# Measurements from very long distance...

#### Externet

Joined Nov 29, 2005
1,960
Do you think it is possible to measure if an object is 1 milimetre closer than other; from 200+ Km away ?

Joined Jul 18, 2013
26,305
The curvature of the earth would have to be allowed for!.
Unless in space?
Max.

#### nsaspook

Joined Aug 27, 2009
10,713
Sure.
https://www.ligo.caltech.edu/page/facts
Most sensitive: At its most sensitive state, LIGO will be able to detect a change in distance between its mirrors 1/10,000th the width of a proton! This is equivalent to measuring the distance to the nearest star (some 4.2 light years) to an accuracy smaller than the width of a human hair!

#### WBahn

Joined Mar 31, 2012
27,925
Do you think it is possible to measure if an object is 1 milimetre closer than other; from 200+ Km away ?
You need to be a lot more specific. Are these two objects next to each other? How big are these object? What part of the object is the measurement to be taken at? Are they static? Are they on the ground? Is there clear line of sight to the objects? Are you willing to spend a few million dollars to make the measurement? Is this just a wild thought thrown out or do you have a specific application in mind?

#### atferrari

Joined Jan 6, 2004
4,672
Do you think it is possible to measure if an object is 1 milimetre closer than other; from 200+ Km away ?
From what I learnt from someone trained by Wild, measurements could be done in situ, so to speak, and then, the resulting values, sent to you. Othertwise, TOF should count.

#### MrChips

Joined Oct 2, 2009
27,702
Do you think it is possible to measure if an object is 1 milimetre closer than other; from 200+ Km away ?
That's about 1 part in 10^9.

LIGO can measure 1 part in 10^21.

#### BR-549

Joined Sep 22, 2013
4,928
A 1 millimeter change in distance will make about 3.3 or 3.4 picoseconds difference in EM travel time.

And times two for the bounce. A picosecond counter is required.

"1 picosecond – time taken by light in a vacuum to travel approximately 0.30 mm."