Mysterious object is being dragged into the black hole at the Milky Way’s center
newsroom.ucla.edu
newsroom.ucla.edu
And if so, wouldn't adherence to HN's context policy require suffixing with a "(23977BC)"?
For all intents and purposes this is happening in real time from our local frame of reference.
The speed of causality varies depending on the speed of the causal particles and waves which propagate from an event. I was just reading an article on surviving nuclear detonations, and from those you have a few causal events propagating at different rates: the radiation, the shockwaves, the fallout.
And if a tree falls in a forest with no one around, it does still make a sound.
I don't think you'll like the reason why. It was Bishop Berkeley[1], who gave his name to Berkeley CA, that originally posited and answered this question, and his answer was that God is always around and, thus, among other things, hears everything, so the falling tree makes a sound.
[1] https://en.wikipedia.org/wiki/George_Berkeley#Contributions_...
The first person to ask a question doesn't have a monopoly on the answer.
For this particular instance, since the event is about 26,000 light years away, we could as easily say that it happened in 50,000 B.C., since that was the time on Earth when our light traveled to meet it.
All but a 3D place so we could avoid time dilation and experience other planets in real-time.
The whole philosophical debate is the most idiotic thing I have ever heard. Of course things happen even if a human doesn’t observe it. Then they have to make up something about a magical religious figure hearing the tree. All I can think of is the Monty Python skit “..pray that there's intelligent life somewhere up in space, 'cause there's bugger all down here on Earth”
Short of a lazier universe than we're willing to accept, it almost certainly did cause an effect, including harmonic vibrations, ones that are replicated around like echos, etc. It's just unclear if it's sound if there's nobody there to interpret it.
Not saying I buy it one way or the other, just that it's the reason I never thought it was as stupid as you do.
Extending this take back to thread root and then the objection, I always understood "speed of sound" to be more or less "speed of causalty filtered through a substrate", i.e. if you bump into this side, how long does it take that side to be affected by it? "Sound" is just one potential type of bump to be traveling, if we subjectively interpret the vibrations that eventually make it through to the other side that way.
It is probably wrong to assume that God interfaces with the physical universe in the same way as us (and other living beings who perceive sound).
They (God) may perceive vibrations of air particles in a way that we wouldn’t call “sound”.
In fact, I’m pretty sure they have to otherwise they would be overwhelmed by all the noise they hear from everywhere all the time.
[1] https://www.merriam-webster.com/dictionary/sound
[2] https://en.m.wikipedia.org/wiki/Sound
[3] https://dictionary.cambridge.org/dictionary/english/sound (note that "can be heard" != "is heard")
But I agree with that those debates are quite uninteresting, so let's not do that.
A 'fact' that is relative to the frame of reference of the observer.
When you say "there’s nothing less interesting and more irritating than", are you speaking universally, or of your own personal experience?
> Nobody cares about some blowhard’s attempt to generate pointless “debate” over hypotheticals and semantics.
How do you people have knowledge of each others thoughts? Serious question.
https://www.lesswrong.com/posts/a7n8GdKiAZRX86T5A/making-bel...
And yes, the speed of light is defined by the speed of causality, but c is still, among other things, the speed of light.
Quantum entanglement can create causal effects much “faster” than c, but you cannot use such effects to transmit information still.
(Maybe a stupid question, I’ve only a passing interest in this stuff)
There’s no need for FTL if you accept MWI.
For example, if you go on a trip and bring your suitcase with you. You discover on opening it that you packed only one blue sock. You then gain the information that your other blue sock must be at home. You didn't "cause" your sock to appear at home at that moment, you simply gained that information from a correlation that you know must be true. The cause occurred when you were packing. In this case the correlation is entirely local though: you always had that information with you in your suitcase, you just didn't look at it until now.
In quantum entanglement the cause and effect were the entanglement itself. When one of the particles is later observed, you're using correlation to decide what the other end is. The difference is that in quantum theory this correlation can be demonstrated to be non-local -- the information wasn't with you the whole time and was only determined at the moment of observation. But you can't take any action based on it and the other side can't either as you have no way to know who caused the collapse. You can't cause any meaningful effect on the other end with the information you get from your observation or the act of your observation.
