# Help with physics...



## Artanyis (Jul 31, 2012)

Ok, I am an engineer, I understand physics on earth, and that's about as far as it goes.

I'm writing a sci-fi story that involves near light travel, and FTL travel based on a dimensional bridge theory.

Here is my problem, you have earth, (5.97219x10^24 kg) say 1 day (24hrs) has passed.  Now a Carrier ship out in deep space (812,837,527 kg) also spends 1 day (24hrs).  and a fighter craft (2032 kg) spends one day in deep space.

Each point of reference is outside of each others gravitational field.  Just by itself there will be a pretty significant time dilation, now it has been a while since I have actually needed to do calculus, so the math is a little beyond me at the moment, but from what I understand, as a ruff difference, if all three bodies are moving at speeds relatively equal to each other, just the mass alone causes time dilation relative to each other.  So the 24hrs earth time is only like 20hrs carrier time and only 17 hrs fighter time, because of its significantly lower mass it is aging much slower.  (this is a very general almost arbitrary time scale.)

Now considering the carrier uses a dimensional bridge to (think the movie event horizon without hell) it actually does not ever increase speed enough to warrant velocity based time dilation.  But the fighters get launched at .998 relative to the speed of light, coupled with the incredibly low mass, that means a lot of time dilation.

I guess my question is, does all this math and theory matter?  If I just make time a constant in the book will it add or subtract from the story?

And are there any physicists out there that can help with the math and dumb it down to general changes I could work into the book if need be?


----------



## Morkonan (Jul 31, 2012)

Artanyis said:


> ...Each point of reference is outside of each others gravitational field.



A gravitational field is infinite. However, the effects of gravity propagate at the speed of light. So, while we experience the gravity of a rock orbiting a star that is two million light years away, significantly reduced due to the inverse square law, we only experience the effects of two million years ago. If the rock is displaced, we won't notice the very minute and hardly detectable change for two million years.



> Just by itself there will be a pretty significant time dilation, now it has been a while since I have actually needed to do calculus, so the math is a little beyond me at the moment, but from what I understand, as a ruff difference, if all three bodies are moving at speeds relatively equal to each other, just the mass alone causes time dilation relative to each other.  So the 24hrs earth time is only like 20hrs carrier time and only 17 hrs fighter time, because of its significantly lower mass it is aging much slower.  (this is a very general almost arbitrary time scale.)



The difference is probably imperceptible with such masses. Even on Earth, people at different altitudes experience differences in time. You'll live your life a bit quicker if you live at a higher altitude, but it may take quite a long time, relatively speaking, for bottom-dwellers to clock any measurable advantage. While I don't know enough to do the math off the top of my head, there wouldn't be any noticeable hour-by-hour time dilation differences due to the _resting_ mass of the ships, themselves. Relativistic mass differences might be something to think about in terms of micro-seconds of time differentials if all the ships were traveling at the same relative velocity. Still, I don't think it would be any issue worth calculating, even for a hard science-fiction story. But, if you'd like, you can do the maths and submit it to a journal. 



> Now considering the carrier uses a dimensional bridge to (think the movie event horizon without hell) it actually does not ever increase speed enough to warrant velocity based time dilation.  But the fighters get launched at .998 relative to the speed of light, coupled with the incredibly low mass, that means a lot of time dilation.



Whoa, .998 c? You can figure out the actual time dilation, if you wished. Google "relativistic time dilation calculator" and you'll see all sorts that could apply to your specific needs.



> I guess my question is, does all this math and theory matter?  If I just make time a constant in the book will it add or subtract from the story?
> 
> And are there any physicists out there that can help with the math and dumb it down to general changes I could work into the book if need be?



I'm no physicist nor even well-versed in time-dilation differences and moving reference frames. But, I don't think that a few microseconds of difference is worth noting in a story for the sake of accuracy. However, there are some considerations you may wish to take into account as a storyteller.

Science Fiction fans love the idea of relativistic time differences. It let's them know that the story is authentic. Well, at least "hard science fiction fans" love such things. So, you may consider using part of that as a story or plot element. What happens in the months between a ship initiating a light-speed maneuver for a few days and then re-entering the same relative time, more or less, of its destination? Did an enemy vessel do anything particularly nasty within the local frame of reference? Would the fighters launched at .99 c have a ship to return to after their hours of flight which turned out to be days of ship-board time? Do the calcs and see if such differences are appreciable within the setting and events of your story. If they are and if they could provide you with plot hooks or interesting adjustments to your setting that hard science-fiction fans would enjoy, you may want to incorporate them.


----------



## Bloggsworth (Aug 1, 2012)

If you need these micro-second time difference to affect the course the storyline, then you will have to do it properly and speak to Dara O'Briain, the well known theoretical-physicist. If they are not to have a major effect in the telling of the story, then compared to the time taken for communications between the parties involved, they are insignificant, and going into detail for the sake of it in explaining why the message took 23 days and 0.0047934 seconds to reach earth is merely going to distract the reader, if not bore them. If they want to go into that much detail tell them to buy a book on astro physics. Yes, there will be a nerd or two who will send an email telling your publisher that you were 0.00095141457 seconds out in your calculation, to which the correct response is "Get a life, it's fiction..."


