Traveling at the speed of light and time dilation

[DHF]

hey guys,

I need some tech support for a spaceship in a story that is currently in the works :)
Assuming the characters could get their ship up to the speed up light in real space (no hyper drive or warp drives) What would the effects of time dilation be on the crew? Time slows down as you approach the speed of light but what happens when you actually reach that barrier? does time stop for the crew? A trip to Alpha Centuri would take 4 years to the outside world but to the crew how much time will have passed?


Thanks for the help

 

[phinds]

... Time slows down as you approach the speed of light but what happens when you actually reach that barrier?

You don't reach it.

 

[Ryan_m_b]

It's impossible for objects with mass to attain the speed of light. You're question is therefore unanswerable because it essentially asks "if we ignore the laws of physics what do the laws of physical say?"

 

[micromass]

hey guys,

I need some tech support for a spaceship in a story that is currently in the works :)
Assuming the characters could get their ship up to the speed up light in real space (no hyper drive or warp drives) What would the effects of time dilation be on the crew? Time slows down as you approach the speed of light but what happens when you actually reach that barrier? does time stop for the crew? A trip to Alpha Centuri would take 4 years to the outside world but to the crew how much time will have passed?


Thanks for the help

You can't go the speed of light or faster.
That said, you can get to Alpha Centauri arbitrarily fast if you go close enough to the speed of light. So if you go fast enough, then you can get there in just a minute or less. Of course, to people on earth, it will have taken years.

 

[Fieldwaveflow]

The best books

I've read some science fiction

You may want to stick with some of the science fiction I've read. Ships (and the people in them) have an almost instant trip time (t1) while traversing the distance of 4 light years (d) as they move 4 years (t2) into the future.

Hard to comprehend. It's not a standard way of thinking.

 

[DHF]

ok good to know. so reaching the speed of light would never happen but the crew could reach 99.99% the speed of light, in which case from their perspective they would only be in transit for a few minutes.

 

[Bandersnatch]

This handy calculator might be of some use to you:
http://mysite.verizon.net/res148h4j/javascript/script_starship.html

 

[Ryan_m_b]

I've read some science fiction

You may want to stick with some of the science fiction I've read. Ships (and the people in them) have an almost instant trip time (t1) while traversing the distance of 4 light years (d) as they move 4 years (t2) into the future.

Hard to comprehend. It's not a standard way of thinking.

I first encountered this in Ken MacLeod's Engines of Light trilogy. Ships essentially have a jump drive but rather than being instant like most other science fiction universes it takes exactly 1 second to jump 1 lightsecond but is instant from the perspective of the crew.

ok good to know. so reaching the speed of light would never happen but the crew could reach 99.99% the speed of light, in which case from their perspective they would only be in transit for a few minutes.

Actually at 99.99% 4 lightyears would take about 3 weeks. Many science fiction worlds use near-light speed travel but you have to consider things like how these speeds are achieved (try plugging in 0.99c into the relativistic rocket equation and you'll quickly realize how unrealistic the fuel requirements are), what protects the crew from radiation (which gets worse and worse the faster you go) etc etc.

 

[cyberdiver]

If you DID (somehow) reach the speed of light, γ = ∞, which means your mass would be m*∞, the light in coming toward you would be blueshifted by a factor of ∞, the shipboard time would be slowed by a factor of ∞...you get the idea (everything changes by a factor of ∞ relative to whatever you choose to call stationary.)
This website should explain why it's impossible for any matter object to reach the speed of light: http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/Special_relativity_adding/index.html

 

[DHF]

yeah that might not be so cool. I wanted to give them a method of travel without warp bubbles or any of the other classic sci-Fi devices but as you all pointed out, traveling in real space would be packed with obsticals. not just the radiation but I imagine the sadness they would have if they passed through a dust cloud.

thanks for the info and calculators. I will try to figure an alternate method of travel for my intrepid crew.

