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I'm working on a science fiction story involving starships travelling about 95% of the speed of light. I wanted to have a reason for the starships to need to shield themselves against time dilation of any significant amount.

Primarily I want this for story-writing purposes, so that you could have fast starships—fast slower than light ships—without the complications caused by time dilation.

I'd like justify this with reasonably hard science... Could there potentially be any unwanted material effects due to time dilation and/or Lorentz contraction?

One idea is this; when a force—such as thrust from an engine—is applied to any material that force will propagate only at that material's speed of sound. Consequently as a starship accelerates, the atoms in the front will always be travelling a little bit slower than the atoms in the back (or vise versa if the ship has its engines in the front).

This would mean that the atoms nearest the engine would be experiencing slightly more time dilation than those further away. At high speeds with pronounced time dilation, could this lead to a weakening of the starship's structure?

While there might be no, or nearly no effects if space were a total vacuum. In practice space is full of rarefied gas and dust—not to mention the quantum vacuum.

This paper might be helpful in regards to extreme relativistic effects: link

TL;DR

I don't want the complications of time dilation but still want fast sublight starships. Is there any justification to screen out time dilation from a structural integrity stand point? Could time dilation and/or Lorentz contraction cause structural problems?

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    $\begingroup$ One of the things about relativity is that things are relative. If you are at 90% c, you are at 90% c relative to something. Just because you are at 90% c relative to earth doesn't mean that you feel time dilation any worse, even upon acceleration, than if you started at 0 c. Since no such device was needed for the Apollo missions, I think the answer is "no". However, if you would have some magical device putting you into earth standard time speed (or something), it might tear your ship apart. Not my field of expertiese though, I upvoted to read what the experts say $\endgroup$ – Raditz_35 Jul 8 '17 at 23:53
  • $\begingroup$ Oh and PS, there is never enough room in those comments: This is just me being interested in what other people do in their stories: What is your justification to stop at about 95% light speed? I would assume that a spaceship that can go 95% c can also reach 99% c (for example) quite easily. $\endgroup$ – Raditz_35 Jul 8 '17 at 23:57
  • $\begingroup$ Even though time dilation is relative, a time dilated mass will still produce effects apparent in both its own reference frame and some external reference frames, such as blueshifting: link. Also, if a starship is not uniformly accelerating then each chunk of the ship will each have its own separate reference frame. As for why 95% of c; I wanted a speed that produced great enough of a time dilation so that any potential effects were amplified, but still near the speed the ships in the story are actually likely to be travelling. $\endgroup$ – Laharon Jul 9 '17 at 0:20
  • $\begingroup$ I have not studied theoretical Physics, I know many here have. Is this true? You are experiencing a blueshift in your own time frame (the link does not help)? Btw, how fast are you accelerating? From 0 to 0.9 c in under a second so you really feel those effects? In that case, the ship would just, well, nuclear fusion and so on. $\endgroup$ – Raditz_35 Jul 9 '17 at 0:32
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    $\begingroup$ You might like my applet. $\endgroup$ – JDługosz Jul 9 '17 at 0:49
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In general, there are no effects doe to being at speed. We are travelling at 90% c from their point of view — does that suddenly make things happen?

Now moving through space at high speed has real problems because the oncoming gas and dust will be near lightspeed, and lorentz contraction of the path you’re on will make you see a higher density of material.

As for acceleration, you might find Bell's Spaceship Paradox interesting. Instead of two ships and a chain, consider the front and back of a single ship. But again, your high speed has nothing to do with it; it doesn’t get worse as you get faster. Rather, it has to do with acceleration, and you're not accelerating more than 1G I would presume.

And remember the equivalence principle! The same effects of a 1G acceleration are manifest in skyscrapers here on Earth! That is, nothing you would notice without delicate instruments.

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  • $\begingroup$ I like that this answer dispels the basic misconception underlying the question and simultaneously provide a reasonable cause for shielding a near-light-speed ship - maybe expand that bit a little more? $\endgroup$ – G0BLiN Jul 9 '17 at 15:22
  • $\begingroup$ Thank you for the link to Bell's Spaceship Paradox. I have a question about it: if I understand correctly, Lorentz contraction is dependent on velocity—not acceleration, wouldn't this mean that a greater velocity would result in greater stresses? $\endgroup$ – Laharon Jul 9 '17 at 21:35
  • $\begingroup$ Here is an edited version of the PhysFAQ with illustrations I drew. Look at the different times in Figure 2 — as time passes (and the ships continue to accelerate) the distance between them grows and the time dilation becomes more different between them. $\endgroup$ – JDługosz Jul 10 '17 at 3:10
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The whole point of relativity is that there should be no way to tell how fast you are travelling, because there is no absolute speed. There can be no effects from travelling fast, because if there were then relativity would be violated.