----------



## felix (Aug 1, 2012)

Good responses from above. 

I'll just bolster what Morkonan said; it's much better for the story to acknowledge time dilation effects (perhaps in passing on the subject how each ship must automatically calculate and account for such changes) but absolutely accurate time-difference calculations are unnecessary, seeing as a ship travelling at .998c is already hugely impractical for anything close to modern technology or economic means. 
Additionally, if you're using some kind of translight drive then you'll have already ticked off a lot of hard sci-fi fans, so no need to pander to them with arsey time-dilation calculations.


----------



## Newman (Aug 2, 2012)

The physics didn't impede Stargate SG-1. Or Star Trek.

Effects are "calculated into the equations."

Spock, adjust for time dilation. 

I've already done that, Captain.


----------



## felix (Aug 2, 2012)

The Stargate used an artificial wormhole, and the Star Trek warp drive uses the Alcubbiere-drive concept. Both concepts do not have relativistic consequences. 
But I get your meaning. 

If you're looking for space-opera, do as you please. If you want hard sci-fi then by all means adhere to the laws of physics, but it's perfectly fine to 'alter' the laws of physics slightly, or to invent your own discoveries in physics, yet to be made. So long as you keep it consistent, as laws of Physics must be, it's all fair game.


----------



## Bloggsworth (Aug 2, 2012)

In the various Star Treks they had radio waves travelling at a multiple of the speed of light, and remember, it's fiction! You can invent a machine to combat the effects of time dilation if it worries you.


----------



## Artanyis (Aug 2, 2012)

That was a lot more responses faster than I expected.  Thank you all for responding.

There is actually a really good reason Stargate never had time dilation, they almost exclusively traveled through a wormhole, a stable version of an Einstein Rosen Bridge so that technically they never came close to approaching light speed.  Startrek, they never had to deal with time dilation becuase they folded space around themselves and got pulled in the wake, sort of.  Its more like space moved around them instead of the other way around, meaning that they are actually not going faster than light, meaning they do not break relativity.  Read Gateway by Fredrick Phole, he used the same theory, although it's like book 3 or so before he actually describes it.  Now Star Trek should have experienced a small amount of time dilation whenever they were in between stars, where the combined effects of the solar systems gravity wells would be at their weakest.

Morkonan, thank you for your idea of using google to find a time dilation calculator, I had been using google to find papers on relativity and time dilation and learned a lot of interesting things.  I've had to combine results from a couple different calculators to get some of the information I was looking for.

So, the main thing is, the carrier never moves faster than a couple hundred km/s, which is fast but hardly fast enough to cause a noticeable time dilation.  It moves FTL kind of like they did in stargate, by creating a tunnel through time and space where there is no space in between.  The fighter pilots on the other hand, travel at .998C for a time to enter a battle and I was curious if that time dilation would be enough to influence the story, which is doubtful now that I know the dilation compared to being stationary is 15.8s, so for every second the fighter is at .998C the carrier experiences 15.8 seconds.  Which means that if the wing of fighters is in transit at .998c for 1 hour than back on the carrier 15 hours and 48 minutes has gone by.  1 hour also means the fighters traveled 1,077,089.1 km... that's a lot of distance.  That is more significant than I first thought.  Which means it really could influence the story, or I make the battle's closer together.  But that doesn't seem rational or fit the universe I am trying to create, so hours of time dilation it is.  Or at least a bit of happy medium.

The other thing I wanted to know is the math on the time dilation based on the reduction of proximity to major gravity wells.  That I fear will require more research and math that I will find a program to do for me.  The good part of that is since the story almost exclusively takes place on the carrier, it may not matter because I don't really need to refer to earth time, just ship time.

Thank you all for the help and I look forward to anymore constructive information I can get.


----------



## Artanyis (Aug 2, 2012)

Okay... I just answered my question.  General Relativity does not properly apply to the Carrier becuase it has Artificial Gravity equal to Earth.  The time dilation does not occur based on mass but on the influence of gravity.  The carrier uses artificial gravity equal to Earth, so 1G.  So regardless of the mass of the carrier or the actual extent of the gravitational filed around the carrier the gravity that effects the carrier and its crew means that that the time dilation experienced on the carrier is a factor of 0.

Yay, breaking physics, localized gravitational field without mass.  I need to decide how that science works.  Gravity being an effect of mass upon mass, the drawing of strong forces.  So how would you generate a gravitational field without mass?  The concept of using superdense particles is out becuase that simply increases your mass, but not your size.  So we either need to reduce mass without changing the gravitational field or increase the gravitational field of mass.  But it also needs to be contained so as to not have an adverse effect on small objects... like the fighters that enter and exit the ship.

Edit Note:  I had the general relativity of gravitational bodies backwards.  When you move out of the influence of a gravity well time speeds up, which means in deep space time is going faster for you becuase you do not have the slowing influence of gravity.  As basic as it gets, as velocity and gravity increase so does time dilation.  So, the fighters will experience less dilation then previously noted becuase they are small and have no almost gravity.  But it will still be a significant amount of dilation becuase of the time spent at .998c.


----------

