# Thread: Travelling forever at the speed of light

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## Travelling forever at the speed of light

If a human-carrying craft could be devised that could travel at the speed of light indefinitely, and since for the occupants aboard no passage of time is perceived, then would not the occupants be able to travel for an infinite amount of time?

In another thread I suggested that the speed of light is the speed of time. A way I can describe what I mean is to consider that we are in a "wind" moving at the speed of light. If stationary, we "feel" the wind, and we age and everything around us ages as well. But if we are moving "with" the wind (of time), moving at the speed of light, we do not "feel" the wind, and it doesn't affect us, i.e., we do not "age" relative to reference points not moving at the speed of light.

Given that time is a dimension, travelling at the speed of light makes it possible to "move around" in that dimension. Unless we are moving at the speed of light, we cannot "move" in time as we move in three-dimensions. We are "in" the fourth dimension, time, but we can only "flow" along with it ... unless ... we are moving at the speed of light. Moving at the speed of light seems to be the gateway to our moving in the dimension of time. If we could travel at the speed of light indefinitely, it seems we could be called "immortal."

Now, I know that everything is not always as it seems....

2. Time will appear to stop to an external observer, but it does not stop for the people on the craft.

3. Right, the people in the craft still age and everything, despite going the speed of light.

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Respectfully, I disagree. If people travelled to a destination 10 light-years away, and for simplicity let's assume they could instantaneously accelerate to and decelerate from the speed of light, their perception of the passage of time with respect to their families they left behind on Earth would be: they were aware of going into light speed, and their next awareness would be that they arrived at their destination.

Although the people on board the craft have no awareness of time having passed, people on Earth have noted the passage of 10 years. And, the people living at the destination could simultaneously - with their super-powerful telescope - see an image of the craft leaving Earth as the craft arrived in physical form.

They, understanding the speed at which light propigates, know that the captain of the craft was 38 years old when he left Earth 10 years ago. And, they now greet the captain, and he's still 38 years old! Meanwhile, the people on Earth, with their super-powerful telescope, have been watching the craft on its 10-year journey, and they see a 38-year-old captain leave the ship.

I'm saying, that if the occupants of the craft had a ship that was capable of sustaining the speed of light indefinitely, then the occupants could travel infinite distances and the occupants would arrive at the same age as when they left. Earth Thus, in a sense, they, as long as they are moving the speed of light, are immortal, at least, in regards to their awareness, and the actual passage of time on Earth from the date of their departure forward in Earth (area) time.

Once out of moving at the speed of light, they again are affected by the "wind" of time, aging as they always did when not moving with the "wind."

5. Imagine you are sitting in your car and everything around you is in super fast forward, you are still going at the normal rate of time, you age the same and to you everything is normal except your observations.

6. If you travel at the speed of light, how do you stop? You press the "go" button and... That's the last conscious thing you do!

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Originally Posted by Dragon Star
Imagine you are sitting in your car and everything around you is in super fast forward, you are still going at the normal rate of time, you age the same and to you everything is normal except your observations.
That analogy doesn't work for the situation I described, assuming I am correct in my observation.

First I assumed the craft could instanteously accelerate to and decelerate from the speed of light. The occupants' time stops in relation to the world from which they departed. I fail to see that their time proceeds normally to them. Please, yuzuha and Dragon Star, explain your assertion. In my hypothetical situation, the occupants travelled 10 light-years in 10 years according to any viewpoint itself not travelling at an appreciable percentage of the speed of light. Even the people greeting the crew after their 10 light-year voyage know that it took them 10 years of Earth time, that the people on Earth had aged 10 years, while the occupants of the craft were the same age as when they achieved light speed.

Kaptan K adds an element I hadn't considered. How would the travellers know when to come out of light speed once their perception of time - and thus their actions within time - stopped? Reaching the speed of light might be a one-way street. Once you're there, you're there.

Now, what of the craft itself? Does time stop for it, too? Or does it consume 10 years worth of fuel, the engines experience 10 years of wear, the hull hit with 10 years worth of meteors and space junk as the occupants noted no time passing, or does the human occupants' perception of no time passing extend to the craft?

Let's say the craft accelerates gradually to the speed of light. An occupant looking out the window would see things passing by ever more quickly. What would he or she see as the last km/sec was achieved? Or, after light speed was attained?

It's all fanciful thinking, I'm sure.

