Now at 140% of my usual brightness! #Betelgeuse | Hacker News Reader

Now at 140% of my usual brightness! #Betelgeuse

twitter.com

32 pointscalgarymicro12 comments

This is the brightest it's been in some time. Astronomers believe Betelgeuse could explode at any point within the next 100,000 years, which is actually very soon on a cosmic timescale. Betelgeuse is 642.5 lightyears from Earth, so if we see it go supernova in the immediate future, it would have actually happened in 1381.

I'll be pedantic here, because this is so commonly misunderstood... no it would not "have happened in 1381", because the moment in time we call 1381 only exists here on Earth. The most fundamental insight of relativity is that there is no universal "now", and therefor it is actually more correct to say that when we see Betelgeuse brightening (to the point of being visible in the daytime sky) it is going supernova right now. Our now, the now of our reference frame, which is the only now we can sensibly talk about.

Btw., the light doesn't take 642.5 years to get here either, because at light speed time does not pass. From its own perspective a photon is simultaneously here and there.

Would it still be correct to consider it as having happened simultaneous to the life of someone living in 1381?

The supernova occurred when the year on Earth was 1381, and the light from that event took 642.5 years to reach us, just like from our perspective it takes 10 hours to send a transmission to Voyager 1; I don't think anyone agrees it would be 'more correct' to say that Voyager transmitted its reponse in the instant that we receive it on Earth, rather than 10 hours prior. This is how I've seen astronomers routinely describe both of these things, so even if it is a simplification I don't think either of my statements are wrong.

I think the 'on Earth' was implied; I doubt anyone who read that thought I was referring to the passage of time from the perspective of the photon, for example.

> The supernova occurred when the year on Earth was 1381

This is the part that is wrong. The supernova (would have) occurred in Betelgeuse's reference frame which doesn't have such a direct correspondence to Earth's.

> just like from our perspective it takes 10 hours to send a transmission to Voyager

This is different because here we are talking about half a round-trip time for the signal. Ultimately the 'speed of light limit' is about information. And for us to pass information to Voyager and then receive a response takes 20 hours in our reference frame. So when you're talking about round trips it makes sense because the start and finish are in the same reference frame.

> reference frame which doesn't have such a direct correspondence to Earth's

If we choose Earth as a fixed reference point (which astronomers definitely do for cosmic events, I've read too many articles where they do that to think that they don't), it still seems to me that my statement is correct; at the very least, it's the way I've seen every astronomer who's ever discussed it express this, and I've never seen any subject matter experts take such an issue with this framing.

If you wanted, you could scale this problem down a lot, to someone on the ground vs. on the ISS. You could even scale it down to a fiber optic cable unspooling between two cars driving at different speeds. At some point, we're still picking a reference frame to count the seconds in.

> This is different because here we are talking about half a round-trip

We don't have to just talk about round trips though. Voyager can also send us transmissions unprompted, and I still doubt anyone would think it's more correct to say that it begins transmissions in the instant we receive them, rather than 10 hours prior (especially because Voyager timestamps it's transmissions with Earth time, or at least what it thinks is Earth time).

Sure, but we can and do still routinely talk about reference frames other than those on earth. Your pedantry isn’t warranted, and rather comes across as needlessly condescending.

Sorry, I don't want to sound condescending. But it's just wrong, and it's a very common error so I thought I should try to do my little part to help correct it. The physicist Carlo Rovelli belabors this point extensively in his book "The Order of Time"... there is no universal "now".

There is no universal now, but you can translate between time frames, no? Asking as a layman inspired by your comments.

You can translate, although it can be quite difficult, and isn't really useful. In the case of Betelgeuse we don't even know its exact distance (to +/- 100 ly) or proper motion relative to Sol, so it would be a very, very rough approximation.

But really the more important thing here is that such translation is quite pointless... it doesn't tell us anything meaningful. Talking about relative time only becomes meaningful when you're talking about round-trip communication, and there the meaningful part is the complete round trip, and not what moment here corresponds to a moment there.

Is the luminosity variation due to changes in the size and temperature of Betelgeuse or because of clouds of cooling ejected matter obscuring our sightlines?

Astronomers think that the 'great dimming' and subsequent return to normal brightness were indeed due to a mass ejection of enormous scale[0], but we're not yet sure why it's now ramping up in brightness beyond what is typical.

[0]https://www.skyatnightmagazine.com/news/betelgeuse-dimming-e...

Reply on news.ycombinator.com