3 ms·

> The order of events, timing of events, and even causal structure of events are not fixed. You can calculate observed order of events for any reference frame

by Retric 3y ago

> The order of events, timing of events, and even causal structure of events are not fixed. You can calculate observed order of events for any reference frame from any reference frame. Thus they are fixed once everyone agrees to using a given reference frame. It’s not much more complicated than seismic detectors recording the seismic waves from an earthquake at different times based on their location. Sure detector X see earthquakes in order A, B, C and detector Y sees them in order C, B, A they would both agree that detector Z should see them in order A, C, B based on it’s location, the timing of the events, and wave propitiation speeds. It only seems unintuitive because you aren’t used to accounting for such issues.

nyrikki 3y ago
In SR, you have the spacetime interval, it is more complex in GR. For constant velocity motion in flat spacetime: Δτ^2Δs^2 = c^2Δt^2-Δx^2-Δy^2-Δz^2 The spacetime interval is invariant, but notice it is also a scaler that combines space and time, and note that time is multiplied by c^2. Also note Δt is the time as observed but an inertial observer that is present at both events. You have three types of intervals here represented by one scaler. Timelike: Δτ^2 > 0 Spacelike: Δτ^2 < 0 Lightlike: Δτ^2 = 0 In the case of Δτ^2 = 0, that is equivalent to an inertial observer that is present at both events at the same time. Which will seem to be simultaneous. Δτ and Δt are practically the same in situations we commonly experience and an assumption of constant velocity motion in flat spacetime is usually accurate enough. And will an assumption of constant velocity motion in flat spacetime, you can CHOOSE a Δτ that is 0. There is no universal reference frame where Δτ =0. More importantly your observations are of the past for distant observers, and don't have a universal concept of 'now' outside of the local case.
- Retric 3y ago
  You just completely missed my point. GR and curved space time again isn’t an issue as long as everyone knows what space time looks like. Mission Control on earth, a moon base, and mars would all experiencing time at a different rate and those rates would vary based on the orbits of each body. However assuming we needed extreme precision, such as telescope interferometry, people can pick one of them arbitrarily or perhaps due to politics some arbitrary 4th option. Whatever their choice everyone’s timelines based on that choice end up being absolutely identical. The math simply works without any ambiguity. So because there’s an obvious choice based on the actual universe we live in, ie maximum amount of time since the Big Bang, then in theory trillions of civilizations across many different galaxies might all use the exact same reference even if no particle experienced it. Edit: Well not exactly the same there’s uncertainty for such calculations but effectively the same.

by Retric 3y ago

nyrikki 3y ago

In SR, you have the spacetime interval, it is more complex in GR. For constant velocity motion in flat spacetime: Δτ^2Δs^2 = c^2Δt^2-Δx^2-Δy^2-Δz^2 The spacetime interval is invariant, but notice it is also a scaler that combines space and time, and note that time is multiplied by c^2. Also note Δt is the time as observed but an inertial observer that is present at both events. You have three types of intervals here represented by one scaler. Timelike: Δτ^2 > 0 Spacelike: Δτ^2 < 0 Lightlike: Δτ^2 = 0 In the case of Δτ^2 = 0, that is equivalent to an inertial observer that is present at both events at the same time. Which will seem to be simultaneous. Δτ and Δt are practically the same in situations we commonly experience and an assumption of constant velocity motion in flat spacetime is usually accurate enough. And will an assumption of constant velocity motion in flat spacetime, you can CHOOSE a Δτ that is 0. There is no universal reference frame where Δτ =0. More importantly your observations are of the past for distant observers, and don't have a universal concept of 'now' outside of the local case.

Retric 3y ago
You just completely missed my point. GR and curved space time again isn’t an issue as long as everyone knows what space time looks like. Mission Control on earth, a moon base, and mars would all experiencing time at a different rate and those rates would vary based on the orbits of each body. However assuming we needed extreme precision, such as telescope interferometry, people can pick one of them arbitrarily or perhaps due to politics some arbitrary 4th option. Whatever their choice everyone’s timelines based on that choice end up being absolutely identical. The math simply works without any ambiguity. So because there’s an obvious choice based on the actual universe we live in, ie maximum amount of time since the Big Bang, then in theory trillions of civilizations across many different galaxies might all use the exact same reference even if no particle experienced it. Edit: Well not exactly the same there’s uncertainty for such calculations but effectively the same.