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  • 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\'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.}

What happens if an immovable object meets an unstoppable force? Is a popular

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