Astronomy Buff

Don’t Make Me Come Over There: Relativity of Simultaneity

by Tony on January 11th, 2007

Did you know that you can experience events in a different order than someone else, just by virtue of where you are compared to that person? If you are far enough away, you can experience the SAME sequence of events, but in a different order than a friend, say, on the Moon.

This is a relativistic effect that is easily seen when the distances between two people are very large when compared with the speed of light, we don’t notice it at all on Earth because we are so close to each other (compared with how fast light travels).

Differences in the order things happen can be observed because of the fact that light can only travel so fast, it takes time for light to get to you from a far off place. For example, it takes light about 1.2 seconds to get to the Moon from Earth.

Here’s a simple example. I am in the United States and over my head I observe two very bright flashes of lights in the sky: blue over my left shoulder and yellow over my right (very high in the sky). To me, those flashes occur at the same time because they are relatively close to me. Now imagine that you are on the Moon, looking towards Earth and the Moon is low in my horizon, towards my right shoulder. You, looking towards my location, would see the yellow light flash first, followed by the blue. You experienced a different order of events than I did.

A very distant third observer, one off my left shoulder would see the blue light first, followed by the yellow.

The implications of this are really weird if you think about it. Since the speed if light is finite, so is the transport of information about an event, so you might actually see me eat the pie before I cut it, for example. Weird, huh?

Technorati Tags: astronomy, relativity

The only thing that is different about us is our locations and that we are very, very far away from each other. The limiting factor, and what makes this observable, is that light can only get to us so fast. The order of events experienced by each observer changes depending on the geometry of the situation.

OK, my example is very simplistic, and I hope I got the point across, but the reason for this post is that John Walker has just posted a project, taken from a real life example, the Apollo 11 moon landing, which beautifully illustrates this effect. I wish I had thought to do this, it is simply brilliant.

I’ve been reading John Walker’s Blog for over two years now and of all the people in my newsreader, John’s blog and website is the most intelligent, hands down. He is the founder of Autodesk and lives in Switzerland, and is among my favorite critical thinkers on the planet. Run, don’t walk to his blog and site, they are required reading for anyone who wants to know stuff about the world we live in. I love the way he thinks.

I also lost 30 pounds following his Hacker’s Diet, the only approach to losing weight that’s ever worked for me.

Anyway what John did was very clever. He analyzed recordings from the Apollo 11 landing, and time shifted them to account for the fact that it takes light 1.2 seconds to get to the Moon from Earth. All of the recordings we’ve ever heard were from the perspective of Mission Control in Houston. What the astronauts heard in the lunar lander was something slightly different.

I won’t go over the details here, because John has done an amazing job on his own site, click here to see the entire project. Simply inspired, I love this!

The punchline is the pause of Neil Armstrong after saying “Houston, uhhhh…” long pause, “tranquility base here…”. The pause was due to Armstrong being ’stepped on’ by mission control, which interrupted his train of thought, making him pause (like what happens when you’re trying to tell someone something and they keep interrupting you). There was a lag on Earth that didn’t exist on the Moon so Houston experienced differences in the sequence of transmissions than Armstrong did in the lander. Listening to the recordings will make this clearer. Check it out, it’s very, very cool.

Finally, as if that wasn’t enough, he did one more amazing thing. He released a recording of what the transmissions sounded like from inside the lunar lander. As he so aptly put it:

Consider, as you listen to this audio, that apart from other folks who clicked on this link before you, only Neil Armstrong, Buzz Aldrin, and this humble scrivener have ever listened to this event from the lunar surface’s unique perspective in spacetime. The transcript for this sequence occurs between vehicle elapsed time 102:42:25 and 102:47:15 in the Apollo Lunar Surface Journal.

He very kindly gave me permission to serve that file on my blog. I hope you enjoy and thanks John, for the great demonstration of simultaneity.

From John Walker:

In this stereo presentation, Aldrin’s transmissions appear in the centre, with Armstrong’s remarks captured only on the onboard recorder in the left channel and CAPCOM Charles Duke’s transmissions in the right channel.

Here is the descent and landing of Apollo 11 as heard from the lunar lander, oddly enough, my favorite thing about this recording is that you can hear the attitude control jets in the background. I’m getting a little teary, ’scuze me a minute…

Please feel free to email me with any questions.

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