News for hackers. Stuff that matters.

A comment on Milky Way’s Black Hole Just Flared, Growing 75 Times as Bright for a Few Hours. Parent comment

Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by internet_person on Wednesday August 14, 2019 @01:31AM (#20692929)
There is at least one known exception: neutrinos from a supernova. (Don't get too excited, this doesn't contradict Einstein, as we'll see.) In the hot, dense core of a supernova, photons will scatter around for a while before escaping. Neutrinos, which interact much less often than photons, and travel also more or less at the speed of light, get out basically right away. The difference can be as much as a few hours! Of course it's the high-energy photons that are more dangerous, so the neutrinos are really more like an early-warning system.

See e.g.: https://en.wikipedia.org/wiki/SN_1987A

[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by saberdancer on Wednesday August 14, 2019 @04:21AM (#20693629)
This is very misleading. In fact, neutrinos are not going faster than speed of light (c), they are going faster than photons that are emitted from the supernova.

Not just that, but lights when it passes through a medium slows down depending on it's refractive index (speed of light in water is about 225 000km/h while in vacuum it's 300 000km/h). This is important distinction as recently we had an uproar when it looked like neutrinos are slightly faster than c (in the end, it was a measurement error).

You can observe effects of particles going faster than speed of light in a medium if you look at photographs of Cherenkov radiation.

[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by lmilcin on Wednesday August 14, 2019 @08:07AM (#20694614)
The correct way to say it is that the speed of light is the speed of causality only in vacuum. In any medium the speed of light is slower but the causality or other fields (for example gravity field) that are not impeded by matter can and will propagate faster than the speed of light.
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by blueprint on Wednesday August 14, 2019 @12:43PM (#20697289)
Where does there exist a vacuum?

How does a particle or its environment measure the particle's traversal of a truly empty space?

[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by Erlich_Bachman on Wednesday August 14, 2019 @04:53AM (#20693761)
That comment never wrote that neutrinos were faster than the speed of light?
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by pavs on Wednesday August 14, 2019 @07:25AM (#20694371)
No, but he did write "There is at least one known exception", the misleading part.
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by tromp on Wednesday August 14, 2019 @08:13AM (#20694667)
> in vacuum it's 300 000km/h

300 000 km per second

[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by mcdevilkiller on Wednesday August 14, 2019 @08:28AM (#20694799)
Are you sure it's km/s? :D
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by tromp on Wednesday August 14, 2019 @09:00AM (#20695052)
As https://en.wikipedia.org/wiki/Speed_of_light puts it:

"Its exact value is 299,792,458 metres per second (approximately 300,000 km/s"

[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by phaemon on Wednesday August 14, 2019 @09:01AM (#20695057)
Yes, it is.
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by Lukas_Skywalker on Wednesday August 14, 2019 @07:59AM (#20694561)
I'm sure it's a typo, but for others: speed of light is 300'000 m/s in vacuum, not km/h.
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by Cyberspy on Thursday August 15, 2019 @04:54PM (#20709561)
It's not a typo - it's 299,792,458 m/s - approx 300,000,000 m/s or 300,000 km/s
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by oAlbe on Wednesday August 14, 2019 @04:04AM (#20693548)
That's still not an exception though. Neutrinos escape first by virtue of the fact that they don't interact with matter and don't bounce around, while light does. They aren't travelling faster than light, just travelling a shorter distance.
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by xmprt on Wednesday August 14, 2019 @04:16AM (#20693608)
The question wasn't about how fast neutrinos can travel but whether we can detect something before we see it.
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by onetimemanytime on Wednesday August 14, 2019 @05:02AM (#20693797)
So x event happened 550,000 years ago and the radiation from it might kill us. You say that we may get a warning from neutrinos before the harmful stuff kills us?
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by chrischen on Wednesday August 14, 2019 @05:30AM (#20693920)
But we detect things with particles other than just light.
[ Parent | on HN ]
Re:Milky Way’s Black Hole Just Flared, Growing 75 Times as B... by throwaway2048 on Wednesday August 14, 2019 @04:21AM (#20693634)
Its possible to exceed the local speed of light in many classes of material.

https://en.wikipedia.org/wiki/Cherenkov_radiation

[ Parent | on HN ]