My name is Daniel Töyrä and I am a PhD student within the Gravitational Wave Initial Training Network (GraWIToN), funded by the European Commission under the Marie Skłodowska-Curie actions. I'm hosted by the University of Birmingham in the United Kingdom where I work with computational modelling of advanced laser-interferometric gravitational wave detectors.

As a member of the LIGO Scientific Collaboration (LSC), one of the goals of my project is to contribute with computational models and simulations to the two Advanced LIGO detectors built mainly by Caltech and MIT. Each of these state of the art devices consists of two perpendicular arms of equal lengths forming an L. If a gravitational wave passes by the detector, it will stretch one arm as it contracts the other. The magnitude of the length change is proportional to the total length. Advanced LIGO uses 4 km long arms, which would make a fairly strong gravitational wave induce a length change in the order of 10-18 m, which is less than one-thousandth of the radius of a proton. However, the devices are capable of measuring length shifts all the way down to 10-18 m.

The length measurements are performed by using laser interferometry. That is how it works: a laser beam is split up into two equal parts by a beam splitter before the beams are sent into different arms. Each arm is a long vacuum chamber with two mirrors inside, one in each end. This constitutes an optical cavity (Fabry-Perot cavity) where the beam is trapped (it bounces back and forth) for some time before it leaks out and returns to the beamsplitter, where it is recombined with the beam returning from the other arm. The system is designed in such a way that the two beams cancel each other (destructive interference) if the arm lengths are equal. But if something, hopefully a gravitational wave, slightly changes the arm lengths, the beams do not cancel each other anymore and an output beam can be detected.

In Birmingham I'm part of a group working with gravitational wave optics. We all have our own main projects that we are responsible for, but we are also working as a team by involving the whole group in all projects. I find the working environment in the group and at the university very inspiring: there are many experienced and enthusiastic scientists, which I feel privileged to work with.

My time as an early stage researcher in GraWIToN started with one-week introductory course on the field of Gravitational waves, held at the University of Birmingham. This was a nice way to get a broader understanding of the topic before diving into my own sub-field. The course was also good for getting to know my new colleagues. Since then I have mainly been learning the basics of the Frequency domain INterfErometer Simulation SoftwarE (FINESSE) that I will use, maintain and develop during my time in Birmingham. Together with a fellow, a first year PhD student, I have performed simulations of a table top experiment involving an optical cavity that one of our colleagues is working on. This has been a good way to make us feel somewhat useful and involved in the group activities while still working on becoming useful for real.

I think this is a very interesting time to work in the field of gravitational waves. The field is expanding rapidly, Advanced LIGO and Advanced VIRGO are soon to be operational, and the forecast for a direct gravitational wave detection during my time as a PhD-student is looking promising. Furthermore, I am grateful for the opportunity to experience how it is to live and work outside my native country of Sweden.

Thank you,
Daniel Töyrä

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