/ http://www.slideshare.net/images/logo.gif / 際際滷Share feed for 際際滷shows by User: jaimetulip https://cdn.slidesharecdn.com/profile-photo-jaimetulip-48x48.jpg?cb=1523510374 An object is not free to move around spacetime at will; it must always move forward in time and cannot change its position in space faster than the speed of light.