 

[cyberdiver]

Relativistic travel can be quite fun. For example, there's Stephen Baxter's short story Pilot available in the public domain here which explores the real extremes of relativistic travel.

Edit: The dust problem can be solved with a dust shield, but a dust shield can only erode so much.

 

[Ryan_m_b]

I will try to figure an alternate method of travel for my intrepid crew.

What's the general plot of the story? Perhaps we can help come up with a suitably scientific plot device. For example; if the story takes place in another system you could have them getting there very slowly (i.e. in a ship that's only boosted up to 1% light speed) whilst traveling in some form of suspended animation.

 

[DHF]

the plot of the story is about the first manned flight to Alpha Centuri. I haven't set a date for the tale yet but I figured it was several hundred years from now. The story will center around the characters's reactions to being separated from their native time. the concept of only a few days passing for them yet years or decades have escaped their notice on Earth. I didn't want warp drives or other magical forms of tech because I didn't feel we would be there yet. I wanted to give Earth advanced tech by our standard but still grounded by our current understanding of physics. That being said, thanks to the Calculator Bandersnatch provided (thanks man you are always helpful) I realize the utterly absurd amounts of energy required, so unless the crew has a ship powered by their own personal sun, I don't think my original idea will work.

Two possibilities I can think of involve :
- pushing the ship into an alternate dimension during travel where it would not be subjected to the laws of physics.
- Wrapping the ship in some some sort of energy field that gives the ship 0 mass during the acceleration period.

Both of those ideas however dance happily into the realm of magical technology so I wanted to avoid that.
Taking your advice Ryan_m_b I could put them in suspended animation and move them at 2% of c. that would get them there in about 400 years. it would require a near sentient computer system to pilot the ship on such a long journey but I think that is a lot more feasible then getting a ship to the speed of light.
Although if I go that route I would have to figure if the government of the time would be willing to spend the money and resources for a venture that would not benefit their nation for centuries. Of course it also opens the door for the crew to spend 400 years in stasis and when they wake up, Humanity is already their, having invented faster means of travel while the astronauts slept.

possibilities.

 

[cyberdiver]

Perhaps the government could be motivated by some threat in Sol, and so start throwing together colony ships to spread humans around.

 

[Fieldwaveflow]

dust cloud

...sadness they would have if they passed through a dust cloud.

thanks for the info and calculators. I will try to figure an alternate method of travel for my intrepid crew.

dust... The ship would sort of be in a time bubble. Anyone looking at the ship might see it as a small, fast moving black hole. Maybe since the ship is near light speed the ship density would be altered to allow it to plow through anything non-relativistic. Or maybe it would just be a very quick trip into oblivion. I'd like to see how a ship near c would decompose on impacting an object. May just look like what happens in atom smasher, not just vaporized but decomposed to destabilized particles of energy. How would you ever turn with that much momentum? What power source could possibly produce the energy needed? You'd have to carry tons of extra mass as fuel even if you could get every last proton of the material to convert to energy. You couldn't see anything, let alone dust, because most of the visible light would be reduced to a little point in the front of the ship from lensing.

 

[DHF]

that's an interesting idea, The ship moving at near light speed would have near infinite mass but does mass translate into density? and the dust and particles they plowed through,seeing as how they would be impacting the hull at near light speed relative to the ship, would those particles impact them with the same amount of energy as they were spending to move forward?

 

[cyberdiver]

The ship would have 'near-infinite' mass relative to a stationary observer. About the particles, probably not (1 hydrogen atom/cm^3).

 

[Karden]

If they are successful at someday building this "warp drive": [http://www.newscientist.com/article...ising-a-starship-warp-drive.html#.UhLUIWT70pc ], wouldn't this counteract the effect of time dilation to some extent? I've always thought of time dilation as related to the reduction of space-time density as speeds increase, but since this drive would compress space-time in front of the ship, seems like it would reduce the time dilation effect to some extent.