8. Originally Posted by Flying Deuces
First I assumed the craft could instantly accelerate
Well that changes EVERYTHING, yes, you have it right.

A photon in a vacuum never knows it existed. Quite true.

9. You ought to read "Tau Zero" by Poul Anderson. It's about a bussard-spacecraft directed to a planet 35 lightyears away, but then something goes wrong, and the crew is forced to accelerate more and more, while the ages and eons pass outside... (by the way, you are right - at v = c, tau is 0, the time passing for someone experiencing this kind of relativistic time dilatation is reduced to zero - no aging at all. But there's no way to reach light speed itself, but only 99.99999...% of it - so the passengers experience time going by as normal, but the world outside passing by / aging faster and faster).

10. Originally Posted by Flying Deuces
I'm saying, that if the occupants of the craft had a ship that was capable of sustaining the speed of light indefinitely, then the occupants could travel infinite distances and the occupants would arrive at the same age as when they left.
How do they travel infinite distance, yet arrive somewhere? Don't you mean very great distance rather than infinite distance?

11. You ought to read "Tau Zero" by Poul Anderson.
Agreed!

12. I get the premise and I do not feel the need to debate the semantics. Of course the whole "accellerate instantly" thing is the problem. The famous muon experiment where more reach the surface than they should if not for relativistic effects is a good demonstration of the point. But, for a photon, it does not "age" as it travels yet its wavelength will stretch as it crosses the universe. Is that a form of aging? Let's face it, the whole concept of the four (?) dimensions of space expanding is a conjecture to explain what we measure.

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Anderson - I haven't read any of his writings in a while. I like old science fiction, in fact, I somehow convinced my struggling parents (back in the early 60s) to allow me to join the Science Fiction Bookclub. It's been too long since I read any new science fiction. I'll find the book.

01101001, I was saying that AT the speed of light it would take the craft's occupants as "long" to travel 10 light years as an infinite number of light years. (Pardon the use of infinite this way, please).

In relation to the time frame of the planet Earth from which they departed, a voyage of infinite length (or heck, of 10 billion lightyears) would appear to take an infinite amount of time (or 10 billion years in the latter case). And no one's commented to the contrary, so I assume this difference in time scales would apply in comparison to every other person throughout the universe (if there are any, but I'm making this up anyway...) who was not moving at speeds near or at the speed of light (if that were possible).

Thus, the occupants, compared to everyone else, would be immortal, or virtually so at 99.999999% of the speed of light.

As I asked, would this stoppage of time apply to the craft itself?

14. Originally Posted by Flying Deuces
As I asked, would this stoppage of time apply to the craft itself?
Yes, and it would be a lot shorter from an Earth perspective.

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Originally Posted by jlhredshift
Yes, and it would be a lot shorter from an Earth perspective.
Ha, I forgot about that aspect.

This is where it sounds nuts, well, one of the places anyway, but back to the imaginary spacecraft, if time stopped for it too, then we are left with the crazy idea that to travel 10 billion light years would require very little fuel, aside from accelerating to the speed of light which would require an infinite force, I presume.

Thanks, y'all for indulging my fanciful flight. It's good recreation for my mind. This universe and we living in it are more fantastic than fiction.

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## not understanding

I am not understanding something crucial here. Someone may have to try to explain basically some concept of general relativity. First off, why does time seem to slow down to either a person travelling close to the speed of light or to an observer. Why does the speed of light have anything to do with time? Why does mass increase as you approach the spped of light. I'm picturing light as something that moves at a speed that can vary (as has been proven, light can be slowed down in certain conditions.) What does light have to do with time? I was under the impression that time exists because of the motion of objects, so if there is no light, no photons, then we would still have the concept of time. I don't see why something cannot travel faster than the spped of light. If light can be slowed down, it is not constant, then surely anything travelling faster than this new speed would be travelling faster than light, but that doesn't mean time is altered in any way. So again, what does speed of light have to do with time. I'm missing something either very obvious or I just don't understand something.

17. light goes slower through glass... the index of refraction is the speed of light in a medium divided by the speed of light in a vacuum.... this latter value is the constant "c". If you are inside a ship travelling near the speed of light, you'd be in a local reference frame and your clocks would tick normally as far as you can tell, and you would still measure the speed of light as "c". If an outside observer left on earth could watch you leave, they would see your ship red-shift and to them it would look like your time is running slow. To you it would look like the earth is red shifting and the earth clocks are running slow.

