While the conclusions of the observation are interesting, what really stood out to me was the observations themselves. That black hole is 20 million light years away, in another galaxy! And we have multiple high enough resolution observations to conclude how the gas orbits the black hole and gets dragged along behind it. That's nuts.
What blows my mind is that 20 million light years also means that what we're observing something that happened 20 million years ago. We're looking into the very distant past, at something that might well no longer exist.
Turn that around and there well could be other lifeforms observing the Earth with dinosaurs roaming around on it. Perhaps observing the impact of a giant meteor onto the earth.
The light of that meteor impact on Earth is still roaming the universe. If only we could travel faster than that light.
What is even more crazy is that we can actually look as far as the beginning of the universe just because of how long it takes for particles from the other side of the universe to reach us.
I’m not a science person but I think I understand the crux of what you’re saying here - have a question though: are those particles finite? And those are light particles, right? Is the number of those particles increasing the longer they travel?
Light is not a particle, nor is it a wave. However, under some circumstances it does behave like a particle - we even have a name for that (photon). However for purposes of this type of discussion - how it travels long distances - we need to look at the wave properties of light. The particle properties are more interaction-based.
Light moves at the speed of causality - C - so it does not experience time. Therefore light never changes. However, for distant extra-galactic objects, the space that the light is traveling through is expanding! Thus the wave property of light is stretched out as well. So what was once visible light is now so stretched out (in our reference frame) that our eyes can't detect it (that's why the night sky is black).
So yes, the light is finite. It itself experiences no change. The number of particles heading in our direction only changes when they encounter some types of mass, such as interstellar or intergalactic gad clouds.
If you want to know why light is not a particle (and also not a wave) check out the dual-slit experiments. Fascinating stuff.
is wrong. A photon has a momentum, which can increase or decrease as it travels through free space.
Using general relativity, it's straightforward to define an affine time on a null geodesic (even if one cannot define a proper time on one), and a momentum that is a function of the affine time at each point.
The Lyman-alpha forest is a straightforward "laboratory" for this approach, which can calculate the Lyα absorption lines seen when a bright distant quasar or luminous Lyα emitter has atomic hydrogen clouds between the distant source and us (the absorption lines indicate photons with a very narrow range of momenta participated in hydrogen atoms' electrons transitioning from ground to the first excited state, with the later relaxation photons radiating in random directions). The pattern of such dark spectral lines tells us how redshifted the background light and earlier absorption lines are at each gas cloud along the way.
Philosophical speaking, I wonder what the folks will think of our theories of light, time and space in the not too distant future.
Much as the flat earth theories or the universe rotates around the earth, I suspect we don’t really get it yet and we’ll have other ideas in 300 years from now.
Just reading here about the theories of light, fundamentally we don’t understand what light truly is. Sure we have this theory and that idea, but these seem too complex and fragile to be really it - imho.
I mean no disrespect, and everyone can have their theories and opinions. But we shouldn’t forget the arrow of time continues and it’s still a long way to go.
I feel like we have advanced technology to the point that we can't be that far off from the truth, or at least we would look more charitably back on our views now since they're grounded in observations made with the best tools we have available to us as opposed to e.g. the flat earth where you can actually see and experience the curvature of the earth without needing any specialized tooling at all.
The commenter is referring to the Cosmic Microwave Background (CMB). Whilst most of the universe is a dark void, if you tune a radio to a certain frequency, there's a faint, static buzz in the microwave part of the spectrum. It is heard no matter which direction you point the dish. This is a left-over from the Big Bang and started out as light but due to the expansion of space, the wavelength has increased, and that's why it's now received in the microwave part of the spectrum. There are lots of great explainers on the CMB and the history of its discovery is entertaining too. One of the easier concepts in cosmology, and one of the most important.
> What blows my mind is that 20 million light years also means that what we're observing something that happened 20 million years ago.
Well... Not really. It took the entirety of the book A Brief History Of Time to even grasp a cusp of this, so I know I won't convince you in a short HN comment, but "now" is the collection of events that currently affect us. Due to causality, events outside our light cone have not occurred yet. They have no more influence than does an egg falling off the counter that has yet to hit the floor. The egg breaks only when it hits the floor - events only occur in our reference frame when they could affect outcomes in our reference frame. C is the propagation of causality, the fact that light travels at that speed in a vacuum doesn't affect that.