That would seem to modify the amount of information in the universe during the duration of the flight (for reasons that are too complicated for me to go into, but if you get it, you get it), so seems like there's some special circumstance math that would have to be included in order to calculate time dilation effects for this type of spacecraft .

Another thing that bugs me about the idea of warp drives like this are that the density of space-time is correlated with mass. If they are compressing space-time in front of the ship, are they circumventing the "infinite mass" problem?

 

[Karden]

Just realized another thing that bugs me about this particular warp drive design.

If it compresses space-time in "front" of the ship, and decompresses "behind" the ship, I think this would actually propel the ship *backwards*. Not only does nature abhor a vacuum, but so does space-time. It demonstrates this by propagating the information that represents coherent structures (like spaceships and atoms) from denser regions of space-time to less dense regions, across vast distances. This attribute of information propagation through the switching fabric of space-time is often referred to as Gravity.

 

[DHF]

Relativistic travel can be quite fun. For example, there's Stephen Baxter's short story Pilot available in the public domain here which explores the real extremes of relativistic travel.

Edit: The dust problem can be solved with a dust shield, but a dust shield can only erode so much.

Thanks CyberDriver,

That was a good read. It did a good job of trying to stick to realistic concepts. One of the weak spots I noticed however was fuel. The author accounts for this by noting that the fugitives are cannibalizing their planet for fuel but the missile on the other hand has no such fuel source so how does it continue to pursue them?

The other inconsistency I noted was their acceleration. They say the missile is using an Earth made drive which limits it to 1 G acceleration. the missile later updates itself to 2Gs, we can accept this because the dialog suggests the missile is reinventing itself beyond its original limits. but then our fugitive heroes casually accelerate to 2 Gs to match the missile, later there is talk that they are traveling at 1000 Gs acceleration. You might say that they can afford such acceleration because they have massive amounts of fuel but that leads me back to my question about how the missile can keep up.

 

[cyberdiver]

Exactly. It illustrates relativistic effects well, but it just surpasses my willing suspension of disbelief threshold by several orders of magnitude. When they mention accelerating at 1000 Gs, I forgot to drop my jaw as well.

 

[DHF]

Yeah 1000Gs was pretty ridiculous, he could have played the same story at 1G, with the Missile increasing by small increments. 1.1G would still allow the missile to catch them, so with much effort they manage to go 1.15, and the missile eventually moves just a little faster ect.

At 1000Gs time looses all meaning for the crew. The main character speaks of the chase lasting years and I assume she means outside time because relatively the entire journey would only last a few days. The trip all the way to Andromeda would only take 15 days or so. at that speed time bogs down so much that going to the next galaxy or going a hundred billion light years would only be a matter of days to their reckoning.

Yet at the end of the story then mention the journey to Andromeda taking 2 months, that wouldn't be correct if they were still accelerating at 1000Gs and by the end they are going several magnitudes faster then that because of the black hole.

...
really, the solution was there from the beginning. when they noticed that the missile was accelerating towards them at 1G they could have just tossed a chunk of ice directly in their path. it sounded like neither the missile nor the dwarf planet had any real maneuverability so if they chucked a bolder behind them as they were going .25G, the missile, traveling at 1G would have smashed into the bolder and likely been destroyed.

my story would have been over very quickly. I suppose his way was a lot more interesting :)

 

[Fieldwaveflow]

I'm wondering

To an observer; what would be the apparent size of the relativistic ship? Aside from not being able to see the spaceship due to high energy concentration space-time. The place where it is would be would appear very small from length contraction.
From the missile's perspective...well, the sensors would need to correct for the light warping effects when traveling near the external c by using some de-lensing algorithms for the onboard computer and sensors. Then it might see a faint dark spot in a bright back ground to go after. You may need to take into account how much light is coming out of the back of the ship from propulsion.

