>Of course, it's important to bear in mind that "just about to explode" means something different in cosmic terms versus human terms. It could happen at any time in the... next few hundred thousand years.
I don't know what sort of luminosity a GRB would have at 8k LY. But if it's strong enough to be an ELE, that's still a pretty short timeframe, in case the expected GRB jets will be pointed at us. We'd need to not only get off the planet, but to another star system entirely.
Think about the odds of the beam being perfectly aligned and aimed to hit the planet with dead-on accuracy though. Even if the spread of the beam is as wide as Pluto's orbit, we'd have to be pretty unlucky to get hit.
If it's as narrow as the arc of a season, one quarter of the Earth's orbit, it could hit us during Autumn or miss us during Winter, or not at all if it's not pointed into the path of our orbit.
Out of curiosity, just what would happen to a person standing on Earth, staring up at the night sky right at the moment that a GRB jet hit the planet?
Also, how wide, in terms of arcseconds, is the jet? Is it a narrow laser beam that has no chance of actually hitting a planet, or is it wide enough that anything within 30 degrees of north is going to be sterilized?
Most likely (as in, very most likely) not bad [1], though it's theorized that the second largest mass extinction event on Earth may have been due to a GRB [2].
Ironically, in the specific case of a imminent gamma ray burst, we might actually have better luck staying on the planet, with eight thouand miles of rock and metal to act as ablative shielding. I'm not sure how far underground we'd need to be to survive the backscatter from eg the moon, though, and I doubt the planet would be habitable afterward anyway.
It's not the gamma rays that kill you. In particular even with a supernova strength source, it's unlikely that the gamma rays would even have enough energy to get through the atmosphere to the point of meaningfully heating the ground. However, they would butcher and greatly change the atmosphere which, in turn, is what kills you - probably through a mechanism of massive amounts of UV and other high energy radiation now happily making its way down to us. As an example, a gamma ray burst is one possible hypothesis for the Ordovician extinction.
Best case for survival would be to become multiplanetary. In particular the conditions that would be created would be survivable by the exact same technology that will be the baseline for human colonization on Mars.
I have to admit, I was not expecting to have been underestimating the intensity of a gamma ray burst.
> even with a supernova strength source
In my defence, my reference power source was "supermassive black hole falling into another supermassive black hole" (probably due to having heard either GRBs in general or one specific one attributed to the collapse of a quasar).
Depends primarily on how far away it is. Scientists are pretty sure they're only 2-3 degrees wide at the source, so the closer you are, the less distance you would need to travel. Of course, the farther you are, the less energetic it is.
So, if GRB are 2 degrees wide at the source, and it occurs 8000LY away, and is pointed directly at us, we'd need to travel 136LY away.
The solar system is moving at 1/1300 the speed of light. I'm not sure how Apep is moving relative to us, but assuming it's stationary, we'd need ~175k years to move out of the crosshairs by sitting here on the planet.
In particular, the sciencealert article mentions "very old" Wolf-Rayet stars, which is a little strange given that they only last a few million years, and certainly are not old on any scale used for stellar lifetimes.
I’m wondering what sort of physics research this opens up before the burst. I imagine it’s still producing energy levels otherwise difficult to create.
It's not click-baity. Any headline like this can only mean "about to on a cosmological scale", and I understood it as such immediately, and I also felt excited, and the excitement was warranted because the news really is exciting.
-I had the unexpected pleasure of sitting next to a geologist on a domestic flight a few years ago; he explained about an unstable mountainous area he’d surveyed, stressing that it would come crashing down at some point.
I inquired as to /when/ we could expect this to happen, as it sounded imminent.
«Oh, any day now - definitely in the next 30-50,000 years!»
And here I am, with project deadlines measured in hours...
The whole site has a trashy click-bait vibe. They're even using the classic click-bait word "jaw-dropping" in the story title.
I've noticed other news outlets who picked up this story used the alternate click-bait word "breathtaking" to describe the image. It's usually one or the other.
If you want to interpret it that way. If you have some idea of cosmological scale and time frames you're probably willing to read the headline in those terms.
After all at 8,000 light years what we're seeing right now "just happened"... near the beginning of recorded human history. Does absolutely everything need to be written from the perspective of default human biases?
That is called "not happened yet" in our lightcone.
It's an easy concept to grasp, but many people resist considering an event to have not yet happened when it has not yet crossed our lightcone. If you are really interested, then I suggest Stephen Hawking's book A Brief History of Time.