> So a hypothetical photon from the big bang (era?) gets observed now means the big bang is happening now?
Yes. More specifically, the big bang would be happening now, at the location from where we perceive that photon. Hey, this stuff is not intuitive.
However the early universe absorbed all the big bang photons. The furthest back we can observe photons from is the CMB. And yes, the phase change is happening now, there... Complicated by the Hubble expansion of course.
> A dinosaur decays to bones and now gets observed means the dinosaur died just now - not that it died millions of years ago?
No, that is incorrect. But you already knew that. Analogies don't work for light and for time and for relativity - it is too different from our everyday experiences for analogies to hold.
But that 20m year old light is in our past light cone; we can say it is in our past. If it is not in our light cone, we cannot communicate any temporal info so we cannot say if it’s now, past or future in reference to us as observer.
I think "something that happened 20M years ago" is still wrong. One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. In other words, if we went back 20M years on Earth we could not meaningfully say the black hole event is happening "right now".
If an astronaut goes to Pluto and films herself doing activity A and then broadcasts that back to Earth, it would take around five hours for the broadcast to reach us.
So you’re saying that it is not correct to say her recorded activity happened five hours ago?
The idea driving this conclusion is called the Relativity of Simultaneity - There is no absolute notion that two events happened "at the same time". You want to say that a distant event occurred at the same time as a nearby event that occurred 5 hours ago. However, a distant third party observing both events may not agree.
This becomes painfully apparent when dealing with black holes. Person A falling into a black hole passes through the event horizon normally and reaches the singularity in finite time. However, Person B observing from outside never sees Person A pass through the event horizon. From Person B's perspective, it doesn't make sense to ask what happens simultaneously with Person A reaching the singularity - it takes infinite time just to reach the event horizon. Even asking the question becomes nonsense.
If we imagine a third observer in a space craft traveling close to he speed of light, it's possible to construct the scenario in such a way that the pilot of the space craft would observe the two events happening at the same moment. Time is relative
As an approximation it can be fine to speak that way in practical matters, but it's dangerous to go drawing any deep conclusions while doing so. Approximations shouldn't be allowed to go outside their operational parameters.
The approximation is the belief that things can happen simultaneously. It is approximating along the fact that we only need to concern ourselves with a single reference frame.
I think the difference in the scenario you proposed is that we can conceive of a practical situation in which someone on Pluto sends a message and waits for a reply. We can then say that they are going to have to wait at least 10 hours to receive a reply. This has meaning.
But when the distances become too great the urge is to resist believing that things have an existence beyond the interactions of particles. There is no practical reality to that object beyond our ability to observe it when the light reaches us.
I get what you're saying but you're also being incredibly dismissive to the common understanding which is still correct, otherwise most astrophysics wouldn't work, because it wouldn't be possible to predict where another planet will be at a later time. It's possible to have two alarm clocks go off at the same instant on two planets, though obviously each will observe the other going off later, after a delay.
It's obviously fair to say they happened at the same time in this scenario.
> One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart.
No. Special relativity says that simultaneity is relative for things moving at different velocities. The classic example is a fast-moving train and a platform disagreeing on simultaneity, even when they're right next to each other. Distance, and light travel time, has nothing to do with it; that's a separate (and mostly less interesting) problem.
Relativity absolutely lets you calculate what was happening simultaneously with a given past event within your own reference frame (i.e. other things moving at roughly the same velocity you are). In fact, you can think of that process, figuring out a consistent version of events after you've gathered all the data, as being exactly what it means to "observe" something in relativity. This is as opposed to "seeing it", which is what happens the exact moment the photons hit your eyes/sensors.
It's a semantic saying that misses what people typically mean when we speak about simultaneous events.
Someone sends an email or speaks on a voice call on the other side of world and a receiver still gets that note at the speed of light, there is slight latency. It's still completely reasonable to measure that latency as we can effectuate improvements to our machines and networks when it increases. The same as if someone wrote a letter in the 1860s and the horses were sick, causing a delay. Simply because information was received by us, the common understanding is that the notification was sent previously, because that is how cause and effect work.
It's not semantics, it's an approximation. Events that happen on the same planet are not relativistically distant, so such approximations are mostly fine in our daily lives; the same as how Newtonian gravity is fine to use in many situations. But today we're talking about astrophysics and a proper understanding of Relativity matters.
