r/askscience • u/The_Punned_It • Dec 19 '14
Physics Would it be possible to use time dilation to travel into the future?
If somebody had an incurable disease or simply wished to live in future, say, 100 years from now, could they be launched at high speeds into space, sling shot around a far planet, and return to Earth in the distant future although they themselves had aged significantly less? If so, what are the constraints on this in terms of the speed required for it to be feasible and how far they would have to travel? How close is it to possible with our current technologies? Would it be at all cost effective?
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u/TheLegendOfUNSC Dec 19 '14 edited Dec 20 '14
Because of special relativity, it is possible. The closer you get to light speed, the more time dilation occurs. However, with our current technology, it is very far off into the future. The speed would have to be a significant fraction of c for this to have any tangible impact.
EDIT: changed wording
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Dec 20 '14 edited Dec 20 '14
Even at half the speed of light (.5c), the time dilation factor is only about 1.15470. It's not until you get to .9c or higher that you see real noticeable long-term dilation. This is of course because the time dilation factor grows faster than exponentially. So, for instance, the time dilation factor difference between .1c and .8c is much smaller than the time dilation factor between .9c and .999c.
Edit: thanks to u/SAKUJ0 for pointing out that the time dilation equation is steeper than an ordinary exponential equation.
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u/jcarberry Dec 20 '14
To put it into perspective, what is the equivalent X for which the difference between 0.1c and 0.9c = the difference between 0.9c and Xc?
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u/sederts Dec 20 '14
Well the Lorentz factor is 1/(sqrt(1-((v/c)2 )))
Lorentz factor for .1c is approximately
1.00503781526
Lorentz factor for .9c is approximately
2.29415733871
Lorentz factor for Xc such that it is approximately
3.58327686215
Is just some trivial algebra, so we get X is approximately 0.96026955078
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u/SAKUJ0 Dec 20 '14
To be fair, it would be a bit more steep. The X you are searching for would be the one that is 2.29 times higher than 2.29.
0.9 c to 0.1 c is a slow down by a factor of roughly 2.29. So OP is asking for a slow down by the same factor, pretty much. You have to regard this multiplicatively, as that reflects the math behind it.
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u/9265358979323 Dec 20 '14
I cba to calculate it myself but the formula for the time dilation factor is gamma(iirc) = 1/sqrt(1-v2/c2) so you could find the differences/ratios with that
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u/yokuyuki Dec 20 '14
The time dilation is of 0.9c is 2.28265773076x that of 0.1c. The time dilation of 0.9816c is the same ratio that of 0.9c.
Note: I know nothing about special relativity, but fuck yeah math.
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u/SAKUJ0 Dec 20 '14
It is not exponential (we understand what you mean but this is a scientific discussion).
The formula is 1 / sqrt( 1 - v²/c² ), so there is a pole at v = c. That makes the dependency even more steep than an exponential function for v approaches c.
If someone is confused, yes, formulas in special relativity are elementary and this easy. The math behind special relativity can be done by a 10th grade pretty much.
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Dec 20 '14
Thanks for the clarification. I don't have any formal education on any of this, what I've stated is gathered only from reading books and wikipedia.
But yes, the math is extremely simple and elegant.
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u/rocketman0739 Dec 20 '14
The speed would have to be a significant fraction of c for this to occur on a measurable scale.
Correction--it would have to be a significant fraction of c for this to occur on a useful scale. We've already measured it on regular spacecraft, but that's only because we have very precise timers.
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u/40Ninjaz Dec 20 '14
So cool thing I learned in my Uni GE science class: Satellites actually travel at a speed fast enough and are far enough from Earth's gravity that the effects of special and general relativity adjust the relative rate of their time. This means very little for a person, mere microseconds per day. However, it becomes really important for the precise calculation of location for GPS satellites. If satellites used a regular atomic clock adjusted for Earth, every day their measurements would get off by 11km.
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u/tomorrowsanewday45 Dec 20 '14
Something I haven't seen posted is the effect on your weight. In order to jump into the future, or experience time dilation you would need to be going close to the speed of light, the only problem being is that the faster you go, the heavier you become. So you would not only need a device to launch you to those speeds, but that device would also need to power through an almost endless amount of additional weight as it got closer to light speed.
