Well, if one was on a ship that could travel at, say 95% of the speed of light, time would run much slower locally than it would on the earth. Let’s say a mission to Alpha Centauri, five years worth of exploration, and the trip back, took 20 years locally. Since time passed much slower during the fifteen years of travel time (give or take), 100 years might have passed on earth. So, when they arrive, the effect as related to them is that they traveled 80 years into the future, but the effect for those on earth is that they were gone for 80 years and are some relic from their past. Boggles the mind, doesn’t it?
Yes…if you’re allowed to go back in time, which leads to causality violation. We were talking about what would happen if you could travel into the future. Traveling into the future doesn’t lead to causality violation. But if you’re allowed to travel into the future and then go back, that’s travel into the past, because when you arrive in the future it’s no longer the future, it’s the present.
Anyway, the point is that travel into the future is absolutely possible according to the laws of physics, and not just in the trivial sense of one second/second. Travel to Tau Ceti nearly as fast as light, and then come home, and you’ve aged 22 minutes while Earth has aged 22 years. That’s time travel into the future. It’s only when you arrive on Earth in 2040 and want to go back to 2018 that we have a problem.
If you travel to Tau Ceti and back, you don’t expect to meet yourself on Earth when you get back, do you? Of course not. If you have a time machine that only travels into the future, you won’t meet yourself in the future.
It’s that return part that seems to cause issue.
A thought:
If you use a time machine while on an aircraft, you’ll re-appear in mid-air, right?
Well, what happens if you use a time machine on a planet orbiting a sun, that’s orbiting the center of a galaxy, that’s moving on an expanding balloon of space-time? Maybe time travel works, but every time you go, you end up in the middle of space.
But that’s just the observation that we’re not going to have a time machine with a big dial on it and you just turn the dial and go whenever you want. A time machine also has to be a space machine.
Not quite. The really boggling notion is that time always runs exactly one second per second everywhere, both on earth and on the ship. And that although both clocks are running perfectly accurately, they don’t match when the ship gets back. Time never runs slower or faster.
That’s also why this is not a good reference for time travel. In this example time behaves locally in a perfectly ordinary fashion. In time travel to the future a break in the one second per second must occur. If I visit 3018 then time has passed at 1000 years/per second, a wildly different event.
Figuring out a way to keep time from running one second per second is the doom of all time travel suppositions.
Maybe…
Depends.
This only works if the method of time travel you are using involves de-materialisng in one location in space-time and re-materialising in another. In an arbitrary system like that, you are obliged to somehow dictate the location of your re-materialisation in time; you may as well dictate the location of re-materialisation in space as well. As Lemur866 says, you need a space-time machine.
But most of the semi-practical methods of time-travel I am aware of involve travelling through a wormhole, with temporally-displaced mouths. This means you turn up at a particular location in time and space, and you know exactly where, and when you are. You can go back and forth through the wormhole at will, since it connects two discrete locations in space/time, and there is no question of suddenly appearing in empty space.
One way to look at the passage of time is the rate of increase in entropy. When you get in a spaceship traveling at .99c and we say “time slows down for you”, what we’re really saying is that entropy here on Earth increases at a predictable rate but entropy on your spaceship will increase at a slower rate. Consequently, all chemical reactions happen slower (so your brain has fewer thoughts) and mechanical energy dissipates slower (so the clock hands make fewer revolutions). If we could figure out how to build a stationary box where entropy increases slower inside the box than outside (something resembling a Faraday cage, perhaps?) then that would essentially be a one-way time machine into the future.*
And it would cause no paradoxes, for the exact same reason that being a spaceship at .99c causes no paradoxes.
- For example, if entropy inside the box increase at 1/3600th compared to the outside of the box, anything inside the box that registers the passage of time would show just one second elapsing inside while an entire hour passes outside.
You could do this by suspending your box just above the event horizon of a black hole. If you had unfeasibly strong string, you could suspend this box so that it were stationary. Mind you, your feet would age more slowly than your head (actually, that already happens on Earth, and no-one notices).
