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  #51  
Old 12-30-2019, 10:26 AM
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Some rather astonishing supernova "trivia" (that doesn't seem like the right word) from xkcd What If:

Quote:
Which of the following would be brighter, in terms of the amount of energy delivered to your retina:

A supernova, seen from as far away as the Sun is from the Earth, or

The detonation of a hydrogen bomb pressed against your eyeball?
Answer halfway down here:
https://what-if.xkcd.com/73/
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Old 12-30-2019, 11:26 AM
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No foot in the mouth. As I say, Betelgeuse has done this before. People just aren't aware of its frequent changes in brightness.
Curious, then, that both of the articles I cited in the MPSIMS thread made exactly that point, to put the current dimming in context:

CBC: Astronomers are wondering whether Orion's shoulder will soon explode

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But Betelgeuse is also a variable star, meaning its brightness rises and falls periodically. But we've never seen it like this.

"Maybe 300 years ago, Betelgeuse was dimmer than what we're observing now, but we don't have data," Kafka said.
...
It's not quite clear why exactly Betelgeuse dims periodically, but one of the possibilities is that, like the sun, it has cooler and hotter parts. If one of those cooler parts swung into our line of sight, that could make it seem like the star had dimmed.

In 2018, Betelgeuse had a couple of dips in its brightness, Kafka said. What we're seeing now could be that same spot, or possibly another.
Forbes: Is Betelgeuse About To Explode?

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Recently, in just the past few weeks, its brightness has dropped tremendously, knocking it out of the top 10 brightest stars for the first time in many years.
...
In fact, if you look beyond the previous decade and instead go back to the past century, you'll find that Betelgeuse has been this dim many, many times in the past. If you look beyond the photosphere of the star itself, you'll find that there are enormous radio emissions that reveal the presence of expelled gas out beyond where the orbit of Neptune is around the Sun.

Similar dimming events have occurred before, reducing the brightness of Betelgeuse below even what it currently is at. But to see a dimming event occur this rapidly and this severely really hasn't been seen before over the past century at all.
It's almost as if the journalists who wrote these articles were aware of Betegeuse's pattern of dimming and wanted to inform their readers of that! But naw. "People just aren't aware of its frequent changes in brightness."
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Old 12-30-2019, 11:35 AM
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Let us know when the light from the first twenty posts in this thread reaches you, then we can work out how far away you are.
I don't think you've thought this through.
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Old 12-30-2019, 01:44 PM
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The variations in Betelgeuse's brightness are due to multiple factors. One factor is in fact starspots, darker regions of the surface which we believe are due to similar phenomena as sunspots. Another is because its size isn't constant, but pulsating. Betelgeuse is one of the very few stars with a large enough angular size that it's possible (albeit still difficult) to resolve it into more than a point source; such observations would be able to tell whether the current dimness is due to pulsation, starspots, or a combination of both. I'm guessing that nobody has had a chance to make such images yet.
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Old 12-30-2019, 02:55 PM
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I don't think you've thought this through.
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Old 12-30-2019, 05:07 PM
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I didn't claim that there was. I am just suggesting (or rather, supporting what I think septimus was getting at), that there is no absolute time. It is misleading to talk about what time an event "really" happened because all we can ever do is pick a reference frame and say when the event took place in that frame. [...]
Right! Chronos is right in the astronomical convention, what he says is practical. But in a physical way, there is no real time. It is, literally and figuratively, relative.
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Old 12-30-2019, 05:14 PM
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Re frame of reference for the time measurement:

There is a natural frame of reference when talking about the age of the universe, that's the time measured on a clock carried by a co-moving observer, i.e. an observer at rest with respect to local space, moving only with the Hubble Flow, with the expansion of space itself. The frame in which you are at rest wrt to local space is the frame in which the universe is isotropic - it looks the same in all directions. [...]
Are you postulating a natural universal frame of reference for time and the age of the Universe? And that the coordinate origin of the referece frame is distributed everywhere homogeneously (or where ever the Hubble Flow happens to take you drifting?) If so, I disagree.
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Old 12-30-2019, 05:36 PM
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The tricky part with the locally co-moving frame is that it isn't a frame of reference. It's a whole bunch of frames of reference, a different one for every point in the Universe.

