4 ms·

How does the length of the burst depend on the size of the event?

post-it 4y ago
Not an astrophysicist, but I think the assumption is that all of the photons are emitted at once in an explosion, and of course they all travel at the same speed. So if the burst is two minutes long, the near side of the explosion was two light-minutes closer than the far side of the explosion.
keketi 4y ago
Imagine observing a lightning bolt strike the ground. The bolt travels through air almost instantly and emits sound in every direction throughout its path. First you will hear the sound waves emitted from the point that it touches the ground, because that is the closest. Then you will hear the sound waves from 1 cm off the ground, 2 cm, 3 cm and so on all the way up to the lightning bolt's point of origin in the sky. By measuring the duration of the noise you can tell the length of the lightning bolt (assuming it's perpendicular to the ground).
- denton-scratch 4y ago
  Is that true? Can you cite? (I don't mean to question you, I'd just like a practical procedure)
  - dredmorbius 4y ago
    It's basic properties of propagation of light and sound. In practice, the duration of thunder from a given lightning bolt might be affected by echos and other phenomena, though you could probably get at least a rough estimate of bolt length by the duration of thunder. The largest lightning bolts ever measured are hundreds of kilometers long. The longest I'm aware of was over 760 km long (477 mi), measured in February of this year: <https://www.noaa.gov/stories/worlds-longest-lightning-flash-on-record-captured-by-noaa-satellites https://www.noaa.gov/stories/worlds-longest-lightning-flash-...> That probably roared for a while, though attentuation of sound would likely occur within 16--32 km (10--20 mi), or a minute or two. Otherwise, it would take nearly 40 minutes for sound to propagate from one end of the strike to the other.
yaks_hairbrush 4y ago
The length of the burst tells us the maximum size of the event. Imagine the event takes place instantly at the source, and that the source has some size. If we observe the event over two minutes, then the event size was two light-minutes. In reality, these aren't completely instantaneous events. But any event duration at its source is going to decrease the event size. Take the extreme case: the event actually took place at its source over the course of two minutes. And we observed it over two minutes. We would have to conclude that the event had zero size. Of course, the reality is that we have no idea how long the event took place at its source. All we know is that we observed it over two minutes. But that's enough information to conclude that the event we're seeing certainly occurred within a two light-minute sphere. If it had occurred over a larger sphere, we would have observed it over a longer period of time because of the time light takes to traverse the length of the source.

Not an astrophysicist, but I think the assumption is that all of the photons are emitted at once in an explosion, and of course they all travel at the same speed. So if the burst is two minutes long, the near side of the explosion was two light-minutes closer than the far side of the explosion.

keketi 4y ago

Imagine observing a lightning bolt strike the ground. The bolt travels through air almost instantly and emits sound in every direction throughout its path. First you will hear the sound waves emitted from the point that it touches the ground, because that is the closest. Then you will hear the sound waves from 1 cm off the ground, 2 cm, 3 cm and so on all the way up to the lightning bolt's point of origin in the sky. By measuring the duration of the noise you can tell the length of the lightning bolt (assuming it's perpendicular to the ground).

denton-scratch 4y ago
Is that true? Can you cite? (I don't mean to question you, I'd just like a practical procedure)
- dredmorbius 4y ago
  It's basic properties of propagation of light and sound. In practice, the duration of thunder from a given lightning bolt might be affected by echos and other phenomena, though you could probably get at least a rough estimate of bolt length by the duration of thunder. The largest lightning bolts ever measured are hundreds of kilometers long. The longest I'm aware of was over 760 km long (477 mi), measured in February of this year: <https://www.noaa.gov/stories/worlds-longest-lightning-flash-on-record-captured-by-noaa-satellites https://www.noaa.gov/stories/worlds-longest-lightning-flash-...> That probably roared for a while, though attentuation of sound would likely occur within 16--32 km (10--20 mi), or a minute or two. Otherwise, it would take nearly 40 minutes for sound to propagate from one end of the strike to the other.

yaks_hairbrush 4y ago

The length of the burst tells us the maximum size of the event. Imagine the event takes place instantly at the source, and that the source has some size. If we observe the event over two minutes, then the event size was two light-minutes. In reality, these aren't completely instantaneous events. But any event duration at its source is going to decrease the event size. Take the extreme case: the event actually took place at its source over the course of two minutes. And we observed it over two minutes. We would have to conclude that the event had zero size. Of course, the reality is that we have no idea how long the event took place at its source. All we know is that we observed it over two minutes. But that's enough information to conclude that the event we're seeing certainly occurred within a two light-minute sphere. If it had occurred over a larger sphere, we would have observed it over a longer period of time because of the time light takes to traverse the length of the source.