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?
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.
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.
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.
> 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 casualty, 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 casualty, the fact that light travels at that speed in a vacuum doesn't affect that.
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 third party observing both events would 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 consider what happens simultaneously with Person A reaching the singularity.
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.
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.
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?
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.
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.
We haven’t even explored all the oceans on earth and can barely predict the weather and yet an astrophysicist will tell you (completely invent) the chemical composition and weather patterns of planets light years away.
And then like 10 years later they’ll go “oh actually that is not a planet”
Yeah
So why did you fully lie and do TV shows and shit based on it lmao
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))
33 comments
[ 0.22 ms ] story [ 13.4 ms ] threadTurn 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.
Sorry if questions don’t make sense…
https://en.wikipedia.org/wiki/Electromagnetic_spectrum
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.
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.
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?
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 consider what happens simultaneously with Person A reaching the singularity.
https://en.wikipedia.org/wiki/Relativity_of_simultaneity
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?
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.
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.)
[0] https://www.youtube.com/watch?v=pTn6Ewhb27k
And then like 10 years later they’ll go “oh actually that is not a planet”
Yeah
So why did you fully lie and do TV shows and shit based on it lmao
Bullshit at all levels