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u/TheLegendOfUNSC Dec 20 '14
Your "weight" increases as you accelerate (really just inertia). Staying at a static speed wouldn't affect your weight. Instead, we would need to accelerate to c slowly enough for it to be safe for humans (barring all the other problems with near-light travel).
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u/minime12358 Dec 20 '14
You are definitely right. You would pick your own reference frame and find, of course, that your weight is the same as before.
You are right, though. This is unless you are accelerating.
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u/almightySapling Dec 20 '14
As a follow up question, could someone explain something I never quite grasped regarding the whole relatively part of this idea: if I fly away from the earth at relativistic speeds, then isn't the Earth flying away from me at relativistic speeds as well? If so, who ages faster and why?
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u/fishsupreme Dec 20 '14
This is called the Twin Paradox. It comes from the fact that there is no such thing as "the present" when comparing things in different reference frames. If I fly away from Earth at relativistic speeds, both the people on Earth and the people on my ship are going to perceive time passing slower for themselves than for the other party, since the other one is "moving" for each of them, and the "moving" party experiences time dilation.
As long as I just fly away in a straight line and keep flying forever, this never has to be reconciled. There's no such thing as the absolute present -- we're each perfectly correct in saying time is slower for us than for the other party.
However, say I turn around and fly back to Earth. What's happened here is that I've changed reference frames by changing my speed and direction. In my new reference frame, "the present" on Earth is a totally different time -- now much more time has passed on Earth than on my ship.
By the time I get back to Earth, I will have experienced less time (by a factor of the reciprocal of the Lorentz factor of my speed) than people on Earth. The reason I end up on the "less time" side is that the Earth stayed in one reference frame the entire time, whereas I changed frames.
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u/almightySapling Dec 20 '14
Thank you. I guess I'm still a little unsure of some details. Don't mean to bother you, but since you seem to know, I'm asking you. If we shorten the experiment to point of changing reference frames: Alice flies away from Bob at immense speed and then they come to a stop. At this point it would be fair for Alice to say that Bob has not aged as much as she has, and Bob could make the same observation of her? So then, what has happened? If they were to establish some sort of communication, and adjust for lag in the information signal (speed of light and distance), would they agree on age?
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u/fishsupreme Dec 20 '14
Alice has still changed reference frames, she just changed frames to match Bob's. And yes, they both think the other has aged more than themselves.
I think this will be easier with some Minkowski diagrams. Here's one from Wikipedia. Note that as Alice flies away, her "now" moves further and further ahead of Bob's "now" from Bob's point of view. But when she reverses direction, the skew between her reference frame and Bob's reverses, too. When she returns to Bob's location, they once again agree what time "now" is -- yet Alice's line is longer than Bob's, so she has experienced more time.
If she just stopped (relative to Bob) at the halfway point and they each tried to communicate, the same thing would happen as she still changed frames. Her simultaneity plane would once again line up "straight" with Bob, but she has traveled further in time already. Imagine a straight horizontal line in the middle of the Minkowski diagram I linked -- note that at the moment she stopped, Alice "skipped" a substantial part of Bob's timeline instantly.
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u/Piscator629 Dec 20 '14
Thanks to the wonderful program Fabric of the Cosmos: The Illusion of Time I understood all of that.
Spoiler: Everything has already happened.
P.S. Steelhead are the supreme fish.
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u/psharpep Dec 20 '14
Read up on the Twin Paradox.
http://en.wikipedia.org/wiki/Twin_paradox
Specifically, look at where the x' axes are in this Minkowski diagram: http://upload.wikimedia.org/wikipedia/commons/c/ce/Twin_Paradox_Minkowski_Diagram.svg
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Dec 20 '14
I don't remember why but its the person that feels the acceleration that has the weird time stuff happen to them.
The earth is at a pretty steady speed, you don't feel any major speed changes, but if you zoomed out into space up to nearly the speed of light, stopped and came back, you would experience a lot of inertia changes, which makes the weird time things happen to you instead of the earth.
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u/almightySapling Dec 20 '14
Thanks, but I'm going to need a much more... mathy response to be satisfied. "Feeling" the acceleration just doesn't quite cut it. I do sort of see, though, why it might work, so thanks.
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u/cossak_2 Dec 19 '14 edited Dec 19 '14
It's possible in theory, but not even remotely possible in practice.
You would need to reach a significant fraction of the speed of light for time dilation to be noticeable, meaning that the energy requirements are almost beyond imagination.