Things get hairy when you start comparing one reference frame to another. You have a star 10 light years away, get on a ship and go at .8c and come back at the same speed, a person on earth would time your round trip at 25 years. However, when you are on the ship, the lorentz contraction factor comes into play and to you the star is only 6 light years from earth (different reference frame) and your round trip only takes 15 years.

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Originally Posted by pranzo
I am not understanding something crucial here. Someone may have to try to explain basically some concept of general relativity. First off, why does time seem to slow down to either a person travelling close to the speed of light or to an observer.
Because observers moving relative to each other when one observer is moving near the velocity of light will disagree on measurments of physical properties.

Originally Posted by pranzo
Why does the speed of light have anything to do with time?
Because it's the ulimate speed that can be used to make measurments of physical properties.

Picture you and a friend synchronizing two strobe lights (one for you, one for her) to flash at 1 second intervals so you can measure that the same amount of time has passed for either of you. It doesn't have to be strobe lights. It can be your heart beats, the breakdown of Cesium isotopes, anything: As long as you both agree the time interval by which you'll signal each other is the same.

Now one of you is somehow moving at 99% the speed of light past the other one. Say further that as she moves past you both of your signaling devices flash together. This is the last time either of you will agree that the two signals happened at the same time.

As she travels by at .99C and continues to signal you at 1 second intervals, you see her intervals as more than 1 second apart because the signal from her to you has to travel farther. She sees your 1 second interval signals as being more than 1 second apart because your signal also has to travel more distance to reach her.

Both of you would disagree that the other was signaling at 1 second intervals. The best that either of you could say is that the other is signaling at a slower rate than at 1 second intervals.

That's Special Relativity. Is it real? Is she living slower than you? Are you living slower than her?

As far as either of you can tell: Yes.

Originally Posted by pranzo
Why does mass increase as you approach the spped of light.
That's General Relativity. GR describes the consequences of how the effects of SR are realized (I think that's what it is, but no way am I sure of this...).

GR describes what must happen to measured physical attibutes by all observers for all observers while the individual who does the accelerating to .99C accelerates to .99C.

Is she living slower than you? Are you living slower than her? GR defines who does the what, therefore describing who gets how old when.

SR is kind of easy - in no way do I understand GR.

Originally Posted by pranzo
I'm picturing light as something that moves at a speed that can vary (as has been proven, light can be slowed down in certain conditions.) What does light have to do with time? I was under the impression that time exists because of the motion of objects, so if there is no light, no photons, then we would still have the concept of time. I don't see why something cannot travel faster than the spped of light. If light can be slowed down, it is not constant, then surely anything travelling faster than this new speed would be travelling faster than light, but that doesn't mean time is altered in any way. So again, what does speed of light have to do with time. I'm missing something either very obvious or I just don't understand something.
The speed of light seems to be the top speed that any information about anything can be transmitted from one point to another in this universe.

That speed limit gives us the reference point for measuring the rest of the universe as we perceive it. Time is a measurement. Does time measurement between different observers differ?

Oh, yeah...

(And that's prolly' completely wrong....)
Last edited by DALeffler; 2006-Jul-21 at 04:01 AM.

19. pranzo,

That is some good insight. The ultimate speed of light is considered to be that which is travelled in a vacuum (free space), devoid of all matter. The reason it is the ultimate speed is because all matter is believed to be simply the "trapped" energy of light, E=mc2. Therefore, matter cannot travel faster than that which makes it up, which is light. Of course there is much, much more to it, but this seems to be the basis of it.

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Originally Posted by Flying Deuces
They, understanding the speed at which light propigates, know that the captain of the craft was 38 years old when he left Earth 10 years ago. And, they now greet the captain, and he's still 38 years old! Meanwhile, the people on Earth, with their super-powerful telescope, have been watching the craft on its 10-year journey, and they see a 38-year-old captain leave the ship.
If this were true, and if he doesn’t age, then he doesn’t move, and nothing inside him or the ship moves. His molecules are going to stop vibrating too. So won’t he freeze to death if every motion on his spacecraft stops? If he strikes a match then the match couldn’t burn out because nothing movies and nothing ages. In fact, he couldn’t move to strike a match. Your scenario doesn’t make any sense.

