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I think you’ve confused the notion of nothingness, which may or may not even be real in practice, and spacetime, which is definitely a thing. Gravity is a resul

I think you’ve confused the notion of nothingness, which may or may not even be real in practice, and spacetime, which is definitely a thing. Gravity is a result distortions in that thing.

colordrops 9y ago
Ok, so if that's the case, why can't objects be relative to spacetime rather than other objects, as discussed in the article?
- pizza 9y ago
  Short answer: because tachyons don't exist (i.e. haven't been found) Slightly longer answer: To be relative to spacetime would (most likely) mean having a means of transmitting information faster than the speed of light, but information cannot travel faster than the speed of light. * "The world line (or worldline) of an object is the path of that object in 4-dimensional spacetime, tracing the history of its location in space at each instant in time." [0] Something that would exist relative to spacetime (as in, would not be confined by the laws that dictate spacetime) would have to be outside of a light cone [1]; "In flat spacetime, the future light cone of an event is the boundary of its causal future and its past light cone is the boundary of its causal past." https://en.wikipedia.org/wiki/World_line https://en.wikipedia.org/wiki/World_line https://en.wikipedia.org/wiki/Light_cone https://en.wikipedia.org/wiki/Light_cone * I don't know very much about quantum entanglement, so if anyone would like to comment as to whether entanglement might appear to violate transmission of information faster than c, that'd be neat, but i have a suspicion that it doesn't because either because of (A) a reason depending on the fact that qubits aren't like bits or (B) that the separation of two entangled subsystems would constitute the actual transmission of information but would still necessarily be no faster than c.
  - BlackFly 9y ago
    This isn't true. Given a manifold that satisfies the vacuum Einstein equations and given a world line in that manifold, it can be shown that the world line is undergoing acceleration using only local calculations/measurements. The worldline of a particle on the edge of a gyroscope will have a non-vanishing acceleration and this can be computed using local calculations/measurements. You can describe the manifold using a coordinate system corotating with the gyroscope, the worldlines will still have a non-zero acceleration. There is absolutely no need for any matter field satisfying hyperbolic PDE's with faster than light characteristics.
- Retra 9y ago
  The only thing that is not an object (as such) that remains in spacetime are fields, whose state evolves according to the principle of relativity. Thus ensuring that you cannot measure your position relative to spacetime without getting "you are right here" as a pretty-much useless answer. For what it's worth, the reason we know spacetime exists and is not 'nothing' is because it carries energy around the universe. That's pretty much the main common property shared by things that exist.

I think you’ve confused the notion of nothingness, which may or may not even be real in practice, and spacetime, which is definitely a thing. Gravity is a result distortions in that thing.

colordrops 9y ago

Ok, so if that's the case, why can't objects be relative to spacetime rather than other objects, as discussed in the article?

pizza 9y ago
Short answer: because tachyons don't exist (i.e. haven't been found) Slightly longer answer: To be relative to spacetime would (most likely) mean having a means of transmitting information faster than the speed of light, but information cannot travel faster than the speed of light. * "The world line (or worldline) of an object is the path of that object in 4-dimensional spacetime, tracing the history of its location in space at each instant in time." [0] Something that would exist relative to spacetime (as in, would not be confined by the laws that dictate spacetime) would have to be outside of a light cone [1]; "In flat spacetime, the future light cone of an event is the boundary of its causal future and its past light cone is the boundary of its causal past." https://en.wikipedia.org/wiki/World_line https://en.wikipedia.org/wiki/World_line https://en.wikipedia.org/wiki/Light_cone https://en.wikipedia.org/wiki/Light_cone * I don't know very much about quantum entanglement, so if anyone would like to comment as to whether entanglement might appear to violate transmission of information faster than c, that'd be neat, but i have a suspicion that it doesn't because either because of (A) a reason depending on the fact that qubits aren't like bits or (B) that the separation of two entangled subsystems would constitute the actual transmission of information but would still necessarily be no faster than c.
- BlackFly 9y ago
  This isn't true. Given a manifold that satisfies the vacuum Einstein equations and given a world line in that manifold, it can be shown that the world line is undergoing acceleration using only local calculations/measurements. The worldline of a particle on the edge of a gyroscope will have a non-vanishing acceleration and this can be computed using local calculations/measurements. You can describe the manifold using a coordinate system corotating with the gyroscope, the worldlines will still have a non-zero acceleration. There is absolutely no need for any matter field satisfying hyperbolic PDE's with faster than light characteristics.
Retra 9y ago
The only thing that is not an object (as such) that remains in spacetime are fields, whose state evolves according to the principle of relativity. Thus ensuring that you cannot measure your position relative to spacetime without getting "you are right here" as a pretty-much useless answer. For what it's worth, the reason we know spacetime exists and is not 'nothing' is because it carries energy around the universe. That's pretty much the main common property shared by things that exist.