If we are using "cause" in the colloquial sense then yes I see what you're saying, it "causes" it. But it's important to be specific with domain terms here because entanglement does not violate causality, which has a specific definition.
Thanks for taking the time to comment. How could I go about learning more about the specific definition of causality you’re referring to here?
As the wikipedia article touches on, there's not an agreed global definition and it can be used differently in multiple contexts (hence why it's important to be specific).
This stack exchange answer gives a good list of terminology: https://physics.stackexchange.com/a/34675
And this article puts a far more technical description on terms: https://www.mprl-series.mpg.de/media/proceedings/3/9/Proc3ch...
Anyway, chrisfosterelli already elaborated on why thinking of the collapse at A "causing" the collapse at B is not a good idea from the perspective of quantum mechanics, either.
Not sure about calling this "real time". If you actually wanted to do something about it (like saving the poor, mysterious object), then any help you send will face a completely different situation from what we see now.
In the distance, there was another civilization that didn't have the technology to see us, or any other way to communicate with us. We also didn't have lasers or any other tech to send messages to them, other than travelling there (which would take some years), but we could see what was going there in almost real time.
Wouldn't the same still apply, even without the limitations of the speed of light?
And when should the family back home celebrate?
Defining the actual timing of events based on the presently available methods to perceive or communicate them seems odd if not also a bit impractical.
The family back home should celebrate when it's relevant. If there's a wormhole, then the kid can probably make a call through it, so they can call on their (self-perceived) birthday. If the wormhole is a one-way trip, then the people back home should wait an additional 4.3 years to celebrate, because they won't have any idea until then.
If your friend tells you today about how they did X yesterday, would you also argue X happened today from your frame of reference?
They're both valid interpretations.
If that friend was 671 million miles away, I was say it happened yesterday. If it was a much smaller distance, I can intervene in the event because the relative effects do not prevent me from interacting.
> They're both valid interpretations.
That's true if you want to accept interpretation without restraint. I prefer consistent interpretations that are compatible with known physics. YMMV
The photon you're seeing that came from 671 million miles travelled to your eye instantly, from its "perspective."
Of course there are. You're confusing the "relativity" part in "General Relativity" with "anything goes" or "anyone can interpret things however they want, without any rules". I have elaborated on this in my other comment to you[0]:
> GR is concerned with objective, well-defined statements. Sometimes a statement might depend on the observer (this is relativity after all) but other observers will still be able to predict what that observer will say because there are clear rules to translate observer-dependent statements between observers. So subjective observer-dependent statements can still be objectively true in this sense.
And no, you wouldn't deny it happened yesterday. If you were watching a live event in front of you you'd have the ability to interact with it, such as by asking your friend to stop. If you're viewing something that already happened, you don't have that ability to interact.
Quantum theory and relativity aren't tools to decrease our specificity and understanding of the universe so they shouldn't be used to justify "all interpretations are valid" type statements.
I'm not trying to do some new-agey "everything is valid" thing here. If I, like the photons, travelled from the source 671 million miles away to Earth at the speed of light, time would not have advanced for me at all. I would be telling you that what you are seeing is happening right this instant. And I would be right!
And of course time on Earth has advanced considerably while I was traveling, from Earth's perspective. This is also true.
It is a remote viewing device because we're discussing using it to view remote events. And you're trying to say "just the same" despite that there are obviously some differences so it cannot be just the same. Try "somewhat analogously".
> If I, like the photons, travelled from the source 671 million miles away to Earth at the speed of light, time would not have advanced for me at all. I would be telling you that what you are seeing is happening right this instant. And I would be right!
No, because it took you 1 hour to get here, and if you went back you wouldn't be there at the time you left, so in no way are the events simultaneous.
You're only describing subjective experience - and someone who travelled much more slowly in stasis would have the same subjective experience despite their report of objective simultaneity being even more obviously incorrect.
If he is right in front of me, it won't take a day for the light to reach me. So, no, I wouldn't.
> They're both valid interpretations.
I wasn't talking about subjective interpretations. I was saying that General Relativity clearly distinguishes between the event X of something happening and the event Y of information about that event reaching an observer. X and Y will necessarily be at a timelike or lightlike distance. Therefore, unless this distance is zero, time will necessarily pass between those two events from the perspective of the observer and, in fact, from the perspective of any other observer you could possibly ask, where "observer" is more or less defined by a coordinate system (whose time axis will be the observer's worldline).