It is debatable whether or not that is the correct way language should handle whether or not an event has happened. Counterexamples in two situations:
1) The cosmic microwave background is, in short, the light still traveling from an early stage in the big bang when the universe turned transparent (380,000 years after the big bang, when the universe became cool enough to form hydrogen atoms)
You can see the big bang happening. So when would you say this phase transition to transparency happened? Here you would say it happened 13.5 billion years ago. At the source of the CMB photons you can see, you would say that is happening now.
With that language construct you have an event which happened across the universe simultaneously happening both 13.5 billion years ago AND right now, depending on location. Which, I think, is absurd.
2) Talking to an astronaut on Mars. Mars is between 3 and 22 light-minutes away. If you want to say hello to your Martian friend, it will take between 6 and 45 minutes for your radio to play the sound of your friend saying hello in response.
What is happening while you are waiting for the response? (how do you discuss the time, what "now" means) To your friend your message has "not happened yet" while to you it already happened.
It is silly to have a definition of "to be" that must ignore the practicalities of simultaneous events which involve non-trivial light transit time. It is very easy to talk about a "now" where both Earth and Mars have the same timeline dispensing with the light cone and talking about light travel taking time. It is not so easy the other way around.
All these ambiguities disappear if you choose a reference frame. From our reference frame, the explosion hasn’t happened. From the stars’, it might have. The issue is popular imagination, and thus common language, deals with reference frames sloppily.
> common language, deals with reference frames sloppily.
I deals with it fantastically, unless otherwise stated it's implied we are talking in the local reference frame. That convention gives us nearly infinite information density because we don't have to state it.
You make terrific points. The problem arises, for both points, with believing that time is a background against which everything happens. For most practical purposes this is fine, and it is very difficult for humans to accept that time is not actually the background against which everything happens. I'll do my best to address your points, but as a layman I'm really not the best person to do that.
Regarding the CMB, I have no problem stating that the Big Bang is happening now in a distant part of the universe. That is an interpretation that is easy for humans to understand, and it's not really wrong since we are now receiving the photons. As a very poor Newtonian analogy one could imagine it to be similar to how a flame front in a combustion chamber propagates.
As for near-real-time two-way communication, I don't think that humans will ever be developed enough to accept the non-agreeing simultaneity. We will continue to say that Mark Watney said something 15 minutes ago when we hear his voice. For practical purposes, it is the best way to deal with the situation, even if it is technically wrong. There is precedent with knowingly using terms incorrectly, just off the top of my head we (metric-using people) typically answer with our mass, not our weight, when asked what we weigh. If I were to tell someone that I weigh 700 Newtons they would have no idea what I'm talking about.
As far as I understand it, there is no such thing as simultaneous events that don't occupy identical locations. There is no "now" that is shared and can be discussed meaningfully. Simultaneity can only exist within a spaciotemporal frame of reference. E.g. events that appear simultaneous on Earth will usually not appear simultaneous on Mars.
This may seem silly, but this is how reality works and as humans adjust to opperating in multiple spaciotemporal frames of reference, our language will have to adapt
>There is no "now" that is shared and can be discussed meaningfully.
This is only true if you spend too long navel gazing.
Relativity may have made a strict, absolute, universal "now" and "simultaneous" obsolete or non-existent, but language can't and won't adapt to the absurdity of claiming they don't exist. No one was making grand claims about absolute certainty of simultaneity before relativity, saying it doesn't exist has really only minor effects.
Language is contextual and with context it is indeed silly to claim "now" doesn't exist. Language can understand frames of reference and acknowledge disagreements without throwing out everything.
Unless photons and black holes start having conversations.
I'm afraid it is true regardless, at least in cosmic scales. "now" is entirely dependent on your reference frame, and it is possible to construct two frames of reference such that if they could instantly communicate they would receive eachother's responses before the original messages were sent. See: Tachyonic antitelephone [0].
> This is only true if you spend too long navel gazing.
Not at all. "Now" can only have meaning withing a spaciotemporal frame of reference. We are used to operating within a single such context and our language assumes that.
Your point 2) explicitly brings up people from two different contexts discussing what "now" means.
While language that assumes a single spaciotemporal reference frame will continue to be used and to be useful, that doesn't make it useful for discussing systems with multiple frames of spaciotemporal reference.
It's not the light cone that matters for simultaneity, it's the reference frame. Everyone in the same reference frame (or close enough) is going to agree about the order of events once they compare notes when the dust settles. I don't think this system is moving fast enough relative to ours for relativity to screw things the common sense meaning of "happened yet". At least, this is how I was taught in my college physics class.
