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 blows my mind is that 20 million light years also means that what we're observing something that happened 20 million years ago.
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...
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.
Sorry if questions don’t make sense…
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.
C is coulombs
And casualty is a dead person
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'm not an historian but the curvature of the world was probably explained away by some other phenomenon because, after all, the world is flat (for those experience curvature in times of flat Earth).
So to "experience" the curvature of the Earth, you first have to know the Earth is round. Much as our "bug free" software is only buggy once we find a bug, before that, the bug was a feature ;)
Similar the stars were explained away as holes in a black sphere surrounding the Earth: the light points in a night skies didn't lead to a universal acceptance of the Universe. It was telescopes that made us realize that those "holes in the sphere" were actually galaxies.
So my understanding would be, that it was Magellan circumnavigating the Earth that made folks actually "experience" the curvature of the Earth.
This might be being pedantic but I am trying to point out that "experiences" are related to understanding and perspective. Experiences are different in different contexts: hence my experiences are different to all those around me.
My interpretation of "experiences" differ according to understanding. An apple falling on my head might well have more to do with evilness of various gods than gravity before the invention of gravity. But even gravity is just another interpretation/explanation/myth to explain the "experience" falling apple.
> 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
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?
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.
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.)
> So a hypothetical photon from the big bang (era?) gets observed now means the big bang is happening 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.
> A dinosaur decays to bones and now gets observed means the dinosaur died just now - not that it died millions of years ago?
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.
I'll repeat: reference frames in relativity are about motion, not any kind of distance, spatial or temporal.
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.
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.
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.
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
Please, pedantically correct my phrase "break the reference frame". I'd like to know how to phrase that properly. "Change reference frame" maybe?
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.
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?
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.
I'll not say that Wikipedia is a particularly good source, I'm happy to be corrected if you have something that contradicts that.
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.
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.
It's obviously fair to say they happened at the same time in this scenario.