But the locally co-moving frame for Earth is almost exactly the same as the locally co-moving frame for Betelgeuse, both of which are almost exactly the same as the rest frame for Betelgeuse, the rest frame for the Earth, and the rest frame for the Galaxy as a whole. And so, while we could, with perfect validity, use some reference frame that is very different from any of those, there's no practical reason to.
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Old 12-30-2019, 06:23 PM
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The tricky part with the locally co-moving frame is that it isn't a frame of reference. It's a whole bunch of frames of reference, a different one for every point in the Universe.
Indeed, but on Earth there is only one frame of reference that makes sense: the local one

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But the locally co-moving frame for Earth is almost exactly the same as the locally co-moving frame for Betelgeuse, both of which are almost exactly the same as the rest frame for Betelgeuse, the rest frame for the Earth, and the rest frame for the Galaxy as a whole. And so, while we could, with perfect validity, use some reference frame that is very different from any of those, there's no practical reason to.
Yes, but they are over 600 Light Years apart, there can not be an event that takes place in one of the two frames of reference that is viewed simultaneously in both frames, they are incompatible. There is no now here and in Betelgeuze, so there is no now at all. We use our local frame of reference because there are indeed practical reasons to do so, but this frame is not at all the same as Beltegeuze's. And it is not absolute.
I wonder if we are talking about the same thing, perhaps this is an argument about words? What am I not getting about your argument? I thought we were discussing the possibility that Beltegeuze "has exploded 640 years and one day ago"? That makes no sense: if it was so we will only be able to know it tomorrow, so it did not happen 640 years ago, it will happen tomorrow. Or later. We can not know before the signal arrives, so it has not happened. Do you mean something different by "happened"?
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Old 12-30-2019, 06:47 PM
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Our frame of reference is not the same as Betelgeuse's, but it's incorrect to say that it's "not at all the same". It's very, very close to being the same. The relative velocity between Betelgeuse and Earth is approximately 34 km/s. That's 0.0001c, for a gamma factor of 1.000000006. There's no measurement we can make of Betelgeuse that has anywhere near that level of precision, so for all practical purposes, they're the same reference frame.
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Old 12-30-2019, 08:59 PM
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Astronomers know all about light travel time. It is called "aberration", aka light-time correction, and has been taken into account for centuries.
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Old 12-30-2019, 10:01 PM
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I don't know what it means to say that they are the same reference frame, or nearly the same, when they are separated by so much space

In this shared coordinate space, where on the Time dimension am I marking the events "Betelgeuse goes supernova" or "Someone posts the #61 post into the thread 'Betelgeuse losing brightness
'"?
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Old 12-30-2019, 10:08 PM
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Quote:
Originally Posted by Riemann View Post
Some rather astonishing supernova "trivia" (that doesn't seem like the right word) from xkcd What If:
Quote:
Which of the following would be brighter, in terms of the amount of energy delivered to your retina:

A supernova, seen from as far away as the Sun is from the Earth, or

The detonation of a hydrogen bomb pressed against your eyeball?
Answer halfway down here:
https://what-if.xkcd.com/73/
My important take-away is that if the sun goes supernova, then when all those photons finally reach us, it will be fast and painless. That's all I need to know.
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Old 12-30-2019, 11:46 PM
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My important take-away is that if the sun goes supernova, then when all those photons finally reach us, it will be fast and painless. That's all I need to know.
There is no mechanism for (the nearby) sun to explode like that, unless I'm missing something.
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Old 12-31-2019, 12:56 AM
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The sun is not going to supernova. Nor is Sirius or any other nearby star. Betelgeuse is probably the closest star that is going to supernova. It's certainly the closest red supergiant, although it's not required for a star to be one in order to supernova.
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Old 12-31-2019, 09:49 AM
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Rigel, a blue supergiant, is also nearby and may go supernova in the not-too-distant future, but it will first transform into a red supergiant. (Was the red supergiant Betelgeuse once a blue supergiant?)

Betelgeuse, BTW, is the answer to the trivia question: Which star has the 3rd largest apparent diameter as viewed from Earth? The #2 slot is held by R Doradus, a variable red giant 200 ly distant. Which star holds the #1 record?
SPOILER:
Our own Sun.
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Old 12-31-2019, 10:03 AM
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Being separated in space is completely irrelevant to reference frames. If two objects are at rest relative to each other, they are in the same reference frame. Both will agree on time intervals between events located at either, or at anywhere else in the same reference frame.
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Old 12-31-2019, 11:18 AM
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Being separated in space is completely irrelevant to reference frames. If two objects are at rest relative to each other, they are in the same reference frame. Both will agree on time intervals between events located at either, or at anywhere else in the same reference frame.
Yes, they will agree on time intervals for events that happen in one place, but not on time sequence for events that are located at different points and they will not agree on what they respectively call now. And that, I think, is the point we are talking past each other.
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Old 12-31-2019, 11:31 AM
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They will agree on time sequences of all pairs of events everywhere. And they won't agree on "now", which is a single event in both time and space, but they will agree on the question of whether two events occurred at the same time.

