# Monthly Archives: March 2015

## SR #11: Simultaneous, Not!

Last time our friend Al used lasers and timers to create a regular grid-like map of the space and time near him. The map allowed him to assign space-time coordinates to events in his frame of reference (even if it takes time for him to see light from those events).

An important concept is the idea of simultaneity — of events in different locations happening at the same moment according to some observer (who has to wait for the event’s light to reach their eye).

So far the events weren’t moving relative to us. What if we — or the events, same thing — are moving (and moving fast)? It turns out, this changes the picture!

## SR #10: Simultaneous Events

In the last two weeks I’ve covered relative motion as the ancients understood it (Galilean Relativity), touched on how light doesn’t follow those rules, and introduced time-space diagrams that we can use to visualize motion. I also introduced the topic of space-time events, which are simply locations in space at a given time.

In particular, I showed how our friend Al can use a laser to determine both the location and the time (relative to himself) of an event. This allows him to map his nearby space and time using a system of regular (that is, grid-like) space-time coordinates.

Today we continue with that idea.

## SR #9a: Extra Diagrams

A couple of readers have asked about the diagrams in this series of Special Relativity posts. I created them with the freeware 3D ray tracing application, POV-Ray. The diagrams are actually three-dimensional “scenes” designed to be viewed as flat pieces. If some of the “dots” look more like little spheres, that’s because they are!

I wrote some introductory posts a while ago (here, here, and here). You can read those if you want more details about the application.

For a little (optional!) Friday fun, I thought I’d share some POV-Ray images that have a bit more “dimension” to them.

## SR #9: Light Diagrams

My Special Relativity “icon”!

This week I’ve introduced you to time-space diagrams. They’re the foundation of everything that follows in this series, so I hope you’re feeling very comfortable with them.

I also introduced you to space-time events, and I apologize for any confusion in calling the diagrams “time-space” and the events “space-time.” Six of one, half-dozen of the other. I wanted to stress the time component of the diagrams, whereas space-time is the more usual general term.

Today we wrap up the week with some important diagram details.

## SR #8: Space-Time Events

Last time I introduced you to the idea of a space-time event. In physics, an “event” has the same meaning as when Hollywood blares out about a “major motion picture event” — that is to say, nothing at all special — just something that happens at a specified location and time.

If you attend a social event, it has a location and a time. When we talk about space-time events, all we mean is a specific location and a specific time (hence the name, space-time event).

Today we’ll explore some interesting aspects of such events.

## SR #7: Dueling Diagrams

The last two posts introduced and explored the concept of time-space diagrams. This time I’ll complete that exploration by using them to consider motion from two points of view. This will be an exercise in application of our diagrams.

I’m going to connect that application with something I stressed last week: that motion has a symmetrical component. It’s perfectly valid to think of the world moving past the train as it is to think of the train moving through the world.

It happens that here our dueling points of view are resolved by something else I discussed last week. See if you spot it before I mention it.

## SR #6: More Diagrams

3D holograms! Me want!!

Last time I introduced you to the idea of a time-space diagram, which is a kind of map used to describe motion. As with many maps and diagrams, we choose to use a flat, two-dimensional representation. Someday hologram technology may advance to casual use of three-dimensional images, but so long as we use paper and display screens, we’re stuck with two.

Motion is movement in both space and time, so we want to use one of our two dimensions to represent time. That leaves us with only one remaining dimension for space, so our diagrams exist in a reduced one-dimensional world.

Today I’ll explore that world in more detail.

## SR #5: Diagrams!

Last week I introduced you to the idea of relative motion between frames of reference. We’ve explored this form of relativity scientifically since Galileo, and it bears his name: Galilean Relativity (or Invariance). Moving objects within a (relatively) moving frame move differently according to those outside that frame.

I also introduced you to the idea that light doesn’t follow that rule; that light moves the same way to all observers. This is what makes Special Relativity different. It turns out that, if a frame is (relatively) moving fast enough, some bizarre things happen.

Time-space diagrams will help us explore that.

## How Big is God?

“Space is big. Really big.”

When I started blogging here, one of the first bloggers I followed was Robin, of Witless Dating After Fifty. Over the years, she’s several times mentioned a great question her dad often posed when discussing religion with someone: “How big is your god?”

Last week my buddy and I were having our weekly beer- and gab-fest and our (typically very meandering) conversation came to touch on the problems with young Earth creationism — the Christian fundamentalist idea that the universe is only thousands of years old.

In fact, there’s a pair of real whopper problems involved!

## Niven & Pournelle

Science fiction authors Larry Niven and Jerry Pournelle collaborated on about a dozen SF novels, at least one of which is highly regarded as a classic in the genre (and an oft-named favorite). Ironically, that one — The Mote in God’s Eye — was the very first book the two of them wrote together.

Rereading it is a task I have queued for this summer (along with the sequel they wrote almost 20 years later: The Gripping Hand). But this past week or so my relaxation reading took me back to their second and third collaborations, the latter of which I just now finished.

Being that it’s Sci-Fi Saturday I thought I’d share those two with you (along with an entirely different series by an entirely different author).