One of the many places I’ve been traveling to recently is a bit unusual: the Linda Hall Library in Kansas City, Missouri. For one thing, it’s a private library; like the Huntington Library in Pasadena, it’s supported almost entirely by private funds. For another, Linda Hall is completely dedicated to science, technology, and engineering. While visiting, I asked what they considered their peer institutions to be — the other science libraries they might be compared to. Nobody could think of any. It seems to be a completely unique place.
I got to tour deep into the bowels of the building, where stacks of journals and scientific reports seem to stretch for ages. The library does a brisk job lending books and articles to other institutions; when you need a technical note from 1923 that tells you how a certain bridge was put together, this is the place to go. There is also an amazing rare-book collection, some of which was being put on display as part of an exhibition entitled “Thinking Outside the Sphere: Views of the Stars from Aristotle to Herschel.” I got to leaf through a first edition of Newton’s Principia, which I have to say was pretty awesome. I didn’t find any mistakes, but my Latin is a bit rusty. Here are the three Laws of Motion, right near the beginning of the text.
The library also adds to the intellectual life of Kansas City by sponsoring public lectures. I followed Sara Seager and Seth Shostak in a series about extraterrestrial life. Not my area of expertise by any means, but they asked me to talk about time travel, which I do know something about. (At least by the standards of other human beings, for which neither “time travel” nor “extraterrestrial life” are subjects of true expertise anywhere.)
Dr. Sean Carroll – The Paradoxes of Time Travel from Linda Hall Library on Vimeo.
Of course I also had some BBQ while in KC. One does not live by the life of the mind alone.
Would love to check that out next time I’m in KC. Got a chance to see Newton’s personal copy of the Principia on display at the Huntington a couple years ago. Very cool!
Now if they just wouldn’t smoke around you while you were eating that BBQ, life would be pretty grand.
Well, I’m envious as all hell!
Yes, that’s quite a place. I was lucky enough to attend several small astrophysics conferences there (sponsored by UMKC) as a grad student and got a tour of the book collection a couple years ago. If anyone else needs something to do there, I was blown away by the astonishing Toy and Miniature museum that I visited on a lark not far away and the WWI museum downtown. *Really* impressive.
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Thanks Sean,
Great talk. Though there a question asked at the end that I also have had. I think I know what the guy was asking and I don’t think you answered it (or maybe you did and I’m just not understanding). If a space ship is moving at 99% of c and I am stationary, you say that a clock on the space ship with be moving slower, relative to my clock. But a person on the spaceship would observe themselves being stationary and me moving at 99% of c. Right? It seems like this argument privileges the space ship’s clock over my clock, and I thought that was wrong, based on special relativity. Of course I know I haven’t found a flaw with Einstein’s theory(!) so I wonder if you could explain where my thinking has gone haywire.
BTW I am really enjoying the book, but I’m not finished yet, so maybe you get to this point.
Thanks.
Hi Brendan,
I’m completely unqualified to answer your question (and in fact I don’t even have a sastifying answer), but I’ve spent some thinking about the exact same problem and I thought I would offer you the explanation I eventually came up with.
I think it comes down to the relativity of simultaneity. Let’s say a space ship is moving at 99% of c and you’re stationnary. When the ship passes you by (i.e. when you’re at the same position), you both set your clocks to 0. Now, say you wait a second in you reference frame. Then you will observe that time moves more slowly on the ship; its clock only indicates 0.14 s (if I’m not mistaken) instead of 1 s :
Your reference frame:
Your clock: 1 s
Ship’s clock: 0.14 s
But, see, the problem is that you’re implicitely assuming that you’re mesuring the time on both clocks at the same time. However, simultaneity is relative; there is not way to synchronize the two events — you looking at your clock and the ship looking at its clock — for both reference frame. Therefore, the people on the ship will NOT agree that when their clocks indicate 0.14 s, yours is at 1 s. Instead, they will find that:
Ship’s reference frame:
Ship’s clock: 0,14 s
Your clock: 0.0199 s
I came this far on my own, but then found out that Wikipedia already had a (much better) explanation:
http://en.wikipedia.org/wiki/Time_dilatation#Time_dilation_due_to_relative_velocity_symmetric_between_observers
Anyway, beyond this point, my thinking is still fuzzy. I’m currently of the opinion that I should try to understand the Lorentz transformation first and foremost, and see all the rest (time dilation, length contraction, relativity of simultaneity, etc.) as special cases or consequences. Every time I’ve tried to understand special relativity, I always end up hitting a wall because the concept seem to make no sense on their own… they always lead to the kind of problem you’ve brought up.
But I still hope I’ll understand it one day.
Just got done reading your book and found it very good, I sincerely believe this science will eventually move the world and the human race forward in a very dramatic way.
Question:
Much of what I here spoken about time travel seems to relate more to historical time, in other words moving from the past to the present and then on to the future. The arrow of time as you said.
Where I’m having difficulty is trying to connect this to the time fluctuations experienced on board a moving space ship when it passes through gravitational waves.
Great talk, thanks.
Brendan– there is no privileging. When two clocks are moving past each other, a person moving along with one of the clocks sees the other one moving more slowly. The symmetry is only broken when one clock turns around to catch up with the other one — in that case, the one that turns around (experiences acceleration) will feel less total time passing than the one that simply continued on.
I sure am glad we got rid of that Latin requirement for publishing.
Thx Sean, your talks are quite remarkably clear, it’s always a pleasure.