# Near Light Speed Travel & Time Dilation I don't get it.

#### Seiryu

I recently came across an article relating to Gliese 581 g and some theoretical information about getting there. It said something along the lines of "while it is 20.3 light years from Earth, passengers aboard a vessel traveling at nearly the speed of light would perceive the journey as nearly instantaneous." This raised a bunch of questions for me that I had never thought about before as I had not previously heard of the concept of time dilation.

I always assumed that time was, well, time. I'd taken the term "light years" to mean that that is how many years the distance would take to travel if going at the speed of light, but now I am just confused. If travel at the speed of light appears to be nearly instantaneous for the traveler while time continues to pass for those that remain stationary then how does it work?

For example let's say that someone travels to Gliese 581 g at near light speed, for them it is no time at all while 20.3 years pass on Earth. Now let's say that after reaching Gilese 581 g that traveler returns to Earth, again at near light speed. Now 40.6 years have passed on Earth while the round trip time for the traveler was nothing at all, right? Or does the return trip somehow reverse the time dilation to make it balance out, in the sense that it would now seems like 20.3 years for the traveler while it is instantaneous for Earth?

I've never really put much thought into this type of thing before and don't know much about physics, so obviously this is all confusing to me. I've tried to research this on my own and I think what I am describing sounds like the Twin Paradox, but I still don't understand it after reading an explanation of what that is. How is it possible for two entities to each perceive the other to be slower than themselves?

Basically what I am trying to figure out is this. If near light speed travel appear instantaneous to the traveler, therefore making them appear to cease aging during travel while everything around them continues to be affected by the passage of time then doesn't near light speed travel essentially allow you to go into the future? Would going 20 light years away from Earth then turning around and coming back make you find yourself 40 years in the future without having aged?

I suppose that since this is all highly theoretical that a definitive answer is impossible, but I am hoping to be able to at least improve my understanding a little since thus far everything I have read has seemed very vague and only raised more questions rather than answers.

Related Special and General Relativity News on Phys.org

#### Dale

Mentor
Hi Seiryu, welcome to PF

I think you might like this page on the relativistic rocket: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]

It doesn't assume that the rocket starts at light speed, but accelerates up to speed at a constant rate. But the essential point is that, yes, if you make a round-trip at relativistic speeds you will come back younger than if you had stayed.

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#### DaleSwanson

You seem to have a pretty good grasp for someone who just discovered time dilation. A key point you didn't mention is length contraction. In addition disagreeing about the rate at which time passes the two observers (one stationary on Earth, one traveling at near c towards Gilese 581), would disagree about how far away Gilese 581 actually is. The moving observer will measure Gilese 581 as being much closer than 20 light years away. That is how they will reconcile arriving there in much less than 20 years and never reaching the speed of light.

Also, you have to remember that the travel time will never be instant. It will get closer and closer to zero as the traveler approaches c, but will never be zero. Also acceleration should be accounted for. Particularly if you are thinking about humans traveling on ships if the acceleration is too high they will not survive. It is common to think about a ship that accelerated at one g (9.8 m/s2) constantly throughout the journey. For the first half the ship would accelerate towards the destination, and then for the second half would accelerate towards the origin. I made a http://daleswanson.org/things/speed.htm" [Broken] for this type of journey when I was trying to better understand relativity and javascript. Playing around with it may give you a better idea of specifics. Keep in mind you have to manually figure for deceleration if you want to be stopped at your destination. You do that by just entering half the distance or time, and then doubling the resulting answers.

Basically what I am trying to figure out is this. If near light speed travel appear instantaneous to the traveler, therefore making them appear to cease aging during travel while everything around them continues to be affected by the passage of time then doesn't near light speed travel essentially allow you to go into the future? Would going 20 light years away from Earth then turning around and coming back make you find yourself 40 years in the future without having aged?
Yes, that is how it would work. One could travel millions of years into Earth future by spending a few years at near light speed.
How is it possible for two entities to each perceive the other to be slower than themselves?
The two observers would both observe the other as moving faster, themselves moving slower. They would both be right, too. However, once one accelerates to turn around and head back to the other things change. Once they meet up again they will both agree that the observer that turned around moved through time slower.

