# Time travel



## i just want luv (Feb 13, 2011)

Haven't researched this much, but do you really believe if you travel at the speed of light you'd travel into the future?

I dont see how thats possible.

And even if it is, isn't it impossible for mankind?

**LETS SAY we had a ship big enough, and fast enough to reach the speed of light**

It's basically a suicide mission because of all the obsticals in space, the ship would eventually crash into a meteor, planet, comet, caught in a supernova.
Also would a human even be able to survive traveling in the speed of light? Even if you some how completed that mission wouldn't you still be stranded in space to die afterwards?


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## Johny (Dec 21, 2010)

Time passes slower when your velocity increases, so if you traveled at a great speed (so that the passage of time difference is noticeable) for x years and then stopped it would have felt (ie. you would have aged) like y years, x>y. I don't think traveling into the past is theoretically possible due to the universe not allowing such crazy paradoxes (ex. stopping yourself from traveling into the past before you do) to exist.


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## SilentWitness (Dec 27, 2009)

Discover a wormhole.

http://www.pbs.org/wnet/hawking/strange/html/wormhole.html


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## cafune (Jan 11, 2011)

Johny said:


> Time passes slower when your velocity increases, so if you traveled at a great speed (so that the passage of time difference is noticeable) for x years and then stopped it would have felt (ie. you would have aged) like y years, x>y. I don't think traveling into the past is theoretically possible due to the universe not allowing such crazy paradoxes (ex. stopping yourself from traveling into the past before you do) to exist.


I've never been able to wrap my head around a concept like that. How can time slow down? How can you quantify time in any other way than we've already done? Stopwatches count every millisecond, it counts without gaps, so how could time slow down? Would the gap between each millisecond increase as the velocity increased?

I don't think it'd be possible to travel back in time because of my belief that time is a continuous line. If it was a continuous line, how would be possible to traveling backwards? It'd be like bringing the dead back to life... would it not? Maybe if time consisted of a series of lines which coexisted it'd be possible, although there is no proof for that.


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## uffie (May 11, 2010)

Live Laugh Love said:


> I've never been able to wrap my head around a concept like that. How can time slow down? How can you quantify time in any other way than we've already done? Stopwatches count every millisecond, it counts without gaps, so how could time slow down? Would the gap between each millisecond increase as the velocity increased?
> 
> I don't think it'd be possible to travel back in time because of my belief that time is a continuous line. If it was a continuous line, how would be possible to traveling backwards? It'd be like bringing the dead back to life... would it not? Maybe if time consisted of a series of lines which coexisted it'd be possible, although there is no proof for that.


Space and time are connected. Lets look at the time dilation equation T=(1-(v^2)/(c^2))^(1/2) with v being velocity and c being the speed of light. As the velocity increases the dilation of time decreases. Global positioning systems or GPS rely heavily on relativity. Without taking in the time dilation of the satellites, the gps system wouldn't work. Atomic clocks are different then just the standard normal wrist watches; they rely on the vibration of atoms. An atomic clock would move slower the faster its travels.


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## Johny (Dec 21, 2010)

Live Laugh Love said:


> I've never been able to wrap my head around a concept like that. How can time slow down? How can you quantify time in any other way than we've already done? Stopwatches count every millisecond, it counts without gaps, so how could time slow down? Would the gap between each millisecond increase as the velocity increased?
> 
> I don't think it'd be possible to travel back in time because of my belief that time is a continuous line. If it was a continuous line, how would be possible to traveling backwards? It'd be like bringing the dead back to life... would it not? Maybe if time consisted of a series of lines which coexisted it'd be possible, although there is no proof for that.


So (paraphrasing from book) you can use anything that cycles to measure time. One thing you can use is a photon bouncing between two mirrors
-----
o
----- (---- is a mirror, o is a photon). Light travels at the same speed (always), so if the mirrors are motionless we can measure one motionless photon second (using a motionless normal watch) as the watch seconds it takes the photon to leave the bottom mirror and arrive at it's original spot (if two measures of time are experiencing the same velocity their measures will be the same). If you send the mirrors and photon in a spaceship travelling very fast, the photon has to travel a greater distance to return to it's original spot (if horizontally the photon travels at an angle, if vertically since light's speed doesn't change (even if bouncing off a moving object) it still has to travel a greater distance (think of the limiting case when the mirrors are traveling near the speed of light)) so on the spaceship time as measured by the mirrors and photon is passing slower than if we were to measure it with motionless mirrors.


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## cafune (Jan 11, 2011)

Live Laugh Love said:


> If it was a continuous line, how would be possible to traveling backwards?


That was atrocious, I don't know what I was thinking when I typed that.



uffie said:


> Space and time are connected. Lets look at the time dilation equation T=(1-(v^2)/(c^2))^(1/2) with v being velocity and c being the speed of light. As the velocity increases the dilation of time decreases. Global positioning systems or GPS rely heavily on relativity. Without taking in the time dilation of the satellites, the gps system wouldn't work. Atomic clocks are different then just the standard normal wrist watches; they rely on the vibration of atoms. An atomic clock would move slower the faster its travels.