This is not up for debate.
No, this is where I disagree. From the perspective of the observer, they are seeing what they are seeing exactly when they see it. They don't perceive any time having passed at all, because we can't perceive an event's age.
What's not up for debate is how long it would take to do round-trip communication between the two places.
What's also not up for debate is that is impossible to show that these far away events are not happening at the time we see them.
How you interpret this is absolutely up for debate, because it's not some intrinsic physical fact.
You are twisting my words and once again interpreting the words "observer" and "event" in a way that deviates from their standard definition in General Relativity.
Yes, of course we (as beings with pairs of eyes) will perceive something happening in the sky when we actually see it happening or, more generally, when we first hear about it. This is trivial. That's not a good way to define when something happened, though, because it is highly subjective – depending on whether you received the information through light, gravitational waves, sound, or carrier pigeon, your notion of when something happens will differ greatly. In fact, given that information from the event will typically reach you in multiple ways at the same time (think of lightning & thunder) and sometimes you might focus on one, sometimes on the other, and your brain might process things at different speeds, even if we're just talking about you, this is not a great and consistent definition.
Moreover, other people could not predict what times you would ascribe to certain events.
This is why General Relativity does not define it that way. GR is concerned with objective, well-defined statements. Sometimes a statement might depend on the observer (this is relativity after all) but other observers will still be able to predict what that observer will say because there are clear rules to translate observer-dependent statements between observers. So subjective observer-dependent statements can still be objectively true in this sense.
> What's also not up for debate is that is impossible to show that these far away events are not happening at the time we see them.
This is simply not true. How do you think we know that they are actually not happening at the time we see them? Would you also say that it's impossible to prove that the events which cause thunder don't happen at the time you hear the thunder, but earlier, i.e. shortly before you see the lightning?
> How you interpret this is absolutely up for debate, because it's not some intrinsic physical fact.
You are of course free to interpret all the things you see in the sky or on Earth however you like. The problem is: Given the aforementioned issues with such subjective interpretations, they are essentially useless. We won't be able to agree on anything and won't be able to make reliable predictions.
In any case, I was responding to OP who was saying[0]
> For all intents and purposes this is happening in real time from our local frame of reference.
"Frame of reference" has a precise mathematical meaning: It's a coordinate frame with coordinates that an observer would use to map out their local region of the universe, with some precise rules as to what "mapping out" means. Now this coordinate frame assigns different times to the event X of something happening and the event Y of you receiving light that was emitted at X (again, provided that the distance between the thing that's happening and you is not zero but that's essentially always the case). So from the perspective of our local frame of reference, whatever we see in the sky is not happening in real time.
In hindsight, I should have maybe omitted the second half of my comment
> If your friend tells you today about how they did X yesterday, would you also argue X happened today from your frame of reference?
I was merely trying to hint at the fact that the definition "It happens when I perceive it" is, as discussed above, completely and utterly useless, let alone ill-defined.
> This is simply not true. How do you think we know that they are actually not happening at the time we see them? Would you also say that it's impossible to prove that the events which cause thunder don't happen at the time you hear the thunder, but earlier, i.e. shortly before you see the lightning?
No, and this is exactly where the analogy between the speed of sound and speed of light breaks down, because they are fundamentally different. The speed of sound is not subject to (significant) relativistic effects.
You can easily prove the speed of sound by measuring the time between the lightning flash and when you hear the thunder. There is no analog with the speed of light.
I agree that this is the crux of the argument:
> For all intents and purposes this is happening in real time from our local frame of reference.
But you have not shown this to be false.
> But you have not shown this to be false.
Once again you have ignored how General Relativity defines "local frame of reference" – which I have elaborated on extensively –, replaced it with your very own and, as discussed, inconsistent definition, and assert that I am wrong based on this made up definition of yours.
Take gravitational lensing. Light from one event will reach you in multiple (in fact, an infinite number of) ways, at different times. What now? How do you define the time at which an event occured?
Take the fact light often propagates at speeds v < c, where c is "the" speed of light, an effect that all light rays you see in the sky are subject to to some degree because they are passing through interstellar medium et cetera. (For an extreme example see slow light[0].) How do you reconcile your definition with the fact that the speed of (such) light will no longer be the speed that bounds whether two events are causally connected?
