down on the site where this event was supposed to take place. It was really beautiful and we could have spent a couple of hours or maybe even days just enjoying the view. This is just one of the beautiful places planned for on this "Cities of Antiquity" cruise.
|Palmia Observatory Resident Astronomers are shown with the proposed birthplace of Aphrodite
on the island of Cyprus in the background
I also had a chance to read a little astrophysics on this cruise. Remember, I brought a half dozen books along to read if and when I found a bit of time. For you smart alecs who argued that the number of martinis ordered and consumed would easily exceed the number of pages read, I can truthfully say that you are completely wrong. I have only had two martinis on this cruise so far and have ready 20-30 pages out of the "Astrophysics for Physicists" textbook. What is going on with that? The answer seems to be that with us keeping so busy going to various tours and events and with wine service included with lunch and dinner, the opportunity for more martinis just did not arise.
Anyway, I wanted to share the insight I gained for understanding general relativity. The shipboard internet connection didn't really support my doing the first general relativity class lessons or assignment, so I am getting behind in that class. Maybe one of you can let me know how that is going? Yes, this means you Searching for Gravity Waves, Dr. Gary! Anyway, I did get an "ah-ha" moment from the text book reading.
The issue was this. Remember how most descriptions of the expansion of the universe rely on the evidence of doppler shift and how most galaxies are red shifted, indicating that they are receding from us at higher and higher velocities proportional to the distance. Also many introductory gravity and general relativity textbooks build on this concept and say that even though the galaxies are moving away from us they should also be experiencing a slowing down or decelerating effect because of the Newtonian gravitational attraction of the other galaxies. These textbooks are often based on these analogies and argue from this Newtonian dynamics approach that the galaxies are moving away at high velocity without any discussion about how or what force is causing this velocity. We know that when astronomers looked to see if the expansion rate was slowing down, it was actually found to be speeding up. All of this is known and expected when mysterious dark energy is included in the analysis. But this is not the issue I was troubled by.
The real issue was that we come to understand that most general relativity explanations describe the expansion of the universe as an expansion of the space between galaxies and not as the result of the galaxies actually receding at higher and higher velocities. That is the space between galaxies is expanding, not that the galaxies are moving faster and faster.
So, why is it that the red shifts and the same equations of Newtonian deceleration are valid descriptions of what is going on with galaxies? It just turns out, and this is the insight that I got by reading the textbook, that in the weak gravity case for the expanding universe, the solutions to Einstein's equations results in the same equations as could be calculated just assuming Newtonian expansion and dynamics and high velocities of galaxies moving away from any observer in the universe.
So, even though the measurement of light from receding galaxies is calculated as a doppler shift, the explanation by Einstein and Newton are completely different. Einstein says that the space between galaxies is expanding, Newtonian dynamics would say the galaxies are receding. The red shift measurements are the same, but the Einstein explanation is the more complete description, especially since it includes the effects of real and dark matter and radiation and strange, mysterious dark energy.
So, the mystery is solved. It becomes apparent once you start trying to work through the mathematics of Einstein's field equations and then recognizing that the solution calculated by Newtonian dynamics is very close to the correct solution, when we are operating in weak gravitational fields like that in the expanding universe. For higher gravitational fields, like near a black hole, for example, the Einstein equations are very non linear and a Newtonian dynamics solution would not be close to a good approximation at all.
Wow, ok, now I am worn out. Maybe now it is the time for a martini?
Until next time,
Resident Astronomer George