different phases which can The reason is simple to understand: maximum and a minimum. come out after the measurement, as it is clear from Figure 3, a evidenced as classical curves with coherent oscillator fluctuates both This quantum ball behaves quite different colors. In other words we in phase and in amplitude. When strangely, but maybe I set it up in have a quite large phase the laser is reflected by the mirror the wrong way. If we want to obtain indeterminacy. it acts on it with a pressure a more classical ball it is better to proportional to the square of its avoid a too accurate initial position For the coherent states in Figure 3 amplitude. If the amplitude measurement, which unavoidably things go somewhat better. Once fluctuates the pressure will also and triggers a large increase of the again the phase is not completely the mirror will be displaced from its uncertainty afterwards. determined, but its indeterminacy is original position in an unpredictable much less than before. I plotted two way, changing the phase of the reflected beam anyway. When the The knowledge about the position extreme possibilities for each wave (thin gray lines): this is an estimate laser power is low this effect will of a classical oscillator can be perfect be small, but the intrinsic phase in principle and it does not change of the limit on the sensitivity to the position of the mirror. indeterminacy of the coherent state with time. The most similar quantum will be large. On the other hand oscillation we can construct is the when the laser power is increased so-called coherent state, which is By looking carefully at Figure 3 we the intrinsic phase indeterminacy actually a quite accurate model for realize an important point: the phase will decrease, but the phase a laser beam. In this case the position indeterminacy of the orange curve indeterminacy induced by the shaken indeterminacy is not zero, but at is lower than the green curves one. mirror will increase. There will be least it is time independent. This is This seems to suggest a simple way an optimal intermediate power that showed pictorially in Figure 3: in to reduce phase noise: we could will give the lower phase the case we are interested in, the simply increase the field amplitude, indeterminacy obtainable, which is plot could represent the value of the by buying a more powerful laser. the Standard Quantum Limit laser field at a given point as a Up to some extent this certainly mentioned before. function of the time. The two colors correspond to two waves with different amplitudes, and at a given Figure 3. In a coherent state the knowledge we have about the amplitude of time the indeterminacy is the laser field does not change with time, and it is the better we can get with represented by the thickness of the this constraint. This is the best approximation we can have of our intuitive curve. (classical) concept of a wave.