An astronomer on Earth, working in Earth's reference frame, would say that Betelgeuse's explosion happened 642 years before it was observed on Earth. An astronomer near Betelgeuse, assuming he survived, and working in Betelgeuse's reference frame, would also say that Betelgeuse's explosion happened 642 years before it was observed on Earth. If there were a mirror halfway between Betelgeuse and Earth, such that the astronomer at Betelgeuse could see the reflection of the explosion, then both astronomers would agree that the astronomer at Betelgeuse saw the reflection at the same time that the astronomer at Earth saw the original explosion.
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Old 12-31-2019, 11:35 AM
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Yes, they will agree on time intervals for events that happen in one place, but not on time sequence for events that are located at different points
There is no such thing as "two events that happen in one place".
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Old 12-31-2019, 11:50 AM
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There is no such thing as "two events that happen in one place".
I should have written "two events that take place in the same place but at different times", and I hope that is possible. Of course, if you argue that no place remains the same because pantha rhei and so on and therefore no event can take place there anymore because there no longer exists, well, yes. Thanks for helping me improve the precision of my written expression.
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Old 12-31-2019, 12:55 PM
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I should have written "two events that take place in the same place but at different times", and I hope that is possible. Of course, if you argue that no place remains the same because pantha rhei and so on and therefore no event can take place there anymore because there no longer exists, well, yes. Thanks for helping me improve the precision of my written expression.
I think you understood me but I'm not sure because your initial clarification still doesn't work. How can you determine whether two events are in the "same place"? It doesn't mean anything without an absolute frame of reference. Two events can seem to be in the same place relative to an inertial observer, but a different inertial observer will not necessarily agree. This is just a consequence of Galilean relativity, not even Einsteinian.

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Old 12-31-2019, 02:06 PM
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So you first specify a frame of reference, and then say that in that frame of reference, they're in the same place. No problem.
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Old 12-31-2019, 02:48 PM
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I'm confused by this whole discussion. Differences in frames of reference arise from relative velocities, not from physical distance. As mentioned, if two bodies are at rest wrt each other they are in the same frame of reference. The fact that one observer may not become aware of an event for while because of significant physical separation, is irrelevant, we can calculate time of the event based on speed of light/information. Two observers who are motionless wrt each other will agree on any simultaneous events, even if they have to wait to see both events. i.e. "that explosion happened 600 years ago in our mutual frame of refence, I did not know that until I just saw it now, 600 years after you did."

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Our frame of reference is not the same as Betelgeuse's, but it's incorrect to say that it's "not at all the same". It's very, very close to being the same. The relative velocity between Betelgeuse and Earth is approximately 34 km/s. That's 0.0001c, for a gamma factor of 1.000000006. There's no measurement we can make of Betelgeuse that has anywhere near that level of precision, so for all practical purposes, they're the same reference frame.
There will be a slight difference in perceived "simultaneity" of distance separated events between us and the Betelguesians, because of that gamma factor, but very slight - essentially, we are in the same frame of reference as Betelgeuse.
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Old 12-31-2019, 02:54 PM
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Are you postulating a natural universal frame of reference for time and the age of the Universe? And that the coordinate origin of the referece frame is distributed everywhere homogeneously (or where ever the Hubble Flow happens to take you drifting?) If so, I disagree.
So, obviously I misstated matters when I said there's "a" natural frame of reference for the universe. There's a natural comoving coordinate system, and a natural comoving local frame at each point, but (as Chronos noted) it's a different frame at each point, albeit the difference is only significant at intergalactic distances.

That implies a natural way to measure the age of the universe - the elapsed time on a clock that has always been at rest in the local comoving frame. And that would be the natural way to talk about the current age of the universe (for us, here, now), and the natural way to describe the time that light was emitted from a distant galaxy that we see - the age of the universe for that galaxy at that time.

I don't think there's any contradiction between this and how we think about spacetime under SR.

Last edited by Riemann; 12-31-2019 at 02:57 PM.
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Old 12-31-2019, 03:12 PM
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That's certainly one valid way to discuss cosmological measurements. But you have to be careful, on cosmological scales, to specify exactly what you mean, because a lot of things that we think of as having well-defined meanings cease to do so when used on cosmological scales. There are a half-dozen different ways to define "distance", for instance, and on small scales, they all agree, but on large scales, they're all different.
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Old 12-31-2019, 03:27 PM
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That's certainly one valid way to discuss cosmological measurements. But you have to be careful, on cosmological scales, to specify exactly what you mean, because a lot of things that we think of as having well-defined meanings cease to do so when used on cosmological scales. There are a half-dozen different ways to define "distance", for instance, and on small scales, they all agree, but on large scales, they're all different.
That's what redshift measures in extragalactic observations, right? It corresponds (under a GR model) to the age of the universe when the light was emitted. Astronomers don't tend to talk about the distance of observations at that scale, because it's not so clear what that means.