Things happen at the earliest possible moment I could know about them.
I was able to get that information about my friend yesterday, I just didn’t.
I couldn’t have learned about this 26,000 years ago.
> Things happen at the earliest possible moment I could know about them.
You're of course free to define the word "happen" this way but among physicists you'd be the only one in doing so.
The speed of light is not always c. That’s why refraction happens. Because light slows down in certain mediums below c. It’s kid of a coincidence that the speed of light happens to be c in a vacuum (actually I’m not sure about this…maybe there’s an organic reason why electromagnetic waves will be c in a vacuum that is fundamental).
I do think it’s interesting that as humans our primary sense is to light which does happen to travel at c. I wonder how different our species would be if we couldn’t sense waves traveling at c, and for example, were only capable of detecting sound.
Alternatively it may be that a species which could not sense waves traveling at c would not be capable of becoming as sophisticated as humans.
More seriously, I don't think speed-of-light has much biological relevance. It has consequences for microchips, though.
about makes this suggestion completely impractical
https://www.aanda.org/articles/aa/abs/2019/05/aa35656-19/aa3...
(So "24650BC" for... context.)
And when measurements improve or are revised, everything would need to be recalculated. So, for posterity, it would be 24570-24730BC (2023 measurements).
Or, 2023 is simpler.
I imagine core civilisations would have a good laugh at someone on the outer arms of the galaxy attempting to impose their frame of reference as preferential.
There is no way to reach some state where you could still exist in a universe where the event hasn’t happened. If you traveled at the speed of light to the event location, you would see it has already happened.
Since the speed of light is the fastest possible movement, there is no way to arrive at the event location faster than that, any faster movement means you would be time traveling back into the past in attempt to reach the event before it happens.
Any possible event that happens in our daily lives starts off with two possible states: happened or didn’t happen. When one of those states is eliminated, the event status is resolved and reality is updated. Usually we see these updates occur damn near instantly. In this case, in this universe, the “didn’t happen” state has been thoroughly eradicated. But at these distances, we don’t see updates yet, but it has happened.
Therefore, you can consider the event as having happened. It was… inevitable.
I don't quite get the time travel bit. I've seen too many sci fi and fantasy shows where time is effectively frozen while supernatural entities move about. Other than the supernatural entities moving about, the instant is the same everywhere, it doesn't go back.
And if I see an event at Alpha Centauri, and immediately teleport there, the light wave is still at Earth. I haven't traveled back in time to the origination of the light wave at Alpha Centauri. And my light wave will not reach Earth until another ~4.3 years has passed after the first light wave that prompted my journey to Alpha Centauri. So the observer will not see anything unusual. I will not have rewritten the light pattern they saw.
Also I think astronomers do see events which appear to be violations of causality, but are actually just gravitational distortions of the light traveling from an event. So interference with the travel of light from an event can't really be seen as traveling back in time.
If you wanted to see it up close, you would have to travel into the past, since your teleportation would have to include a translation of time coordinates, not just of space coordinates.
Also, although order of events is variable, causality is (I believe) currently assumed to be absolute, so while two observers can differ on which came first between events A and B, this is only possible if A and B’s light (really, information) cones do not intersect such that one precedes the other causally.
Suppose A does not intersect B by light cone, and you can instantly teleport. Then you can interact at A, teleport and interact at B, then immediate teleport everywhere in the universe, causally intertwining all frames before causality can physically propagate to those coordinates. Then you have time traveled to such an extent that I have no idea how the universe reconciles such a thing.
Assuming a finite speed limit to information transfer exists, exceeding that limit means you can create a potentially unlimited number of contradictions in state.
It would be like watching the waves do what they do, but none of it happened, because no matter what you think you saw, there is at least one contradiction (due to the teleporting meddler). Like if you imagine any one state, and you meddle it to be impossible, as if doing Cantor’s diagonalization on the potential state space (if you can do literally instant teleportation and action with no cooldown).
I honestly don't understand what you're talking about with respect to contradictions. You aren't traveling back in time by instantaneously jumping. Is the universe supposed to be one cohesive thing? What are you talking about when you reference states in relation to the universe?
