transcript: What happens if an immovable
object meets an unstoppable force? Is a popular 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 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 - 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. 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.
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