Time Travel... the fly in the ointment

[Jimmy Higgins]

For whatever reason, I was thinking about time travel. You hear about time travel, going from one time to another, but it occurred to me about one other thing, which I don't recall ever hearing about in a tv program. Time and space are linked, no?

So if one were to go back in time, wouldn't they be limited to when the position they are currently in, is relatively the same? IE, if you go back in time from your position, to six months ago, aren't you royally fucked as Earth is located on the other side of the sun? Heck, wouldn't just a minute put you in great danger as well?

 

[Loren Pechtel]

For whatever reason, I was thinking about time travel. You hear about time travel, going from one time to another, but it occurred to me about one other thing, which I don't recall ever hearing about in a tv program. Time and space are linked, no?

So if one were to go back in time, wouldn't they be limited to when the position they are currently in, is relatively the same? IE, if you go back in time from your position, to six months ago, aren't you royally fucked as Earth is located on the other side of the sun? Heck, wouldn't just a minute put you in great danger as well?

James Hogan, Thrice Upon A Time

It's a time-communication story where this is definitely relevant. Their time transmitter actually has an infinite range but it's only useful within a fairly short window because the signal shows up in the past at the location of the transmitter in the present--but the Earth is moving through space, thus putting them far apart.

 

[skepticalbip]

For whatever reason, I was thinking about time travel. You hear about time travel, going from one time to another, but it occurred to me about one other thing, which I don't recall ever hearing about in a tv program. Time and space are linked, no?

So if one were to go back in time, wouldn't they be limited to when the position they are currently in, is relatively the same? IE, if you go back in time from your position, to six months ago, aren't you royally fucked as Earth is located on the other side of the sun? Heck, wouldn't just a minute put you in great danger as well?

Position, like speed, is tricky to specify. The term position (or speed) doesn't really mean anything without specifying with respect to what. With respect to the Earth, the Earth doesn't move. However, if position (and speed) is defined with respect to something else then you get a different answer. A reference point of Jupiter, the Sun, galactic core, center of mass of the local group, or some specific quasar will all result in very different descriptions of Earth's position (and speed).

Any particular chosen reference point is as valid as any other. It is just that the math is a little more convoluted with some.

 

[Keith&Co.]

Time and space are linked, no?

Some scifi assumes that this link could be used to navigate time travel. Others assume that the earth's movement must be corrected for in navigating the time machine. It all depends on how much time you want to spend lecturing the reader about how difficult the time travel element is.

In Star Trek IV, the one with the whales, the method involved high speeds around the edge of a gravity well, and appears to assume that the time traveling object remains centered on that gravity well throughout the time traveling. I would imagine that if you attmepted to travel to a time that this gravity well didn't exist, you'd either be unable to pass the point of the star's accumulation or you'd spin off in a random direction when no longer anchored by the well.
Might be a Superman plot there....

 

[Peez]

I think that the old 'what happens if you kill your father/mother before you were conceived' problem is a bigger issue with backward time travel.

That and grammar.

Peez

 

[Jimmy Higgins]

For whatever reason, I was thinking about time travel. You hear about time travel, going from one time to another, but it occurred to me about one other thing, which I don't recall ever hearing about in a tv program. Time and space are linked, no?

So if one were to go back in time, wouldn't they be limited to when the position they are currently in, is relatively the same? IE, if you go back in time from your position, to six months ago, aren't you royally fucked as Earth is located on the other side of the sun? Heck, wouldn't just a minute put you in great danger as well?

Position, like speed, is tricky to specify. The term position (or speed) doesn't really mean anything without specifying with respect to what. With respect to the Earth, the Earth doesn't move. However, if position (and speed) is defined with respect to something else then you get a different answer. A reference point of Jupiter, the Sun, galactic core, center of mass of the local group, or some specific quasar will all result in very different descriptions of Earth's position (and speed).

Any particular chosen reference point is as valid as any other. It is just that the math is a little more convoluted with some.

Perhaps I'm looking at this too Cartesianly. At the present moment, a person is at a universal point X,Y,Z,time. Ten seconds later, the X,Y,Z,time have all changed as the Earth moves about the sun... and rotates along its own axis. I suppose if one can move in time, it would be just as "easy" to shift the position as well. But, from a universal coordinates standpoint, you would need to adjust for a shift in space, as much as you would for time.

