If the twin paradox is a real physical possibility then how does modern science explain the coupling between speed and the chemical reactions taking place in the human body that cause aging (that is, how does mere speed alone cause the body to slow down the aging process)?

eliasen

The answer is that *everything* slows down. The clock on a rapidly moving spaceship slows down only *as perceived by a stationary observer.* For the twin on the spaceship, everything seems normal. All reactions proceed as normal. It's just that when the twin on the spaceship gets back to earth, the journey took less time (as measured by the clock on the spaceship) than the time as measured on earth. Time is not absolute--it *did* take less time for the person on the spaceship. That's why he didn't age as much. May 18 02, 5:48 PM

Yea, but what is the current thinking that explains how speed effects the rate at which chemical reactions occur? Molecules react when their movement brings them in close enough proximity to one and other for electromagnetic forces to influence which atoms will hold onto which other atoms more strongly. That collission rate isn't going to change simply because the entire system is moving at the speed of light. So, how do scientists figure that the rate at which chemical reactions (and, ultimately, the aging process) takin place in the body are going to change simply because the body is traveling extremily fast? May 18 02, 6:28 PM

eliasen

Let me repeat. *Nothing* apparently changes for the person in the travelling spaceship. All reaction rates proceed exactly as they would otherwise. Any measurement of any physical process done by the person on the spaceship would work just exactly like it would for for someone on earth. But the time measured during the journey in the spaceship *would be less* than for the person on earth. The time taken simply *is* less. Aging isn't 'slowed,' that person actually lived through less time. May 18 02, 6:56 PM

Gnomon

As the spaceship approaches the speed of light, all the atoms in the human body increase in mass, so their reactions are all slowed. The individual particles in the atoms increase in mass as well. All other physical processes are slowed down by exactly the same amount so it is as if time itself slows down on board the spaceship. May 27 02, 12:14 PM

eliasen

Gnomon's analysis isn't right. From the point of view of someone on the spaceship, their atoms have exactly the same mass as always. Reactions are *not* slowed. The person on the spaceship just lives through less time than the person on earth. May 27 02, 4:51 PM