[maxerickson]

If you are confident you can hit a 4d coordinate, just point your device out into space at your present time.

(if I got the numbers right, we are 108 years * 390 km/s = ~0.14 light years from the location of the Tunguska collision)

[bane]

I've always wondered why pondering about time travel devices with only the temporal dimension as a setting seemed to neglect the fact that things move about in the universe! Traveling backwards or forwards in time at the same point in space means that all the stuff you're interested in is likely to not be where you think you left it.

[ars]

You would have to retain any momentum you have when traveling through time, so you would move more or less together with the earth.

Interestingly messing with momentum as you travel through time (i.e. take your momentum with you and it's lost from the old time) would be a bigger problem for physics than time travel itself.

On top of that having your gravitational influence just disappear and reappear elsewhere would be a huge problem. (Gravity is never created or destroyed, it just moves - there are never any discontinuities with gravity.) So when you travel through time your gravity would have to also influence things throughout all the time you transit.

The net result of both those things is you end up exactly where you would if you had just sat there and moved through time the natural way.

So time travel fiction that has the machine end up exactly in the same spatial location as it started are more physically accurate than those that try to talk about the change in location (and have some kind of compensator that moves it).

[kijin]

Depends on your method of travel.

You may be right if you just fast-forward yourself through all of the intermediate 4D coordinates, but what if time travel is not fast-forward? Git merge timelines and oops, one million merge conflicts.

For example, constructing a wormhole might require precise knowledge of the location of both endpoints relative to a specific reference frame. Misplacing your destination by as little as 3 meters can cause you to end up in the ground and suffocate to death.

[ars]

> but what if time travel is not a fast-forward?

That's my point - it kind of has to be, otherwise you violate conservation of momentum, energy, and gravity.

Violating those would be a bigger deal than time travel itself.

Incidentally that's why wormholes can't actually exist - they violate all of the above.

[kijin]

Could a time machine redirect, dissipate, or convert most of that energy into another form, such as a massive explosion in the middle of Siberia, leaving the traveler with very little momentum to worry about?

[maxerickson]

Maybe your time machine uses a bad design.

Which is a bit tongue in cheek, but I don't see how you can speak so concretely about it without having a working time machine.

[ars]

> but I don't see how you can speak so concretely about it without having a working time machine

Because the laws of physics are what they are. A time machine would have to fit inside those laws.

This constrains the design of the time machine - you can't just do anything you want.

Because of that, it's possible to narrow down how a time machine would work, if it could work.

[bane]

Well, maybe this is all true for your time machine, but I'm definitely using a different design.

[rollthehard6]

This was used to great effect as a weapons device in the 2000AD comic story Strontium Dog - http://www.musicbanter.com/members-journal/79558-trollhearts...

[mirimir]

Joe Haldeman gets that right in The Accidental Time Machine :)

[stouset]

Based on which reference frame?

[dmd]

https://en.wikipedia.org/wiki/Comoving_distance#Comoving_coo...

[akiselev]

The solar system's rotation is about the center of the galaxy so that would be the natural pick for an inertial reference frame, or the supermassive black hole believed to be in the center.

[kachnuv_ocasek]

Rotating frames of reference are not inertial.

[akiselev]

The center of the galaxy isn't rotating, it's just a point, no? Picking the criteria for such a point would be difficult and tracking it even more so, but what's the alternative?

[bashinator]

The center of the galaxy is almost certainly rotating, and is a supermassive black hole, not a point.

[kijin]

A black hole is a point. Its size is zero, regardless of its mass. Its event horizon has a size, but we don't need to care about that.

We do, however, need to care about the all the stuff that's revolving around that point, because even a slight imbalance will cause the galactic center of mass (barycenter) to move away from the black hole. Things get pretty wobbly out there.

[akiselev]

That's just the massive object at the center of our galaxy.

[ridgeguy]

Minimum Doppler shift of the cosmic background radiation integrated around the entire sky?

[semi-extrinsic]

Why not just go (a long way off) in front of a Mars rover? No harming any people, guaranteed to get noticed, no military worries.

[Terr_]

That reminds me of the novel "Pandora's Star", where two eccentric physicists surprise the first astronauts to land on mars:

> Wilson was already moving, glide walking as fast as was safe in the low gravity, making for the rear of the Eagle II. He knew they were close, and he could see everything on this side of the spaceplane. As soon as he was past the bell-shaped rocket nozzles he forced himself to a halt. Someone else was standing there, arm held high in an almost apologetic wave. Someone in what looked like a home-made space suit.

> [...] Behind the interloper was a two-metre circle of another place. It hung above the Martian soil like some bizarre superimposed TV image, with a weird rim made up from seething diffraction patterns of light from a grey universe. An opening through space, a gateway into what looked like a rundown physics lab.

> The other side had been sealed off with thick glass. A college geek-type with a wild afro hairstyle was pressed against it, looking out at Mars, laughing and pointing at Wilson. Above him, bright Californian sunlight shone in through the physics lab’s open windows.

[wlievens]

Loved that book, and everything else Hamilton writes

[Terr_]

I do feel like he tried a little too hard to put every character and plot into one single universe though.

Terry Pratchett gets away with it because his universe doesn't have to make sense :P

[therein]

What if your time travel lab is stationed at Mars already?