Why can’t you go back in time (well, at least turn back the calendar) by crossing through the International Date Line, west to east, several times in one day?
Or, conversely, go forward multiple days by going E->W?
(My apologies if this has been covered already…)
This is intuitively obvious, and it seems like it should have an easy explanation, but I can’t seem to figure it out. My best guess is that the IDL is just a convenience factor, and shouldn’t be treated as rigorously as time zones.
Imagine you could circumnavigate the globe in two minutes. You leave Japan on Friday afternoon, traveling east, and you arrive in California on Thursday afternoon. You keep flying, and since it takes but two minutes to circle the earth, you cross the IDL again, and it becomes Wednesday afternoon. And so on.
Maybe we should ask someone on the space shuttle what day it is…
If you went farther east, say New York, it would then be Thursday evening, right? And then in Europe, it’d be even later–Friday morning. And so on until you come back to Friday afternoon in Japan.
Then, you’d start over. That’s why the dateline is there–to make things simple.
Easier situation: Imagine you are at the South Pole or the North Pole. Run around the pole 365 times (366 if it’s a leap year). You have now gained a year, or lost it, depending on which way you ran. Great! You’ve got a time macine! Run around enough times and travel into the past, or the future. You’ll either see things from long ago (or far ahead), or else you’ll age so you can’t run any more, or disappear into an embryo.
Something is clearly wrong with this line of thought. You can gain insight into the problem by reading Jules Verne’s “Around the World in Eighty Days” (or Edgar Allen Poe’s “Three Sundays in a Week”, or the story by the guy Poe stole this idea from.). To a person doing the travelling you GAIN a small amount of time by travelling from West to East, and you LOSE a small amount going the other way. Usually these creep up on you so slowly you don’t notice it, Like Phileas Fogg, who ended up in London a day early, but didn’t realize it. The International Date Line “erases” these errors by subtracting/adding a day when you cross it. When you get really close to the poles, or you travel really fast, however, you don’t have time to notice the passage of a “day”, even though you really ought to be subtracting it or adding it. Thus, every time you circumnavigate that barber-pole stuck in the pole you should be recognizing that, in your little reference system, you should have added a day. Then, when you cross the IDL you subtract that day, and end up even. The flaw in the IDL Time Machine is that, especially when you don’t see the sun rise and fall, you miss the fact that every circumnavigation of the earth you make ought to be counted as a “day”, which crossing the IDL simply wipes out.
Welcome the the SDMB Ben! Well, the answer to your question is obvious, but it can be a little hard to conceptualize. Really, it goes into the very reason we have an International Date Line in the first place. You understand the concept of time zones I’m sure. As you cross one travelling east, suddenly it becomes an hour later. There is no special reason for this to happen, except that humans have decided that it makes more sense for noon to be in the middle of the daylight hours, even if this skews times around the globe. We could just have easily have decided that it made more sense for dawn that hits Greenwich at 7am to be at 11pm here in San Diego. But we didn’t. So we have these different time zone divisions around the globe. Now if it was one hour later every time you crossed one, then you would find it 24 hours later if you travelled around the whole earth and returned to your starting point, even if you took no time in doing so. Obviously that’s impossible. For the system to work therefore, there has to be some line that you cross going east where it becomes a day earlier. That line is the International Date Line. Tempting though it seems, you are not actually travelling through time. You are only gaining back the time you lost hour by hour going through the time zones.
On April 18th, 1857, Lewis Carroll (author of ‘Alice in Wonderland’) was moved to ask a very similar question. The SDMB didn’t exist then, so instead he posted it to The Illustrated London News. You can read his version of the question on this page.
If you visit this page, you will need to use your browser’s ‘Find on this page’ tool (or equivalent) to search for ‘Where does the day begin?’, which is how Carroll phrased his question.
His question got a lot of people talking, until in 1884 an International Conference deemed that there would be a single Universal Day and that this would begin at midnight at Greenwich.
For a nice diagram of the Internationasl Date Line, see this page.
Okay, I may be completely off base, but since the actual concept of time is man made wouldn’t the idea of time travel based on the International Date Line be impossible?
I mean, time zones where created, if I remember correctly, based on the idea of train travel.
Greenwich Mean Time (or whatever) could be placed anywhere in the world, therefore in my view the proposed idea in the OP could not be logical. In addition, we define years, 1900, 1950 etc, not based on logic right? There are other calendars which had a different concept of time.
I think the question has been pretty well answered, but I’ll chime in with my own version. Sometimes multiple, different explanations of something help me a lot…
Actually, it’s the International Date Line that prevents this from happening.
Well, sort of. You would not actually travel in time, but you’d be faced with a problem of having spots on the earth be at different days depending on how you travelled to reach them.
Say you’re standing just east of the IDL, and that the local time there is 3:30 p.m. on Monday. Suppose you’re near the North Pole, so that you can “walk around the world” in under a minute by travelling in a circle about the pole. Start walking east.
Your local time will start getting later: 4:30, 5:30, and so on. Soon you’ll be at 11:30 p.m. Monday, and then you’ll cross midnight somewhere and it’ll be 12:30 a.m. Tuesday. As you continue moving east, the time will continue to get later. When you come to a point just west of the IDL, the time will be 2:30 p.m. on Tuesday.
Now we see why the IDL is there: with it, if you were to continue all the way back to your starting point, the time would suddenly jump from 2:30 p.m. Tuesday to 3:30 p.m. Monday, which is good: you haven’t gained a day.
Suppose the IDL were not there. After walking completely around the circle, you’d be at your starting point at 3:30 p.m. on Tuesday. This is a problem - the very one that you mention in your OP. You would appear to “gain a day” with each lap around the earth, which would cause all sorts of problems.
