4 ms·

The reason the scenario posits an invasion from so far away is that that's about how much distance it takes for a small speed difference here to translate into

The reason the scenario posits an invasion from so far away is that that's about how much distance it takes for a small speed difference here to translate into a day long time difference there.

awb 5y ago
You could imagine an event happening a day later from a traveling car, but that’s about it. Two people can’t actually observe an event a day later from the same reference point (Earth), and naturally the event occurs at a singular time at Andromeda’s reference point. Same with any point between Earth and Andromeda. So, if you can’t observe it at different times from the same reference point did it actually happen at different times? From the Wikipedia page on the thought experiment, check out Criticisms: https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument#Criticisms https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argu... > The interpretations of relativity used in the Rietdijk–Putnam argument and the Andromeda paradox are not universally accepted. > Howard Stein[5] and Steven F. Savitt[6] note that in relativity the present is a local concept that cannot be extended to global hyperplanes. > Furthermore, N. David Mermin[7] states: That no inherent meaning can be assigned to the simultaneity of distant events is the single most important lesson to be learned from relativity. — David Mermin, It’s About Time
- goblin89 5y ago
  A philosophical idea may not have meaningful implications within the scope of physics, but that doesn’t make it useless. Rietdijk–Putnam argument, for example, appears to sort of elevate the idea of differing planes of simultaneity, taken from special relativity, to an idea of a four-dimensional universe. It’s a curious thought experiment and I wonder how it reconciles with a model where the universe is considered to be 4D with the fourth dimension being time. My intuition doesn’t work all that well beyond three dimension so I can’t tell if those are essentially the same or not. It’s not falsifiable, but neither is string theory.
Retric 5y ago
That’s not how it works. But even beyond that you have the math wrong. Andromeda galaxy is only 2.5 million light-years from Earth. To get a 1 day difference in time dilation across 2.5 million years would take 1 in 913 million difference which someone traveling at even 100MPH (45m/s) isn’t going to see. (1 - (45^2/((3x10^8)^2))^0.5 simply isn’t enough.
- penteract 5y ago
  I did the calculations from https://en.wikipedia.org/wiki/Special_relativity#Lorentz_transformation_and_its_inverse https://en.wikipedia.org/wiki/Special_relativity#Lorentz_tra... and it comes to ~0.25 years if the person in the car is moving towards Andromeda at 30m/s. γ is indeed very close to 1, so the humans' watches would still agree closely after 2.5 million years, but the humans would be a quarter of a light year apart at that point, so they need to disagree about something to get the same observed speed of light from Andromeda.
  - deleted 5y ago
    [deleted]
  - Retric 5y ago
    Direction of travel was undefined in the example, just a car on the earth. Suppose they where traveling 30m/s away from andromeda vs towards it suddenly the timing switches. Chose a specific vector and both observers exactly agree with the events timing. Essentially the only thing they disagree about is what Andromeda’s location and velocity is. (Edited this several times, but it all simplified to choice of vectors.) But even that’s assuming spaceships rather than “one stationary and one travelling past in a car” the guy in a car is never going to end up light years distant from the stationary person.
feoren 5y ago
So as I'm driving, the debate is going on, but as soon as I slam on my brakes, it's already happened! They're already on their way! Oh but now the squirrel is out of the road and I'm late so I speed way back up and the debate hasn't even started yet now. No. The only thing that is "happening right now" on Andromeda from my point of view is the causality wave (light) that is hitting me right now. It's nonsense to talk about anything happening "right now" if its causality wave hasn't reached me yet.

The reason the scenario posits an invasion from so far away is that that's about how much distance it takes for a small speed difference here to translate into a day long time difference there.

awb 5y ago

You could imagine an event happening a day later from a traveling car, but that’s about it. Two people can’t actually observe an event a day later from the same reference point (Earth), and naturally the event occurs at a singular time at Andromeda’s reference point. Same with any point between Earth and Andromeda. So, if you can’t observe it at different times from the same reference point did it actually happen at different times? From the Wikipedia page on the thought experiment, check out Criticisms: https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument#Criticisms https://en.m.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argu... > The interpretations of relativity used in the Rietdijk–Putnam argument and the Andromeda paradox are not universally accepted. > Howard Stein[5] and Steven F. Savitt[6] note that in relativity the present is a local concept that cannot be extended to global hyperplanes. > Furthermore, N. David Mermin[7] states: That no inherent meaning can be assigned to the simultaneity of distant events is the single most important lesson to be learned from relativity. — David Mermin, It’s About Time

goblin89 5y ago
A philosophical idea may not have meaningful implications within the scope of physics, but that doesn’t make it useless. Rietdijk–Putnam argument, for example, appears to sort of elevate the idea of differing planes of simultaneity, taken from special relativity, to an idea of a four-dimensional universe. It’s a curious thought experiment and I wonder how it reconciles with a model where the universe is considered to be 4D with the fourth dimension being time. My intuition doesn’t work all that well beyond three dimension so I can’t tell if those are essentially the same or not. It’s not falsifiable, but neither is string theory.

Retric 5y ago

That’s not how it works. But even beyond that you have the math wrong. Andromeda galaxy is only 2.5 million light-years from Earth. To get a 1 day difference in time dilation across 2.5 million years would take 1 in 913 million difference which someone traveling at even 100MPH (45m/s) isn’t going to see. (1 - (45^2/((3x10^8)^2))^0.5 simply isn’t enough.

penteract 5y ago
I did the calculations from https://en.wikipedia.org/wiki/Special_relativity#Lorentz_transformation_and_its_inverse https://en.wikipedia.org/wiki/Special_relativity#Lorentz_tra... and it comes to ~0.25 years if the person in the car is moving towards Andromeda at 30m/s. γ is indeed very close to 1, so the humans' watches would still agree closely after 2.5 million years, but the humans would be a quarter of a light year apart at that point, so they need to disagree about something to get the same observed speed of light from Andromeda.
- deleted 5y ago
  [deleted]
- Retric 5y ago
  Direction of travel was undefined in the example, just a car on the earth. Suppose they where traveling 30m/s away from andromeda vs towards it suddenly the timing switches. Chose a specific vector and both observers exactly agree with the events timing. Essentially the only thing they disagree about is what Andromeda’s location and velocity is. (Edited this several times, but it all simplified to choice of vectors.) But even that’s assuming spaceships rather than “one stationary and one travelling past in a car” the guy in a car is never going to end up light years distant from the stationary person.

feoren 5y ago

So as I'm driving, the debate is going on, but as soon as I slam on my brakes, it's already happened! They're already on their way! Oh but now the squirrel is out of the road and I'm late so I speed way back up and the debate hasn't even started yet now. No. The only thing that is "happening right now" on Andromeda from my point of view is the causality wave (light) that is hitting me right now. It's nonsense to talk about anything happening "right now" if its causality wave hasn't reached me yet.