> Everyone in the same reference frame (or close enough) is going to agree about the order of events
Not necessarily. On a macro scale for general human interactions, for the most part, but the only thing observers have to agree on is space time interval and causality. Space, time, and order (if not important to causality) might be different.
That's because humans are mostly all in the same reference frame. And hey, so is this binary system, within astronomy-
sized error bars anyway. I'm not sure the terminology of "observer" is totally consistent, but it's definitely true that you can only disagree with another observer on ordering of events if you're in a different reference frame, basically if you're moving at a (noticeably) different velocity.
In the presentation of SR I saw, "observers" are basically identical with reference frames. What is "observed" is exactly the sequence of events pieced together from instruments in that reference frame, close enough to "see" all the relevant events with negligible lightspeed delay, all with synchronized clocks. "Seeing" a distant event, the time when light from that event reaches a particular point, is an entirely different question that, frankly, we mostly ignored.
I see confusion about reference frame. It is perfectly okay to say something is happening at 1 million light years away. And there is no ambiguity within our presumed Earth reference frame. Of course we won't know for another million years if such a claim is true. But if it is, it is perfectly sane to say it happened a million years ago when we do observe it. The point is given a reference frame, it is perfectly fine to specify an event with location and time, so long as you specify both location and time. These have nothing to do with light cones.
[Add] To give another more relevant example if we observe a signal today coming from a location one million light years away, there is no ambiguity to say it happened a million years ago at such and such location. Anyone in a different frame can compute the time and location of the event unambiguously for their own frame, knowing the relative velocity between the frames.
> It is perfectly okay to say something is happening at 1 million light years away.
Well, maybe. Maybe not. Carlo Rovelli would argue that time doesn't exist like that (or at all) and that saying that something is happening now when it is beyond our lightcone would be a meaningless statement, and a lot of physicists would agree with him. (A lot would disagree, too.)
Reference frames only clear this up if time does exist in a meaningful way and 'now' is more than a construction of our own observation.
Rovelli's position is that time is not fundamental, not that it does not exist. It emerges from something more fundamental.
That is, he argues that there exists a theory (or family of theories) from which one can derive both General Relativity and Relativistic Quantum Field Theory in their effective limits, and that such a theory does not need to be paramaterized by time (or more broadly, does not need to explicitly reference the notion of time at all).
More precisely, he and Connes propose[1] that statistical regularities in a generally covariant quantum system (our block universe) are just as much "laws" as dynamical relations like F = ma or F = dp/dt = \gamma(v)^3 m_0 a_parallel + \gamma(v)^1 m_0 a_perpendicular. There are macroscopic constraints and a low-entropy initial condition in the universe that takes our system away from being in just any configuration with equal probability, and towards a configuration with an entropy gradient. From that entropy gradient we can recover a notion of time, and even coordinatize it. Time emerges in a block universe (and indeed most subregions thereof) arranged in this way.
> something is happening now when it is beyond our lightcone
Galaxies probably don't cease to exist at the moment they cross out of our causal cone with the metric expansion, or soon afterwards. We exist even though there are observers who saw our ancestors cross out of their causal cones. Do you think the density and spectrum of the CMB evolve very differently for them and for us? Do you assert that such a question is meaningless? Do you think Rovelli does? (If so, why do you think that?)
- --
[1] A. Connes, C. Rovelli, "Von Neumann Algebra Automorphisms and Time-Thermodynamics Relation in General Covariant Quantum Theories", Class. Quantum Grav. 11:2899–2918, 1994.
>Rovelli's position is that time is not fundamental, not that it does not exist. It emerges from something more fundamental.
Well, I'm a layperson, and I'm guessing you are a physicist (or at least much more inclined towards it than I am), so I'm hesitant to argue here, but...
If that's what Rovelli means, he really should say that :)
I've got The Order of Time in front of me and in it, as well as plenty of articles and interviews (targeted at laypeople), he says time doesn't exist, uses phrases such as 'A world without time', etc. Good portions of the book are prose where he muses on things like the meaning of life if time doesn't exist. He calls our perception of it an illusion.
> our block universe
In Order of Time he specifically says '[Our world] is not a "static" world, or a "block universe"'.
I feel disconnected here, because I'm definitely not able to argue with what you're saying, but Rovelli's words from a book 23 years newer than your referenced article, seem to directly contradict what you're arguing - he explicitly states he does not believe we live in a block universe.
>Galaxies probably don't cease to exist at the moment they cross out of our causal cone with the metric expansion, or soon afterwards. We exist even though there are observers who saw our ancestors cross out of their causal cones. Do you think the density and spectrum of the CMB evolve very differently for them and for us? Do you assert that such a question is meaningless? Do you think Rovelli does? (If so, why do you think that?)
