# Universal Reference Point question

1. Nov 21, 2008

### Chaos' lil bro Order

If in the future humans master time travel, might it be valuable to know a Universal Reference point that is common to both the present and past coordinates in space and time. For example, say we designated a single point on Earth from which all other relativistic effects are to be measured against. We could set this point as a microdot in a NIST building, sitting on a pedestal and when we created it we'd stamp the exact time, position to the sun and moon and all sorts of other data beside it. Everyone in the world who thought time travel may be possible in their life times invests in an atomic watch with GPS and the watch always calculates your relative position in time and space versus the NIST dot and it takes into the relativistic effects of you moving about in your life 1000s of miles away from the dot. The watch also downloads and tracks the movements of the Earth in the solar system, the solar system in the galaxy, and the galaxy in the cluster? with the goal of making the dot an inertial reference point in time. Who knows what form of time travel if any will be available in the future, this universal reference point may be useful or never, I am not sure.

2. Nov 22, 2008

### Al68

I just got one of these watches yesterday. Best thing is, it also has a built in mp3 player.

3. Nov 24, 2008

### Chaos' lil bro Order

Why should I care, read this again.

4. Nov 24, 2008

### Jonathan Scott

You can't use a point (and set of axes) on the Earth to define an inertial reference frame, because it's not inertial. (Earth rotates on its axis once a day, and moves around its center of mass with the moon, and orbits the sun, and the sun moves around the galaxy, and the galaxies move relative to each other, and so on). You can't use a non-inertial reference point because it doesn't uniquely define times and locations elsewhere, because changing velocity leads to changing simultaneity.

If you ask where you are "now" relative to a reference point, you need to define the frame in which "now" is calculated. If it is the subjective frame, that means the same event would have different coordinates depending on the velocity of the observer. If it is the frame of the "reference point", that has to be inertial, otherwise it cannot uniquely label distant events.

In general, you can't define an inertial frame which is useful both locally and at the galactic scale, because of gravity. A frame which "feels" inertial locally is one which is actually in free fall, so is being affected by gravity, which varies with location. You can at least detect and eliminate rotation locally relative to the fixed stars to a high accuracy, but it is not practical to calculate the linear gravitational acceleration due to all stars, galaxies and so on and define an "inertial" frame in that way, especially as on a larger scale you would find that the expansion of the universe would also affect it.

5. Nov 25, 2008