Yes, but we can say it happened in our past. But agreed, we cannot go back 20m years and say it’s simultaneous with the events here. If GP meant that, then agreed.
I think you misunderstood Hawking. "Now" in the relativistic sense is not the same as our past light cone. Reference frames for simultaneity are defined by relative motion. Galaxies without too big a redshift are close enough to moving at the same speed we are, and thus being in our same reference frame, that simultaneity (and times relative to the present) are reasonable to talk about.
I may have misunderstood Hawking, but my Physics grades suggest that I do understand the principles.
I tried to explain within the confines of an HN comment so that GGP could understand. Any answer that fits in this box will be incomplete. The most complete answer that is correct, would be a simple "No" but I felt that was too dismissive and not informative enough.
Regardless of your past grades, your usage of terms in your comment like "now" and "reference frame" are plainly incorrect. A simple "no" would have been plainly incorrect, to say nothing of being dismissive.
I'll repeat: reference frames in relativity are about motion, not any kind of distance, spatial or temporal.
I agree that my usage of the term "now" was incorrect. "Now" really isn't defined well when the concept of simultaneous events is not valid.
I also used reference frame when referring to distant objects, for the simple reason that the conversation is (was at that point) confined to light traveling between those objects - the light being in a different reference frame from the two objects. I agree, now the morning after, that this was poorly phased and can not be generalized to two arbitrary objects no matter how distant.
As I've stated, an answer in the confines of an HN comment on this topic will always be incomplete. We can answer very specific situations, but those answers can not be generalized to other situations.
You keep using the length of HN comments as an excuse, when you're the one injecting complexity that doesn't apply. Even your "corrections" here are still wrong.
I'll repeat once more: reference frames are about motion, not distance. A nearby (relatively) fast-moving object has a different frame, but a far away object that's (relatively) motionless has the same frame. Simultaneity is defined per reference frame. The reference frame of the light traveling between objects is entirely a distraction for our purposes.
The point I've been trying to make is that one can not travel from one cosmically-distant place in the Earth's reference frame to another cosmically-distant place in the Earth's reference frame without breaking the reference frame. And specific to the GGGGP's question - traveling at the speed of light breaks that reference frame.
Please, pedantically correct my phrase "break the reference frame". I'd like to know how to phrase that properly. "Change reference frame" maybe?
There's no way to phrase it correctly because the concept is incoherent. Using light to send information from one part of a reference from to another is a non-issue. In fact that's the best way to do it, and it's a standard part of the formulation of "reference frames" that I learned in class. We don't care about the light's reference frame along the way, only its externally observable trajectory.
But that has nothing to do with the GGGGGP's question about things happening in the past.
I anticipate your reply that the light is just sending information about what happened. But a moving observer could perceive _our_ now as occurring before the events that we now observe in light from the hypothetical 20 million light year distant object.
There is no "now" and no relative, definite "past" for distant objects, even if they are in the same reference frame. It is not intuitive, I understand that. I didn't make the rules.
I do appreciate you insisting that I improve my terminology. And I do appreciate being made to defend myself, I had to go look some of this stuff up. It is fascinating - especially the pre-Einstein work that left everybody puzzled.
LATE EDIT:
> According to the special theory of relativity introduced by Albert Einstein, it is impossible to say in an absolute sense that two distinct events occur at the same time if those events are separated in space. If one reference frame assigns precisely the same time to two events that are at different points in space, a reference frame that is moving relative to the first will generally assign different times to the two events.
The last clause of your Wikipedia quote is another repetition of what I've been saying this whole time, so Wikipedia is fine with me I guess.
Note that the "separated in space" requirement doesn't require any particular distance scale. A moving observer could disagree with you about simultaneity of events on opposite ends of your desk.
And yet, coming back to the original point, we don't need to worry about the infinity of hypothetically possible moving observers to notice that we and a not-too-distant galaxy have approximately the same reference frame, and thus can approximately establish a shared timeline. That's plenty to make it meaningful to say we're looking at things that happened 20mya. (Please note that I was explicit about this being approximate in my first reply.)
