Relativity III: The “Constant” Nature of Time

Hello, and welcome back to MPC! Last week, we spoke about the strange results of the Michelson-Morley experiment and began to interpret them. Today, we will try to analyze a more fundamental concept: the constant nature of time. Let’s get started!

What does time measure? That may seem like an odd question, but most physical quantities measure something. For example, distance is a measure of how far away something is. What “entity” does time measure?  This is rather difficult to answer, as time is a seemingly abstract concept. Let’s think about some of the common uses of time:

  • It took me 20 minutes to get from my house to the doctor’s office.
  • It took 10 minutes for the soup to cool off.
  • I spent 1 hour reading the first few chapters of a book.

Although the use of time in the aforementioned scenarios may seem very distinct and unrelated, there is one common thread tying them all together: time is measuring change. In the first scenario, the 20 minutes refers to how long it takes for me to move from one place to another, to change location. In the second scenario, the 10 minutes measures how long it takes for the soup to change from a hot state to a cool state. Lastly, in the third scenario, it takes 1 hour for the amount of the book I have read to change from nothing to the first few chapters. As a matter of fact, the second itself is defined below:

“The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom” (

What is important here is that whenever you use the word second, you are referring to a transition, or a change, in some system. Overall, time can be thought of as a measure of change.

So, what is the “constant nature of time” that I alluded to at the start of this post. Let’s just say that it is 6:00 and I tell you to meet me at a restaurant in 30 minutes. Now, nobody is perfect and chances are that one of us will be a little late. Nonetheless, let’s assume that we are “perfect humans” and both arrive at the restaurant in exactly 30 minutes. Why do we meet both arrive at 6:30? This occurs because me and you both have the same perception/definition of 30 minutes — 30 minutes refers to a certain amount of time that is the same, or constant, for both me and you. As another example, let’s say that me and you are measuring how long it takes for someone to run one lap around a track. Let’s make the assumption that we are “human clocks” with perfect precision. Will we measure the same time? Of course! 1 minute is 1 minutes, 30 minutes and 10 seconds is 30 minutes and 10 seconds, etc. There is no way that I will measure 2 minutes and you will measure 3 minutes (if this were to happen, then one or both of us would not have perfect precision). In other words, the constant nature of time refers to the fact that a unit of time to me is the same unit of time to you, which is the same unit of time to your friend, etc.

I apologize that this week’s blog post is on the short side, but we are almost ready to start special relativity. This blog post may seem completely unrelated to other topics that we have discussed in the past, but, as we will see next week, this seemingly constant nature of time and the Michelson-Morley experiment were exactly what Albert Einstein needed to make his major breakthrough. See you then!

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