First of all, Einstein’s Theory of Relativity is not one simple equation, it’s a book. Secondly, although it poses a bunch of theories, it also leaves room for a lot of “what ifs”. Scientists have been exploring those “what ifs” since he wrote it. NASA is addressing one of those “what ifs” right now via the Gravity Probe B. The intent of the GP-B is to measure Earth’s impact on space-time. In other words, it will try to measure how space-time is warped by the presence of Earth ( a large body of mass ).
Always the type to ponder questions, I immediately had this question, if indeed Earth does warp space-time, then how do we know for certain by using the doppler shift how far away things are. I know they account for objects, but if they aren’t sure how large bodies of mass ( huge planets, large stars, etc.. ), then how do they know how much to account for the warped space-time?
Earth and Sky offered this great explanation: ” gravitational lens is a clump of matter that bends space — and thereby distorts light coming from a source behind it. Instead of seeing one star or galaxy through the lens, you might see two to five stars — or arcs of light. Gravitational lenses are hard to find because an enormous amount of matter is needed to create any noticeable effect on light. In 1979, the first gravitational lens — an entire galaxy — was discovered. Here’s Anthony Tyson … they found two quasars — radio quasars — right next to each other and when they looked at the spectrum, they discovered it was the same identical quasar seen two times, separated by six arc seconds on the sky. And that was the beginning of a hugely productive mining of the sky for gravitational lenses. It started in the late 1970s. There are many, many gravitational lenses now.”
That did well to explain how they know that what we see has been warped, but it really didn’t answer my question of how much space-time has been warped.
A friend continued to appease my ignorance, and further added this:
“the amount of space-time bending is inversely proportional to the distance from the massive object. Since the massive object is very far away, the hidden object must appear to be directly behind the massive object in order to get the light-bending effect.
And to answer another question. The time it takes the photon to reach the earth increases due to the bending of space-time.”
I thought that was very insightful, but still didn’t answer the basic question. We went round and round and round and finally another poster, 0x6a74, summarized with an even lengthier explanation of how space is not measured “in” time, but “by” time. And once it was all said and done, finished with this:
by the time one’s looking at these guys, billions of ly away, that something massive in between may have warped space enough so that something that looks to be several thousand ly more than it is …. not much percentage error against all the other possible errors.
In other words, we’re just making best guesses based on what we do know. GP-B could re-establish the age of the universe, again.