The equation is made based on the assumption that velocity and time are related though. But thanks for the explanation!



Johny said:


> So (paraphrasing from book) you can use anything that cycles to measure time. One thing you can use is a photon bouncing between two mirrors
> -----
> o
> ----- (---- is a mirror, o is a photon). Light travels at the same speed (always), so if the mirrors are motionless we can measure one motionless photon second (using a motionless normal watch) as the watch seconds it takes the photon to leave the bottom mirror and arrive at it's original spot (if two measures of time are experiencing the same velocity their measures will be the same). If you send the mirrors and photon in a spaceship travelling very fast, the photon has to travel a greater distance to return to it's original spot (if horizontally the photon travels at an angle, if vertically since light's speed doesn't change (even if bouncing off a moving object) it still has to travel a greater distance (think of the limiting case when the mirrors are traveling near the speed of light)) so on the spaceship time as measured by the mirrors and photon is passing slower than if we were to measure it with motionless mirrors.


Wow, thanks! I'm not going to lie, that took a while to completely understand. So because the mirrors travel near the speed of light, it makes to difficult for the photon to reach it's previous position because the point it's attempting to reach is traveling at almost the same speed that it is traveling in. So you measure time by looking at something that travels in cycles... so if you applied the idea of the photon bouncing between two mirrors on Earth you'd end up with different results than what happens while they're traveling at nearly the speed of light, which means that time passes by faster here than it does while moving at high velocities. Thanks again!


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## ripwog (May 4, 2011)

I've read before where someone was pointing out that time travel would be useless anyway, because if you travelled into the past, the earth wouldn't be there since it's constantly moving in space. So even if it were possible to travel in time, you'd have to simultaneously reach the position the earth was in when that time occurred.


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## Classified (Dec 7, 2004)

ripwog said:


> I've read before where someone was pointing out that time travel would be useless anyway, because if you travelled into the past, the earth wouldn't be there since it's constantly moving in space. So even if it were possible to travel in time, you'd have to simultaneously reach the position the earth was in when that time occurred.


I had never thought about that. Not only is the Earth spinning around the Sun, but the solar system is spinning around the galaxy as well.



Bluepanda said:


> The easiest way to time travel i think is to orbit a black hole and then fly away after x time has passed.
> 
> As long as you don't get too close or cross the event horizon it should be fairly easy to jump in and out of orbit.


As for Black holes, except for possibly being tears in the space-time continuum, I don't believe they would have the effect that you want.


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## Choci Loni (May 12, 2011)

It's relatively easy to "travel into the future" as the principles of relativity states that time slows down (from an outsiders point of view) as you approach the speed of light. Time dilation even occurs in a very small extent while traveling by plane for example. All we need is a to reach a very high speed.










As you can see in this graph though, we need to reach an extremely high velocity to achieve a factor of two.

Time travel into the past is another issue altogether. Even if we miraculously somehow managed to create and utilize a tunnel in spacetime for example, we don't really know what'll happen. It's interesting to discuss. Is the grandfather paradox valid for example? Is a parallel timeline created when you travel into the past? There are a lot of bizarre questions the concept gives rise to.

I guess only time can tell (haha).


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## Classified (Dec 7, 2004)

It still gets really confusing when dealing with space, light, and time. I'm not sure what you would feel if you were to go that fast. Would time just slow down for you, while the rest of the universe stayed constant moving at a normal rate? In 1 year, it might feel like a very long time (or a very short time) for the astronauts, yet normal to those outside the spacecraft.


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## Classified (Dec 7, 2004)

http://science.nasa.gov/science-news/science-at-nasa/2011/04may_epic/

http://en.wikipedia.org/wiki/Gravitational_time_dilation

I guess it happens, but I don't think it will be very practical anytime soon.

I guess this is why I'm not an astrophysicist.

http://en.wikipedia.org/wiki/Twin_Paradox

This stuff gets kind of crazy. But, I would think that if a space ship traveled at the speed of light, the time behind you looks like it stopped. But, the light/time in front of you would be going much faster it would seem.


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## ValiantThor (Dec 6, 2010)

fly out into space at the speed of light for 20 years, take a wormhole back to earth and you will be 5000 years in the future, i remember hearing this somewhere a while ago


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## i just want luv (Feb 13, 2011)

You must have incredible sense of directions if you can travel billions of miles away from earth into pitch black then turn around and head back the exact same way in pitch black travel billions of miles back and find Earths location. All this while avoiding planets, astroids, comets, meteors, explosions,and other black holes. 

I doubt we know everything about the universe so I'm betting other things we've never known about will be in affect once traveling so far aswell.


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## Quantic (May 30, 2011)

Live Laugh Love said:


> That was atrocious, I don't know what I was thinking when I typed that.
> 
> The equation is made based on the assumption that velocity and time are related though. But thanks for the explanation!
> 
> Wow, thanks! I'm not going to lie, that took a while to completely understand. So because the mirrors travel near the speed of light, it makes to difficult for the photon to reach it's previous position because the point it's attempting to reach is traveling at almost the same speed that it is traveling in. So you measure time by looking at something that travels in cycles... so if you applied the idea of the photon bouncing between two mirrors on Earth you'd end up with different results than what happens while they're traveling at nearly the speed of light, which means that time passes by faster here than it does while moving at high velocities. Thanks again!


