Signals from space... what is the meaning of 187.5?

At this deeply depressing time, some interesting intergalactic news for us humans... Last week, astronomers at Canada's CHIME observatory in British Columbia  detected - for only the second time ever - repeating fast radio bursts emitted from within a distant galaxy.

Now, fast radio bursts (FRBs) are in themselves a mystery - ever since the first one was discovered in 2007 (although detected in 2001 it was found only six years later in archival data) astronomers still don't know what causes them. They are radio pulses from a fraction of a millisecond to a few milliseconds in length, originating outside the Milky Way.

Until the first set of repeating FRBs was detected at the Arecibo radio telescope in 2015, they were considered to be one-off events caused by say, a star falling into a black hole or radio waves from a similar cosmic cataclysm. But the repeating FRBs detected in 2015 suggested this could not be the case - the source had to be something different, something that was capable of sending pulses of radio bursts over and over again.

The mysterious nature of FRBs is deepened by the fact that until the 2015 discoveries, all FRBs detected had dispersion measures that were multiples of the number 187.5. This bizarre characteristic led some commentators to suggest that alien intelligence stood behind these bursts. Below: this from the New Scientist, 31 March 2015. [FRBs are numbered by the date of their detection in the format YYMMDD, so 110703 is 7 March 2011]

FRBs detected after 2015 have tended not to have dispersion measures that can be multiplied by 187.5, to the chagrin of alien numerologists looking for meaning in coincidence. But the above graph still shows something of extremely low (1 in 5,000) probability.

A further mystery is that sources of FRBs are believed to be from outside our galaxy - indeed from many different galaxies. Whatever it is that's causing them seems to be distributed widely across the universe and functioning in the same way.

Some UFOlogists have been suggesting that FRBs are signals beamed to us by intelligent beings. However, a more scientifically acceptable hypothesis is that they are pulses coming from spaceships using some kind of propulsion system that uses fast radio bursts. As the signals come from within galaxies one and half to three billion light-years from earth, the chances that aliens are signaling to us are remote; after all, one and half billion years ago, life on Earth had just evolved to simple, multicellular organisms. The spaceship theory is more readily acceptable!

Astronomers at CHIME are no doubt sharing their data with scientists around the world and work on understanding FRBs has recently (as of last week) become enriched with these new observations. Until someone comes up with a solid theory of what causes FRBs, in particular repeating FRBs - please, accept the mystery!

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