The problem that leads to the famous clock paradox is that the captain sees himself as aging from 38 to 48 but he sees nothing age on the planet he leaves and the planet he arrives at. So which is it when he arrives? Is he 38 or 48? Which aged, him or the people on the planet he flew to, who saw themselves age but didn’t see him age. When he arrives, how is he going to be both 38 and 48 at the same time?

21. concerning
light goes slower through glass...
I believe that in fact the speed of light is constant. It appears to travel slower through a medium but this is due to it interacting with matter, being absorbed and then re-emitted. The number of interactions (air < water < glass ) determine the observed effect however in between theses interactions it travels at c, no more and no less.

22. Originally Posted by max8166
concerning I believe that in fact the speed of light is constant. It appears to travel slower through a medium but this is due to it interacting with matter, being absorbed and then re-emitted. The number of interactions (air < water < glass ) determine the observed effect however in between theses interactions it travels at c, no more and no less.
I don't think so.

The speed of light is measurably slower through any medium other than vacuum.

As for refraction and reflection and Snell's law - have a look at Fermat's "Principle of Least Time". You can use any P2P application to download an excellent lecture by Feynmann on this subject.

clop

23. Originally Posted by Sam5
If this were true, and if he doesn’t age, then he doesn’t move, and nothing inside him or the ship moves. His molecules are going to stop vibrating too. So won’t he freeze to death if every motion on his spacecraft stops? If he strikes a match then the match couldn’t burn out because nothing movies and nothing ages. In fact, he couldn’t move to strike a match. Your scenario doesn’t make any sense.

The problem that leads to the famous clock paradox is that the captain sees himself as aging from 38 to 48 but he sees nothing age on the planet he leaves and the planet he arrives at. So which is it when he arrives? Is he 38 or 48? Which aged, him or the people on the planet he flew to, who saw themselves age but didn’t see him age. When he arrives, how is he going to be both 38 and 48 at the same time?
I've always heard it that time goes slower for the person moving at that speed, not for everyone else. So that that paradox isn't an issue, or rather, it is, but in reverse. So that the captain arrives and notices a massive change on the two planets but he, himself, is unchanged.

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Originally Posted by GDwarf
I've always heard it that time goes slower for the person moving at that speed, not for everyone else. So that that paradox isn't an issue, or rather, it is, but in reverse. So that the captain arrives and notices a massive change on the two planets but he, himself, is unchanged.
No, the Captain would see himself as aging during the trip. He will gradually age from 38 to 48 and he will have the wrinkles on his face to show that he aged. According to the theory, the motion is “relative”. No object can be considered absolutely stationary in the universe. All motion between objects is relative as long as the objects aren’t accelerating. To the Captain on the ship, his ship is “stationary” the whole time, and it is the two planets that are “moving”, and the Captain “observes” the clocks on the two planets slowing down but his own clock doesn’t slow down. To the observers on the two planets they see themselves as “stationary” and the Captain as “moving”, and they observe the clocks on the Captain’s ship slowing down but their own clocks don’t slow down. No one observes their own clocks slowing down or themselves as “moving”, both groups observe the other relatively moving objects as “moving” and the relatively moving clocks as “slowing down”. They all observe themselves as aging normally.

25. Originally Posted by Sam5
No, the Captain would see himself as aging during the trip.
True.*

Originally Posted by Sam5
He will gradually age from 38 to 48 and he will have the wrinkles on his face to show that he aged.
False. If he travels a distance that we measure to be 10 light years at an appreciable fraction of the speed of light, he will age significantly less than ten years.

Originally Posted by Sam5
According to the theory, the motion is “relative”. No object can be considered absolutely stationary in the universe. All motion between objects is relative as long as the objects aren’t accelerating. To the Captain on the ship, his ship is “stationary” the whole time, and it is the two planets that are “moving”, and the Captain “observes” the clocks on the two planets slowing down but his own clock doesn’t slow down.
True. How do we resolve this apparent contradiction? Remember, special relativity predicts not just time dilation, but length contraction. If we (assumed to be stationary with respect to two stars) measure the distance between those two stars as ten light years, then the captain (who sees the stars rushing toward him very quickly) will see the distance between those stars as significantly smaller. So he'll say that very little time elapsed, not because he felt like time was going more slowly, but because the distance he travelled was much less than ten light years by his measurements.

