Hacker News Reader

Comment by pso | Hacker News Reader

Pure speculation, isn't the most interesting time period in the signal the transition from signal to no-signal (and back again)? How high is the resolution of the sensors? Is it possible that some frequencies are blocked fractionally earlier than others? At a fine enough temporal resolution, there might be more clues regarding the source.

View on HN

Good question. For many FRBs there is access to the original antenna time streams (which are essentially voltages samples at 800 MHz), so superficially that might give you 1.25ns resolution. However, the intrinsic width of the pulses seem to be more like ~1ms. That tells you something about the emission source, giving you a rough upper bound on it's size of c * 1ms ~ 300km.

However, lots of interesting plasma physics effect occur between us and the FRB. The dominant thing that happens is that low frequencies arrive later than high frequencies (as they travel slow in a plasma). This causes the FRB to be smeared over many seconds rather than a millisecond. There's also multi path interference effects and an whole bunch of other stuff that happens. So actual the temporal structure of the burst tells you much more about the intervening medium than it does about the source.

I think of multipath being about signals bouncing off other objects (reflection) and bending through media (refraction) before reaching the receiver. At the cosmic scale, I'm guessing multipath also involves gravitational lensing. Correct?

There is some possibility of gravitational lensing of FRBs and people are definitely searching for it. However, in this case it is refraction caused by variation in the density of the ionised gas between us and the source (within our galaxy, the host galaxy, and at lower densities in the intervening medium).

A broader question that puzzles me, not specific to this phenomenon. Pulse length is often used to bound the size of a source in space (it cannot be smaller than light takes to traverse the object). But it’s possible that two phenomena can be correlated without being close, if there is a delayed action (akin to setting two alarm clocks and separating them in space). How can we confidently use this as evidence of size?

> Pulse length is often used to bound the size of a source in space (it cannot be smaller than light takes to traverse the object).

Sorry for the possibly noob question, but why is this? I can't think of an intuitive explanation

I guess the assumption is that the phenomenon starts at a point, spreads across the object sized l at c, and lasts for only a period of time t at any location. Therefore the pulse ends after l/c + t = T. By measuring T you can put an upper bound on l.

Example of this inference, from the Wikipedia on Fast Radio Bursts (but I have seen in many contexts):

> The sources are thought to be a few hundred kilometers or less in size, as the bursts last for only a few milliseconds

Does that make sense?

I just realized what I described is not giving a lower bound.

You just made me realize that it would be cheaper for an advanced civilization to send a message light years away by altering the medium through which the signal passes than to control the source directly, e.g. a neutron star.

The Three Body Problem trilogy by Liu Cixin might interest you

That book is garbage and the author doesn't understand science, let alone writing. Just heads up for all the recommendations spammed on here.

Though his creative interpretation of higher dimensions was interesting (sending the tiny probe that unfolds on earth).

Reply on news.ycombinator.com