No, I am saying specifically that saying something is happening /now/ when it is outside of our light cone is a meaningless statement. It's not a matter of whether or not something exists when outside of our light cone (Though in some ways, as it can never effect us, it might as well not), but whether or not something distant can happen now. It doesn't even need to be outside of our light cone - just far off. 'A present that is common throughout the whole universe does not exist. There is a present that is near to us, but nothing that is "present" in a far-off galaxy. The present is a localized rather than a global phenomenon."
Early on in the book, Rovelli argues that the concept of "now" or the present really only applies on a scale that's about the size of the Earth.
As for what I believe, I'm not sure. I think Rovelli makes compelling arguments. But I could say the same about Smolin and others. I think I lean towards Rovelli's interpretation, or at least what I (hopefully!) understand of it.
I'm perfectly willing to accept that you could be totally correct here and that I am wildly off base - but what I don't understand is why Rovelli's non-science paper writings seem to argue very differently than what you are saying. Am I misinterpreting them? Is he dumbing things down for the layperson? Why does he explicitly say we do not live in a block universe, and that time doesn't exist?
I haven't read Rovelli's pop-sci book. Could you glance through the table of contents and/or the index and look for, among other things, "thermal time" and "time emerges in a world without time"? I would be astonished if he did not discuss these ideas at length in his book.
> Rovelli's words from a book 23 years newer than your referenced article, seem to directly contradict what you're arguing
Ok, let's deal with that. It's not like he stopped writing on the topic or substantially changed his position (rather than elucidating it).
You can click on arXiv:1505.01125, arXiv:1407.3384, and arXiV:0903.3832 in the search below; each paper directly supports what I wrote. The first two digits of any arXiv ref is the year of submission.
however not all of those are directly relevant to this particular argument.
> he explicitly states he does not believe we live in a block universe
Can you extract from the book the couple paragraphs around where he says that explicitly? I bet it'll be easy to clarify. I'll even take a preemptive guess.
We'll have to digress briefly into (hopefully scientific) philosophy.
Unfortunately philosophers have several different versions of "block universe" with varying degrees of determinism. The growing block universe, for instance, holds that the past exists, that the present is an objective quantity, and that the future does not yet exist (as in it is not fully determined) at the the present of the growing block. Is it possible that Rovelli was rightly criticizing that, and you or he or his editor missed a "growing" in "growing block universe"?
Returning to his Quantum Gravity book, I'm happy to recommend §2.4 generally, although it is far from pop-sci. In 2.4.4 (p 58, pdf page 76) he distinguishes ten different notions of time. Clearly not all of these can be equally fundamental; his argument is that none of them is fundamental in that they are demanded by all possible theories compatible with General Relativity in its classical limit. However, as some of the ten are directly measurable, one cannot (and he does not) argue they they do not exist -- rather they emerge from correlations between observables.
Now we can leave the philosophical.
The scientific content of his Quantum Gravity book's central conjectures are that not only is it feasible to write down physical behaviours without recourse to time as a fundamental quantity, those behaviours' descriptions take an especially simple form when one does so in the way he proposes through the course of the book. The first of these points is not really scientifically controversial.
> No, I am saying specifically that saying something is happening /now/ when it is outside of our light cone is a meaningless statement.
There is no single correct definition of now in a general curved spacetime; there is instead a democracy of "now"s. Each microscopic portion of your body has its own separate causal cone, and at any particular instant the ones on microscopic things near your scalp are spacelike separated from the ones near the soles of your feet. Does that make stubbing your toe meaningless?
> "The present is a localized rather than a global phenomenon"
It's an idiosyncracy rather than a phenomenon at all. By the time you have perceptual awareness of your toe-stub it is ancient history on the scale of electron interactions between your skin and the leg of the table or whatever, and indeed the daughter products of the scatterings (there will be at least some photons and gravitational waves shed) will have raced past your brain to as far as outside the orbit of the moon. You have free choice of "nows", and no fundamental reason to choose which of them to apply.
> localized
From a purely technical perspective, we can always apply Fermi coordinates to anything freely falling in a Lorentzian spacetime (as ours is, to the best of our ability to test directly or via astrophysical observation). This guarantees a minimum definition of "local". However, the coordinates are peculiar to the object; although we can certainly extend Fermi coordinates into a https://en.wikipedia.org/wiki/Local_reference_frame?oldforma... , frames of reference do not determine the physical behaviours. As you and I drift past each other in deep space, there are only personal reasons to choose a set of Cartesian coordinates with you at the origin over a set of Spherical coordinates with me at the origin.
