What happens if an
immovable object meets an unstoppable force?
Of course, relativity
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 (principle of relativity: there is no absolute or “preffered”
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 object” 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 is, when you divided F by M you
always get zero. Of course, as I have said, not being able to accelerate an
object does not 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 will stay not
moving. If it’s moving at 100 miles per hour, then it will 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, bosons, gluons, or gravitons) that interact with an
object and changes its momentum—the only way to NOT be affected by a force is
not to 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 photons that hits your body changes you momentum a tiny
little bit, and there is nothing you can do about it other than avoid light
altogether or become transparent. So, all forces is already unstoppable.
But my impression is
that the phrase “unstoppable force” isn’t literally 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 be accelerated. 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 require infinite energy, I don’t know of anything in the universe that
behaves like this, not the least because it would be automatically be a black
hole so big that everything in the universe would already inside of it. But what
if we ignored gravity and imagine there WERE an un-acceleratable object? Well,
first it would be a source of infinite free power and would allow us to live in
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 this 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 possiblity is that they must pass
right through each other with no effect on each other at all.