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#### Simplyh

First: the traveler is traveling at .9c relatively to which reference frame? Earth, Gliese or his own frame?

Let's first isolate acceleration: it can be a quick acceleration/deceleration from 0 to .9c or a slow acceleration/ deceleration for half the travel or something in between. This is GR effect of a gravitational field over time and has nothing to do with relative movement.
Letś assume that between two certain points of the journey the ship does not suffer any acceleration.
First of all we must ask: relatively to what does the ship move? Because if the ship runs at .9c relatively to Gliese than it can run at .01c relatively to another ship running on the same direction and runs at 0 relatively to the ship itself.
According to SR, movement has no sense, only movement relatively to a certain reference frame. So it has no sense saying that someone is moving at .9c! We must say that he runs at .9c relatively to some reference frame.
For Gliese's frame the traveler is moving at .9c and his time run's slower then Gliese's. But the same applies to the traveller: in his frame, he is stationary and Gliese approaches at .9c. So, on the traveller's frame, Gliese time flows slower than his.
SR time dilatation is totally symmetrical and can not have the meaning you are giving it.

On the traveller's frame, Gliese is 20,3 light years away and approaching at .6c; Gliese will take 25,3 years of the traveller's time to reach him; but, from his point of view, Glieseans will only get old 20,3 years older, meanwhile.

#### 1MileCrash

I recently came across an article relating to Gliese 581 g and some theoretical information about getting there. It said something along the lines of "while it is 20.3 light years from Earth, passengers aboard a vessel traveling at nearly the speed of light would perceive the journey as nearly instantaneous." This raised a bunch of questions for me that I had never thought about before as I had not previously heard of the concept of time dilation.
I'm glad you've taken an interest, because it's well, interesting.

I always assumed that time was, well, time. I'd taken the term "light years" to mean that that is how many years the distance would take to travel if going at the speed of light, but now I am just confused. If travel at the speed of light appears to be nearly instantaneous for the traveler while time continues to pass for those that remain stationary then how does it work?
Okay, put it this way. A light year is certainly the distance traveled at the speed of light within an amount of time equal to one earth year. However, this is for an observer, stationary in reference to whatever is traveling at the speed of light. In other words, if a flash of light was emitted from Gliese 581, it would reach us at earth 20 years later. However, if we were on Gliese 581 and "hitched a ride" with that flash of light we would hit earth in less than an instant. Immediately.

For example let's say that someone travels to Gliese 581 g at near light speed, for them it is no time at all while 20.3 years pass on Earth. Now let's say that after reaching Gilese 581 g that traveler returns to Earth, again at near light speed. Now 40.6 years have passed on Earth while the round trip time for the traveler was nothing at all, right? Or does the return trip somehow reverse the time dilation to make it balance out, in the sense that it would now seems like 20.3 years for the traveler while it is instantaneous for Earth?
Nothing at all passed for the person on the ship except the buckling of the seatbelts. They have traveled 40.6 years into the future. (assuming they traveled exactly the speed of light.)

I've never really put much thought into this type of thing before and don't know much about physics, so obviously this is all confusing to me. I've tried to research this on my own and I think what I am describing sounds like the Twin Paradox, but I still don't understand it after reading an explanation of what that is. How is it possible for two entities to each perceive the other to be slower than themselves?
The twin paradox assumes that both bodies experienced a symmetrical event, which is incorrect. Although we cannot define absolute motion, we can define who is really accelerating.

Basically what I am trying to figure out is this. If near light speed travel appear instantaneous to the traveler, therefore making them appear to cease aging during travel while everything around them continues to be affected by the passage of time then doesn't near light speed travel essentially allow you to go into the future? Would going 20 light years away from Earth then turning around and coming back make you find yourself 40 years in the future without having aged?
Yes, that's exactly right.