If we drift past each other with high enough relative velocity, then even when we are extremely close to each other -- much closer than the scale of the earth -- your wristwatch now and my wristwatch now will not generally agree. (Assuming they are identically constructed, if they agree at some instant, they will disagree at other instants). This is true even if at all relevant times we are well within each other's causal cones.
So some ideas of "now" fail at scales much smaller than that of Earth's radius.
Others might survive at scales much larger: we've been doing Einstein-synchronization experiments since the Viking missions to Mars, and we can straightforwardly predict https://en.wikipedia.org/wiki/Barycentric_Dynamical_Time anywhere in the solar system. So we can say at time X TDB Mars-based spaceprobe should activate instrument Y. Although it might take us a couple minutes to verify that spaceprobe activated Y at X, we can be pretty confident before those two minutes expire as our TDB clock registered X, its TDB clock also registered X.
Likewise, we can synchronize in principle with a distant observer of the Cosmic Microwave Background such that we each agree to do something when the CMB's dipole-free spectrum is closest to a blackbody of X kelvins. Assuming honesty, we can be pretty confident that millions of years later we will see that their "CMB clock" registered X kelvins when they began the agreed action.
These are among the infinite supply of idiosyncratic "now"s that can be useful even if they are in no way fundamental. Utility surely implies meaningfulness, even if neither utility nor meaningfulness implies universality.
> As for what I believe, I'm not sure.
Good. Neither am I. Neither is Rovelli, if he is honest. There is a lot of room to research the various problems of time.
> I don't understand [...] why Rovelli's non-science paper writings seem to argue very differently than what you are saying.
As I said, I don't know what the non-science papers say exactly; if you feel like pasting extracts in reply, we can try to sort this out. Any of your guesses in your final paragraph could be right, or it could be something else.
As an aside to my earlier direct reply, these are foundations-of-physics issues. Most working physicists don't really care what are in the foundations, to the great annoyance of many philosophers. (Tim Maudlin comes instantly to mind.)
But annoyed philosophers don't change the results from numerical relativity.
"The very foundation of general covariant physics is the idea that the notion of a simultaneity surface all over the universe is devoid of physical meaning. Seems to me that it is better not to found hamiltonian mechanics on a notion devoid of physical significance."
Sure, nobody should disagree that such a surface is unphysical. However, 3+1 decompositions are at the bottom of successful results -- in particular you will be hard-pressed to find numerical relativity projects that don't split spacetime into spaces organized by some arbitrary time coordinate, where each space is a large enough simultaneity surface to trigger his fundamental objection. Yet we have good matches to real astrophysical results, for example Ransom, Archibald et. al, https://arxiv.org/abs/1401.0535 ( http://www.astron.nl/~archibald/video.html )
A complete theory from which CQG emerges in weak gravity is a good goal, and Rovelli is pursuing it, while trying to keep the good bits of modern physics (in particular that we've made practically all laws of physics generally covariant or at least relativistic, and that this is not just useful, but reflects something real about the universe).
More power to him. But his success in his project has practically no impact on the success of modern uses of General Relativity (in various formulations), or its complete accord with all available evidence accumulated so far. Seriously, there is no counter-evidence. It is mathematically complete. The only thing left for it is to study the mechanisms that generate the metric and the microscopic details of sources. (Maybe we can combine the two to study what metric large quantum systems (~ milligram rest masses) actually source when prepared in superpositions of position, for example. We think there's some problem of some sort there because semiclassical gravity -- relativistic quantum field theory on a "background" of standard General Relativity -- predicts something nonsensical; maybe Rovelli's work in the foundations will find a way to make a reliable non-nonsensical prediction there by the time we can prepare such a large quantum system. FWIW, progress on that front is being made: http://sciencenordic.com/can-large-objects-exist-quantum-sta... ).
Wow I am just floored by the thoroughness of your answers. Just want to let you know that someone enjoyed your effort though tbh I may never fully digest all you said here (and I do plan to read what you wrote a couple more times).
"Most working physicists don't really care what are in the foundations, to the great annoyance of many philosophers."
I was in physics grad school over twenty years ago. Your statement is accurate as far as I can tell. I did and do care about certain foundation issues. In particular I care whether free will is even fundamentally possible. However I never find any philosophical discussion capable of advancing my understanding "meaningfully", as in "to a physicist".
> I did and do care about certain foundation issues.
I meant more that while many foundational questions are interesting, the absence of conclusive answers don't stop most working physicists from making progress in their areas of professional interest.
