100 years of Relativity!

Yesterday was Thanksgiving, and many of us were hanging out with our relatives, but that’s not the kind of relativity I’m talking about.  100 years ago, November 26, 1915, Albert Einstein published his General Theory of Relativity.  If you’re a physicist, you know all about this.  If you’re one of the rest of us, then let David Tennant explain it to you.  😉

If you still think the warping of spacetime is a nutty concept that can’t possibly be true, well, the Universe is probably having a bit of a chuckle about your ignorance:

This is a Chandra X-ray Observatory image of the Cheshire Cat Group of galaxies, named for its whimsical smile.  The group really only consists of the bright "eyes" of the cat; the arcs that form its face and smile are actually much more distant background galaxies whose light has been severely bent by the powerful gravity of the two massive elliptical galaxies.

This is a Chandra X-ray Observatory image of the Cheshire Cat Group of galaxies, named for its whimsical smile. The group really only consists of the bright “eyes” of the cat; the arcs that form its face and smile are actually much more distant background galaxies whose light has been severely bent by the powerful gravity of the two massive elliptical galaxies.

There are even wilder examples, like the supernova I mentioned a while back, and the Einstein Cross.  Einstein predicted that an image like this could happen, but believed it was so ridiculously unlikely (since it requires near perfect geometry to arise purely by chance) that no one would ever find one.  Probably he underestimated the increases in resolution that would become possible over the next century, opening up ever more distant reaches of the Universe to observation.  The Einstein Cross is a bright quasar that sits directly behind a very massive galaxy from our perspective, resulting in its image being split four times around the galaxy.

Einstein_cross

In fact, not only did he ultimately (and posthumously) lose the bet that no one would find an Einstein Cross, but we’ve actually found several now.  And also Einstein Rings (where the galaxy in back is smeared out into a complete ring by the lensing galaxy) and a host of other intriguing abnormalities.  What’s more, lensing via spacetime works the same way as lensing via carefully ground glass, in that it can magnify the distant object, to astronomers now use these as very powerful natural telescopes (which unfortunately cannot be aimed or refocused, and so have a very specific view).

My other favorite little factoid about relativity concerns the GPS satellites, and also the other navigation constellations we’ve got now: GLONASS, Beidou, Galileo, etc.  When the system first went up, the engineers who designed it dismissed the idea that they would need to account for relativity.  But the physicists maintained they did.  Broadly speaking, the satellites transmit the current time and their current position, and ground receivers use that to figure out how long the signal has been traveling and thereby the distance to the satellite.  The problem is, that depends on the time being accurate.  Every GPS satellite has a very accurate atomic clock on board, and ground receivers can pick up the US Naval Observatory atomic clock signal, and so then it should all work out fine, right?  They should be in synch, right?

Well, no.  As the video above pointed out, even going up a very tall building can produce a measurable drift in accuracy.  The clock that is higher in altitude will experience time more quickly. The engineers thought this effect would be too minor to matter, and that they could just adjust the clocks periodically, but that was not the case.   These satellites are about 12,600 miles in altitude; the difference is actually pretty significant at that point.  Within a few weeks, the positions the receivers were calculating were off by meters, and the drift was only going to get worse over time.  What’s more, general relativity was only half the problem; the satellites are orbiting the Earth at quite fast speeds, which is turning beneath them, so there is a significant relative acceleration to think about, which creates time dilation — they speed up because of their altitude, but to an observer on the ground they also slow down.  The physicists were right.  The GPS receivers had to be reprogrammed to account for both of these effects.  Since most of us now have a GPS receiver about our persons at all times (in the form of a smartphone), this may be where relativity impacts our day-to-day lives the most, and we don’t even think about it.

Now that’s cool.  😉

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