Hell, we don't actually have to care about the galaxy's frame at all. Even within our own reference frame, we can calculate what nearby events we observe to be simultaneous with distant events we see (not observe) now, and it'll still be basically the naive time-in-the-past estimate based on light travel time. The worst you can say about that is that it's not wrong, right? We don't need to consider all possible frames before giving a time in the past of our own frame.
> But a moving observer could perceive _our_ now as occurring before the events that we now observe in light from the hypothetical 20 million light year distant object.
Actually, if by "our now" you mean after we've received the photons from the distant event (or really even the same time), I'm pretty sure that would be a superluminal observer. So, uh, lol no. It's kind of important that no real reference frames disagree about causal order.
Also, quit telling me relativity is "unintuitive" like I don't realize that. I've literally done the math on this.
From a practical perspective this makes perfect sense to me. But we do know the photons travelled over a period of time, right? Where those photons came from, ‘now’ is something else. Though it doesn’t matter practically, those events did occur in the past, before they were a part of our now. Or am I missing something?
No, actually, the photons did not travel over a period of time. Photons travel at C, and thus experience no time in their reference frame.
As for _our_ reference frame, I've explained in other comments but a debate has developed between those few who do understand because they've studied physics, and those who think that their intuition is more correct than Einstein. I decline to elaborate further, HN comments are not long enough to cover it all. In short: you are in fact missing something but that is fine, everybody is. Einstein himself stated that in his day maybe three people on the planet understood, and he was uncertain if he was among those three.
You've got some nerve to come in here with self-admittedly barely-understood BS from a pop science book and pretend it's other people's fault for being uneducated when they challenge you.
Quality of the information from the pop-sci book not withstanding, I've also taken physics courses on the subject. Albeit twenty years ago.
But go ahead, you win the internet argument. When I thought this was a discussion, and I could potentially learn something or help somebody understand, I was happy to participate. But I have no interest in an argument consisting of insults and accusations and no exchange of information. You win, congratulations.
Actually we don't know the speed of light in one direction[0]. So we could actually be seeing things as they are in this moment or at any other moment in time for all we know.
Based on the downvotes my comment is getting it seems there's a lot of scientifically illiterate people on this site. So, I'll expand on what I said.
We cannot directly measure the one-way speed of light because doing so requires synchronizing two distant clocks without already knowing how fast light travels.
When scientists state that light travels at 299,792,458 meters per second, they are referring to the two-way speed of light (a round trip from a source to a mirror and back).
To find the speed in one direction, you need a clock at the start and a clock at the finish line. To sync those two clocks, you must send a signal between them but, any signal you use to sync the clocks travels at the speed of light, which creates a circular problem.
Reminds me of https://science.nasa.gov/asset/hubble/runaway-black-hole-nea...
A supermassive blackhole drawing in dust on such a scale that it's creating new stars in its wake as it zips by.
(The picture is my desktop background. I think it nicely illustrates three fundamental shapes of the universe: 1) spheres (gravity dominates), 2) disks (angular momentum winning over gravity), 3) straight lines (no gravity))
My prior experience with science popularizers is that they are typically telling a potential, and either objectively likely or subjectively hoped for, narrative plausibly supported by the evidence.
"Science literacy" is one of those squishy concepts that lets folks give grace and continually learn and improve the view of the universe.
Because like many, I never lost my inner 11 year old and HOW COOL IS IT THAT WE CAN OBSERVE A BLACK HOLE?!?
While the conclusions of the observation are interesting, what really stood out to me was the observations themselves. That black hole is 20 million light years away, in another galaxy! And we have multiple high enough resolution observations to conclude how the gas orbits the black hole and gets dragged along behind it. That's nuts.
What blows my mind is that 20 million light years also means that what we're observing something that happened 20 million years ago. We're looking into the very distant past, at something that might well no longer exist.
Turn that around and there well could be other lifeforms observing the Earth with dinosaurs roaming around on it. Perhaps observing the impact of a giant meteor onto the earth.
The light of that meteor impact on Earth is still roaming the universe. If only we could travel faster than that light.
What is even more crazy is that we can actually look as far as the beginning of the universe just because of how long it takes for particles from the other side of the universe to reach us.
I’m not a science person but I think I understand the crux of what you’re saying here - have a question though: are those particles finite? And those are light particles, right? Is the number of those particles increasing the longer they travel?