For example:
> I care whether free will is even fundamentally possible
We act like it is. Similarly, we predict our past with greater fidelity than we predict our future, and we have a sense of "now".
Are these behaviours tied to something physically fundamental, or are they emergent? Nobody knows.
It'd be nice if we could have a long chat with a grey parrot or an octopus and find out whether it has a different view of past and future. Maybe we'd learn that it remembers in both directions equally well, and so doesn't really distinguish between them. Perhaps our future-directed memory was just an anatomical sacrifice like many others during the food-energy bottlenecks that dot humanity's evolution, and the cost of survival of our ancestors is us having a peculiar attitude towards time (and sports, like Wayne Gretzky's line about skating to where the puck will be) compared to less closely related organisms on our planet. ("Squawk, the future-memory part of your brain is missing, like the part of your brain that would deal with your vomeronasal organ. You will pass me a cracker now, while convincing yourself that you decided to. Squawk."
Since we are unable to have such interspecies conversations right now, we can only discuss this among ourselves, and are all stuck with essentially the same anatomy and no fossil early human brains to compare to ourselves.
I can understand the temptation to think that the very common idea of a fixed past and an undetermined future is a feature of the universe as a whole rather than something like impacted wisdom teeth or an inflamed appendix.
On the other hand, I think outright working (as in a likely self-funded job) on answering free-will-or-not is somewhere between pretty unproductive philosophizing and early symptoms of the sort of obsessions which overtook Linus Pauling, Brian Josephson, William Shockley, or Jack Parsons (among sadly too many others). Yes, that's a dickishly prejudicial thing to admit to, whether or not the attitude was determined by the information on a slice of the universe coupled to physical laws that let one recover the information everywhere else in the universe. As almost all of the content of any cross-universe slice is practically inaccessible to us, blaming eternalism or determinism for my "hot take" seems like a cheat.
In practice, though, none of this is likely to even occur to me consciously as I grind through different field equations, or try to match results from a static (time-independent) spacetime to a mostly-similar dynamical (varying over time) one, for example.
Isn't there two events here: the actual supernova and our observation of the supernova. So the actual supernova event could have occurred but our observation is when it crosses our 'lightcone'?
You are right, there are two points in space-time (the definition of the word "event" in special relativity). But the "space time distance" between them is zero! (another way to say that is that the two points have light-like separation)
All of the weirdness of special relativity stems from that definition of "space time distance", which is just an extension of Pythagoras' theorem:
distance = sqrt(x^2+y^2+z^2-(c*t)^2)
x, y, and z are the differences in spacial coordinates between the two events (in a given coordinate system) and t is the difference in time coordinates (in the same coordinate system). The minus sign is where the interesting effects stem from.
The issue is that "now" is not well-defined since it depends on the reference frame. Therefore what we consider "already happened" another observer may consider "still in the future".
All observers will agree on the order of events if and only if the light from each event traveled to the next before the next happened.
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That is extremely incorrect. If for example we know for sure that the observations we've made so far lead to supernova explosion in 10 years and thus the resulting life-threatening gamma burst reaching Earth in 2028, it would make sense to do something before the actual "observation" of the burst.
In relativistic terms, "the actual supernova" is an event (an x, y, z, t quad). It's a perfectly well understood entity in relativity. The point in relativity is that different coordinate systems put different numbers on x, y, z, and t, but they all are talking about the same event.
When we see it, we will be able to put a perfectly accurate x, y, z, and t on the supernova in our coordinate system, with t being "8000 years ago". All this has nothing to do with relativity, merely with the finite propagation speed of light.
If we call the moment at which the light reaches us t=0, then we will say that the supernova occurred at t=-8000, because we know where it occurred, and we know what the speed of light is.
General relativity provides a toolset for a complete transformation from "our coordinate system" to any other: a diffeomorphism between coordinates, and laws of physics written in generally covariant form and obeying some constraint equations for values on any three-dimensional submanifold.
If the supernova's matter is that of the standard model of particle physics, you get everything after the comma in the previous paragraph.
So any two observers of the supernova have a straightforward mechanism for relating observations against their idiosyncratic choice (or choices, they aren't restricted to just one!) of system of coordinates. The actual observables are independent of the choice of coordinate systems.
You'll find (mathematical) proofs of this in discussions of the initial value problem in General Relativity; (physical) evidence is the same as that from astrophysical tests of General Relativity.
As a practical matter, realistic observers will only measure a tiny subset of the total observables of the supernova, and different observers will measure a different subset, even if they use precisely the same system of coordinates. Obscuration by dust or distance is not caused by the coordinates in which one expresses distance or the size of dust particles.
