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At a constant 1G acceleration it takes a little less than a year to reach near light speed speeds. Time would dilate locally on the ship slowing time for them. Could the time dilation be pushed to 12 times faster than normal time, so 1 light year of travel is only 1 month of local spaceship time? If possible, A star 10 light years away would only take 2 years 9 months to reach from the spaceships perspective. 1 year to accelerate, 9 months in dilation, 1 year to decelerate. It would of course still take 11 normal year to reach.

The closer you get the the speed of light a kind of drag effects the ship stopping it from getting faster and reaching the speed of light, so only so much speed and time dilation is possible with a limited amount of thrusting power. What would be the time dilation for a space craft with 1G thrusting power going max speed? If the engines were turned off at fastest possible speed, would that dragging force slow the ship down or lessen the time dilation or would it coast at the same speed and dilation to save fuel? If the engines were still on, pushing with 1G but the ship was at max possible speed, would the crew still feel the 1G on the ship even if it is not accelerating anymore?

Would the ship measuring the distance to it's destination at 12 times dilation seem like it is traveling 12 times faster then the speed of light from the ships perspective?

Could you go fast enough that the dilation would be 52 times normal so 1 week a light year? or 365 times for 1 day a light year? Is there a limit to the time dilation? do we need faster then light tech to explore space at all? Sure it would take a really long time out side the ship in real time, but to the people on the ship it would be doable.

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    $\begingroup$ "12 times faster then the speed of light from the ships perspective?" - Of course not, because speed of light in vacuum does not depend on reference frame (aka "perspective"). But it may reach destination using less ship time, than light will need "outside time". Is that what you meant there? Because now ot makes little sense. Actually, I believe this may be OK on physics stack exchange with little to no tweaking. They are better at explaining relativity than we are here. $\endgroup$
    – Mołot
    Commented Jun 18, 2018 at 10:46
  • $\begingroup$ There is a lot of popular, easy to understand stuff about relativity out there, on the internet and in the shape of books and even tv shows. You are asking the most basic questions about how relativity works, I don't think an reply here can be a truly understandable, correct and at the same time a satisfying answer to your questions. Someone might still attempt it, but my guess is for every question one answers, there will grow 2 more misunderstandings. Please consider starting at the beginning $\endgroup$
    – Raditz_35
    Commented Jun 18, 2018 at 11:04
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    $\begingroup$ Unrelated 2nd comment: "do we need faster then light tech to explore space at all?" What do you mean by exploring? There is a lot of television about explorers in space. They encounter aliens that talk English and have a problem of the week for them. True space exploration, well, perhaps we do not even need to leave Earth. Why do you need people on a space ship anyways? Do you know how boring it is to sit in a room with 10 people your entire life? I think you need to either rethink that idea or just accept that this kind of doesn't make sense and nobody really cares. $\endgroup$
    – Raditz_35
    Commented Jun 18, 2018 at 11:09
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    $\begingroup$ Welcome to worldbuilding. Can you please clean up and clarify your question to allow understanding what your question is? I am having an hard time understanding it. $\endgroup$
    – L.Dutch ♦
    Commented Jun 18, 2018 at 11:41
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    $\begingroup$ "a kind of drag effects" is totally not what happens, just by the way. From ships PoV, it is accelerating just the same. $\endgroup$
    – Mołot
    Commented Jun 18, 2018 at 12:55

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Wow that's a whole lot of question marks, I'm going to give you the Larry Niven special on 1G travel times. This comes straight out of the essay Bigger Than Worlds which I thoroughly recommend to any serious world builder, according to Niven it takes:

four years to the nearest star, twenty-one years to the galactic hub, twenty-eight to Andromeda galaxy-all at one gravity acceleration

That's ship time and assumes 1G acceleration to the half way point followed by 1G deceleration the rest of the way to the destination.

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  • $\begingroup$ That's accurate, but it leaves out some details...like what kind of drive can do that, and where the energy comes from (just the kinetic energy of the ship exceeds its rest mass by a large factor in longer trips), and how you shield against the radiation and impacts you get plowing at high gamma through even "empty" space...But with your hands waving wildly enough, you can do it. $\endgroup$
    – Mark Olson
    Commented Jun 18, 2018 at 11:05
  • $\begingroup$ @MarkOlson It assumed a Bassard Ramjet because it was written when we still thought those were workable, they're not but if they were they'd be effectively self-shielding. $\endgroup$
    – Ash
    Commented Jun 18, 2018 at 11:25
  • $\begingroup$ Fair enough for fuel and low-energy charged particles, but not for neutral particles and dust. And what's the gamma at turnover on a trip to Andromeda? Somewhere in excess of 100,000 -- which makes for really hard to deflect charged particles! $\endgroup$
    – Mark Olson
    Commented Jun 18, 2018 at 11:31
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    $\begingroup$ The basic Bussard uses magnetic fields which <hand wave>deflect charged particles into a magnetic fusion containment volumes</hand wave> which boosts the ship "for free". Besides the aforementioned hand wave, magnetic fields only work on charged particles, so neutrals just slam right in. But maybe they're ionized by lasers or something. Still, the Bussard field itself acts like a giant wind shield and when going fast enough the interstellar medium creates huge drag. You get a surprisingly slow terminal velocity. $\endgroup$
    – Mark Olson
    Commented Jun 18, 2018 at 13:05
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    $\begingroup$ @MarkOlson Yeah from memory you actually go "backwards" in a Bassard, it has a lower terminal velocity than Sol's galactic core frame velocity. $\endgroup$
    – Ash
    Commented Jun 18, 2018 at 13:09
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Several questions at once, but not too hard to answer...

  1. Yes, with constant acceleration (1G or otherwise) space travel looks much more plausible (from traveler's perspective). There are even calculators on the web, like this one.

  2. It is not clear what you mean by "max speed". In theory, constant acceleration ship can reach speeds arbitrary close to the speed of light. In practice, it would be difficult to achieve, because of energy/fuel requirements, For any kind of engine, the amounts will be huge.

  3. In interstellar space, "drag force" is quite low, we probably need to account for it only on longer, 1000+ light years journeys. The actual problem will be ablation and collisions - ship will encounter interstellar gas and dust at relativistic speed, which will cause any kind of material to degrade, showering the inside of ship with high-energy particles in the process. Collision with a grain of sand can be fatal for the whole ship, unless there is proper shielding.

  4. If the engines are on, crew will feel gravity force, and ship will be accelerating or decelerating. If the engines are off, there will be zero g.

  5. If time is dilated to the factor of 12, the crew will know that it is traveling at "factor 12" speed, but it wouldn't be anything like FTL "warp 12". Due to relativistic Length contraction, destination will be seen as being 12 times closer than in a "normal" reference frame. To the crew (and to an outside observer), the speed would still be sublight. Crew will observe potentially very strong "blue shift" of light at their destination and "red shift" at their departure point.

  6. Can you go faster than 12? Faster than 365? Yes, there is no theoretical limit. Practical limit is another matter, and it will depend on the type of engine that is used.

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  • $\begingroup$ On point 3, I read the Revelation Space books by Alastair Reynolds recently and his 'lighthugger' ships use a thick ablative shield of ice on their 'nose cones'. $\endgroup$
    – mwarren
    Commented Oct 30, 2019 at 15:40
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Given some handwavium for the power source for the constant acceleration drive and shielding from colliding with particles, it works. However bear in mind the consequences - observers on the starting planet would still see the ship travelling at marginally under c. For a 1,000 light year round trip maybe only 10 years have passed for the crew, but 1,000 years have still passed on the starting planet so by the time they returned everyone they knew would be long, long dead.

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