Space, Time and Motion: Course Preview

Understanding Time

This discussion was selected from Week 2 of the AMNH online course Space, Time and Motion, part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators. This is an excerpt from an actual course discussion, but learner names have been changed. Explore more sample resources...

A second is a second is a second, for you or me or anyone else. But in fact, time doesn't behave that way at all. Based on this week's readings, what are some of the challenges of understanding the concept of relative time?

Adrian 8 Oct 5:36 PM

Its perception. While I can understand the concept that time inextricably linked to motion - I can not 'feel that' concept. My senses tell me that time is absolute.

The main challenge in understanding the concept of relative time is that we are all basically 'in the same time' relative to each other. If you are traveling on a plane once around the globe, and I am standing still - I can perceive that you are moving faster than I am, but I can not perceive that time has slowed for you relative to me - it has only done so by a few nanoseconds.

A second is a second based on our agreement of time. We base time the earth's speed around the sun (one year), the earth's daily rotation around the sun (sun up to sun up) - one day 24 hours. A unit of time.

We all agree what a second is because we are all in the same time. But if one of us accelerates to the speed of light time their time slows relative to the one who is not moving. Only if one of us could move at 70% -100% of the speed of light - we would start to notice time slowing down.

In Wired magazine June 2005, there is an article about this student Peter Lynds who proposes that time is an illusion. Time doesn't flow in any direction. Time is merely a sequence of events that happen relative to each other. According to Lynds', our perception of time as a sequence of moments is just a neurological artifact, an outgrowth of the chunk-by-chunk way our brains perceive reality. Again, we are prisoners of our perception.

David 9 Oct 4:58 AM

Time ... Absolute, Relative, or Perception?

Hi Adrian,
Your post stated many of the ideas I wanted to express, so I will just add to yours. Time is "absolute" for those of us living on the surface of the Earth, which makes it very difficult if not impossible to convey the idea of "relative" time to most people.

On Earth we measure "time" by events that repeat themselves at regular intervals ... the Earth moving around the Sun, the Earth rotating on its axis, the vibrations of a cesium atom. These time measurements are absolute and repeatable.

But how do we measure "relative" time. Is it by how far light travels between events? This seems to be the how time is measured in the "train" diagrams in the text book on the "The Relativity of Time, Mass, and Length". The faster an object is moving compared to you, the longer light has to travel between events. But I can see where this could be explained using Galilean relativity ... because if the object is moving away from you, it takes longer for the light to reach you. I'm not exactly clear on these concepts, but I think that in trying to explain relative time, they use simplified explanations and analogies that are not 100% accurate descriptions of the realities involved.

As far as perceptions go ... I think we need to be very careful of our perceptions. I like to show my students optical illusions at times ... just to show them that we can't always believe our perceptions. In science we need to be able to measure things. As stated above, we can measure time absolutely and with precision as long as we are on the surface of the earth. In Essay 1, Dr. Liu mentions "phenomenological time" ... what we "feel" as to the passing of time. I don't think you can accurately measure phenomenological time. But I also think that "time" in general is a human construct. We have simply chosen ways to mark the interval between passing events. This doesn't mean that "time" is an illusion, it simply means "time" is how ever we want to define it and measure it.

Susan 9 Oct 11:56 AM

Well, if we measure time by the events such as revolution around the sun and sun up to sun up, we then have events which change those intervals. For instance, the earthquake and tsunami on December 26 (probably just the earthquake actually) slowed the Earth's orbit just a little bit. Fractions of a second. Now, of course, we don't sense the orbit or infinitesimal changes in its speed, but these things are always happening.

Instructor: Helen 9 Oct 3:25 PM

If a tree falls in a forest ... Susan. You make an excellent point! Just because we cannot always directly sense phenomena does not mean that nothing has happened. ...and does not make these events any less spectacular! Best, Helen

David 9 Oct 3:29 PM

I've also heard that the official timekeepers at US Naval Observatory have to add a "leap second" now and then. I can't remember exactly why. I find it interesting that each year is 365 and a 1/4 days long. And somewhere in the readings it was mentioned that relativistic time effects must be taken into consideration when using the GPS satellites orbiting the Earth at great speeds. So our "absolute time" isn't quite absolute, but it does serve our daily needs.

Scientist: Sean 9 Oct 11:48 PM

This is a good point. You have to be specific about how you measure the passage of time. You're right that because the is moving away from you, the pulses appear to have a larger period for you than they would appear to someone on the ship where they are emitted. This is a separate phenomenon from relativity. This is the doppler effect, which is indeed Galilean. In reality, the pulses received on Earth would appear to have larger periods for 2 reasons: relativity and the doppler effect.

When we talk about relativity alone, we're assuming that the doppler effect has been correctly taken into account.

Instructor: Helen 9 Oct 3:27 PM

Great Article!
Adrian, would you happen to know if this article is available online? If so it would be an excellent source to add to the webliography (accessible by clicking on the webliography tab at the top of the screen).
Best, Helen

Adrian 9 Oct 7:18 PM

The article can be found at
The papers by Peter Lynds can be found on his web site

Scientist: Sean 9 Oct 11:43 PM

Experiencing relativity

Adrian is right that we haven't experience relativity directly. Try the following: Suppose that the speed of light is 20 miles per hour (a fast run). Now try to imagine what the world would look like if relativity were in effect. See if you can describe it in terms of everyday occurrences that are warped because of relativity. Try to make the description as "experiential" as possible.

Gabriel 10 Oct 10:17 AM

Hi Sean
This is a great idea. If I apply this to a lesson plan with students, have the speed of light be some low number that students could grasp. I think high school students would be able to apply this train of thought. I jotted a note for me to try this with 8th graders for latter in the year.

Gordon 10 Oct 12:40 PM

If you paint scenes on a sheet of paper (6'x18') then put that scene roll, with the 18' fused into a loop. (Duct tape is good for making a tight seam.) Now set that loop next to a treadmill, have the students run at various speeds, with the scene traveling at various speeds, while blocking out all other views (turn off the lights in the room with lights only on the scene loop).

The 'cartoon' effect that it will have on perceptions are meant to express relative effects, on stationary perceptions. The treadmill, and movable scene are the relative effects and our matured and established ideas are the stationary perceptions.

Susan 10 Oct 1:58 PM

That's a pretty cool idea. I don't need to spend more time on frame of reference this year, at least I don't think so. If I do, that might be a really cool thing to demonstrate, although the difficulty might be to take your idea and translate it into materials available in the classroom.

Gordon 14 Oct 3:26 PM

With the blink of an eye a world in a shoebox can show giants how to think. One wall can frame the moving scene roll, another can be used for a pinhole camera obscura.