I'm not a relativity physicist, and don't even know if these questions and suppositions are even the right things to be asking. If one could travel faster than the speed of light by teleportation, I don't think time travel or an unsyncing of the fabric of reality seems like it would need to happen. But what do I know. Thought experiments don't always reflect reality.
So let there be a one mile long object, let it travel at nearly light speed for five seconds. As God, freeze time after 5 seconds and place two measuring stations: one touching the head of Object from Observer's point of view, and one touching the tail of Object from Object's point of view (we can teleport around instantly so we can verify that we've got everything set up correctly with no penalty).
Observer Head thinks Object Head and Object Tail are at about 5 light-seconds + 1 mile from origin (because Object is vanishingly short, and Object Head started at 1 mile from origin). But within Object, the Tail is 1 mile behind.
So Object Tail will signal Observer Tail at about 1 mile/c before it reaches where it supposedly is, and Observer Tail instantly signals Observer Head, thus Observer Head receives causality from its signal before it sends its signal.
Not sure how the universe resolves that. Unless the universe can "overclock" and become indefinitely fast at propagating and processing information when need be.
I think my example isn't entirely sound because I forget the exact math (it's been a long time), so you might have to tweak the example to be fully mathematically sound, but the general concept of causal contradiction within one's own frame of reference should be sound, I think.
I've played around with the fantastic idea of instant portals between places (or simple teleportation), and assume this would have unusual effects on gravity when, for instance, portalling from one side of a planet to the other.
I think I lack the theoretical basis to really understand whether this is a problem for the universe as it is. And I don't think a lay explanation can be satisfactory. I appreciate your attempt.
In this case, something is actually (not illusory) happening before it happens, if we assume some actor that can act faster than causality can keep up.
I have also come to the conclusion I have forgotten too much of my math haha, was a fun exercise trying to recover some part of it.
Those representations are inaccurate.
If time is truly frozen, that means photons should be frozen in place, unless photons are somehow exempt due to having no mass and thus no time.
If the frozen photon theory is correct, you would have to see things by moving your eye into rays of light and letting photons hit your receptors and get consumed. But because these photons are being consumed and not replaced, you will leave pockets of darkness where your eyes will see no signal if you pass through there again.
At normal everyday distances you will see everything frozen in place with this method. However, at long distances such as light years, moving into the light source will run its animation over time at the speed you are moving toward it. So you will see the event occur before your eyes right up until you reach the destination where the event has already occurred, and you can observe firsthand what we already knew to be true.
Yes, I know Sci Fi and fantasy are inaccurate depictions of reality.
> So you will see the event occur before your eyes right up until you reach the destination where the event has already occurred, and you can observe firsthand what we already knew to be true.
I get this.
I just don't get how moving faster than the speed of light (or causality) necessarily invokes literal time travel. And while I've read that physical scientists are amazed at how well mathematics model and predict reality, if this time travel is a result of the mathematics, then I need to be convinced that in this case the mathematics really are modeling reality.
Suddenly, some travelers arrive and say they also came to witness the event, but they came from light years away.
How? The light of the event would have only traveled for a few light minutes, and no other type of information would travel faster than that. There’s no way someone light years away would know about it yet.
The only explanation, assuming they didn’t perfectly predict the event, is that they time traveled. From your perspective they covered a distance of light years in the span of time between the event’s start and you first meeting them.
And if they do seem to suddenly appear much faster than you would believe possible, you suddenly know that you're at the bottom of a time-dilation gravity well (or the equivalent). Though you probably could have figured this out beforehand from the red-shift of the light traveling between the two points. Or you know that they observed the event from a closer point of view, instantaneously traveled to a most distant location to change clothes, and then traveled again to your location. Time travel is only one of at least three possibilities.
In general relativity, things get weird and frames are more of a local phenomenon, but as long as you stay away from the event horizon, it’s not that weird.
Having observed many thousands of discussions about "conspiracy theory" or "just wrong" (but the normal person can't explain why it is wrong, in fact) topics, I am very confident in this belief. And now having also had some similar conversations with ChatGPT, which is able to overcome with ease many mistakes that humans make, yet cannot overcome others, I am even more convinced.
Obviously the Earth’s orbit is noncircular. Is that what you mean?
I'm referring to people who think it's like a few thousand feet above the earth.