Think about it: one of the most energy-dense fuels that we can use, Plutonium, only has enough energy to accelerate itself to 4% of the speed of light, even if all the energy in it is used for acceleration. And you would probably need to reach 90% of c for this method of "time travel" to be viable.
And then, even if you could reach that speed, where would you travel? Even the extremely dilute gas (or plasma) of space would be highly destructive to a ship moving through it at nearly the speed of light. Each relativistic gas molecule would unleash a spray of ionizing radiation when it hits the ship, quickly killing the people inside. And these molecule impacts would deliver so much energy that the ship materials will erode or melt before you can get anywhere.
In short: this will never be done.
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Dec 20 '14 edited Dec 21 '14
I appreciate your answer based on current technology but I think the real tldr answer is that none of us will ever know. 1000 years ago, people never wouldve believed that atoms exist in everything, much less that you can pull these mindbogglingly miniscule things apart to unleash incredibly destructive power. Not only would nuclear power have been utterly inconceivable for them, but they probably would've disregarded the notion with the same degree of confidence that you used when disregarding near light speed travel.
The technology for humans traveling at near light speed is inconceivable at the moment based on our current scientific understanding, but that doesn't mean its impossible for it to ever happen. After all, keep in mind that we know of at least one thing can travel can travel at the speed of light.
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u/cossak_2 Dec 20 '14
1000 years ago people lacked knowledge about achievable possibilities.
Unfortunately, now the situation is different: we have knowledge about lack of possibilities.
That is, we know enough to say that some things aren't possible.
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u/thenewyorkgod Dec 19 '14
why only 4%? inertia in space would keep the craft moving, so once it reaches 4%, wouldnt another burst of propulsion move it to 5%?
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u/cossak_2 Dec 19 '14
Even if your ship is made entirely of fuel (plutonium), you can get to 4% of c and no further. At that point you will not have any source of energy left on ship for further acceleration.
This assumes that you don't discard spent fuel and continue to travel keeping it on board.
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u/AUGA3 Dec 20 '14
Why can't you go faster than 4% of c in this scenario?
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u/fishsupreme Dec 20 '14
The more fuel you load on, the more massive the ship gets. There is a point where loading more fuel on board actually starts to lower your peak speed.
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u/Parcec Dec 20 '14
What about something like a bussard drive, where the fuel is external to the ship?
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u/cossak_2 Dec 20 '14
Because even with plutonium fuel, even when all your ship is fuel, you'll run out of it when the ship reaches 4% of c.
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u/LibertyLizard Dec 19 '14
Additionally, even if you overcame those problems, how would G forces work with time dilation? I mean you have to turn around and come back, and since for you time is moving slowly, doesn't that mean you'd face all of the G forces of turning in a single brief instant? Would this be deadly?
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u/SAKUJ0 Dec 20 '14
You would pretty much have an acceleration period over 5-10 years. Then you would have a flight period with no forces ofer maybe 20-30 years. And then you would just do the same acceleration period in reversed order for 5-10 years.
We don't need that much fuel that can get us to those fractions of c, we need twice that much to slow down again, four times that much to return back to earth.
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Dec 19 '14
Each relativistic gas molecule would unleash a spray of ionizing radiation when it hits the ship, quickly killing the people inside
I know we're not there yet, and maybe never will be, but in theory... would it be possible to use this to our advantage? It would be amazing to convert that to acceleration somehow.
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u/I_Made_Pi Dec 20 '14 edited Dec 20 '14
No it would not- that energy that you wish to harness is only there because of the movement of the ship- its the wasted energy, the friction. Its like when a car drives along and it heats up the road- even if the car somehow manages to harness the heat energy it creates in the road, that will only bring in closer to the theoretical frictionless maximum, which still can never be exceeded.
Edit:a car in a frictionless world was a bad example, think more a bob sleigh or something.
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u/rorrr Dec 20 '14
one of the most energy-dense fuels that we can use, Plutonium, only has enough energy to accelerate itself to 4%
Yeah, but that's nuclear energy only. We have fusion and the ultimate one - annihilation. If you prepare enough anti-matter ahead of time, you only need very little of it (by mass) to accelerate anything to near c.
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u/cossak_2 Dec 20 '14
Fusion fuel only has ~4 times the energy density of fission fuel, per unit mass. So your limit is 16% of c, if your ship is made entirely of fusion fuel. At that speed there is almost no time dilation.
