5 ms·

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

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.

Towaway69 20d ago
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.
- himata4113 20d ago
  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.
  - ilt 20d ago
    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…
    - tappaseater 20d ago
      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.
      - ShinyLeftPad 20d ago
        You don't get microwave radio frequency by increasing wavelength of light, it's the other way around.
        _Microft 20d ago
        Microwaves do have much larger wavelengths than visible light. https://en.wikipedia.org/wiki/Electromagnetic_spectrum https://en.wikipedia.org/wiki/Electromagnetic_spectrum
        ShinyLeftPad 20d ago
        Ah my bad, frequency vs length.
      - thenthenthen 20d ago
        Like a doppler shift?
        andrewflnr 20d ago
        Exactly a Doppler shift.
    - dotancohen 20d ago
      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.
      - ButlerianJihad 20d ago
        > Light moves at the speed of casualty - C C is coulombs And casualty is a dead person
        dotancohen 20d ago
        Typo corrected, thank you!
        raattgift 19d ago
        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://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html https://en.wikipedia.org/wiki/Lyman-alpha_emitter 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 https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment
- dotancohen 20d ago
  > 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.
  - anonzzzies 20d ago
    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.
    - grumpopotamus 20d ago
      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".
      - anonzzzies 20d ago
        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.
      - ViktorRay 20d ago
        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?
- ShinyLeftPad 20d ago
  It's crazy only if you use a specific geocentric version of "now".
- phyzix5761 20d ago
  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 https://www.youtube.com/watch?v=pTn6Ewhb27k
  - GoblinSlayer 20d ago
    If you send light around the universe, is it in one direction?
  - phyzix5761 19d ago
    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.

Towaway69 20d ago

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.

himata4113 20d ago
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.
- ilt 20d ago
  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…
  - tappaseater 20d ago
    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.
    - ShinyLeftPad 20d ago
      You don't get microwave radio frequency by increasing wavelength of light, it's the other way around.
      - _Microft 20d ago
        Microwaves do have much larger wavelengths than visible light. https://en.wikipedia.org/wiki/Electromagnetic_spectrum https://en.wikipedia.org/wiki/Electromagnetic_spectrum
        ShinyLeftPad 20d ago
        Ah my bad, frequency vs length.
    - thenthenthen 20d ago
      Like a doppler shift?
      - andrewflnr 20d ago
        Exactly a Doppler shift.
  - dotancohen 20d ago
    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.
    - ButlerianJihad 20d ago
      > Light moves at the speed of casualty - C C is coulombs And casualty is a dead person
      - dotancohen 20d ago
        Typo corrected, thank you!
        raattgift 19d ago
        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://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html https://en.wikipedia.org/wiki/Lyman-alpha_emitter 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 https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment
dotancohen 20d ago
> 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.
- anonzzzies 20d ago
  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.
  - grumpopotamus 20d ago
    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".
    - anonzzzies 20d ago
      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.
    - ViktorRay 20d ago
      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?
ShinyLeftPad 20d ago
It's crazy only if you use a specific geocentric version of "now".
phyzix5761 20d ago
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 https://www.youtube.com/watch?v=pTn6Ewhb27k
- GoblinSlayer 20d ago
  If you send light around the universe, is it in one direction?
- phyzix5761 19d ago
  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.