3 ms·

> This supernova, about 31 million light-years from Earth in the galaxy NGC2146, was detected three hours after the explosion happened. Faster Than Light commu

> This supernova, about 31 million light-years from Earth in the galaxy NGC2146, was detected three hours after the explosion happened. Faster Than Light communication!

sudhirj 5y ago
That's the first thing that stood out to me as well, but it's actually pretty accurate when it comes to light speed communication. It was basically 3 hours after the event observable by us, which is for all practical purposes "when it happened". There is no true universal "when it happened", it's all relative.
- iseethroughbs 5y ago
  Technically light doesn't experience time. Which means there's no such thing as "how much time it takes light to travel from the Sun to Earth". It takes no time at all. Yet we do say "8 minutes 20 seconds". Everyone would claim that's the correct answer. In which case at the point of detecting this light on Earth, if someone asks us "when did the event of it leaving the Sun occur" you'd obviously say "8 minutes 20 seconds ago". So while there's no true universal "when it happened", we're all here on Earth and we've developed a certain way of expressing ourselves about time and light, and broadly speaking we're all sharing a relative point of view in spacetime, relative to something 31 million light years away. So to say "we detected it 3 hours after it happened" when it happened 31 million lights away would be poorly written, simply put.
  - MikeDelta 5y ago
    There is only one event that happened, the question is how much time we will measure between that moment and the photons of the event reaching earth. In the reference frame of earth it is 31 mio years. In frames that move faster and faster to the speed of light, relative to ours, this would take less and less time, an effect known as time dilation. Obviously the article means that 3 hours after the first photons of this event reached earth (and we see it as happened), we managed to aim our sensors towards it and start measuring. It looks a bit clumsy indeed.
    - sudhirj 5y ago
      Another way to think about it would to change the distance and see if the sentence still makes sense. If the event was on the other side of the world, 100 light-milliseconds away, it wouldn't be an issue. If it happened on Mars, a light-minute or so away, also not an issue. If it happened on the Sun, 8 light-minutes away, this phrasing would also be fine, same for Jupiter and even Pluto. Not sure where the line is, but it's somewhere between 10 light-minutes and 31 million light-years, I guess :P
jfengel 5y ago
That confused me, too. It seems to refer to this from the paper: On 2018 March 2.49 (UT dates are used throughout), we dis-covered AT 2018zd13 at an unfiltered optical magnitude of 17.8 in the outskirts of NGC 2146... Combined with our pre-discovery detection at 18.1 mag on 2018 March 1.54, we estimate an explosion epoch of 2018 March 1.4 ± 0.1 (~3 h before the first detection; I'm not entirely sure I understand the date/time format, but the last part suggests that TFA meant "We pointed our telescope at it when the explosion was already in progress. We think it had been going for about three hours at that point." I think "March 2.49" means ".49 of the way through March 2", so 11:46 AM, but that's just a guess. I've never seen that before. If so, that "first detection" was at 12:58PM on March 1, and they figure it actually started at 9:36AM... about three hours earlier.

> This supernova, about 31 million light-years from Earth in the galaxy NGC2146, was detected three hours after the explosion happened. Faster Than Light communication!

sudhirj 5y ago

That's the first thing that stood out to me as well, but it's actually pretty accurate when it comes to light speed communication. It was basically 3 hours after the event observable by us, which is for all practical purposes "when it happened". There is no true universal "when it happened", it's all relative.

iseethroughbs 5y ago
Technically light doesn't experience time. Which means there's no such thing as "how much time it takes light to travel from the Sun to Earth". It takes no time at all. Yet we do say "8 minutes 20 seconds". Everyone would claim that's the correct answer. In which case at the point of detecting this light on Earth, if someone asks us "when did the event of it leaving the Sun occur" you'd obviously say "8 minutes 20 seconds ago". So while there's no true universal "when it happened", we're all here on Earth and we've developed a certain way of expressing ourselves about time and light, and broadly speaking we're all sharing a relative point of view in spacetime, relative to something 31 million light years away. So to say "we detected it 3 hours after it happened" when it happened 31 million lights away would be poorly written, simply put.
- MikeDelta 5y ago
  There is only one event that happened, the question is how much time we will measure between that moment and the photons of the event reaching earth. In the reference frame of earth it is 31 mio years. In frames that move faster and faster to the speed of light, relative to ours, this would take less and less time, an effect known as time dilation. Obviously the article means that 3 hours after the first photons of this event reached earth (and we see it as happened), we managed to aim our sensors towards it and start measuring. It looks a bit clumsy indeed.
  - sudhirj 5y ago
    Another way to think about it would to change the distance and see if the sentence still makes sense. If the event was on the other side of the world, 100 light-milliseconds away, it wouldn't be an issue. If it happened on Mars, a light-minute or so away, also not an issue. If it happened on the Sun, 8 light-minutes away, this phrasing would also be fine, same for Jupiter and even Pluto. Not sure where the line is, but it's somewhere between 10 light-minutes and 31 million light-years, I guess :P

jfengel 5y ago

That confused me, too. It seems to refer to this from the paper: On 2018 March 2.49 (UT dates are used throughout), we dis-covered AT 2018zd13 at an unfiltered optical magnitude of 17.8 in the outskirts of NGC 2146... Combined with our pre-discovery detection at 18.1 mag on 2018 March 1.54, we estimate an explosion epoch of 2018 March 1.4 ± 0.1 (~3 h before the first detection; I'm not entirely sure I understand the date/time format, but the last part suggests that TFA meant "We pointed our telescope at it when the explosion was already in progress. We think it had been going for about three hours at that point." I think "March 2.49" means ".49 of the way through March 2", so 11:46 AM, but that's just a guess. I've never seen that before. If so, that "first detection" was at 12:58PM on March 1, and they figure it actually started at 9:36AM... about three hours earlier.