&quot;The theory holds that gravity is geometry: particles are deflected ... not because they feel a force ... but because space and time around the object are curved.&quot;<p>Can anyone explain to me why it <i>can&#x27;t</i> simply be a force? If you accept that gravity is caused by energy and not mass (which is obviously the case), then I see no reason a photon can not experience a force.<p>So what is the reason to insist it&#x27;s geometry?<p>And related to that, I still have never seen an explanation of how geometry is supposed to cause something to start moving without a force. Explaining how it <i>deflects</i> something, I get. How does it start moving in the first place?<p>The only explanation I&#x27;ve gotten is that things are always moving - through time, and the geometry just transfers some of that motion into physical motion. But that explanation is very very very lacking since all things do not move through time at the same rate (because of the various types of time dilation that are possible), yet the gravitational force is identical.