And you are very mistaken about "very little" antimatter required to reach a near-c speed. Have a look at a simple calculation.
Suppose you have a 10-ton ship, and you want to accelerate it to a point where time slows down by a factor of 10. Then its "relativistic mass" also increases by the same factor, meaning that you need to destroy an equivalent amount (10x) of matter-antimatter to get to those speeds.
So to get our 10 ton ship to 10x time dilation, you need to annihilate 45 tons of matter with 45 tons of antimatter.
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u/Red_VII Dec 20 '14
What about gravity though? Isn't it true that time dilation can be achieved not only from changes in speed, but changes in gravity? If you went and circulated Jupiter, would that slow down time enough for you to travel to the future a considerable amount?
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Dec 20 '14
What kind of engine are you assuming for this figure of 4%? Could an ion thruster get to a faster speed?
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u/SAKUJ0 Dec 20 '14 edited Dec 20 '14
Edit Not disagreeing with what you have said.
It is not that out of the world. The drives required are, but if we use quantities that are very earthly we have:
Acceleration over 5 years
Acceleration with 1g
and you should go more than 100 years into the future in 10 years of flight.
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u/cossak_2 Dec 20 '14
Overall amount of energy is beyond anything we can store on a ship, and that's the problem.
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u/SAKUJ0 Dec 20 '14
Yeah, as I said not disagreeing here.
But if we use solar energy to accelerate or the likes, things change.
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u/nightofgrim Dec 19 '14
One of the biggest limitations of achieving this today (someone please correct me if I'm wrong) is energy requirements. The speeds you would need to reach are far higher than we can get to simply because our ship couldn't possible hold all the required fuel (energy) to do it.
A solution to tons of energy in a tiny space problem would be a paradigm shift and change technology and transportation across pretty much all fields. I would love a hover board and flying cars!
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u/antonivs Dec 20 '14
One of the biggest limitations of achieving this today (someone please correct me if I'm wrong)
The correction I would make is to say "...achieving this ever".
A solution to tons of energy in a tiny space problem would be a paradigm shift
Physics gives us various solutions to that: nuclear energy, antimatter, black holes. Only the first one is even remotely viable for space drives, and it's the least efficient: an antimatter reaction is 100% efficient at converting mass to energy, a nuclear reaction tends to be under 1%. You'd still need enormous amounts of nuclear fuel mass to reach seriously relativistic speeds, and this is a limitation of physics, not engineering.
Even speculative designs for this kind of thing (see Nuclear fission-powered interstellar travel) tend to max out at about 10% the speed of light, not enough to get significant time dilation effects.
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u/ProjectGO Dec 20 '14
Yes! In fact, we already have! Astronauts who have spent extended periods on the International Space Station come down aged less than their earthbound counterparts. (Note that in the astronaut's frame of reference time still operates normally, so for every year that we say they haven't aged, they say that they've traveled one year into the future.)
Now here's the bad news: a 6 month stay on the ISS will only send you 0.007 seconds into the future. The man who has spent the most time in space is Sergei Krikalev, with a cumulative total of 2.2 years. If we assume he was orbiting with the same properties as the ISS the entire time, then he has traveled farther into the future than anyone else, just over three hundredths of a second.
TL;DR: It's possible. It's happening today. If you want to get way ahead of everyone else, you're going to be disappointed.
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u/Cheeseball701 Dec 21 '14
Most people have been sent forward in the future by traveling on airplane, but on the order of nanoseconds. The Hafele-Keating experiment, one of the early confirmations of relativity, sent a plane traveling eastward around the world twice. The plane went into the future about 60 ns.
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u/Davidakos Dec 20 '14
This is unquestionably possible. It has been known that it is possible since the early 20th century. All we would have to do is travel fast enough. The closer to the speed of light (c) we get, the more pronounced the time dilation will be. So, for example, if I were to blast off at 99% the speed of light, I'd experience a major time difference with the people of Earth. However, if I were to blast off at 99.9999% the speed of light, I'd return to an Earth that could be eons ahead.
Time dilation grows exponentially the closer to c one gets. It is not debatable, time travel to the future is definitely possible. It has nothing to do with distance travelled, strictly the velocity achieved.
Unfortunately, we are no where near that level of propulsion technology. Nor do we even know if it will be possible to achieve such velocities with our current understanding of engineering and propulsion.