That is cool. The idea of a missile trying to catch up to something near light. The missile would need to put out more energy to the point of going faster in time to catch the ship. And of course the ship would need to be traveling to a far distant destination to allow the missile time for it's faster rate of time to catch up to the ship before it reached the destination...The ship would have to decelerate at some point. That would mean doom. Unless they had enough fuel to outrun the missile or maybe alter course to put an asteroid between the ship and the missile. I wonder if the missile stayed at near c when it got to a planet destination if it wouldn't just pass through the planet since it would be so small and the density of a planets mass would be like a gas compared to the missile. The missile might see itself just zip right through a planet while those on the planet saw a black hole headed their way for hundreds of years.
Sorry, hard not to think of the other implications of a missile doing 1000g while trying to catch a ship near c.

 

[DHF]

Fuel is something that the story does not take into consideration at all. The protagonists's ship is a dwarf planet and they are using the mass of the planet itself to fuel their acceleration. there is no mention of what the missile is using for fuel but it is mentioned that it is using the same type of engine so the dialog clearly indicates that large amount of mass have to be fed into the drive to continue accelerating. by the end of the story the main character describes her world as having been reduced to a splinter after years of feeding the engine. yet the missile is never mentioned as having any sort of unlimited fuel.

This story does bring up another interesting question though. If a ship is traveling to Andromeda at just under the speed of light, in actuality it would take 2 million years to get there. Relatively however it would take days. Would the ship need 2 million years worth of fuel to feed the engines or would they only need to keep the engines fueled for 15 or so days, assuming you could accelerate to and survive the 1000Gs proposed in the story.

 

[Drakkith]

This story does bring up another interesting question though. If a ship is traveling to Andromeda at just under the speed of light, in actuality it would take 2 million years to get there. Relatively however it would take days. Would the ship need 2 million years worth of fuel to feed the engines or would they only need to keep the engines fueled for 15 or so days, assuming you could accelerate to and survive the 1000Gs proposed in the story.

Fifteen days seeing as how the trip lasted that long for the ship. And that assumes the engines are constantly being fed the same amount of fuel. If not, then you cannot measure the fuel in "days".

 

[cyberdiver]

Here's an alternative way to think about their post-flyby predicament: They are not moving; Andromeda is rushing toward them at an obscene fraction the speed of light. To put this into perspective, scientists had a problem once: they calculated that muons created in the upper atmosphere should be decaying before they reached the ground. The solution was that time dilation was slowing time down for the muons (relative to the scientists), allowing them to reach the ground. From the muon's perspective, the Earth would be rushing toward them at an obscene fraction of light, and therefore would be Lorentz-contracted, so from its perspective, would be a shorter distance to travel to the ground than without relativity. This site contains a visualisation of the effect.

Time would run slower aboard Chiron, so all processes occur slower relative to stationary. Therefore, the engines would only need fuel for the amount of on-board time they are burning. I guess the story implies that they burnt up everything at the black hole, though.

 

[DHF]

Time would run slower aboard Chiron, so all processes occur slower relative to stationary. Therefore, the engines would only need fuel for the amount of on-board time they are burning. I guess the story implies that they burnt up everything at the black hole, though.

That would make a lot of sense but then she ruins it by saying that years of reaction mass plundering left their world little more then a splinter. yet I read the story over and they spent very little time at the lower accelerations, there is reference to a few months of chase before they were forced to accelerate at 1000Gs. the rest of the story takes place at that acceleration which means they should have reached their maximum fraction of c pretty quickly. at that speed the entire chase would last days.

The time issue is the only thing I guess you can really nitpick because the fuel and acceleration dance so far beyond disbelief that we just have to nod and accept that its what makes sci-Fi go. after all moving a planet at the speed of light is hardly the craziest thing I have seen in sci-fi. Blowing up a sun with a single missile has to take the crown for that one.

 

[cyberdiver]

after all moving a planet at the speed of light is hardly the craziest thing I have seen in sci-fi. Blowing up a sun with a single missile has to take the crown for that one.