Sorry if questions don’t make sense…
Light is not a particle, nor is it a wave. However, under some circumstances it does behave like a particle - we even have a name for that (photon). However for purposes of this type of discussion - how it travels long distances - we need to look at the wave properties of light. The particle properties are more interaction-based.
Light moves at the speed of causality - C - so it does not experience time. Therefore light never changes. However, for distant extra-galactic objects, the space that the light is traveling through is expanding! Thus the wave property of light is stretched out as well. So what was once visible light is now so stretched out (in our reference frame) that our eyes can't detect it (that's why the night sky is black).
So yes, the light is finite. It itself experiences no change. The number of particles heading in our direction only changes when they encounter some types of mass, such as interstellar or intergalactic gad clouds.
If you want to know why light is not a particle (and also not a wave) check out the dual-slit experiments. Fascinating stuff.
> Light moves at the speed of casualty - C
C is coulombs
And casualty is a dead person
As long as we're nitpicking, casualties include injuries as well as deaths.
Actually C is for Cookie. :P
Well, that’s good enough for me.
Typo corrected, thank you!
Additionally,
> light never changes
is wrong. A photon has a momentum, which can increase or decrease as it travels through free space.
Using general relativity, it's straightforward to define an affine time on a null geodesic (even if one cannot define a proper time on one), and a momentum that is a function of the affine time at each point.
The Lyman-alpha forest is a straightforward "laboratory" for this approach, which can calculate the Lyα absorption lines seen when a bright distant quasar or luminous Lyα emitter has atomic hydrogen clouds between the distant source and us (the absorption lines indicate photons with a very narrow range of momenta participated in hydrogen atoms' electrons transitioning from ground to the first excited state, with the later relaxation photons radiating in random directions). The pattern of such dark spectral lines tells us how redshifted the background light and earlier absorption lines are at each gas cloud along the way.
https://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html
https://en.wikipedia.org/wiki/Lyman-alpha_emitter
And over much-shorter-than-cosmological distances, https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment
Philosophical speaking, I wonder what the folks will think of our theories of light, time and space in the not too distant future.
Much as the flat earth theories or the universe rotates around the earth, I suspect we don’t really get it yet and we’ll have other ideas in 300 years from now.
Just reading here about the theories of light, fundamentally we don’t understand what light truly is. Sure we have this theory and that idea, but these seem too complex and fragile to be really it - imho.
I mean no disrespect, and everyone can have their theories and opinions. But we shouldn’t forget the arrow of time continues and it’s still a long way to go.
I feel like we have advanced technology to the point that we can't be that far off from the truth, or at least we would look more charitably back on our views now since they're grounded in observations made with the best tools we have available to us as opposed to e.g. the flat earth where you can actually see and experience the curvature of the earth without needing any specialized tooling at all.
The commenter is referring to the Cosmic Microwave Background (CMB). Whilst most of the universe is a dark void, if you tune a radio to a certain frequency, there's a faint, static buzz in the microwave part of the spectrum. It is heard no matter which direction you point the dish. This is a left-over from the Big Bang and started out as light but due to the expansion of space, the wavelength has increased, and that's why it's now received in the microwave part of the spectrum. There are lots of great explainers on the CMB and the history of its discovery is entertaining too. One of the easier concepts in cosmology, and one of the most important.
You don't get microwave radio frequency by increasing wavelength of light, it's the other way around.
Microwaves do have much larger wavelengths than visible light.
https://en.wikipedia.org/wiki/Electromagnetic_spectrum
Ah my bad, frequency vs length.
Like a doppler shift?
Exactly a Doppler shift.
> but "now" is the collection of events that currently affect us.
Just to see if I understand what you’re saying…
So a hypothetical photon from the big bang (era?) gets observed now means the big bang is happening now?
A dinosaur decays to bones and now gets observed means the dinosaur died just now - not that it died millions of years ago?
(Dinosaur = photon from a star or something. Decay = red shift. Or a real dinosaur— guess it’s the same now.)
However the early universe absorbed all the big bang photons. The furthest back we can observe photons from is the CMB. And yes, the phase change is happening now, there... Complicated by the Hubble expansion of course.
No, that is incorrect. But you already knew that. Analogies don't work for light and for time and for relativity - it is too different from our everyday experiences for analogies to hold.[flagged]
“Now” is weird. It doesn’t exist in a cosmic sense. Check out book “the order of time”.