Last edited by Riemann; 12-31-2019 at 03:28 PM.
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Old 12-31-2019, 05:17 PM
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Well, that, and even if you're clear on what notion of distance you mean, you still need to calculate it, and that depends on precisely how the Universe has been expanding since then, which has some uncertainties. But the redshift is clearly and directly measured, to a high degree of both precision and confidence.
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Old 12-31-2019, 09:01 PM
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Where's the kaboom? I was expecting an Earth-shattering kaboom!
You may have to settle for that neutrino pulse.
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Old 01-06-2020, 01:40 AM
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Just pretty lights, or a catastrophe?


Just how dangerous to us would Betelgeuse going supernova be? Would it cause health or climate effects dangerous to us or infrastructure we depend on? Would the delivered thermal energy contribute to global warming? Would the radiation cause massive deaths from radiation poisoning, a more moderate effect in increased cancer rates or birth defects, problems with livestock, mutated supergerms, a zombie apocalypse? Would it crash the Internet or screw up my TV reception? And how long a delay would there be between when the visible light that lets us see the event arrives and the following storm of radiation arrives?
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Old 01-06-2020, 02:22 AM
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No danger at all.
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Old 01-06-2020, 08:36 AM
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No danger at all.
Well, the thing would be about as bright as the moon, so I guess maybe somebody could crash their car staring at it. Also, maybe increased incidence of werewolves?

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Old 01-06-2020, 08:45 AM
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I wouldn't say no environmental effects at all. There are probably some living things for whom moonlight levels are relevant, and the addition of another light source comparable to the full moon might be disruptive to them.

But that's about it.

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And how long a delay would there be between when the visible light that lets us see the event arrives and the following storm of radiation arrives?
Light is radiation. Whatever gamma rays and X-rays we get from the supernova (and there will be some, just not enough to worry about) will arrive here exactly as quickly as any other sort of light.
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Old 01-06-2020, 10:40 AM
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I wouldn't say no environmental effects at all. There are probably some living things for whom moonlight levels are relevant, and the addition of another light source comparable to the full moon might be disruptive to them.
Corals use the full moon to synchronize their spawning. But they also use other environmental factors, such as water temperature, too. And they only do it once a year. If the supernova happens at the wrong time, it might change the timing. It's possible that that might delay it too much and the spawning won't be very successful.

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Light is radiation. Whatever gamma rays and X-rays we get from the supernova (and there will be some, just not enough to worry about) will arrive here exactly as quickly as any other sort of light.
Likely he meant high energy particles from it. Neutrinos will get here before the light does. We don't know the exact speed of neutrinos, but it's really, really close to that of light. But it also will escape from the supernova immediately, while the light will take a few hours to work its way out. In fact, there's a Supernova Early Warning System, a collaboration of various neutrino observatories, that will alert astronomers to supernovae based on the detection of large numbers of neutrinos.

Other stuff will get here at various times depending on how much energy they have. There's no specific speed of electrons, protons or other particles in space.
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Old 01-06-2020, 11:49 AM
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Which in turn means that those other particles would be even less dangerous. Even if the supernova produces a sharp pulse of, say, protons, they'll be at a wide range of different energies, and hence speeds, and hence arrive here over a very long period of time instead of being a sharp, sudden pulse. Like, if one proton is going 99% of the speed of light (a quite impressive speed for a proton), and another is going at 98% of the speed of light, there will be over six years between their arrivals.
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Old 01-06-2020, 12:09 PM
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Another thing is that for charged particles (electrons, protons, alpha particles, etc.), the less enegy they have, the more their path is modified by the galactic magnetic field. So those will take a longer path to travel here, which of course, takes more time.
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Old 01-06-2020, 12:11 PM
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Most countries have gamma, neutron and alpha detectors placed at ports to screen incoming traffic and passengers for radioactivity.

Am I right in thinking that those detectors will malfunction if said giant goes supernova?
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Old 01-06-2020, 04:15 PM
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Neutrons won't make it here at all. They have a half-life of around 15 minutes, far too short, even at ludicrous time dilation factors, to make it 640 ly.

Alpha particles could make it here, but really, really slowly (by radiation standards), and they're stopped by almost anything.

