Things that can be observed in a snapshot view of the universe

  • Level: Undergrad 
  • Thread starter Thread starter Jehannum
  • Start date Start date
  • Tags Tags
    Observed Universe
Join the discussion
Registration is free. Start your own thread to ask a follow-up.
Sign up Log in
3 replies · 1K views
Jehannum
Messages
102
Reaction score
26
TL;DR
I'm interested in separating out things that can be observed by looking at a picture of the universe at a single moment in time, and things that can't
If we took a snapshot of the entire universe (or if we stopped time) but still had the power to move around and measure, we could observe static properties such as:

Mass
Force
Position

From the above, we would be able to infer acceleration. But we couldn't observe present velocity and therefore kinetic energy or momentum.

Therefore the state of the universe isn't fully encoded in its own three dimensions. Yet, if we were to restart the universe, objects would move at their previous velocity.

Where is this extra information "stored"?

What is the fundamental difference between properties that can be measured in the snapshot (e.g. mass) and those that can't (velocity)?

Is the answer simply "spacetime"?
 
Physics news on Phys.org

Discussion

Hi,

Wrong starting point
Jehannum said:
From the above, we would be able to infer acceleration. But we couldn't observe present velocity and therefore kinetic energy or momentum.
The redsdhift would give us velocity. But taking a snapshot with time stopped is difficult 😉
 
By “snapshot” do you mean a specification of the position of every particle in the universe? So that we have that information as well as the properties (charge, mass, ...) of each particle? And other things that we can measure by “moving around”, such as the current value of time-varying electromagnetic fields?

Jehannum said:
From the above, we would be able to infer acceleration.
We don’t have enough to infer accelerations because magnetic forces depend on velocities.

More generally, in classical physics a particle has six degrees of freedom: position in each of three dimensions and velocity (or momentum) in each of three dimensions. A “snapshot” as I’m understanding what you mean only captures three of them so we don’t have a complete specification of the system state.

Thus the “extra information” we need is the velocities, which are the first derivative of position with respect to time, and the instantaneous rate of change of the field values , which are also first derivatives of measurable quantities. That’s what cannot be found from a “snapshot”.
 
Nugatory said:
By “snapshot” do you mean a specification of the position of every particle in the universe? So that we have that information as well as the properties (charge, mass, ...) of each particle? And other things that we can measure by “moving around”, such as the current value of time-varying electromagnetic fields?

Yes, this is essentially what I mean.

I'm trying to understand how/why it is that some things like position and mass are "encoded" in a static 3-D picture (which is in effect the universe itself) simply by dint of being there, but other properties (e.g. velocity) have to be specified externally.

I know this all seems a little vague - it's a half-formed idea that I need a bit of help crystallising: stepping back from assumptions and going to first principles. Thanks for the replies so far. They have been helpful.