Q: “Is it 12 hours earlier or 12 hours later on the other side of the globe?”
A: “It depends: travel west and it’s 12 hours earlier; travel east and it’s 12 hours later.”
The point, basically, is that this is was the IDL has been created to prevent.
To easily answer the OP: There are two date lines. The other date line moves and is wherever midnight is. If you travel around the world, you cross them both and you’re right back to the same day you started at.
The whole Earth is at one point in Time. It’s our calendars & watches that you need to keep resetting. You can reset your watch or flip through your calendar, but you aren’t travelling through time (except in the conventional sense of one second at a time). Similarly, you can bounce back and forth across the international date line (or any time zone) and Time is unaffected…you’ll just need to calibrate your watch/calendar to match the local folks’ measurements of time.
You’re right about this, of course. The problem that lack of an IDL causes is not actually time travel.
What we want to be able to do is look at any given spot on the earth and assign it a local time/day relative to Greenwich Mean Time (or whatever global standard you wish to use).
Are we going to declare the local time in Los Angeles to be 8 hours earlier, or 16 hours later, than GMT? We can’t do both - they’re the same time, but different days. Neither one is inherently wrong, but to avoid confusion we need to pick one and stick with it.
The IDL removes this ambiguity in assigning local times. The location of the IDL is arbitrary, but it would be a world of much confusion if we didn’t assign one at all. There’s nothing to stop us from moving the IDL to the boundary between US Central and US Mountain times, except for sanity reasons.
I have a sort-of related question that’s been bugging me at a low level for a while, but since this thread’s here, and there seems to be some folks with a much better grasp of this concept than I around, I’ll ask.
About a year ago I took a flight from Los Angeles to Taiwan. The flight left LAX at around 11 PM Monday night, and arrived in Taipei at about 6 AM…Wednesday.
So where in the hell did that Tuesday go in my life? Should I start celebrating my birthday one day later?
I’m in the eastern time zone, say 1:00pm. It takes me five minutes to get to the central time zone, and it is now 12:05pm. Am I 55 minutes younger? Nope. I’m 5 minutes older. You may call it “time travel”, I call it wasting 5 minutes. Then again, I don’t believe that time actually exists further than being a concept in our minds. The IDL is there so you don’t get confused. If you cross it, then you set the time 23 hours in the opposite direction you would normally do. (heading east, you set back 23 hours, west set forward 23 hours). Like someone else said, you’re just changing your watch. The earth will keep on spinning with or without you.
You should celebrate your birthday when it is your birthday in the place you were born. So yeah, if you’re still in Taipei, then your birthday is the next day.
They actually tried this once, but it was a miserable failure. They set up a sort of centrifuge right at the south pole. They put a couple guys in it and ran it clockwise for about 150 thousand revolutions. Unfortunately, when the guys came out, they were wearing doublets and speaking Middle English.
Seriously, there’s a misconception rampant in this thread. People are speaking of the IDL as if it were an actual entity, encoded in international law or something. This is wrong. The International Date Line does not exist!
The IDL you see on your globe or map of the world is just a cartographic convention. It’s drawn between those areas whose legal times are about 24 hours apart[sup]1[/sup]. Or, in the open ocean, along the 180[sup]o[/sup] degree of longitude. But it has no legal existence. It has even less existence than a line of latitude, if that’s possible.
[sup]1[/sup] Give or take a few hours, notably between Kiribati and some of its neighbors.
I think I agree with you, but just to clarify by trying to restate your point:
The IDL sort of materializes automatically between those regions that have decided to be ahead of GMT and those that are behind it, right? It wasn’t handed down from above by global edict, but simply comes about as an inevitable consequence of locals deciding their own relation to GMT. To the extent that it “exists” at all, it need not be a single line at all.
The western part of the US could, all by itself, create an International Date Closed Curve by declaring itself to be not GMT-8 but GMT+16, right? The boundaries of the Pacific Time Zone would then involve not just a 1-hour shift, but also a day shift, as well. For example: 12 noon Monday GMT; 5 a.m. Monday Central Time; 4 a.m. Tuesday Pacific Time (and, if western Canada didn’t go along, 4 a.m. Monday in Edmonton).
Stupid? Yes. But we could do it, just to get to the weekend a day earlier…
Having read all of your very interesting and knowledgable replies, I would like you to examine my “position” on this question: Taking the International Date Line out of the equation, would it be correct to say that “your” time travels with you, no matter what (or how many) time zones you cross? (For instance, I never reset my watch when I travel to Germany - a 6 hour time difference)
Using the original premise, I believe that trying to convince someone you are a visitor “from the future” would probably land you in the booby hatch. I may be taking the OP too literally, but I have always been interested in the concept of time travel and its paradoxes.
Thank you for your warm welcome, and thank you for graciously setting me straight. Looking back on it now, I was being a bonehead, and forgetting to advance an hour upon travelling through each succesive eastward timezone.
My bad.
-Ben
P.S.: Just for clarification, I didn’t really think I found a loophole in physics – I just thought I had uncovered a discrepancy in the time-zone conventions. Lesson learned: never try to think while at work.
Heck, why spend all that money in travelling and airfare and such, and all that strain on the body? Time travel is even easier than that – every autumn, you set your watch back an hour as you go off Daylight Savings Time. Well, why not just set your watch back an hour EVERY NIGHT before you go to bed? You would thus be time travelling, gaining an extra hour of lifetime each night.
Suppose you missed a TV show you really wanted to see. Why, just set your watch back a few hours and you could watch it!