But that 20m year old light is in our past light cone; we can say it is in our past. If it is not in our light cone, we cannot communicate any temporal info so we cannot say if it’s now, past or future in reference to us as observer.
I think "something that happened 20M years ago" is still wrong. One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart. In other words, if we went back 20M years on Earth we could not meaningfully say the black hole event is happening "right now".
Wait then what about events that happen on Pluto?
Pluto is around five light hours away from us.
If an astronaut goes to Pluto and films herself doing activity A and then broadcasts that back to Earth, it would take around five hours for the broadcast to reach us.
So you’re saying that it is not correct to say her recorded activity happened five hours ago?
The idea driving this conclusion is called the Relativity of Simultaneity - There is no absolute notion that two events happened "at the same time". You want to say that a distant event occurred at the same time as a nearby event that occurred 5 hours ago. However, a distant third party observing both events may not agree.
This becomes painfully apparent when dealing with black holes. Person A falling into a black hole passes through the event horizon normally and reaches the singularity in finite time. However, Person B observing from outside never sees Person A pass through the event horizon. From Person B's perspective, it doesn't make sense to ask what happens simultaneously with Person A reaching the singularity - it takes infinite time just to reach the event horizon. Even asking the question becomes nonsense.
https://en.wikipedia.org/wiki/Relativity_of_simultaneity
A quantum entangled data stream of video between Pluto and earth would mean your now observing it in real time
Quantum entanglement does not enable FTL communication: [1].
[1] https://en.wikipedia.org/wiki/No-communication_theorem
If we imagine a third observer in a space craft traveling close to he speed of light, it's possible to construct the scenario in such a way that the pilot of the space craft would observe the two events happening at the same moment. Time is relative
Ah yes that’s indeed true.
But from the Earth’s reference frame wasn’t the Pluto activity I mentioned earlier simultaneous with what happened 5 hours ago on Earth?
So couldn’t you say from Earth’s reference frame this black hole’s activity happened simultaneously with events 20 million years ago?
As an approximation it can be fine to speak that way in practical matters, but it's dangerous to go drawing any deep conclusions while doing so. Approximations shouldn't be allowed to go outside their operational parameters.
But it’s not an approximation right? In our reference frame it is the truth.
The approximation is the belief that things can happen simultaneously. It is approximating along the fact that we only need to concern ourselves with a single reference frame.
I think the difference in the scenario you proposed is that we can conceive of a practical situation in which someone on Pluto sends a message and waits for a reply. We can then say that they are going to have to wait at least 10 hours to receive a reply. This has meaning.
But when the distances become too great the urge is to resist believing that things have an existence beyond the interactions of particles. There is no practical reality to that object beyond our ability to observe it when the light reaches us.
I get what you're saying but you're also being incredibly dismissive to the common understanding which is still correct, otherwise most astrophysics wouldn't work, because it wouldn't be possible to predict where another planet will be at a later time. It's possible to have two alarm clocks go off at the same instant on two planets, though obviously each will observe the other going off later, after a delay.
It's obviously fair to say they happened at the same time in this scenario.
> One of the consequences of special relativity is there is no such thing as "simultaneous" events very far apart.
No. Special relativity says that simultaneity is relative for things moving at different velocities. The classic example is a fast-moving train and a platform disagreeing on simultaneity, even when they're right next to each other. Distance, and light travel time, has nothing to do with it; that's a separate (and mostly less interesting) problem.
Relativity absolutely lets you calculate what was happening simultaneously with a given past event within your own reference frame (i.e. other things moving at roughly the same velocity you are). In fact, you can think of that process, figuring out a consistent version of events after you've gathered all the data, as being exactly what it means to "observe" something in relativity. This is as opposed to "seeing it", which is what happens the exact moment the photons hit your eyes/sensors.
It's a semantic saying that misses what people typically mean when we speak about simultaneous events.
Someone sends an email or speaks on a voice call on the other side of world and a receiver still gets that note at the speed of light, there is slight latency. It's still completely reasonable to measure that latency as we can effectuate improvements to our machines and networks when it increases. The same as if someone wrote a letter in the 1860s and the horses were sick, causing a delay. Simply because information was received by us, the common understanding is that the notification was sent previously, because that is how cause and effect work.