You’re looking at it from Earth’s reference frame. From the pov of the spaceship, the entropy on earth increases at a faster rate while that inside the ship remains at the normal rate. All reference frames are equal. You can’t privilege Earth’s. If you could there wouldn’t be any “paradox”. It would be a normal happening.
I thought it was the other way around. At least my feet smell like it should be.
But isn’t that effectively FTL travel? I mean, that way you can travel to a point in spacetime one second in the future and 100 light years away.
One of the biggest hurdles to time travel does would still will have been verb tenses.
Right. As you know, Bob, let’s say you’re on that spaceship headed for Tau Ceti. Hey, I happen to like Tau Ceti. Anyway, you’re zoooooooming away at a high fraction of the speed of light. Yeah, but only from the reference frame of the Earth. In another reference frame, Earth and the rest of the galaxy are zooming away from you at NAFAL, and you just slowed down.
So when you break out your hyper-telescope to observe the watch on the wrist of your twin back on Earth, it turns out you observe that his watch is going more slowly than your watch. You haven’t sped up, you’ve slowed down and he’s the one who sped up. But it turns out your twin has a hyper-telescope too, and when he looks at the clock you hung on the tailfins of your ship, he sees that your clock is going more slowly than his watch. He hasn’t sped up, you’re the one who sped up. From his reference frame.
Both of you observe the same phenomenon, that the other guy has gone screwy while you’ve stayed the same.
The only way to resolve the dispute is for both of you to return to the same reference frame and compare notes. So you turn your ship around after buzzing Tau Ceti, and head back for Earth. When you do that, you find that it was your clock that was slower. If instead of you returning to Earth’s reference frame, he had gotten in a second ship and zoomed after you and docked with you, you’d find that it was his clock that was slower.
I accept that time travel is impossible in the sense of going back to the past and changing actual events. IMHO, the only way out of that is if there are parallel universes, so a new timeline opens up in another universe if you went back. The logical problem: which timeline would you be in? Could you be in both at the same time? In which case, what happens to your awareness?
Just possibly it might be possible to go back into the past - but only as an observer. Could this explain some paranormal phenomena? The question might be whether whether you could get back again to your temporal starting point. If there is a way to do this, then at our present understanding I would guess that it would be done somehow with quantum mechanics.
Observing the past is trivially easy. Everyone does it all the time.
I don’t know if the phrase “probability macadam” is yours or not, but it’s a fantastic. It’ explains it so nicely.
Nobody does it none of the time. People only observe the present.
We do have memories and visual and aural records of the past. Of course those may have just been created by a trickster god.
I’m betting that no real understanding of time travel will occur before we have a real understanding of time. We’re not even close to that.
Thing is this: the time dilation effects of SR are based on acceleration. The “traditional” way to get to Taos Eddy’s place is to accelerate at a constant rate for about 110Pm, then hit the attitude jets to flip around and decelerate for the remaining 110Pm, arriving there at about the speed you left at. Meanwhile, you have been moving in an open curve while your Earthling friend has been doing epicycles year in and day out, so that adds a touch of muddle to the math.
And it would in fact look the same if you were flying in a vacuum. Space is not at all like a vacuum (well, some parts are extremely empty, but). You can both look around and spot some “standard candles” to cross-reference your reference frames – like, even, Tau Ceti.
Thus, you can observe what the Earth’s velocity is relative to the standard candles, and the Earthlings can observe what your velocity is relative to them as well. If, at the mid-point, you have reached a super-high sub-light speed relative to your destination, both you and the Earthlings will observe that difference.
Of course, it will take a ridiculous amount of fuel to accomplish, because SR says that as you approach c, your relative mass approaches infinity (mass which includes the fuel you also need to push along). And, depending on how your drive system is powered, you might have issues getting back, because Tau Ceti is seen to be “metal poor”: you might well not be able to refuel at all.