On the Nimble Theory Blog I read that nothing can travel faster than the speed of light. I’m also reminded by the blogger of Brian Greene’s statement (from The Fabric of the Cosmos, p.49) that “the combined speed of any object’s motion through space and it’s motion though time is always precisely equal to the speed of light.”
I’m familiar with the fact that time slows down the closer you get to the speed of light. You all remember the story of the one twin going off to Alpha Centauri at something like light speed and coming back after 80,000 years having only aged 3 months since he left.
I read Greene’s words to mean that all of us, and you too, everything, your computer, the meal you’re about to eat, the books in your library, the tools in your shop, everything there is, everything all about you is traveling at the speed of light.
Why is this so? Because everything is traveling through something called, since Einstein, spacetime, consisting of (at least) three spatial dimensions plus time. It is the total movement through space and time that will always equal the speed of light —the result being that when you increase your speed through space you decrease your speed through time, and vice versa, not being able to ever reach a combined spacetime speed faster than light.
Light has a special place in all this. Light moves only through space and at light speed. There is no movement through time, with the result being that every photon that has ever “lived” is ageless. The universe ages, light does not.
Now if you’re still with me, let me share with you the following discussion, much of this I’ve taken from Andrew Fraknoi’s blog, The Universe in the Classroom. To begin Fraknoi asks how fast are you moving when you are sitting still?
For when we are sitting still, much of the time for most of us, as I am now, at my computer, and when I’m not on a fast train taking me to Aix en Provence, or on a cruise ship heading for Spain from Fort Lauderdale, or on a catamaran doing 9 knots between Les Marquesas and Hawaii, I am still moving, and moving much more and much faster than in any one of the situations I mention.
For while sitting, or while sailing or doing anything else, we are fully a part of the following motions:
1. With the earth that once a day turns on its axis, moving at close to 1000 miles per hour at a point near the equator, and slowing down somewhat as you move north.
2. With the earth again as it revolves around the Sun. We are approximately 93 million miles from the Sun, and at that distance it takes us one year to make the full circle, our path being close to 600 million miles, which means an orbital speed of 66,000 miles per hour, a speed that would take you from San Francisco to Washington DC in 3 minutes.
3. With the members of our local group including the Sun in the Milky Way Galaxy, a kind of “milling about” speed, but still moving at 43,000 mph, roughly in the direction of the bright star Vega in the constellation of Lyra.
4. And we move along with the entire Milky Way Galaxy itself as it spins, with us in it, like an enormous pinwheel. Now it takes our Sun, and therefore us along with it, approximately 225 million years to make the trip around our Galaxy. This is the “galactic year.” And since the Sun and the Earth first formed about 20 galactic years have passed, meaning that we have been around the Galaxy 20 times. On the other hand, throughout all of recorded human history, we have barely moved along this path.
How fast do we have to move to make it around the Milky Way in one galactic year? It’s a huge circle, and the speed with which the Sun has to move is an astounding 483,000 miles per hour and the earth, anchored to the Sun by gravity, follows along at that same fantastic speed.
5. Finally we are moving with the entire Galaxy through empty space. To understand this we will need to bring into our discussion the Big Bang, that enormous explosion that marked the beginning of space, time, and everything else, the entire universe.
As the early universe grew it expanded, that which was, and still is, a kind of stretching of space itself. And as space stretches, creating more distance between the galaxies, these are seen to be moving apart. How fast is our Milky Way Galaxy expanding? This speed turns out to be an astounding 1.3 million miles per hour!
Now all these motions, all motions that we are part of even while sitting in our favorite chair, — rotating on the earth’s axis (1000 mph), about the sun (66,000 mph), within our galaxy (43,000 mph), following the path of the galactic year (483,000 mph), and being a part of the expansion or stretching of the universe (1.3 million mph) — all these motions together do not come even close (much less than 2 million mph given the multiple directions of the motions) to the speed of light which is tops at 670 million miles per hour.
So, what can we now say about Greene’s statement, that the combined speed of any object’s motion through space and it’s motion though time is always precisely equal to the speed of light?
Well it’s clear that our motion, while alive on this earth, is nearly 100% (perhaps 99.9) motion through time. We are time creatures. And it’s true that time for us is much more important than space.
While as Green points out we may be traveling at light speed, nearly all of this movement consists in our growing old, and, if you will have it that way, at the speed of light.