It's not semantics, it's an approximation. Events that happen on the same planet are not relativistically distant, so such approximations are mostly fine in our daily lives; the same as how Newtonian gravity is fine to use in many situations. But today we're talking about astrophysics and a proper understanding of Relativity matters.
Yes, but we can say it happened in our past. But agreed, we cannot go back 20m years and say it’s simultaneous with the events here. If GP meant that, then agreed.
You are getting downvotes, but you are correct. People are voting by intuition, not by physics. Relativity is not intuitive.
No, I took a physics class that covered relativity of simultaneity and that's how I know this is BS.
I think you misunderstood Hawking. "Now" in the relativistic sense is not the same as our past light cone. Reference frames for simultaneity are defined by relative motion. Galaxies without too big a redshift are close enough to moving at the same speed we are, and thus being in our same reference frame, that simultaneity (and times relative to the present) are reasonable to talk about.
I may have misunderstood Hawking, but my Physics grades suggest that I do understand the principles.
I tried to explain within the confines of an HN comment so that GGP could understand. Any answer that fits in this box will be incomplete. The most complete answer that is correct, would be a simple "No" but I felt that was too dismissive and not informative enough.
Regardless of your past grades, your usage of terms in your comment like "now" and "reference frame" are plainly incorrect. A simple "no" would have been plainly incorrect, to say nothing of being dismissive.
I'll repeat: reference frames in relativity are about motion, not any kind of distance, spatial or temporal.
I agree that my usage of the term "now" was incorrect. "Now" really isn't defined well when the concept of simultaneous events is not valid.
I also used reference frame when referring to distant objects, for the simple reason that the conversation is (was at that point) confined to light traveling between those objects - the light being in a different reference frame from the two objects. I agree, now the morning after, that this was poorly phased and can not be generalized to two arbitrary objects no matter how distant.
As I've stated, an answer in the confines of an HN comment on this topic will always be incomplete. We can answer very specific situations, but those answers can not be generalized to other situations.
You keep using the length of HN comments as an excuse, when you're the one injecting complexity that doesn't apply. Even your "corrections" here are still wrong.
I'll repeat once more: reference frames are about motion, not distance. A nearby (relatively) fast-moving object has a different frame, but a far away object that's (relatively) motionless has the same frame. Simultaneity is defined per reference frame. The reference frame of the light traveling between objects is entirely a distraction for our purposes.
This isn't actually that hard to explain in a comment. See also https://news.ycombinator.com/item?id=49691488
The point I've been trying to make is that one can not travel from one cosmically-distant place in the Earth's reference frame to another cosmically-distant place in the Earth's reference frame without breaking the reference frame. And specific to the GGGGP's question - traveling at the speed of light breaks that reference frame.
Please, pedantically correct my phrase "break the reference frame". I'd like to know how to phrase that properly. "Change reference frame" maybe?
There's no way to phrase it correctly because the concept is incoherent. Using light to send information from one part of a reference from to another is a non-issue. In fact that's the best way to do it, and it's a standard part of the formulation of "reference frames" that I learned in class. We don't care about the light's reference frame along the way, only its externally observable trajectory.
But that has nothing to do with the GGGGGP's question about things happening in the past.
I anticipate your reply that the light is just sending information about what happened. But a moving observer could perceive _our_ now as occurring before the events that we now observe in light from the hypothetical 20 million light year distant object.
There is no "now" and no relative, definite "past" for distant objects, even if they are in the same reference frame. It is not intuitive, I understand that. I didn't make the rules.
I do appreciate you insisting that I improve my terminology. And I do appreciate being made to defend myself, I had to go look some of this stuff up. It is fascinating - especially the pre-Einstein work that left everybody puzzled.
LATE EDIT:
That's from here: https://en.wikipedia.org/wiki/Relativity_of_simultaneityI'll not say that Wikipedia is a particularly good source, I'm happy to be corrected if you have something that contradicts that.
The last clause of your Wikipedia quote is another repetition of what I've been saying this whole time, so Wikipedia is fine with me I guess.
Note that the "separated in space" requirement doesn't require any particular distance scale. A moving observer could disagree with you about simultaneity of events on opposite ends of your desk.
