# Can one ever be in the same place more than once?

Given that I am planted on a rotating Earth, which is orbiting around the Sun, which occupies the outer reaches of a spinning galaxy, which is itself moving within an expanding universe, is it fair to say that – though I’m sitting still as I type this – I will never again occupy the exact point in space that I am right now, or that I was one second ago?

It depends on your point of reference. From the one you gave, no.

The OP didn’t actually give a point of reference, and I’m not sure you can even infer one from the post.

To elaborate, there is no such thing as an absolute fixed point in space, like there is in a mathematical set of axes where the point (12, -13, 52) never changes. You can talk about position only with respect to some other object. In daily ho-hum life we think of our position with respect to some point on the surface of Earth. You could also use the Sun, center of the Milky Way, etc., depending on your purpose. But once you leave our galaxy things get a little dicey–there is no center of the universe to use as a reference point.

However, I agree that based on the scope of what’s described in the OP, you will never occupy the same point in space relative to pretty much any reference point you choose except a point on Earth.

He said “outer reaches of a spinning galaxy”, so I took that as a point of reference.

You might relative to the sun.

Is this one of those “You can’t step into the same river twice” sorts of things?

If you wanna be a killjoy bring up this point while watching a time-travel movie like The Time Machine, or better yet any of the Back To the Future films. Barring any extra phoney-baloney pseudo-science they’d all require an Aristotelian universe…

This actually came up in an old SDMB question, someone wanted to know what if a time machine had no positional movement capability would work. And the conclusion is no. If you building a time machine, it must be able to travel in space somehow. Either Doc’s DeLorean is in someway “synched” to the Earth as it’s position changes in time, or the only traveling you’re going to do is what the Tardis occasionally does – leave the time-stream and end up a blue box rocketing through the atmosphere to a rough stop.

The tv series Seven Days DID take this into account. The time machine of the show did not stay motionless relative to the earth. Instead, in the process of moving back in time, it also left the earth’s surface and came back down on the earth of a week ago. In fact, the chrononaut was not merely a passive passenger, but had to steer the craft to a reasonable landing spot on the earth of the past.

…but you’re older.

In a legal context, it might be possible to be in two jurisdictions at once if you are, say, straddling a city limit line or a state line, with one leg on one side of the line and one leg on the other. E.g., one might go up into Appalachia and straddle the Virginia/West Virginia line and say that they are legally in Virginia (e.g. to the point where any incidents would be under the jurisdiction of a Virginia court) as well as legally in West Virginia.

The OP seems to be talking about being in the same place at two different times whilst you seem to be talking about being at two different places at the same time.

I’ve been in the same place my whole life. Everything just keeps moving around me.

I found at an early age that I can move the whole world backwards with my feet. Just this evening I moved it a couple of miles (the dogs were helping, of course, but they only have little feet, so I reckon I was doing most of the work).

Of course if I want to move the world more quickly I put my car around me (I have to bring it over to me first, using my feet), and use the power of the engine and the wheels of the car to move the world at high speed in the direction of my choice. I like to bring different parts of the world to me so I can see them up close, or talk to the people there. Then when I’m finished I put the world back where I found it and bring my house back so I can sleep there.

There are even machines that allow me to push the whole world down thousands of feet and rotate it underneath me, thousands of miles in a single day. I hope you all don’t mind.

Why is this always misquoted? Heraclitus never said “the same river twice;” he just said “the same river.” To step in the same river *twice, *you’d have to step into it three times. (And of course, to Heraclitus, it still wouldn’t be the same river.)

Oh How can you be in two places at once when you’re not anywhere at aaallll?

In English, it has to be “the same river twice.” Otherwise, you get, “Can you step into the same river?” That isn’t well defined.

I also disagree with you that “stepping into the same river twice” means stepping into it three times.

Sorry for the highjack… (But…you did ask a question, and I attempted to answer it…)

(And, following from Bmalion, “The Old Same River! You can’t get there from here.”)

It’s right out back, here’s the key.

There are no priviliged frames of reference. It all depends on what frame your “time machine” is anchored to as you transit a temporal interval. However, the Solar System has an orbital speed about the galactic center of about 250 km/s, and the Milky Way Galaxy has a speed relative to the local cosmic microwave background of about 550 km/s, which is about the closest thing that exists to an absolute reference.

However, there is a larger problem with space travel. Because your time machine needs to account for not only displacement through time but also the change in kinetic and potential energy states along your path in space-time, you have to make up the difference between your momentum vector at the start and end of your time travel. This means you have to apply a momentum change and the corresponding energy difference that exceeds the solar escape speed at the Earth’s orbit by an order of magnitude times the interval you transit (not accounting for any other relativistic effects). So, basically, your time machine also has to have sufficient impulse to be a credible interstellar spaceship. Mr. Fusion isn’t going to cut it. You need some kind of unholy energies that are only available by opening a portal and invoking the aid of one of the Great Old Ones, and we know where that path leads…

Stranger

So that is what that damned buzzing noise is…