Interference from human activity has always been a sticking point in astronomical observations. Radio astronomy is notoriously sensitive to unintentional interference – hence why there are “radio silent” zones near telescopes where cell phones are banned. But gravitational wave astronomy is affected to an even worse degree than radio astronomy, according to a new paper pre-published on arXiv by Reed Essick of the University of Toronto, and it’s not clear there’s much we can do about it.
The paper, entitled “Can LIGO Detect Daylight Savings Time” is one of the best papers ever at getting to the point. Its first sentence is “Yes, it can.” Simple enough of an answer – but why? Daylight savings time is a human construction that not even every country participates in – what possible impact could it have on gravitational waves caused by merging black holes billions of light years away?
It doesn’t – but it does have an impact on the human activity near the detectors that make up LIGO. In fact, Dr. Essick found that there were noticeable patterns in the sensitivity of LIGO that revolved around the times of typical human activity.
Fraser discusses the new features of LIGO and Virgo after a recent upgrade.
To do this, he used a resource called an “injection campaign” where scientists intentionally introduce data that looks like a gravitational wave overtop of the usual background noise the detector experiences. The two injection campaigns he used were for observing run 3, between 2019 and 2020, and the beginning part of observing run 4, between 2023 and 2024, which was the most recent observational run available.
There was a weekly cadence where the sensitivity of the system would drop for five days straight and then not see a significant drop for two days – a weekday/weekend pattern.
In particular, he noticed that the sensitivity dropped significantly on Tuesdays and Wednesdays as well, when the instruments were undergoing maintenance. Typically that happened on Tuesdays, and the detectors were taken offline, but that maintenance could also extend to Wednesday, lowering the overall sensitivity then.
Fraser discusses the future of gravitational wave detection.
Dr. Essick also found a daily pattern of a day/night cycle. On weekdays, the sensitivity significantly decreased during working hours, when humans were most active. There was then a noticeable bump in sensitivity levels past 6 PM, when human activity at the site had died down. Most importantly for the title of the paper, this daily cycle changed time when the locations of the LIGO observatories shifted to daylight savings time and back. He noted there was a 74 minute difference in the sensitivity changes at the site when it was on daylight savings time compared to when it wasn’t – almost exactly in line with the expected shift of the hour-long time change.
This isn’t necessarily a surprising finding – human activity has been messing with astronomical observations for years. Seismologists have also noticed a weekly and daily cadence in seismological noise caused by people’s daily commutes and activity patterns. Even the LIGO sites themselves had previously been noted to have these weekly cycle patterns of activity.
Interestingly, the paper looked at data from more than just LIGO, which has facilities in Washington and Virgina. It included background noise from Virgo, located in Italy, and KARGA, located in Japan. However, since the weighting of the US-based LIGO detectors in the overall network was weighted so highly in the overall system, the weekly and daily patterns corresponded most closely to the US’s work schedule.
This discovery is more than just a neat nuance of gravitational wave astronomy – it has implications for the types of gravitational waves the system is able to detect. Gravitational waves are impacted by the direction they come from, so data from one could be skewed based on both the time of year and time of day, and even what day of the week it is. Accounting for these changes will be key to understanding the complete picture of gravitational wave astronomy – unless we end up building one of the gigantic space-based interferometers that doesn’t care about human activity patterns. But until we do, it will remain a challenge to correctly detect gravitational waves without bias.
Learn More:
R. Essick – Can LIGO Detect Daylight Savings Time?
UT – Gravitational Wave Detectors: How They Work
UT – For Their Next Trick, Gravitational Wave Observatories Could Detect Collapsing Stars
UT – Physicists Figure out how to Make Gravitational Wave Detectors “Hear” 6x More Universe