3 ms·

There is no objective concept of 'now' even at distance scales of Alpha Centauri with reasonable curvature and speeds. If you want to stick to special relativi

There is no objective concept of 'now' even at distance scales of Alpha Centauri with reasonable curvature and speeds. If you want to stick to special relativity, the Andromeda paradox will show the problem with simultaneity with distance, and Einstein's train thought experiment about trains will show the problem of simultaneity with high speed. The order of events, timing of events, and even causal structure of events are not fixed. Even the Conventional Celestial Reference System is only quasi-inertial, only inertial like over small periods of time. The incredible smoothness of the earth approximating a co-rotating frame with a high error tolerance is the only reason we can treat time as absolute on the surface of the Earth. But to derive the new definition of the Kilogram requires lots of corrections as do modern atomic clocks. Don't confuse the ability to choose a reference frame with the Newtonian concept of universal reference frames. When making measurements SR is a framework that allows you to pick any reference frame that is convenient, but the speed of causality(light) prevents the existence of a universal reference frame.

numpad0 3y ago
I don’t get it, can’t you just set off neutron bomb every second on Earth and use it as a monotonic counter signal? Isn’t that like, speed of light or wavefront of event propagation in the universe isn’t consistent but unavoidably potato shaped - but if so, wouldn’t it be at least difficult to have multiple potatoes intersecting, or potato being shaped like Klein bottle? Self intersecting potato shaped space-time would be cool, though.
- nyrikki 3y ago
  The problem is that they aren't 'potato shaped' rotations, they are better thought of as hyperbolic rotations. Simultaneous distant events in Lorentz transformations will all lay on a shared hyperbola not a distorted circle. Really this is beyond typical human intuition and requires math, but here is a quick minutephysics video that quickly demonstrates fairly well it without me typing out a bunch of math that no one wants anyway. https://youtu.be/SrNVsfkGW-0 https://youtu.be/SrNVsfkGW-0
- whimsicalism 3y ago
  in short, no. not if you want hyperaccurate time keeping
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.

numpad0 3y ago

I don’t get it, can’t you just set off neutron bomb every second on Earth and use it as a monotonic counter signal? Isn’t that like, speed of light or wavefront of event propagation in the universe isn’t consistent but unavoidably potato shaped - but if so, wouldn’t it be at least difficult to have multiple potatoes intersecting, or potato being shaped like Klein bottle? Self intersecting potato shaped space-time would be cool, though.

nyrikki 3y ago
The problem is that they aren't 'potato shaped' rotations, they are better thought of as hyperbolic rotations. Simultaneous distant events in Lorentz transformations will all lay on a shared hyperbola not a distorted circle. Really this is beyond typical human intuition and requires math, but here is a quick minutephysics video that quickly demonstrates fairly well it without me typing out a bunch of math that no one wants anyway. https://youtu.be/SrNVsfkGW-0 https://youtu.be/SrNVsfkGW-0
whimsicalism 3y ago
in short, no. not if you want hyperaccurate time keeping

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.