8

While getting to the past obviously requires some effort or accident, why (in-universe) are the crews going to such lengths to recreate the time-travel circumstances instead of simply travelling around at almost the speed of light for a while and exploit time dilation to get to the future without much of their own time being spent?

Zommuter
  • 4,585
  • 2
  • 34
  • 66
  • Accelerating to almost the speed of light takes a lot of energy. – Mr Lister Oct 01 '15 at 15:49
  • I don't think they can achieve near light speed with impulse. And even when, you have (from your dilated point of view) to accelerate to 1000 times light speed (somewhere at 99,x% Lightspeed for an stationary observer) to have 1000 times dilation. – Hothie Oct 01 '15 at 15:50
  • 3
    @Hothie: This is just technically wrong. T = T0 * (1-v^2/c^2) ^ -1/2 – ThePopMachine Oct 01 '15 at 17:26
  • On a side note, in TNG A Matter of Time Rasmussen wanted to go back in time to return home, instead of forwards. – Xantec Oct 01 '15 at 20:48
  • 1
    Great Scott! It's because Marty's got to save his kids! – BBlake Oct 02 '15 at 14:07
  • @ThePopMachine I argued from the point of view aboard the ship. They can(must) accelerate with 1g (subjective for them) for 100 years to reach 100times dilation. For a stationary observer their acceleration will drop more and more (due to dilation, mass increase and length contraction) as they approach light speed slower and slower. If they where accelerated with 1g from the outside (ot pov of the stationary observer) up to 100times dilation, the crew would expierience 100g acceleration at the end from their pov. – Hothie Oct 09 '15 at 09:03
  • @Hothie Were do you get all those numbers from? I'm afraid either we misunderstand you or you misunderstood relativity: you cannot accelerate to 1000 time light speed, you can approach light speed more and more but that's it. But maybe you're using a non-physical analogy I'm currently not aware of, which frankly is very misleading, if not not even wrong... – Zommuter Oct 09 '15 at 19:29
  • You misunderstood me. And yes, my numbers where wrong. But my point was, that you need a long personal time to get to high relativistic speeds. I found and interesting article about that: https://en.wikipedia.org/wiki/Space_travel_using_constant_acceleration – Hothie Oct 11 '15 at 19:26
  • @Hothie Oh, you tried a lie-to-children ;) Interesting article you linked to (though 1g is hardly the limit a starship would achieve). I guess you could actually expand this into a full answer if you want to – Zommuter Oct 12 '15 at 10:33
  • I really thought that time dilation and lorentz contraction are "scaled" that way, that if you accelerate 100 years with 1g (from the pov of the starships crew) you reach exactly the speed to end up with time dilation and lorentz contraction of factor 100. But the article says that after 12 years with 1g you will cross the galaxy. I just tried to get an answer on physics SE but have not got one until now. – Hothie Oct 12 '15 at 11:31
  • @Hothie that one I guess? Let's wait for an answer - though my gut feeling says it can't be so convenient numbers, 12 years to cross the universe is also rather much. Then again, 9.81 m/s² for 3.8e8 seconds is a lot... – Zommuter Oct 12 '15 at 11:36
  • Ok. Answered myself. Seems that the effects of time dilation and lorentz contraction stacks up. So it's not newtonian for the crew. You can reach time dilation factor 100 in roughly 5 years with constant 1g acceleration. But to come back to the original question a "time dilation back to the future" trip will cost you several years, if you can't afford more than 1g acceleration. – Hothie Oct 12 '15 at 13:52

3 Answers3

10

Ships did not seem capable of moving fast enough for meaningful time dilation to occur. For example, from https://memory-alpha.fandom.com/wiki/Impulse_drive:

The 'Star Trek Voyager Technical Manual' page 13 has full impulse listed as ¼ of the speed of light which is 167,000,000 mph or 74,770 km/s.

At full impulse, they would still require about 97 hours to travel 100 hours into the future. (sqrt(1 - (0.25)**2) is approximately 0.968). Given that the typical problem is being thrown years or decades (or longer) into the past, time dilation is not a practical solution to the problem of returning to your original point in time.

