# Q & A: Is the whole universe affected when we move even a finger of our hand?

Q:
Sir, i have heard that the whole universe is affected when we move even a finger of our hand. Is this actually true? If it is so then in day to day life, every person changes its position in every second then why the effect on the universe can't be seen. pls give complete explanation about this concept.
- geet (age 19)
haryana
A:

This is really hard to imagine, but it is somewhat true. The forces (or potentials) in general decay with distance but have different distance dependencies. Strong and weak forces decay so sharply that their effect is only important in nuclear contexts, whereas gravitation is a longer range force decaying by 1/r2. Let's focus on gravitation: the 1/r2 function only becomes 0 at r=infinity, i.e. there is no finite distance from your finger at which its force on an object is 0. That means changing your hand's position should effect everything else. Everything contributes to everything else's trajectory, which makes detailed prediction impossible. Worse, many parts are "chaotic", which means that if you can't predict the details you can't predict even many of the big events in the long run. To appreciate the (computational) hardness of such a problem, you do not need to focus on such a big system. Equipped with contemporary supercomputers, we cannot even predict the trajectories of a million atoms in a single protein longer than milliseconds.

But in practice this is of course not the case for most purposes, because its effect will be negligible at distant objects. The thermal jiggling (Brownian motion of particles) become important at small scales.  Whatever you do with your hand will have a small effect at a distance compared to that already random-looking jitter. We may attempt to estimate the gravitational force between a human body and the nearest celestial object-the Moon, which is about a million fold less than Earth's gravitational force on you. Now you can do this for your finger tip and the star Proxima Centauri. The universe is so big.

Tunc +(small mods mw)

(published on 04/21/2015)