> There is no "the actual supernova". That's the whole point of relativity.
No, coarsely, there is a 4d-worldtube along which one finds a WR star, a supernova of type Ib or Ic, and a remnant. Or more finely, there are worldtubes for large numbers of particles that are bound together gravitationally and through standard-model interactions into these more macroscopic objects. In some compact region of spacetime a collection of these particles is best described as the start of a Ib or Ic supernova; their worldtubes diverge (possibly spectacularly) elsewhere in spacetime. Or, if you like, we take a roughly 4-cylindrical section through the spacetime-filling fields of the standard model such that somewhere in that 4-cylinder the excitations of the fields are best described as the start of a Ib or Ic supernova. Really, it doesn't matter, because we know mathematically how to relate each of these descriptions, and have a good idea about why we might choose one description over another when asking various questions. That is the whole point of relativity.
Yep, if we haven't observed it, it is effectively not happened.
Same as finally observing some of the first galaxies created after the big bang. Speed of light ~= speed of information. If we haven't seen indication of it directly or indirectly, its at best an inference.
That's a bit like saying "Going 50 miles an hour is a meaningless concept, you should say 50 miles an hour relative to the surface of the earth". There is an implied reference frame here.
Since it would be impossible for us to know given information can only be transmitted at the speed of light, I think it's fair to say it hasn't happened yet, as far as we're concerned.
Seems like there's either a speed of light and things can happen before we see them, or light's transmitted instantaneously. Doesn't make sense to talk about light's speed if the thing producing the light doesn't "happen" until the light reaches the observer.
I'm not sure what you mean here. Talking about the "speed of light" makes perfect sense when there's a propagation delay between when the light is emitted and when it is perceived by an observer. And how can something be said scientifically to occur if it hasn't been observed yet? Whether or not things are actually happening but we can't observe them is moot, because it's not possible for us to observe them before we see them.
I'm not sure where the confusion is, except that you're trying to apply a common-sense notion of "time" and "events" as things that happen independently of the observer. But in fact Special Relativity makes some very bold statements about how integral the observer is to the process of observing both time and events.
I can understand the desire to apply common-sense definitions to things, but please understand that "event" and "observer" and "time" have very rigorous definitions in science, and that these definitions are constructed into a system of equations(a model) which has great predictive power but which does not always line up with common-sense notions of these words.
I think the confusion here is related to the idea that you can infer something happens even if you cannot observe it happening. If you know it takes light 8K years to reach you, and you can right now see a countdown process which will go off in 4K years, it is reasonable to assume the event already occurred in the process frame of reference. Of course, the certainty of that hinges on the inevitability of the process, but that could viably be on the same scale as the certainty of a direct observation itself.
Basically, our ability to accurately predict the future is not confined to events in our own frame of reference, so the difference between what is known to have happened and what is predicted to happen need not be very large.
I’m not enough of an astrophysicist to do this math myself, but what would this kind of burst look like from Earth? Observable w a telescope? W the naked eye? Enter a bomb shelter and save yourself?
If it does generate a gamma ray burst, then that is a more interesting phenomena. Depending on whether or not it is pointed in our direction, we might see more or less energy from it directly. After travelling 8000 light years and passing through all the dust between us and the planet, it would be seriously attenuated and unlikely to cause any destruction or even noticeable effects on earth.
[1] Astrologers on the other hand, it could be the sign that the end of the world is nigh :-)
I enjoyed Phil's article on it, thanks for the link. The take away is pretty good, a lot of things would have to align for it to hit us, and if it did, the effects range from 'none' to 'extinction'. Which is true for a lot of things (like asteroids)
It suggests another interesting plot for a science fiction novel, an alien attack force is coming to invade the planet, and a GRB event goes of across the galaxy, missing Earth completely but killing everything on the alien armada. Which slowly drifts toward the inner solar system.
You can read the Wikipedia article here: https://en.wikipedia.org/wiki/Gamma-ray_burst but to summarize the burst is believed to be a 'jet' this forms as part of the explosion of the spinning masses.
Can you point me to a source for that "about 30 degrees away from us"? I haven't seen it.
And, how directional? All the energy is spread across half the sky? Only 10 degrees? Or only one degree? Is 30 degrees enough for it to completely miss us? Mostly miss us? Or are we still in the danger cone?
Finally, in a binary star system, is that 30 degrees going to change as the stars orbit each other? (Worse, IIRC, there's a third, more distant star. Can it change the orbit of the other two in a way that shifts that 30 degrees?)