The speed of light is constant, space and time are not (something has to give.)

However, the time dilation and length contraction, as cool as it's results would be, are also why we cannot reach light speed travel. The speeds don't add because of time dilation and length contraction.

Say we're in that space ship nearing the speed of light. We decide to add 600 mph to our speed, we're closing the gap on reaching C. Yet, 600 mph to us is no longer 600 mph to a stationary observer. We are traveling. A mile could be a millimeter, an hour could be a year, so we're just adding 600 millimeters per year to our speed in the eyes of an observer (these numbers are not correct, they are merely here to demonstrate.). And, as we get closer and closer to C, the amount of time dilation and length contraction will just keep increasing and increasing. Say we eventually add 1 MPH to our speed, despite the length contraction and time dilation. The next 1 MPH is going to be a real *****, because last time 1 MPH for us was equal to 1 millimeter per year, well now it's more like one nanometer per decade.

I'd relate it to trying to reach the horizon by running toward it. This is why objects with mass can NEVER reach the speed of light, and why objects (particles) without mass MUST travel the speed of light.

#### Simplyh

Nothing at all passed for the person on the ship except the buckling of the seatbelts. They have traveled 40.6 years into the future. (assuming they traveled exactly the speed of light.)

Say we're in that space ship nearing the speed of light. We decide to add 600 mph to our speed, we're closing the gap on reaching C. Yet, 600 mph to us is no longer 600 mph to a stationary observer. We are traveling. ...
I don't think you are right on these two observations.
It is not generally accepted that jump into the future. If you want to make such a statement you should explain yourself better. This way it looks like sciencefiction.

"We are nearing the speed of light" makes no sense in relativity. There is no movement at a certain speed; only relative movement makes sense. So, in your frame (that's where you are), you are travelling at zero speed. You may be speeding relatively to Gliese or Earth, or whatever. Relatively to you, you are at complete, total rest; unless you accelerate. But then you must apply GR, not SR. So, your next considerations can not apply.

#### 1MileCrash

I don't think you are right on these two observations.
It is not generally accepted that jump into the future. If you want to make such a statement you should explain yourself better. This way it looks like sciencefiction.
At speeds nearing the speed of light, time dilates (relative to another frame of reference.). At the speed of light, time cannot be experienced. (relative to another frame of reference.)

Since YOU are motionless from YOUR reference, you feel no changes in time. Which means it feels like a perfectly normal instant, but it's still just an instant to you.

"We are nearing the speed of light" makes no sense in relativity. There is no movement at a certain speed; only relative movement makes sense. So, in your frame (that's where you are), you are travelling at zero speed. You may be speeding relatively to Gliese or Earth, or whatever. Relatively to you, you are at complete, total rest; unless you accelerate. But then you must apply GR, not SR. So, your next considerations can not apply.
"Nearing the speed of light" makes perfect sense. We are at an arbitrary speed near the speed of light (.99C) and continuing to add speed. If you want me to assign an inertial reference frame to which I am measuring this speed (which I did not feel was necessary in this simple example) I will go with anything in the universe hypothetical or that actually exists that is traveling a constant speed that yields the relativistic measurement of our spaceship traveling at the speeds I mentioned.

For fun, we'll say it's a puppy dog.

(what I mean by this is that when someone mentions a body traveling a certain speed, it is safe to assume that on a physics forum, they understand the elementary facts of special relativity and expect you to assume that this speed is measured off of SOME reference frame. There is no such thing as an absolute speed, just as there is no such thing as an absolute reference frame. It can be whatever I want, or whatever you want, a rock, a spec of dust, whatever, just assume it is something that would result in the mentioned speeds if measured from that reference frame.)