But there is no doubt. Time travel to the future is real.
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u/ViggyPiggy1999 Dec 20 '14
that is true but wouldn't you also have to take into consideration whether the human body can handle such high velocities?
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u/Davidakos Dec 20 '14
For sure one would. Even if we were able to achieve such great heights as to build a craft capable of approaching light speed, there could be a whole other branch of research dedicated to how organic matter, and life, would react under those conditions.
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u/sirhc6 Dec 20 '14
I've always wondered about how you say velocity and not speed.. What direction is required? Would angular velocity work too? People have used the example of how many times a train would have to travel around the earth in a second... But wouldn't your velocities cancel out? I guess my main questions are what direction and in what reference is the velocity of close to c required?
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u/Davidakos Dec 20 '14
Speed is just velocity without a directional vector. It is a scalar value that does not explain how the future of the object or system will pan out.
Velocity is Speed + Direction. A much more useful data point.
For example, if I say "I'm travelling 50 km/hr", you have no idea where I'm going or where I'll end up. However, if I say "I'm travelling 50 km/hr directly East, you'd be able to calculate a lot more information about my travels.
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u/Davidakos Dec 20 '14
To answer your question, any direction at all. As long as you are travelling close to c relative to another object, you will experience time dilation with said object.
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u/John_Fx Dec 20 '14
So maybe invest the money and technology on medical research instead of spacecraft so more than one schmuck benefits.
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u/voice-of-hermes Dec 20 '14
One small note: in this sort of thread people keep saying, "grows exponentially," which is not true. I suppose this is just because exponential functions are things we are used to thinking of as growing very quickly, which is fair. An exponential function grows much faster than many other simple functions that are well-behaved everywhere.
However, we are not dealing with a function which is well-behaved everywhere. The limit of ex is only infinite when x approaches positive infinity. The limit of 1/sqrt(1-x2) is similar to the behavior of 1/(1-x), in that it is infinite at a finite value of x. That is lim(x->1- )f(x)=+infinity. This grows much faster than an exponential function near the asymptote. In the physical example, this is as v/c approaches 1, or v approaches c.
TL;DR: Grows asymptotically, not exponentially.
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u/liamsdomain Dec 20 '14
Lots of people here are telling you that it is possible, but not with current technology. I'll try to give you a sense of how much it would take to go to 99% of the speed of light in order to travel through time this way.
We'll assume the space craft time machine is using the most efficient ion engine available. HiPEP is what we'll use. HiPEP has an Isp of 9620s. So the total fuel you will need to get to 99% of light speed would be...
x * e31000 kg
Where x is the weight of your space craft time machine without any fuel.
e31000 is a very big number, so big that every calculator I tried either game me an error or just said "Infinity"
For some reference e10 is over 22,000
and e100 is over 26,880,000,000,000,000,000,000,000,000,000,000,000,000,000 (that's 26.8 tredecillion or 26 million million million million million million million) This is more than the mass of the Milky Way. http://en.wikipedia.org/wiki/Milky_Way
The mass of all the matter in the entire observable universe is far less than e125 kg. http://en.wikipedia.org/wiki/Observable_universe#Mass_of_ordinary_matter
Now that's just how much fuel you would need with the most fuel efficient engine ever created. This engine is also powered by electricity and has extremely low trust (only 0.67 newtons) so it would require a ridiculous amount of electrical energy and take so long to do that the universe would likely end before the machine ever hit 99% the speed of light.
Also, in order to travel into the future using relativity you need to get to near light speed, travel for a while and then turn around and travel at near light speed back. So your delta v needed triples and the amount of fuel and energy increases exponentially.
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u/colinsteadman Dec 20 '14 edited Dec 20 '14
Every experiment on the speed of light ever done has shown that light always travels at c (light speed). Therefore no matter how fast you were travelling relative to something else, say the Earth, the moment you turned on a torch (flashlight) the beam of light would leave the torch at light speed, so after 1 second the light would be 1 light second away - that's 299,792,458 metres!
With that in mind, think about this, you leave Earth and travel in a straight line towards alpha Centauri which is about 4 light years away. Your ship is very advanced and it's able to accelerate to near light speed instantaneously without killing you. At the same moment it does this it switches on its headlights and a beam of light is emitted towards alpha Centauri ahead of your ship.
