8 ms·

Maybe the explosion has already happened? But we still wouldn’t be able to see it yet due to the distance and speed of light...

dotancohen 8y ago
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.
- mitchty 8y ago
  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.
  - ssijak 8y ago
    Quantum entangelment?
    - mitchty 8y ago
      Still gated by the speed of light. It might be spooky action at a distance, but it will still take as long as light/gravity would take to observe.
- bsmitty5000 8y ago
  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'?
  - arnarbi 8y ago
    Yes, but the point here is that the first event is unobservable and thus not worth considering.
    - trhway 8y ago
      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.
  - btilly 8y ago
    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.
    - deleted 8y ago
      [deleted]
      - sctb 8y ago
        Most of your comments break the guidelines by being uncivil and/or unsubstantive, so we've banned the account. If you'd like to start using the site as intended you're welcome to email us at hn@ycombinator.com.
  - krastanov 8y ago
    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.
- andrewflnr 8y ago
  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.
  - gregdunn 8y ago
    > 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.
    - andrewflnr 8y ago
      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.
- deleted 8y ago
  [deleted]
- colechristensen 8y ago
  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.
  - hgdsraj 8y ago
    Relativity of silmulntaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
  - shkkmo 8y ago
    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
- Certhas 8y ago
  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.
- fspeech 8y ago
  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.
  - gregdunn 8y ago
    > 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.
    - raattgift 8y ago
      > (or at all) 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. https://arxiv.org/abs/gr-qc/9406019 https://arxiv.org/abs/gr-qc/9406019
nkrisc 8y ago
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.
- ashrk 8y ago
  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.
  - jschwartzi 8y ago
    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.
    - Retra 8y ago
      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.
deleted 8y ago
[deleted]

dotancohen 8y ago

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.

mitchty 8y ago
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.
- ssijak 8y ago
  Quantum entangelment?
  - mitchty 8y ago
    Still gated by the speed of light. It might be spooky action at a distance, but it will still take as long as light/gravity would take to observe.
bsmitty5000 8y ago
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'?
- arnarbi 8y ago
  Yes, but the point here is that the first event is unobservable and thus not worth considering.
  - trhway 8y ago
    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.
- btilly 8y ago
  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.
  - deleted 8y ago
    [deleted]
    - sctb 8y ago
      Most of your comments break the guidelines by being uncivil and/or unsubstantive, so we've banned the account. If you'd like to start using the site as intended you're welcome to email us at hn@ycombinator.com.
- krastanov 8y ago
  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.
andrewflnr 8y ago
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.
- gregdunn 8y ago
  > 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.
  - andrewflnr 8y ago
    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.
deleted 8y ago
[deleted]
colechristensen 8y ago
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.
- hgdsraj 8y ago
  Relativity of silmulntaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity
- shkkmo 8y ago
  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
Certhas 8y ago
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.
fspeech 8y ago
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.
- gregdunn 8y ago
  > 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.
  - raattgift 8y ago
    > (or at all) 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. https://arxiv.org/abs/gr-qc/9406019 https://arxiv.org/abs/gr-qc/9406019

nkrisc 8y ago

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.

ashrk 8y ago
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.
- jschwartzi 8y ago
  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.
  - Retra 8y ago
    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.

deleted 8y ago

[deleted]