That's so bizarre: Stand here, thing A happened before thing B. Stand over there, thing B happened before thing A. Stand at this other place, now they did happen at the same time, but only for you! Nobody's more right than the other. Universally, "same-timeness" (simultaneity) does not exist.
So, by asking for:
> to know things that are happening right now or relatively recently by our standards
You're really only asking for your particular flavor of "right now or relatively recently".
A--1--B--2--C
Imagine 1 and 2 are satellites and observer B launches a rocket at both of them at the same time. From B's perspective, both satellites explode at the same time.
A, however, will see 1 explode before 2. C, on the other hand, will see the reverse, that 2 explodes first.
There's no right ordering here, just different reference frames.
So yes, Alice, Bob, and Charlie can all calculate the timestamps relative to the other positions, and they could collectively decide together that one of those options will count as the "true" measuring point. But this is still a measurement decision that they have made.
I’m an idiot and would love to know how this would be possible.
But please do ignore the people confusing the delay in seeing distant events with relativistic simultaneity. It definitely did happen already in our reference frame. Just think of all astronomical events as being implicitly timestamped by observation time, not actual time. After all, distance is hard to measure in space, which means observation time is the more solid reference point.
Grammar becomes complicated when relativity dictates that there’s no universal ordering of events/timeline.
Thus given location X you can get a universally agreed upon ordering. Or alternatively you can devise locations from various sets of orderings. A related example might be using multiple audio recordings to determine when and where various shots were fired.
There are no universal time across the universe due to relativity, so the only time that make sense and can be determined with any degree of accuracy is when it happened from our perspective.
I do appreciate the HN joke though.
> because the light we see is ~25k years old from the center of the galaxy, we are seeing it as it was 25k years ago. However, in astronomy we do not worry about this and instead just use the time at which the light reaches Earth- firstly there is just no way to know what is happening there literally now
so in their view, it seems assuming the event as present is the norm.
*: https://old.reddit.com/r/science/comments/11bk0u1/a_mysterio...
Speaking about something beinng N lightyears away from us just means both the distance in space and time. What we are seeing happening there and then is not from our past: it could not affect our past because the light from that event hadn't arrived to us when and where our past was happening, and thus could not affect our local past. We see it happening "before" our past, but observers elsewhere may see it happening "at the same time" or "after" our past, in their frames of reference.
It might be as much today for it as it is for us.
But the whole point of relativity is that there is no absolute scale of time. Time differs based on the observer. So any time that we describe should be based on an observer on earth, because the overwhelming majority of HN readers are on earth.
“One possibility is that X7’s gas and dust were ejected at the moment when two stars merged,” Ciurlo said. “In this process, the merged star is hidden inside a shell of dust and gas, which might fit the description of the G objects. And the ejected gas perhaps produced X7-like objects.”
This is pretty interesting, so much ejection due to a merger that the light is no longer visible.
Space stories like this always melt my mind because this has all happened already , but we're observing it now.
In a real sense, it only happens when the light from it reaches you. Reality propagates at the speed of light.
I'm having difficulty parsing your "in a real sense". Are you refering to Relativity here or something deeper like 'subjective reality'?
Without the light (or radiation), all you can say is that "something may have taken place everywhere".
Whether/when I "know" about it is separate, and not what they're talking about.
Then there’s ER=EPR to try to conceptualize.
The OP was referring to when a event happened though.
For the sake if the argument let's allow this body to be an light-hour away from us when it happened.
I think your logic is very weak here.
Things happen without us knowing about it, and without light from such events ever reaching us. We don't need to know it has happened for it to have happened.
An element of light traces out a worldline. On any worldline we can apply whatever labelling-of-points we want since relativity is a coordinate-independent theory. We can label the points of a worldline with greek letters, hieroglyphs, roman numerals, natural numbers, real numbers, whatever we like and in any order we like.
One can build an infinity of calculationally-useless or misleading sets of coordinates on these worldlines for things heavier/slower than light. But one can also build an infinity of calculationally-useful and non-misleading coordinates for them, and many of those make use of the invariant spacetime interval. The same applies to coordinates for massless things / things that move at the speed of light, even though the invariant spacetime interval for light in free-fall is always 0, even if it is in free-fall for billions of years (like light from distant quasars, or the cosmic microwave background).