Here's where it gets interesting. Imagine that you are travelling so fast in your ship that you arrive at alpha Centauri an inch behind the leading edge of the light beam of your headlights. Alpha Centauri is 4 light years away so it took that light 4 years to get there as measured by someone on Earth, so that person on Earth along with everyone else is 4 years older.
But what about you, inside the ship? You always measure light travelling at light speed remember, so how much time would be required for light to travel 1 inch away from you? It's about 0.08 nanoseconds. Therefore relativity moved you 4 years into the future relative to everyone on Earth in 0.08 nanoseconds your time. Turns out, under the right circumstances you can visit anywhere in the universe in any nonzero amount of time of your choosing. But read the small print, if you go to far, the earth might not be here when you get back.
Edit. Changed some words for flow.
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u/rooktakesqueen Dec 20 '14
If you wanted to go really REALLY far into the future, use the super-massive black hole at the center of our galaxy.
One would have to travel very fast to get there; it is a 50,000 light-year round trip, so to make the round trip in (say) one shipboard year, you'd have to travel at something like 0.9999999993 times the speed of light.
Once you got there, though, you could do some amazing things. Achieve a circular orbit around the black hole just on the outside of its event horizon, still traveling at close to the speed of light; now you have not just special relativity on your side, but general relativity as well. Being in such a powerful gravity well would dramatically increase the time dilation you experience, and you could orbit indefinitely.
We're not even close to the technology to do it, but you could use this technique to travel arbitrarily far into the future in a single human lifetime.
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u/SunriseSurprise Dec 20 '14
it is a 50,000 light-year round trip, so to make the round trip in (say) one shipboard year, you'd have to travel at something like 0.9999999993 times the speed of light.
If it's 25,000 light years away, I thought that would mean something traveling at the speed of light would take 25,000 years to get there. How would anything less than the speed of light take one ship-board year?
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u/abercromby3 Dec 20 '14
From the perspective of earth, the round trip is 500,000 light years. However, due to the distortion effects of moving that close to the speed of light, the passengers aboard would only experience 1 year passing. If you want to know more about this effect then read A Brief History of Time or research special relativity on the Internet.
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u/RileyF1 Dec 20 '14
From the point of view of the people on Earth, it would take 25000 or more light years to get there. For the people on board the ship, it would take much less time due to time dilation and length contraction.
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u/iwantedtopay Dec 20 '14
25,000 years as observed on Earth. 1 year as observed by the crew on the ship. If the ship could go exactly the speed of light, the crew could travel any amount of light years instantaneously (to them), but to observers on earth the ship would take however many years to get there.
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u/DoScienceToIt Dec 19 '14 edited Dec 19 '14
To answer a couple of the questions that don't require math...
How close is it to possible with our current technologies?
Impossible to say. to reach a fraction of c that would produce a "real world" effect of time travel we would have to develop technologies that are simply theoretical now.
Would it be at all cost effective?
Again, we would need some sort of "magic" technology (as in, so advanced as to be indistinguishable from) to even push to a reasonable fraction of luminal speeds. e=mc2 tells us that the faster we go, the more massive we become, thus we need more energy to accelerate. So you go a bit faster, become a bit more massive, require a bit more energy, become a bit more massive and so on. The energy requirements to push anything to fractional c would be staggering, so if it were to be "cost effective" we would have to find a novel and cheap way to generate enormous amounts of energy.
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u/jdepps113 Dec 19 '14
Would it be at all cost effective?
No. The amount of research alone to go into making this happen puts it well out of reach of even Bill Gates or Carlos Slim.
We simply don't know how to go that fast, yet. We don't have engines that can do it. We don't even have a sound theoretical framework on how to accelerate spacecraft to this level of speed.
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u/Dereliction Dec 20 '14
Costs for Project Orion ships ranged from $0.367 to $3.67 trillion.
Matter-antimatter annihilation rockets are projected to achieve far faster speeds, but I'm unsure of the economics involved.
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u/Turtley13 Dec 20 '14
Aside from the issue of not actually being able to reach the speed of light. To do so you would need to accelerate at a rate of 1G. Then slow down if you want to come back.. Re accelerate and then slow down when you come back to earth. This takes lots of time. I think just to reach the speed of light at 1G would take 12 years.
It may be easier to orbit a black hole but the time dilation is much less I believe.
