Would Alice have been aware of this slowing of time? No! In no way can Alice “feel” this slowing of time. For example, suppose both Alice and Bob have heart rates of exactly one beat per second when they are standing next to each other. Along her walk, if she bothered to measure, Alice would have found that her heart continued to beat exactly once per second (seconds according to her watch; also, we are assuming that the exertion of walking does not affect her heart rate!). In fact everything about her: her biological clock, her speed of thought, or any other conceivable measure of time, is exactly in synch with her watch. She doesn’t notice anything out of the ordinary until she arrives back at Bob’s side. It is only when they compare watches that they realize something strange must have been happening during her walk. On comparing watches they both agree that Alice has, for some reason, experienced less elapsed time than Bob; her heart has beat fewer times than Bob’s; she has actually aged less than Bob during her walk. Her walk has, quite literally, transported her into Bob’s future!

Of course the effect is quite small for a situation like this, but it can be dramatic if we imagine, instead of Alice going for a walk around the block, getting into a very fast spaceship and making a round trip to a distant star and back. Suppose the trip takes, say, twenty years according to Bob. It may take only a few days or less for Alice, depending on how close to the speed of light her spaceship was moving. This is the famous “twin paradox”, which is not a paradox at all—it is easily understood within the framework of the peculiar geometry of spacetime that Einstein discovered (actually, the mathematician Hermann Minkowski was responsible for pointing out the interesting type of spacetime geometry that lies behind this and other effects). This time dilation effect is easy to observe experimentally using accurate atomic clocks, and is universally accepted as a basic—albeit somewhat strange—property of our universe. The really interesting thing is to now follow Einstein’s train of thought and apply this basic fact in the context of our rotating space station to see what we can learn about the nature of gravity.

So let’s imagine that Alice and Bob have identical watches as before, with Bob floating freely “at rest” outside the space station (he is in a spacesuit doing a spacewalk), watching Alice revolving round and round inside the spinning space station. As Alice comes around, she puts her watch up against the window and at the instant she passes by Bob, he sets his watch to read the same time as hers. She then continues around once (just like walking around the block) and the next time they meet Bob looks at Alice’s watch through the window and notices that less time has elapsed for her than for him. Time is moving more slowly for Alice than it is for Bob, although, as emphasized earlier, she herself does not experience anything out of the ordinary. It’s only when they compare watches that they both notice something strange is happening.

The effect becomes increasingly dramatic the faster the space station spins. If the space station is spinning so fast that Alice is whizzing by Bob at nearly the speed of light, time is almost at a standstill for her relative to Bob’s experience of time. In fact, because of this, the rotating space station can be used as a time machine for traveling into the future. Suppose that Alice and Bob are both 20 years old, and one day she climbs into the space station and stays there for 30 years according to Bob. According to Alice, maybe only 30 days or 30 minutes has passed for her, depending on how fast the space station is rotating. She climbs out of the space station still 20 years old, but finds a 50 year old Bob, and indeed a whole universe that is 30 years into the future of the universe she knew. From Alice’s point of view, she simply climbed into a strange spinning contraption, sat there for 30 minutes, say (albeit in a very strong gravitational field), and climbed out to find that she has travelled 30 years into the future!

So what does this tell us about the nature of gravity? Einstein’s thinking went like this: Let’s forget everything we think we know about gravity and start from scratch. The slowing of time effect in this artificial gravity example is clearly a special relativity effect (time dilation) associated with Alice’s motion (rotating space station). But according to the equivalence principle, Alice is equally entitled to consider herself to be at rest (non-rotating space station) in a real gravitational field produced by some masses existing somewhere outside the space station. If there really is no way for Alice to tell the difference between these two situations, then whatever properties she notices about artificial gravity must also be true of real gravity. Thus Einstein was led to the idea that gravity, whatever it is, is likely to be intimately connected with a slowing of time.