Or is looking at the universe in an X,Y,Z,time sort of way incorrect?

 

[Shadowy Man]

Or is looking at the universe in an X,Y,Z,time sort of way incorrect?

Yes, because you need x,y,z relative to something. There is no 'universal' frame of reference. The closest we come is the CMB.

 

[Kharakov]

For whatever reason, I was thinking about time travel. You hear about time travel, going from one time to another, but it occurred to me about one other thing, which I don't recall ever hearing about in a tv program. Time and space are linked, no?

So if one were to go back in time, wouldn't they be limited to when the position they are currently in, is relatively the same? IE, if you go back in time from your position, to six months ago, aren't you royally fucked as Earth is located on the other side of the sun? Heck, wouldn't just a minute put you in great danger as well?

James Hogan, Thrice Upon A Time

It's a time-communication story where this is definitely relevant. Their time transmitter actually has an infinite range but it's only useful within a fairly short window because the signal shows up in the past at the location of the transmitter in the present--but the Earth is moving through space, thus putting them far apart.

I think you probably brought this one up before, but can't they simply put the transmitter in a satellite and relay the signal back and forwards in time as they see fit (to future and past transmitters)?

I imagine some type of technological singularity appearing centered at the original transmitter location.

 

[Loren Pechtel]

I think that the old 'what happens if you kill your father/mother before you were conceived' problem is a bigger issue with backward time travel.

That and grammar.

Peez

Yes, but you can have the position problem even when you only have backwards communication.

 

[Jimmy Higgins]

Or is looking at the universe in an X,Y,Z,time sort of way incorrect?

Yes, because you need x,y,z relative to something. There is no 'universal' frame of reference. The closest we come is the CMB.

The origin... relative to the origin.

 

[bilby]

Not only is position in space dependent on specifying a reference frame; so is position in time. Separated clocks need not run at the same rate, and there is no such thing as a universal 'now', on which all observers can agree, any more than there is a universal 'there' whose location is the same for all observers.

Perversely, this might make the OP issue less problematic; movement in time is fundamentally no different from movement in space, so the rules may be similar. If I travel from here to New York, I don't have to worry about my position relative to other reference frames; despite the very fast and complex pattern of motion I need to undertake to fly from here to New York, I can simplify the problem to just following a great circle path at Mach 0.82 and FL380. Airline pilots would go insane if they had to try to understand their navigation relative to an observer on one of Jupiter's moons (or any other vantage point in the universe) who is arbitrarily defined as 'stationary'.

If I was writing the SciFi, I would assume that similar simplistic navigational rules should apply to space/time travel as to those that apply to space travel with a fairly constant time vector. Momentum in any of the four dimensions would be conserved, unless a force is applied; The only thing that prevents us from time travel right now is that we don't know how to apply a significant force in the T dimension, so we carry on moving towards the future and away from the past at a rate that remains very close to one elapsed second per experienced second. This can be changed by accelerating in spacetime, but the change is minuscule in the T dimension compared to the change in X, Y and Z, because c² is so large. Time-travel therefore requires a) a very large energy source; and b) a way to direct that energy into a force on the T axis, while avoiding wasting too much of it in generating forces acting in the other three dimensions.

IIRC, there was a sci-fi tale in which an FTL drive used by long-lived aliens was found to in fact be a sub-luminal drive, coupled to a time machine. The aliens would spend, say, ten-thousand years, flying to a nearby star, and then skip back in time to a time shortly after their departure, giving the same overall effect as travelling thousands of light-years almost instantaneously. This was a very convenient plot device to explain why humans couldn't duplicate the feat, and to allow for invasions over huge distances; but as is usual with such 'clever' ideas, it ultimately leads to more plot-holes than a Swiss cheese.

 

[Bronzeage]

If anyone is interested in time travel, I can make them a good deal on a used time machine.

 

[DBT]

If you are going back in time, you must necessarily go back to the earth's position in space at the time you are traveling to...winding back the years in time, space and position.

 

[bilby]

If you are going back in time, you must necessarily go back to the earth's position in space at the time you are travelling to...winding back the years in time, space and position.