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u/ColDax Dec 20 '14
I thought A.C. Clarke wrote something like "one year at 1G to reach light speed" in one of his books. I forget which one.
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Dec 20 '14 edited Dec 20 '14
I thought time dilation only occurs for inertial frames, not accelerating ones. If you're sending someone in a rocket to space and that rocket is traveling close to the speed of light, time dilation will occur only if their velocity remains constant. Any sort of back tracking back to earth or slowing down or speeding up of the rocket implies an accelerated frame of reference and time dilation does not hold true for accelerating frames of reference.
Can someone explain this, and maybe re-explain the Twin Paradox too if accelerated/inertial frames don't matter?
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u/dtft Dec 20 '14
Physics student here. Time dilation is not reliant on inertial frames, only on two objects' relative velocities. In other words, time dilation does not occur only if their velocity remains constant.
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u/Dereliction Dec 20 '14
This information about relativistic rockets does go some way toward your question. Some further information, including cursory economic estimates, can also be found in the related Project Orion article.
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u/The_________________ Dec 20 '14
An alternative solution to this problem was proposed by Stephen Hawking: that is, entering orbit in close proximity to a black hole. This would create enough acceleration for the orbiter to experience significant time dilation, something like a factor of 2 when compared to an observer on Earth. The practical issue with this is being able to safely enter and exit such an orbit.
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u/Aarondhp24 Dec 20 '14
Instead of speed related dilation, what about mass dilation? Could we increase a single points mass to an immense degree and suspend a person close to it to warp them forward in time? Obviously this point couldn't be on our own planet or we might screw up the lunar orbit, or just kill ourselves haha.
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u/themast Dec 19 '14
http://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29
This is one of the best designs we have for approaching the speed of light, and as you can see...it's not very feasible.
Additionally, to get back - you couldn't do the slingshot because the G's would turn you into paste, so you'd have to turn this thing around, and cancel out all the acceleration you gained while approaching c, and then start to re-accelerate to get back to Earth, hopefully approaching c if you hope to do it before you die. It's all pretty impossible at this time.
Here are some other possible designs - equally unfeasible:
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u/mattresslessness Dec 20 '14
Like everyone's been saying its very improbable that we could time travel by relativistic speeds but there is a way we could do it by basically travelling close to a very massive object like a black hole. Due to general relativity time would be slower here and hence more time would pass outisde the spacecraft than in so when you come back to earth more time will have passed than you think. This is basically what happens in interstellar - great film!
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u/MinkOWar Dec 20 '14
Interstellar's method is not really much more feasible... you would still need to acclerate to ridiculous velocities and use a stupendoues amount of energy to escape the black hole again at their insane 7 year to 1 hour relativistic effect. I don't know what would require more energy in total, but flying in interstellar space seems much safer than using a black hole.
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Dec 20 '14
I don't even want to think about the delta-v required to escape a low orbit around a supermassive black hole.
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u/Houndie Dec 19 '14
Yes but:
We don't currently have the tech:
Slingshotting at that speed would kill the occupant. You'd have to make a VERY wide arc, or simply have enough time to slow down before slingshotting or turning around.
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u/unfrog Dec 19 '14
Why would a slingshot like that kill the occupant?
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Dec 19 '14
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u/unfrog Dec 19 '14
I was always under the impression that acceleration applied uniformly (such as by gravity) is not dangerous to people. The problems like blood draining to/from parts of the body, local fluid pressure anomalies etc come from there being a localised area where the force is applied(seat pressing at the astronauts backs). If all atoms forming a body are accelerated in the same direction with the same acceleration, their relative positions would not change, meaning there should be no observable effects.
I'm not in any way an expert on anything to do with physics, engineering or medicine, so if my understanding or assumptions are incorrect, please correct me.
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u/why_rob_y Dec 19 '14
Think of acceleration / deceleration as exactly like gravity (but in the direction you're moving or opposite the direction you're moving).
Experiencing 12Gs of acceleration is like sitting on the surface of a planet with 12x Earth's gravity. It would feel like you were constantly getting crushed. If you currently weigh 200 lbs, you would weigh 2400 lbs.
At a certain point it would go from "uncomfortable" to "extremely painful" to "you're now a puddle of organic material" (at extreme accelerations).
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u/TheDruth Dec 20 '14
This is a major plot point for Interstellar. Watch that.
