## Sunday, August 20, 2017

### But that is just a theory...

In the world of online discussion, concepts like evolution get dismissed as "but that is just a theory". Everything that science deals with is only a theory, the physical sciences have no axioms. Consider the theories of gravity:

#### Observation

• When I release an object, it falls to the ground
• When I release an object farther from the ground it hits harder
• The object that hit the ground was the same object I released

#### Accepted Knowledge

• An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
• Acceleration is produced when a force acts on a mass. (F = m*a)

#### Theory

• The object changed from rest to falling, so there must be a force applied.
• The object landed with a greater net force (more F) and so either the mass changed or there was acceleration. The mass was observed to have not changed.
• Conclusion:
• "gravity" - the force applied to my object which caused acceleration.
Newton explained how gravity worked in his theory. It was generally successful for modeling how the world worked and could be used to compute future behavior. The scientific value of Newton's theory of gravity was that physical interactions of visible objects and the planet earth could be predicted.

Newton's theory of gravity provided a explanation for how Kepler's descriptions of planetary motion occurred, and in doing so showed that the physical forces which cause the planets to move are the same forces which we see pull an apple off a tree. It had corner cases though where Newton's equations failed to describe observed events, such as the precession of the perihelion of Mercury.

This does not mean Newton was *wrong*, as his equations still successfully model and predict *a lot*. It means the theory was incomplete. Einstein refined the equations with his theory of gravity, the theory of general relativity. For the situations that Newton had observed, Newtonian gravity provides a close enough approximation to the computations of general relativity, but general relativity holds up in cases where the predictions of Newton failed.

We also already know that there are corner cases where general relativity fails to predict events, such as the field of quantum physics. In time there will be a unifying theory which explains both object level motion and particle level motion, but for today we have our current level of incomplete theory.

So yes, you can throw out the "Evolution is just a theory", but to do so ignores the actual weight behind those words. All scientific proposals are theories to be replaced when a more conclusive theory is proposed.