A calculationally-useful ordering applies a monotonically increasing order from the start to the end of a worldline in a time-orientable manifold (our universe is time orientable: smaller and denser in the past, bigger and sparser in the future). For timelike worldlines (i.e., anything that is always slower than light), almost always the most useful ordering is proper time.
But we cannot calculate proper time on a null (lightlike) worldline, so we will want some other monotonically increasing ordering function on the worldline, and ideally one with which we can solve the geodesic equation. Such a family of orderings is not only known, but has been textbook material since 1970 (Spivak's introduction to differential geometry). It's the affine parametrization.
For lightlike observers there is thus a useful and well-defined notion of time: the affine (parameter) time. This is different from but analogous to the proper time available to timelike observers. We can do standard vector physics on a photon using affine time, e.g. we can calculate its phase at various points along its trip from point A to point B. (Indeed, talking about a photon's quantum wavefunction, the affine parameter is proportional to its phase). We can also take the derivative of position with respect to affine time as a momentum that accurately captures the gravitational redshift or blueshift between two points on the null worldline.
"Is"?
https://old.reddit.com/r/science/comments/11bk0u1/a_mysterio...
The fact we can determine -anything- is mind blowing.
> "...the Sun - in fact, our whole solar system - orbits around the center of the Milky Way Galaxy. We are moving at an average velocity of 828,000 km/hr. But even at that high rate, it still takes us about 230 million years to make one complete orbit around the Milky Way!"
It takes a vast amount of energy for a stable orbiting body to reach the center of mass of the object it is orbiting around, for example that's why the small Parker Solar Probe (launched 2018) required the massive Delta 4 heavy rocket to provide the boost energy:
https://www.youtube.com/watch?v=AlyuSwRSVHU
So, 'dragged' isn't really the right way to think about it, is it? If two stars collided there must have been some massive energy input into the X7 object which accelerated it towards the black hole (it might instead have been accelerated away from the black hole, depending on the dynamics of the collision). Hence, 'it was shoved towards the black hole' is perhaps a more accurate view than 'it is being dragged by the black hole'.
If the Earth smacked dead-on into another Earth orbiting in the opposite direction around the sun, I imagine most of the combined mass would start falling into the sun due to net loss of momentum, although jets of material might get accelerated further out into the solar system, as well as inwards. In that case 'dragged' might make more sense.
The case for two stars seems more complicated, they start orbiting each other before merging, which could be a very energetic event if they were large enough, i.e. a 'merger-triggered core collapse supernova'.
When you think of orbiting as "falling and missing continuously", the lowest-energy way to stop missing is to stop moving forward.
Probably not…
If big object A pulls small object B with gravity, wouldn't B just speed up and then miss, ending up in some kind of orbit? Like, if you are in space and throw a rock at the Sun, it won't hit unless you perfectly counteract the relative speed, right?
Hitting the Sun is HARD. https://www.youtube.com/watch?v=LHvR1fRTW8g
You are correct that it is not easy to make things collide in space. But it's (obviously) not impossible. The trajectory doesn't have to be perfect, just close enough that the lowest point in the orbit is less than the radius of the object being orbited. In the case of a supermassive black hole, that's a pretty big target. The event horizon of Sgr A* is about 50 million kilometers in diameter, and that is surrounded by an accretion disk that acts kind of like an atmosphere, so anything that comes close gets slowed down through friction, and then it eventually falls in.
(Also, near black holes, general relativity makes things kinda weird.)
> if you are in space and throw a rock at the Sun, it won't hit unless you perfectly counteract the relative speed, right?
If you are out in space in orbit around the sun this is true. But there are lots of ways to be "out in space" not in orbit around the sun. The reason that most of the stuff we see is in orbit around the sun is that the stuff that is not in orbit around the sun doesn't stick around very long. It either flies off into deep space, or it falls into the sun.
Assuming of course that you could "throw" that rock hard enough to counter our massive rotational velocity.
They're essentially counteracting that relative speed over time by bleeding energy to all sorts of drag (one exotic example that comes to mind is with things like neutron stars and black holes bleeding out vast amounts of energy in gravitational waves - literal waves in space-time we can detect from billions of light years away - in the moments before they crash into each other).