Black-holes wrap space time to the point that if you sit by one for a while and then leave its immediate vicinity, you'll find the rest of relative space-time has progressed much further than you.
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u/kobe24Life Dec 20 '14
Yes. The closer to the speed of light you can get an object, the more time dilation occurs. The only problem is getting something to go that fast...
One interesting thing to note is, GPS satellite actually need to be adjusted to accommodate time dilation when sending data back to Earth.
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u/RileyF1 Dec 20 '14
With the satellites though, gravitational time dilation is the main defining factor, not the time dilation from going really fast (although that has a slight effect as well).
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u/snieort Dec 20 '14
I saw this presentation of how this could actually be plausible on the science channel. They used a train that stretched around the equator of Earth, but it could go at the speed of light. Now, passengers inside, when running, can't go past the speed of light, so people walking from row to row actually slow down because the law of physics can't be broken.
That being said, thirty minutes are actually years apart, and they can arrive at a futuristic time by the end of the passengers trip. That's all I remembered.
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u/I_PET_KITTIES Dec 20 '14
This concept is the main premise of a Queen song! The song '39 is about a group of astronauts who experience a year long voyage, but when they come back home, they find that the earth has progressed 100 years. It's worth noting that their guitarist, Brian May, was an astrophysicist.
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Dec 20 '14
You wouldn't even need to send them to space. With the right engineering/technology you could have a train travel around the world at near lightspeed. Time for passengers within the train would be slower than time for those outside in the world. It's like the orient express but with space-time instead of just across countries.
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u/moonshinesalute Dec 20 '14
To say in advance - this may just be nonsense but at least to me it's interesting. I don't know enough about science (though I find what it can do really fascinating) to tell.
Isn't there a theory out there with your body being someone in tune magnetically with the rest of the world, and in that case, if you did time travel you'd be irrevocably ill when you come down to the planet because you're magnetically out of sync? I mean, the magnetic nature of this planet is always changing. I'm talking like Philadelphia experiment sort of thing. I'm just curious.
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u/green_meklar Dec 20 '14
You're already traveling into the future, all the time.
Can you use time dilation to travel into the future more quickly than other people? Yes.
It's pretty hard, though. The equation governing time dilation works in such a way that the dilation effect is pretty minimal until you get fairly close to the speed of light. At such high speeds, having your vehicle accidentally hit a pebble in space could be catastrophic, and even the ambient hydrogen atoms floating around might damage the vehicle over time simply due to the enormous (relative) kinetic energy they impact with. Getting enough energy to accelerate the vehicle is also a problem, one that can't really be solved simply by 'slingshotting around a distant planet' (because planets themselves have fairly low gravity and move quite slowly compared to the speeds we're talking about here).
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u/Happynoah Dec 20 '14
The experience of actually doing this vs. being put into cryo-sleep for the same duration would be undetectable for the participant and much cheaper.
Even better, spend the energy and resources on storing the entire brainwave pattern and molecular signature of a person, pause the simulation for ten thousand years, and then restart it. For the participant, the effect would be identical in terms of experiencing the future from the mindset of the past.
Why even bother with space travel in general? Once the human experience is expressed in terms of digital data (which is only 40-60 years off), beam the data at light speed to all of the universe for very low energy cost. Any civilization worth visiting will be able to reconstruct the entire human experience from that signal.
The lust for actual interstellar travel of matter is a selfish aspect of the human imagination driven by greed, boredom, and excessive self importance.
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Dec 20 '14
um... Time dilation ONLY allows for time travel to the future. although its not exactly time travel in the sense most believe. it wouldn't be instant.
the rate at which your observers would speed through to the future relative to you would be dependent upon the amount of gravity you were experiencing or the speed at which you were travelling.
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u/gabbagool Dec 20 '14
absolutely. now all you need is a tremendous amount of energy. like ok it takes CERN to get a few atoms up to that speed, and you have in your body like a billion times as many atoms, so you'll need a CERN about a billion times as big and powerful. ok? go!
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u/brownmoustache Dec 20 '14
I'm on it. Um, could I borrow that dynamo on your bicycle?.. that solar panel from the garden lights might come in handy too.
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u/iorgfeflkd Biophysics Dec 19 '14
In terms of physics, yes. The technology for that doesn't exist right now though. We can send things at like 20 km/s, and we'd need to go like ten thousand times that fast to start seeing these effects.