What about blowing up a star with a handgun? (Blue Shift, Vacuum Diagrams)
Typical Baxter. Overkilling everything.

Is there any help needed with the story by the way?

 

[DHF]

Given what we covered so far about the difficulties in accelerating to the speed of light, I think I am going to abandon that Idea and go with the suspended animation theme. The story will take place within the next 300-500 years but I think I will leave the date vague to give myself flexibility. The Engine will either be Cold Fusion or Anti Mater powered. not sure which would be more fiesable. Given what we talked about with Baxter, what do you think a reasonable acceleration rate would be? It is Quarter G acceleration within the realms of possibility given a few hundred years of advance? Depending on what I can realistically get away with, I will have the crew in suspended animation, I don't like the idea of cryogenics but I can't come up with any other alternatives. The ship will be automated and run by a near sentient computer system. I have no plans for a HAL, The computer is well aware that it is a machine and has no ambitions or feelings, its just interested in following its directives which include protecting the crew. This might lead to complications of the crew want to do something dangerous that the computer was not programmed to anticipate. depending what kind of acceleration and speed they can achieve, I will decide how long they are in stasis and that will determine what awaits them.

 

[cyberdiver]

A constant 0.01 g acceleration via laser sail (30 MW/kg) should get you up to 0.21c at the halfway point between Sol and Alpha Cen, according to the relativistic starship calculator (posted previously). Then, a magnetosail can be employed to brake against interstellar plasma, then stellar wind. Total trip time: 41 years. That's just one-way though; for a return trip, you could use some kind of antimatter-induced fusion rocket and brake with the magnetosail again. Accelerate at say 0.1 g for 348 days should get you up to 0.1c for return.

 

[typical guy]

I'm a fan of sci fi books and a favorite theme is that of the "sleeper ship" so to speak. They freeze the crew and later thaw them out when they arrive at their destination. The crew arrives at their destination thousands of years later to find that the planet is already populated by billions of people because humans found some loophole that allowed them to "cheat" the speed of light (wormholes or whatever the idea dejour is). The crew of this "highly advanced " ship now have really old technology and no way to make a meaningful living with their skills.

I think the same kind of argument could be made for near light speed travel. If you're traveling for a few weeks your time to go across the galaxy, the rest of the galaxy is experience a great deal more time and if there is some FTL loophole possible, not matter how remote, it will eventually be exploited given enough time. This journey would end rather sadly for the occupants.

 

[cyberdiver]

I wouldn't call it certainty, though.

 

[DHF]

I like the aspect of the crew waking up to find that their civilization had passed them by. I have seen and read it before and I wanted to model aspects of my story after that. A season one episode of Star trek TNG comes to mind where they find a trio of 20th century folks in cryo status, they revive them, pat them on the back and integrate them into 24th century society. personally I agree with you that it wouldn't be that easy. imagine if you transported someone from the 16th century to our age. the culture shock would be physiologically crippling and I think adaption would be difficult. not just for him but for the current age to accept such a throwback.

as I said I wasnt fond of cryogenics and originally wanted to achieve the same effect by having them skim the speed of light for a few weeks subjectively only to find centuries passed on the outside but as we already covered, traveling at such speeds become very unrealistic unless you are pictching the story millennium down the road and limitless forms of energy are at our disposal.

 

[cyberdiver]

If you want to go relativistic, you could use a Bussard ramjet using CNO-catalysed hydrogen fusion. It's been studied, and is probably possible. IMHO, a story that occurs 300-400 years in the future should be allowed a bit of a fudge factor.

 

[DHF]

thanks, that gives me some ideas. What about Radiation? would this be something I would have to develop a mechanism for? I have heard that as you approach c, radiation impact increases but since radiation is impacting a resting mass from all directions doesn't that mean that as you moved forward, the radiation impacting you from behind would be lessened? would the decrease in radiation from behind you offset the increased energy of the radiation impacting you from the front or would the forward radiation's impact increase greater then it was decreasing from behind?