h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

It's an easy mistake to make, but the Earth is stationary. Far from it - it's rotating for a start. Also, it is continually moving around the Sun. The Sun is also moving at an incredible rate, out in one of the spiral arms of the Milky Way Galaxy. And lets not forget, the galaxy is also swirling its way through the universe.

So what's my point? Distance no longer matters, as you could get anywhere you wanted by yesterday if time travel were possible. My point is this: in one of te other threads, someone said that it would not matter if you arrived a day or two earlier (or later) than planned. I think this is completely wrong - in that day or two, the Earth, our solar system, our whole galaxy would have moved millions of billions of miles. Even an error of a few seconds would be fatal. Where does that leave our time traveller? Stranded in a cold nothingness at best. Possibly burned to a crisp in the heart of another star.

Of course, if you move too far forward/back in time, then you can't get to Earth anyway (at least not without loads of travelling). Isaac Asimov once explained it perfectly, but basically, the Earth is moving. There is therefore, as explained above, the requirement to move in space as well as in time.

The problem with this, though, is that if one travels too far in time, the Earth has moved on too far. To catch it up without having to travel once you reach the desired time would require faster-than-light travel. Which may not be possible.

I think that's how it goes, anyway. I'm no physicist.

One of the great things about being a biochemist is that we don't have to worry about this kind of thing.

One thing: Yes, the earth is moving around the sun, and the sun is moving around the galactic center, etc, but there is no defined thing which does not move. There is no reason to say that the earth isn't still, and that the sun doesn't revolve around us (though, one would still have to say that the other planets revolve around the sun). Therefore, you could just set your machine to use the earth as its reference point, and not worry about it.

Or, more likely, you would only be able to travel to an earlier point in the machine's existance, and therefore would be located wherever the machine was at that time.

PhysicsMan

No you couldn't!

Just saying that the Earth is still doesn't change the fact that *the Earth is moving incredibly fast*

As an analogy:
Q. Call an elephant's trunk a leg. How many legs does an elephant have?
A. Four. Arbitrarily deciding to call the 'trunk' a 'leg' doesn't suddenly make an elephant grow an extra leg...

The point is, if the earth isn't stationary, what is? According to Einstein (I think it was; maybe it was Newton), it is impossible to distinguish between an object at rest and an object in constant motion. The laws of physics would apply identically in either situation. So, for our intents and purposes, the earth is not moving. Therefore, the time machine should send us to the same location on earth.

However, I think that this arguement is irrelevent, because you could only travel into the time machine's own past (not before the machine was built), and when you arrived your location would be the same in relation to the time machine as it was when you left.

PhysicsMan

That's exactly the point! The Earth isn't stationary. It doesn't matter if there *is* some stationary point somewhere in the universe - we just know that the Earth is moving.

The laws of physics are *not* the same for a moving and a non-moving Earth - how would you explain things like cyclones if the Earth were not rotating? It is the very movement of the Earth that causes effects such as this.

The whole point of this argument was to show that time travel runs into not just the obvious difficulties of travelling through time, but that huge distances are involved as well - which imply faster-than-light travel, which is impossible (or so theory says). Sending someone to the same location on Earth, but thousands of years into the past or future, would involve transporting them millions, billions of miles through space.

I see your point, and would agree that using a car or phonebooth-like time machine would result in diffucult spacial problems. However, all of the reasonable time travel theories I've seen require that you travel into the time machine's own past. It's sorta like a doorway: you can travel to other times, but only when the doorway existed. Because of this, there would be no spacial problems; you would end up wherever the doorway happened to be at the time you arrived.

PhysicsMan