| ▲ | dotancohen 4 hours 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. |
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| ▲ | steve_adams_86 10 minutes ago | parent | next [-] |
| 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? |
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| ▲ | ckcheng 25 minutes ago | parent | prev | next [-] |
| > 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.) |
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| ▲ | anonzzzies 3 hours ago | parent | prev [-] |
| 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. |
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| ▲ | grumpopotamus 2 hours ago | parent [-] | | 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". | | |
| ▲ | tomrod 44 minutes ago | parent | next [-] | | 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. | |
| ▲ | anonzzzies 2 hours ago | parent | prev | next [-] | | 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 an hour ago | parent | prev [-] | | 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? | | |
| ▲ | philipov 19 minutes ago | parent | next [-] | | 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 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 ask what happens simultaneously with Person A reaching the singularity - it takes infinite time just to reach the event horizon. Even asking the question is nonsense. https://en.wikipedia.org/wiki/Relativity_of_simultaneity | |
| ▲ | herbstein 15 minutes ago | parent | prev [-] | | 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 |
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