About 5 years ago, it was being suggested that the Wolf-Rayet star WR 104 might cause a dangerous gamma-ray burst when it goes supernova, but these things are highly directional, and I think the current consensus is that it is not aligned closely enough to be harmful. Apep is at a similar distance (~8,000 light years), and the article seems to be suggesting it will likely be more powerful than WR 104 is expected to be, but there is at least one news article is claiming that it is probably not pointed at us.
This star system is about 8000 light-years away. For comparison, in 1572 a different star, also about 8000 light-years away, was observed to go supernova. At its brightest, it was reported to be about as bright as Venus. This supernova would probably reach similar brightness levels.
Interesting story. You have to wonder if the "Christmas" star was a supernova like this one. It suddenly appears, becomes visible during the day for a few weeks, then slowly vanishes for about a year.
There was a sci-fi short story about this, in which a team of explorers found the Christmas star...and the remnants of the civilization which wiped out as a consequence.
or, whether it actually happened. no other cultures with astronomical knowledge have ever been found to have reported on such a star ~2000 years ago. if there was a sighting visible in the med region, it was likely an asteroid or, possibly, Jupiter.
I saw that article. If you read a little further, the position of the comet couldn't have led the so-called wisemen to Bethleham (yet another apocryphal tale). So, still no other cultures.
There have been reports of Supernova's (SN 1006) @ a visual magnitude of -7.5 during the day which might be visible by human eyes, however this was a very long time ago.
"Of course, it's important to bear in mind that 'just about to explode' means something different in cosmic terms versus human terms. It could happen at any time in the... next few hundred thousand years."
False; it is not the same time everywhere. Two people travelling at different speeds will disagree about what order events happened in, and there is no privileged reference frame.
They will disagree about what order some events happen in. But those events are causally unrelated to each other, so it doesn't matter what order they happened in.
Don't think GRBs are all that worrisome, by comparison.
It has recently been calculated that there is a 50-percent chance that a GRB-induced mass extinction on Earth has occurred in the past 500 million years. This abstract: ( https://arxiv.org/abs/1409.2506 )
links to a PDF of the entire paper.
So the probability in a human lifetime is magnitudes less than a supervolcano event or big asteroid impact.
According to ( https://phys.org337426962/pdf.pdf ): "The researchers don't believe a GRB striking the Earth could penetrate the atmosphere, but do believe one could destroy the ozone layer".
We started to do that to ourselves more than once, not long ago. We are by far the highest-probability danger to Earth life. Chlorofluorocarbons: check. Nuclear weapons: check.
Back to less-likely (inhuman) radiation sources: In the late 8th century, tree rings in the northern hemisphere 'recorded' a one-year atmospheric increase of 1% in carbon-14 ... 20 times normal. Cause: 'an extremely intense burst of high-energy radiation'.
Cause unknown, but could have been ... a coronal mass ejection (the Sun).
With this close proximity, isn't there a good chance we may be hit directly with the gamma ray burst ? Shouldn't we be alarmed about another possible mass extinction event ?
Generally speaking it is exceptionally rare for a star to point directly at us. This is to an acurracy of millionth's of a degree, as the straight line travel across interstellar space is massive. So getting hit with a GRB is like winning the interstallar lottery.
Secondly, if you cannot control it, or influence it. Worrying about it is just fantasizing. Humans tend to assume overly dramatic events are more likely then they actually are.
Apparently they can clearly detect that the ejection from the poles is much slower than the ejection from the equator, which tells me the poles are probably directed away from Earth and the stars are being seen somewhat side-on. So I'm guessing any such burst will be directed away.
The GRB starts, from my understanding, when the core collapses - and that only takes seconds. And then travels at light speed. The shockwave making its way through the rest of the star can take hours, and you might see the explosion take months. But the GRB would hit us before we would see the shockwave, or explosion.
Without the ability to accurately determine when a supernova will occur, we will effectively have no real warning we're about to be hit.
Protecting ourselves from it is also well beyond our current technology. We'd either need to build enough physical shielding to block it from hitting earth, or we'd need to leave our solar system.
I always get excited when I read these things that I might be lucky enough to see a supernova in the sky in my lifetime (provided the gamma ray burst misses us!).
I feel like being interested in space is an exercise in being simultaneously enthralled and disappointed. This sounds awesome! But, like with basically everything space-related, the scale is too immense for a human lifetime:
> Of course, it's important to bear in mind that "just about to explode" means something different in cosmic terms versus human terms. It could happen at any time in the... next few hundred thousand years.