No where did I mention an absolute speed "relative to space" so none of what you said is really relevant, nor is my post in contradiction with any of it. Our "stationary observer" is one that is moving at the same constant speed and direction that the space ship was before it started it's journey.

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#### ghwellsjr

Gold Member
How is it possible for two entities to each perceive the other to be slower than themselves?
The two observers would both observe the other as moving faster, themselves moving slower. They would both be right, too. However, once one accelerates to turn around and head back to the other things change. Once they meet up again they will both agree that the observer that turned around moved through time slower.
Each observer measures the same relative speed between them, and from that they conclude that the other one's clock is ticking slower than their own.

#### Simplyh

At speeds nearing the speed of light, time dilates (relative to another frame of reference.). At the speed of light, time cannot be experienced. (relative to another frame of reference.)

Since YOU are motionless from YOUR reference, you feel no changes in time. Which means it feels like a perfectly normal instant, but it's still just an instant to you.
If speed only makes sense relatively than you can not say "nearing the speed of light" unless you pick up an absolute reference. In common language we refer speeds to Earth as an absolute frame: that's why we may say "he is speeding at 40 Km/h". If we would be precise we should say "he is speeding at 40 Km/h relatively to Earth" but he is speeding at "1 km/h relatively to that other car". Accepting that there is no absolute frame to which movement can be referred, it is not possible to make the statement:"he is speeding at 40 Km/h" nor "At speeds nearing the speed of light".

Our "stationary observer" is one that is moving at the same constant speed and direction that the space ship was before it started it's journey.
Then, only for that observer, as far as we know, can we say that your speed is .99c. Yet, in your frame, he is the one traveling at .99c and you are stationary: all measurements of the flowing of time will depend on the observer.

Time change, independent from the observer, comes only from acceleration (+ or -), not relative speed. If you want to quantify you must use GR, not SR.

#### 1MileCrash

If speed only makes sense relatively than you can not say "nearing the speed of light" unless you pick up an absolute reference. In common language we refer speeds to Earth as an absolute frame: that's why we may say "he is speeding at 40 Km/h". If we would be precise we should say "he is speeding at 40 Km/h relatively to Earth" but he is speeding at "1 km/h relatively to that other car". Accepting that there is no absolute frame to which movement can be referred, it is not possible to make the statement:"he is speeding at 40 Km/h" nor "At speeds nearing the speed of light".
Again, what about my example contradicts this? I do not need to explicitly state an absolute frame every, single, time I mention a speed. If a speed is mentioned, you can assume it is as it's measured from a frame of reference that makes it true. Yes, I realize that any body can be defined to be traveling any arbitrary speed and that none of these arbitrary speeds are more "correct" or "preferred" than the other.

However, if I say "a rocket ship is traveling 1000 mph" it is traveling 1000 mph. What is the frame of reference you ask? Anything, that makes it true that it's traveling 1000 mph. It doesn't matter. It's also traveling any arbitrary speed you want it to, but for our example, it's traveling 1000 mph, because relative to every single mentioned object in the example, it's traveling 1000 mph, therefore the frame of reference is 100% understood and does not need to be clarified. If I say "A rocket launches from the moon and travels 5000 mph" it doesn't take a world-class physicist to understand that the moon is the frame of reference.

It is completely possible to make a statement "at speeds nearing the speed of light." If there is an understood reference frame we are measuring the speed from. A rocketship that launches from earth measures it's speed relative to earth's location at launch. It is going x speed. Not absolutely, but for our measurement, it is traveling 1000 mph. Any effects from acceleration and speed are experienced only relative to whatever reference frame makes its mentioned speed true.

This is not complicated. If I told you I drove down the highway at 55 mph, I doubt you'd shrug your head in confusion. There is an understood reference frame.

Then, only for that observer, as far as we know, can we say that your speed is .99c. Yet, in your frame, he is the one traveling at .99c and you are stationary: all measurements of the flowing of time will depend on the observer.
Yes, another blatant fact my post was not in contradiction of.

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