Time dilation occurs at any relative velocity, moving "fast" just makes the dilation larger and easier to detect. Below is the picture that illustrates what Johny was describing:










The 2 postulates of special relativity are:

1. _The Principle of Relativity
_ The laws of physics are the same in all inertial frames of reference.
2. _The Constancy of Speed of Light in Vacuum
_ The speed of light in vacuum has the same value _c_ in all inertial frames of reference.

Postulate 2 is basically what accounts for all of the "weirdness" of special relativity; time dilation, length contraction, increasing mass with increasing velocity, loss of simultaneity..

If one accepts that the speed of light is the same in ALL inertial frames you can derive time dilation. An inertial frame is a reference frame that is moving at a constant velocity relative to another frame, so no acceleration is allowed (that is where general relativity comes in).

So the picture above shows a light clock from 2 inertial reference frames. The "owner" frame is a frame attached to the spaceship that the light clock resides in. The light clock is a source laser and a detector. The source emits a photon which travels at the speed of light to the mirror, is reflected and travels at the speed of light back to the detector. When the detector detects a photon it emits a "click" sound.

The "onlooker" frame is the second picture in the middle. If you throw a ball up on a moving train, you see it go straight up and come straight down. But someone standing next to the train tracks see the ball go up at an angle and come back down at an angle. This angled path is the motion of the photon as viewed from the "onlooker" frame.

But there is a conundrum. We've agreed that the speed of light is the same in all reference frames. In the "owner" frame the photon only has to travel a distance 2*d in time t to leave the source and reach the detector. But in the "onlooker" frame the photon has to travel a larger distance to leave the source and reach the detector, because the photon is following the hypotenuse of a right triangle which has a side d, and the hypotenuse is always larger than any of the sides of a right triangle.

If the light ray leaves the source and reaches the detector in the same amount of time in both frames, then the speed of light has to be different for the two different frames; it would have to be larger in the "onlooker" frame if it were to travel the larger distance in the same time. To reconcile this with our postulate that the speed of light is constant in all reference frames, we must agree that the _time_ required for the light to travel the larger distance is _greater_. So it is time that is changing, not the speed of light.

So, relative to the "onlooker", the clock on the spaceship is ticking slower. The light ray is traveling at the same velocity, c, but we would hear the clicks going slower, taking longer; the time is dilated (relative to the onlookers identical light clock that is at rest relative to him).

The owner on the spaceship notices nothing changing about his clock the faster he goes, but when he looks out the window at other spaceships/people on earth/how many vibrations a cesium atom makes relative to his clock, he notices all of them going _slower. _Because _relative_ to him, they are the ones moving at a velocity greater than zero, so their clocks are moving slower.

You are probably wondering about the twin paradox. If everyone's clock is moving slower relative to everyone else, how does the twin on the spaceship return to earth at a younger age than the twin that stayed on earth? The resolution comes from the fact that the twin on the spaceship has to accelerate away from the earth, decelerates to stop, then accelerates again to travel back to earth. I haven't been taught general relativity, but it is the accelerating frames that accounts for the age difference. The twin on the earth never accelerated during the whole time the spaceship twin had to do multiple accelerations. Indeed, astronauts who orbit the earth at 17,500mph for weeks at a time return to earth younger, and we have precise enough clocks to prove it.

It should be noted that you cannot travel at the speed of light as a few posts hint at. Your mass increases the faster you go, so it takes more and more energy to keep accelerating. Your length gets shorter and shorter, you would have zero size "at the speed of light", time would stop, and it would take an infinite amount of energy to move because your mass "at the speed of light" is infinite. Light gets to travel at c because it is massless.


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## WTFnooooo (Mar 27, 2010)

I don't think it's possible.
Subjective time can be experienced, it's what we call *the now*, but _Objective time_ can not be managed, it can be recorded and then simulated or replicated.
For example, you could identify the trajectory of _every _atom and recognize their previous move, once you have all this info you can use it to _reproduce _the past, not live it materially but you could witness it virtually, as a simulation.
You could live it physically but you need to be able to _create _atoms and arrange them accordingly into a _vast expanding space_, a universe. That still wouldn't be the past but an imitation of it.
A lot of work.


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## Invalid Username (May 25, 2011)

I'm not sure if this helps explain time dilation.


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## Uranium (Jun 14, 2011)

I wouldn't call it time travel. Einstein has the theory of relavity. There's this thing called time dilation, and the theory is that, if you are traveling faster than someone else, time is actually slower for you than it is for the person traveling at a slower speed. I saw a video where someone actually prooved it. They had one of those atomic clocks on a plane and an atomic clock on land. They flew around the earth on time, when they landed they noticed that the clock on the plane was not as far as the clock on land, but only by a couple milliseconds. So if you travel at the speed of light to the sun in back, when you got back, your wife may be 90 years old even though you felt like you were only gone for a couple of minutes.


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