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  • $\begingroup$ "There can be no effects from travelling fast" - other than all the interstellar dust smacking into you and all the meteors zipping past you, and eventually all the intense glow of the cosmic microwave background radiation shifted into visible spectrum (not to mention that you need a pretty good UV filters on your windshield to prevent your bridge crew from developing skin cancer). Even at the sluggish speeds Earth is moving at, some 0.001 c or so, we can notice the CMBR being slightly bluer in one direction and slightly redder in the other one. $\endgroup$ – John Dvorak Jul 9 '17 at 9:50
  • $\begingroup$ Sorry, You have it very wrong. The kind og motion relativity You are speaking about whas well known to Galileo, who spoke about throwing balls inside a ship and impossibility to determine if ship is moving or not. Einstein's Special Relativity actually says there's one thing that's not relative at all: the speed of light that is a universal constant and it's measured constantly the same by all observers regardless of their relative speed. Special Relativity deals with accelerations and explains why the spaceship time will slow and not Earth's (no space to go into details here). $\endgroup$ – ZioByte Jul 9 '17 at 12:07
  • $\begingroup$ @JanDvorak But you would see the same effect if your spaceship was stationary and the interstellar dust and microwave background radiation were moving. It's not because of the speed of the spaceship; it's because of the difference in speed of the spaceship and the dust. $\endgroup$ – Mike Scott Jul 9 '17 at 14:31
  • $\begingroup$ @ZioByte I'm afraid you're the one who has it wrong. From the viewpoint of the spaceship, it's time on Earth that's slowed down. $\endgroup$ – Mike Scott Jul 9 '17 at 14:32
  • $\begingroup$ @MikeScott: I'm quite sure of what I'm saying. Acceleration is what makes the difference. Think about a spaceship leaving Earth and then, after two travels near-light-speed comes back. At that point the two times are compared. Who will be "older"? If You are right the spacemen will find on Earth less time passed than on their reference system. This is not true. Less years have gone through on spaceship with respect of "stationary" Earth, in both reference systems. General Relativity is quite clear on this. $\endgroup$ – ZioByte Jul 9 '17 at 20:37
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There are no, absolutely no, structural effects or defects caused by traveling close to lightspeed. The main problem encountering gas or dust in interstellar space. A forward shield or barrier will be needed for astronaut safety.

The main complication with time dilation for a writer is calculating how much time is different between the astronauts in the spaceship and the people who remain on Earth.

The physicist John Cramer in an example about a spaceship travelling close to lightspeed came up with a simple way calculating the time for the astronauts. Assume the spaceship accelerates at one gravity (1 g) for one year and reaches a cruising velocity of 0.867 c (86.67% of lightspeed). Time dilation will be two. Let the spaceship travel 86.67 light years which take 100 years for it to do so. At a time dilation Lorentz factor of two, fifty years pass in the spaceship. It now decelerates for another year at 1 g.

The total time in the spaceship will be two years for the acceleration and deceleration phases combined plus fifty years ship time. A total time of 52 years.

In the relative rest frame of reference for Earth, and presumably for their destination, the time it has taken the spaceship to undertake this trip will 102 years.

You can further assume during the acceleration and deceleration phases the spaceship travels a distance of half light year for each phase. The total distance travelled 87.67 light years.

The alternative is travelling sufficiently fast to go places, but not so far as to large amounts of time dilation. A velocity of about half-lightspeed it achieve this effect. The time dilation is 1.1547. Calculate the time it take a given distance at 0.5 c, which is simple just double the distance travelled, and divide by 1.0635 to work out the time passed for the astronauts. Then another year for the acceleration and deceleration phases.

Time dilation can be the friend of science-fiction writers who want fast, slower-than-light travel. Find a friendly person who knows enough to do the calculations for you.

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  • $\begingroup$ Half lightspeed has a time dilation of 1.1547005383792517 not 1.0625. «Find a friendly person who knows enough to do the calculations for you.» he already has ☺. $\endgroup$ – JDługosz Jul 9 '17 at 10:50
  • $\begingroup$ @JDługosz Rodents! You can't rely on research papers these days. I took a figure given in a paper on relativistic travel and for half-lightspeed the time dilation was given as 16 yrs in motion to 17 in the rest frame. Shows I should have used your calculator instead. $\endgroup$ – a4android Jul 9 '17 at 12:21
  • $\begingroup$ @JDługosz Answer edited for corrected value of time dilation. Thanks for pointing out the error. $\endgroup$ – a4android Jul 9 '17 at 12:26
  • $\begingroup$ So what’s the 1.0635 come from? $\endgroup$ – JDługosz Jul 9 '17 at 19:27

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