question on the internet.

Of course, relativity clearly tells us that there is no such thing as an immovable object

here's why: If you pick any supposedly immovable object, or just something like your house

or the earth, I can make it move. All I need to do is start to move relative to it, for

example I might ride a rocket, and suddenly from my perspective, I'm not moving and the

earth sails by outside. The laws of physics make no preference for inertial frame of reference,

so from my perspective here, I do not stir, and yet it's clear: the immovable object moves!

So because of relativity, "immovable objects" cannot be. But what I think people normally

MEAN by "immovable object" is something that if it's not moving, you can't make it start

moving by Pushing on it. So, not an immovable object, but an "un-acceleratable" one.\

\ Using Newton's second law, we know that an

object's acceleration is equal to the total force on it divided by its mass (though you've

probably seen this as F=ma). So an "un-acceleratable" object would be an object with infinite mass;

an object so massive that no matter how big the total force F is, when you divide F by

m you always get zero.

Of course, as we've said, not being able to accelerate an object doesn't necessarily mean

that the object isn't moving - it just means that you can't directly change its speed - if

it's not moving, then it'll stay not moving. And if it's moving at 100 miles an hour, then

it'll stay moving at 100 miles per hour.

So, what about an unstoppable force? Well, all the fundamental forces in nature are actually

caused by particles (like photons or gluons or gravitons) that interact with an object

and change its momentum - the only way to NOT be affected by a force is to not interact

with it at all (like how electrons don't interact with gluons so they aren't subject to the

strong nuclear force). Even light itself is an unstoppable force - every photon that hits

your body changes your momentum a tiny little bit, and there's nothing you can do about

it other than avoid light altogether or become transparent.

So all forces are already unstoppable\'85

but my impression is that the phrase "unstoppable force" isn't really meant to imply anything

about "forces" like electromagnetism or gravity, but rather, something that you cannot stop

from barreling down on you. That is, an object whose velocity cannot be changed by pushing

on it.

So, IF by an unstoppable force we mean an object moving at a speed that can never be

changed, then that means the object cannot accelerate\'85 but wait a second, this sounds

familiar! Recalling what we learned earlier, an unstoppable force must be an un-acceleratable

object! And that means that an "unstoppable force" and an "immovable object" are really

just the same, viewed from different reference frames!\

Now, since infinite mass requires infinite

energy, we don't know of anything in the universe that behaves like this, not the least because

it would automatically be a black hole so big that everything in the universe would

already be inside of it.\'85 but what if we ignored gravity and imagined there WERE an

un-acceleratable object? Well, first it would be a source of infinite free power and would

allow us to live in a 100% happy utopian society and break the second law of thermodynamics

and probably create portals and time travel, too (you can do a lot with infinite energy).

But more importantly, if two of these infinitely massive un-acceleratable objects were moving

towards each other and collided, then since by definition it's not possible for the velocity

of either of them to change, the only possibility is that they must pass right through each

other with no effect on each other at all.}