Gamma rays will make it here fine, and will be detected by satellites, but the atmosphere is actually mostly opaque to gamma rays. Again, surface instruments won't see much.

Ground-based neutrino detectors will definitely detect it, but nobody but scientists has any reason to care about neutrinos.
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Old 01-06-2020, 04:41 PM
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I wouldn't say no environmental effects at all. There are probably some living things for whom moonlight levels are relevant, and the addition of another light source comparable to the full moon might be disruptive to them.
Moths would get lost, or at least very confused. Likewise any other critters that navigate by moonlight.
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Old 01-06-2020, 07:58 PM
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Every person on Earth would get about two dozen neutrino interactions in their body from a Betelgeusian supernova. Not biologically relevant, but neat.
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Old 01-06-2020, 11:52 PM
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Every person on Earth would get about two dozen neutrino interactions in their body from a Betelgeusian supernova. Not biologically relevant, but neat.
If I get some kind of chance neutrino interaction, might it turn me into some sort of mutant super-hero?
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Old 01-07-2020, 08:27 AM
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Every person on Earth would get about two dozen neutrino interactions in their body from a Betelgeusian supernova. Not biologically relevant, but neat.
Will these neutrinos be particularly energetic? I thought each of us would be struck by only a few quadrillion extra neutrinos when Orion's Right Hand explodes; is that enough to make interaction likely? IIUC, each of us is attacked by far more neutrinos than that over a lifetime, but is lucky (unlucky?) if even one interacts with us.
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Old 01-07-2020, 12:02 PM
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If I get some kind of chance neutrino interaction, might it turn me into some sort of mutant super-hero?
Statistically, I think it's more likely to turn you into some sort of mutant super-villain.

[I haven't done rigorous statistics, but every mutant super-hero seems to have fought multiple mutant super-villains]
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Old 01-07-2020, 12:15 PM
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Moths would get lost, or at least very confused. Likewise any other critters that navigate by moonlight.
What are the critters that navigate by moonlight? The only one listed in wikipedia is the scarabaeus zambesianus, an African dung beetle.
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Old 01-07-2020, 02:50 PM
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Will these neutrinos be particularly energetic? I thought each of us would be struck by only a few quadrillion extra neutrinos when Orion's Right Hand explodes; is that enough to make interaction likely? IIUC, each of us is attacked by far more neutrinos than that over a lifetime, but is lucky (unlucky?) if even one interacts with us.
My estimates:
Of the kajillions of neutrinos passing through you normally (mostly from the sun), one will interact with an atom in your body on average every five years for a lifetime total in the teens. Betelgeuse going supernova would surpass this total, producing 20 or so interactions in your body in just a few seconds. And Betelgeuse is 4x107 times further away from us than the sun. Core-collapse supernovae are... impressive.

Regarding energies: The neutrinos themselves are about as energetic as (or just a bit more energetic than) typical products of nuclear radioactive processes, with energies mostly in the 5 MeV to 40 MeV range.
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Old 01-07-2020, 04:44 PM
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For what it's worth, one of my old professors studies Betelgeuse. I asked him what he thought of the recent dimming, and he says that it's certainly at an unprecedented low, but that he doesn't think that it means the end is imminent (though he reiterated that nobody knows what it looks like that a star is about to blow). He thinks the dimming is most likely due to a drop in temperature due to pulsations in radius, but that some folks are entertaining the notion that it's exhaled a dust cloud that's partly obscuring it.
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Old 01-07-2020, 04:49 PM
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He thinks the dimming is most likely due to a drop in temperature due to pulsations in radius, but that some folks are entertaining the notion that it's exhaled a dust cloud that's partly obscuring it.
I'm surprised that spectroscopic observations haven't ruled out one of the theories by now.
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Old 01-07-2020, 06:12 PM
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I'm guessing that the difficulty there is that Betelgeuse isn't one uniform temperature, so its spectrum will be more complicated than tat of a smaller star. I'm sure it's being studied, but the analysis takes time. Especially since the grad students are probably on break.
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Old 01-07-2020, 06:26 PM
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I'm guessing that the difficulty there is that Betelgeuse isn't one uniform temperature, so its spectrum will be more complicated than tat of a smaller star. I'm sure it's being studied, but the analysis takes time. Especially since the grad students are probably on break.
Grad students take breaks now??

I thought dust was fairly conspicuous in IR, but I suppose if it's a newly produced dust cloud, it wouldn't be in thermal equilibrium and be difficult to understand the observations.
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Old 01-08-2020, 04:59 PM
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No danger at all.
never underestimate the power of astrologers.

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