And yet, coming back to the original point, we don't need to worry about the infinity of hypothetically possible moving observers to notice that we and a not-too-distant galaxy have approximately the same reference frame, and thus can approximately establish a shared timeline. That's plenty to make it meaningful to say we're looking at things that happened 20mya. (Please note that I was explicit about this being approximate in my first reply.)
Hell, we don't actually have to care about the galaxy's frame at all. Even within our own reference frame, we can calculate what nearby events we observe to be simultaneous with distant events we see (not observe) now, and it'll still be basically the naive time-in-the-past estimate based on light travel time. The worst you can say about that is that it's not wrong, right? We don't need to consider all possible frames before giving a time in the past of our own frame.
> But a moving observer could perceive _our_ now as occurring before the events that we now observe in light from the hypothetical 20 million light year distant object.
Actually, if by "our now" you mean after we've received the photons from the distant event (or really even the same time), I'm pretty sure that would be a superluminal observer. So, uh, lol no. It's kind of important that no real reference frames disagree about causal order.
Also, quit telling me relativity is "unintuitive" like I don't realize that. I've literally done the math on this.
From a practical perspective this makes perfect sense to me. But we do know the photons travelled over a period of time, right? Where those photons came from, ‘now’ is something else. Though it doesn’t matter practically, those events did occur in the past, before they were a part of our now. Or am I missing something?
No, actually, the photons did not travel over a period of time. Photons travel at C, and thus experience no time in their reference frame.
As for _our_ reference frame, I've explained in other comments but a debate has developed between those few who do understand because they've studied physics, and those who think that their intuition is more correct than Einstein. I decline to elaborate further, HN comments are not long enough to cover it all. In short: you are in fact missing something but that is fine, everybody is. Einstein himself stated that in his day maybe three people on the planet understood, and he was uncertain if he was among those three.
You've got some nerve to come in here with self-admittedly barely-understood BS from a pop science book and pretend it's other people's fault for being uneducated when they challenge you.
Quality of the information from the pop-sci book not withstanding, I've also taken physics courses on the subject. Albeit twenty years ago.
But go ahead, you win the internet argument. When I thought this was a discussion, and I could potentially learn something or help somebody understand, I was happy to participate. But I have no interest in an argument consisting of insults and accusations and no exchange of information. You win, congratulations.
No exchange of information? I explained the real situation over and over.
Lmfao
It's crazy only if you use a specific geocentric version of "now".
Actually we don't know the speed of light in one direction[0]. So we could actually be seeing things as they are in this moment or at any other moment in time for all we know.
[0] https://www.youtube.com/watch?v=pTn6Ewhb27k
Based on the downvotes my comment is getting it seems there's a lot of scientifically illiterate people on this site. So, I'll expand on what I said.
We cannot directly measure the one-way speed of light because doing so requires synchronizing two distant clocks without already knowing how fast light travels.
When scientists state that light travels at 299,792,458 meters per second, they are referring to the two-way speed of light (a round trip from a source to a mirror and back).
To find the speed in one direction, you need a clock at the start and a clock at the finish line. To sync those two clocks, you must send a signal between them but, any signal you use to sync the clocks travels at the speed of light, which creates a circular problem.
If you send light around the universe, is it in one direction?
Reminds me of https://science.nasa.gov/asset/hubble/runaway-black-hole-nea... A supermassive blackhole drawing in dust on such a scale that it's creating new stars in its wake as it zips by. (The picture is my desktop background. I think it nicely illustrates three fundamental shapes of the universe: 1) spheres (gravity dominates), 2) disks (angular momentum winning over gravity), 3) straight lines (no gravity))
Paper in Arxiv https://arxiv.org/abs/2608.10719
LegacySurvey deep link[1].
Zooming out from there and one gets an appreciation for how (in)significant we really are.
[1] https://www.legacysurvey.org/viewer?ra=195.8160&dec=-17.4268...
Wow, the Westlake University was funded in 2018 as a private research university.
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This seems unnecessarily harsh.
My prior experience with science popularizers is that they are typically telling a potential, and either objectively likely or subjectively hoped for, narrative plausibly supported by the evidence.
"Science literacy" is one of those squishy concepts that lets folks give grace and continually learn and improve the view of the universe.
Because like many, I never lost my inner 11 year old and HOW COOL IS IT THAT WE CAN OBSERVE A BLACK HOLE?!?
Project Hail Mary