All well and good; but will you end up at the same point on the surface, and at the same distance from the Earth's centre of gravity? If I go back in time 10 Hours, starting from Johannesburg, will I wind up on the East Coast of Australia; or worse, 5,700 feet in the air above the East Coast? Or are you saying that if I go back in time ten hours I will wind up exactly wherever I was 10 hours ago, occupying the same space I occupied then? If the latter, then what happens if I travel back to before I was born?

Of course, your answer should take into account that as the author of this tale, you are completely omnipotent, and can dictate any result you feel like...

 

[DBT]

If you are going back in time, you must necessarily go back to the earth's position in space at the time you are travelling to...winding back the years in time, space and position.

All well and good; but will you end up at the same point on the surface, and at the same distance from the Earth's centre of gravity? If I go back in time 10 Hours, starting from Johannesburg, will I wind up on the East Coast of Australia; or worse, 5,700 feet in the air above the East Coast? Or are you saying that if I go back in time ten hours I will wind up exactly wherever I was 10 hours ago, occupying the same space I occupied then? If the latter, then what happens if I travel back to before I was born?

Of course, your answer should take into account that as the author of this tale, you are completely omnipotent, and can dictate any result you feel like...

Maybe I'm wrong but when we travel forward in time, minute by minute, hour by hour, days, months, years, we are 'dragged' through space by our connection with the Earth, gravity keeps us in position....so why would traveling back in time be any different? Our time machine should be bound by gravity while it moves backward at whatever the set rate, you may choose the same sedatery rate in reverse or a rapid rate. Speeding up the rate should not break the time machines connection with the planets gravity, but the lie of the land may vary, layers, erosion, etc.

 

[bilby]

All well and good; but will you end up at the same point on the surface, and at the same distance from the Earth's centre of gravity? If I go back in time 10 Hours, starting from Johannesburg, will I wind up on the East Coast of Australia; or worse, 5,700 feet in the air above the East Coast? Or are you saying that if I go back in time ten hours I will wind up exactly wherever I was 10 hours ago, occupying the same space I occupied then? If the latter, then what happens if I travel back to before I was born?

Of course, your answer should take into account that as the author of this tale, you are completely omnipotent, and can dictate any result you feel like...

Maybe I'm wrong but when we travel forward in time, minute by minute, hour by hour, days, months, years, we are 'dragged' through space by our connection with the Earth, gravity keeps us in position....so why would traveling back in time be any different? Our time machine should be bound by gravity while it moves backward at whatever the set rate, you may choose the same sedatery rate in reverse or a rapid rate. Speeding up the rate should not break the time machines connection with the planets gravity, but the lie of the land may vary, layers, erosion, etc.

Won't that cause issues with displacement of the stuff already occupying the space and time that you move into?

If you travel backwards in time, that could be rather painful for your past self, as your future self tries to (re)occupy the same space...

 

[DBT]

Maybe I'm wrong but when we travel forward in time, minute by minute, hour by hour, days, months, years, we are 'dragged' through space by our connection with the Earth, gravity keeps us in position....so why would traveling back in time be any different? Our time machine should be bound by gravity while it moves backward at whatever the set rate, you may choose the same sedatery rate in reverse or a rapid rate. Speeding up the rate should not break the time machines connection with the planets gravity, but the lie of the land may vary, layers, erosion, etc.

Won't that cause issues with displacement of the stuff already occupying the space and time that you move into?
If you travel backwards in time, that could be rather painful for your past self, as your future self tries to (re)occupy the same space...

No doubt it would. Some authors of time travel novels include a pop or a bang as the air is displaced by the arrival of their intrepid time traveler, including positioning their time machine in spot that is known not to have a hill or mountain at the targeted time of arrival, or during any time between departure and arrival. H G Wells comes to mind, but his time traveler stayed on location while buildings and hills arose and decayed around him and his time machine/time bubble.

 

[Perspicuo]

If there is no absolute frame of reference in the universe, as an author,

(a) I can make time travel relative to the gravity well, i.e. Earth. Or,

(b) I can make it relative to quantum entanglement, i.e. if you time-travel in the N.Y. Public Library in 2015, you will appear in the past at the same library in 1940. The portal is "entangled" with the surrounding structure.

 

[Playball40]

I so wish I understood physics better.

 

[Keith&Co.]

Our time machine should be bound by gravity while it moves backward at whatever the set rate,

Ah, but the effect of gravity is acceleration per second per second. A time machine would change the value of a second, altering or removing gravity's affect on the time machine.