Community
  • 1
chepner
  • 3,825
  • 26
  • 35
  • 4
    And, of course, "warp speed" puts you in a non-relativistic space, so you wouldn't get time dilation. – FuzzyBoots Oct 01 '15 at 16:56
  • To add to this a bit, if the explanation of relativistic time dilation I was given in Physics class many years ago is correct, traveling at the speed of light is only good for a 50% slowdown, meanwhile your energy requirements approach the mass of your ship... – Perkins Oct 01 '15 at 23:38
  • 1
    @Perkins I'm afraid your class was inaccurate, were you to travel exactly at the speed of light (which is impossible since you're not massless) no time would pass for you at all. But as your momentum increases towards infinity then that means for accelerating you'll indeed need more and more energy. Though that last statement of your sounds a bit misleading to me... – Zommuter Oct 02 '15 at 06:06
  • 1
    That's a strange limitation, I'd have assumed in space, mostly without fiction, momentum wouldn't be limited, only acceleration... does the technical manual state anything about the why of this? – Zommuter Oct 02 '15 at 06:09
  • The force required for acceleration is proportional to relativistic mass, which increases with velocity, which is true in the real world, not just Star Trek. :) – chepner Oct 02 '15 at 11:23
  • @Zommuter Looking at it again, I found the logic failure that made it look like 50%. :)

    There's nothing saying that you have to be massless to accelerate to the speed of light, it's just that the energy requirements approach the mass of your ship and, potentially, the inertia of your ship approaches infinity. So you'd need an external power source.

    Given that increase in inertia, a speed limitation for impulse power isn't too ridiculous. Yes, they could continue to accelerate, but the engine efficiency starts to decrease, and their fuel supplies are limited.

     – Perkins Oct 02 '15 at 16:53
  • @Perkins Good point - makes me wonder about a relativistic extension to the Rocket Equation - and the linked article actually has one (just above the section "Terms of the equation") and that tanh in there quite well exlpains what you described in words. – Zommuter Oct 02 '15 at 18:46
  • Hm, according to this post ST:VOY's Timeless mentions 80% of c, which would yield 60%. – Zommuter Oct 02 '15 at 19:05
  • 2
    Suppose you are 100 years in the past. .8c still means a 60-year trip to get back. Even at .9999c, you're looking at a 1.4 years of travel time, ignoring the issue of engine wear-and-tear and fuel expenditure. Trying to recreate whatever phenomenon threw you back in the first place seems like a no-brainer. – chepner Oct 02 '15 at 19:23
1

I have wondered this myself, and I've never seen or read an explanation in (extended) canon to cover this. While chepner's answer does cover that impulse drive alone doesn't achieve the necessary speeds, they could still rig something up to misuse impulse along with thrusters, gravitational slingshots (of the real-world, sub-light variety), and maybe some handwaving with inertial dampeners, static warp bubbles and anti-gravs. These are Starfleet engineers, after all.

Out of universe, I suspect the reasons run toward a combination of dramatic necessity and ignorance of the physics involved.

In universe, one can rationalize. Assuming that they could achieve the speeds needed, they would be traveling a significant distance for an extended period of time, from an outside observers perspective. Even a ship as small as The Defiant or a Bird of Prey would have a hard time not looking like a great big spatial anomaly under those circumstances: they would then run a risk of detection, disrupting the timeline that they are attempting to return to. Their power generation might not be up to maintaining shielding against traveling through the interstellar medium at such high speeds for what might still be an extended period of time from their own perspective. Their ability to course correct at need, and indeed, to brake once they "arrived" at their proper time, is dubious.

None of these things is a greater obstacle than others they have handwaved away in other contexts, but it could give them plausible deniability of the relativistic option if they felt they needed one.

Politank-Z
  • 14,092
  • 5
  • 64
  • 90
1

Getting back to the future isn't so much the problem, the problem is doing it with accuracy. Gravitational slingshot ting works just fine, but a tiny error has compounding consequences in when you land or whether you even survive.

Escoce
  • 4,414
  • 19
  • 27