I was recently speaking with someone who had an argument about whether time exists or not. Time Dilation is often put forward as proof that time exists and that it is not merely a figment of imagination of mankind. But this person argued that by believing this, we are making a self-contained assumption about time. He argued that time is actually just the measurement of change that occurs in an object relative to constant natural phenomena. For instance, atomic clocks measure the microwave emissions of changing electrons, and older clocks measure the degree of the earth's rotation. He suggests that it is a huge and erroneous jump to say that these things measure anything other than what is stated i.e. emissions of changing electrons or the degree of the earth's rotation, and that to say that they measure a force or external entity that is time is simply illogical. And I would be inclined to agree. I watched a Stephen Hawking program where he discussed the possibility of time travel. He spoke about the effect of mass/matter on time, as if time itself is a force. And he spoke about how the earth's mass affects the atomic clocks in global-positioning satellites, and thus how G.P.S. systems have to be adjusted due to the loss of a third of a billionth of a second because of the effect of mass on "time". But really, it seems that mass has an effect on the emissions from the electrons, and not on time itself. (You can see the video, here: youtu.be/02tchltLm3c) He concluded in the program that a twin who travelled on a spaceship and circled around an extremely large mass for an extended period, e.g. a black hole, would come home having aged less than his twin who remained on earth. This is where I get confused. I really did not want this question to focus on time travel, but it looks like I have a few additional questions that pertain to the existence of time itself. If, say, this effect on the atomic clocks is because the matter and the emissions from the electrons have slowed, will the twin actually age slower because, oh I don't know, the changing of the particles in his body are going slower because of the effect of mass, or am I making an illogical jump relating to particle physics? Or, say it is not "time" itself that is being slowed, but merely the emissions from the electrons, is it illogical to suggest that the twin would age slower? From what I read on the internet, it is considered unscientific and almost crackpottery to suggest that time does not itself exist. So when time is given as a measurement, what is actually being measured is the change that occurs in an object relative to some natural phenomena. How is this not correct? The practical implication of this is that instead of viewing time as a force of nature, it is merely viewed as a measurement system, akin to the metric system, and is afforded no special properties with regard to the "space-time continuum". I don't understand why this is not what is accepted.