Originally Posted by Sam5
To the observers on the two planets they see themselves as “stationary” and the Captain as “moving”, and they observe the clocks on the Captain’s ship slowing down but their own clocks don’t slow down. No one observes their own clocks slowing down or themselves as “moving”, both groups observe the other relatively moving objects as “moving” and the relatively moving clocks as “slowing down”. They all observe themselves as aging normally.
This is also correct. And if you work out the math carefully, you will see that all observers will always agree on what someone else's clock should show as the elapsed time between two events, regardless of how they are moving. If that someone else is moving very quickly, that result will not agree with Newtonian mechanics.

* Assuming that he is not travelling at the speed of light, which appears to be impossible for anything with mass, but is travelling very close to the speed of light in our frame of reference.

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All it says is the hyperbolic nature of spacetime does not allow you to construct reference frames for objects travelling at c.

The proper time between any two events on a null worldline is zero, so you could argue that for anyone travelling at the speed of light they do not experince anytime at all between things which happen to them, BUT this argument has the flaw that as the object does not have any refrence frame there is no time coordinate to relate this proper time to.

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Hmmm... My guess is that having a ship that really goes at 1c is trouble. I would expect it is going to have infinite energy in relation to anything it may smack into, no possibility of control, and no way to stop...

I think it's quite likely that time would be infinitly dillated at lightspeed, sure, people in the same refence frame does not notice, but if the flow of one quanta of time inside equals an infinite amount on the outside, it sums up to the same in the end...

Then there is the infinite abberation, and the problem that the ship is , from the ships reference frame, going to be exposed to infinite amount of radiation, and what about gravity, energy has gravity too...

I am not sure how it all would sum up, but I do not think it would be good for anyone on the ship or in the same universe for that matter, after all, the ship would have to use more energy than any universe could possibly contain, wouldn't it?...

I wonder, though, if the universe were to end around a ship that did survive the jump to lightspeed, and is kind of sealed outside time, would it continue to exist... Would such a state be existance at all...

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Originally Posted by Flying Deuces
If a human-carrying craft could be devised that could travel at the speed of light indefinitely,and....
There is no "and". At the speed of light, time doesn't "happen".

Originally Posted by Flying Deuces
...since for the occupants aboard no passage of time is perceived, then would not the occupants be able to travel for an infinite amount of time?
They wouldn't travel for ANY time. It's not that time happens more slowly, it wouldn't exsist at all: It's null.

Originally Posted by Flying Deuces
In another thread I suggested that the speed of light is the speed of time. A way I can describe what I mean is to consider that we are in a "wind" moving at the speed of light. If stationary, we "feel" the wind, and we age and everything around us ages as well. But if we are moving "with" the wind (of time), moving at the speed of light, we do not "feel" the wind, and it doesn't affect us, i.e., we do not "age" relative to reference points not moving at the speed of light.
I'm not sure exsistance without time is exsistance (at least I think that's what I think TrAI said above...).

Originally Posted by Flying Deuces
Given that time is a dimension, travelling at the speed of light makes it possible to "move around" in that dimension. Unless we are moving at the speed of light, we cannot "move" in time as we move in three-dimensions. We are "in" the fourth dimension, time, but we can only "flow" along with it ... unless ... we are moving at the speed of light. Moving at the speed of light seems to be the gateway to our moving in the dimension of time. If we could travel at the speed of light indefinitely, it seems we could be called "immortal."

Now, I know that everything is not always as it seems....
My humble and respectful opinion is that traveling the speed of light closes time as a dimension: It doesn't open it for us to move around in.

Saying there's "no time" equates to saying there's "no up or down": How can we move through something that's not there?

(I reserve the right to be wrong - Always! )

29. Actually, when travelling at the speed of light, time does occur, but the trip appears to be instantaneous to the travellers. It would appear to them that they reached their destination as soon as they pressed the button that allowed them to travel at this speed. So it's not as if time ceases to exist, so they don't just freeze or anything, but that a distance is travelled in zero time in their frame of reference, so they would think they travelled infinitely fast. There is a difference there.

One can imagine it better for some velocity close to but less than the speed of light. The travelers press the button and their time of travel might seem to take only a few seconds in their frame of reference. But to an observer who is stationary to their original rest state (say on Earth), they will appear to be moving extremely slowly as we look through the windows (but still travelling close to the speed of light) and to us the time for the trip would seem much longer.

Just as an exercise, I decided to determine at what speed relative to Earth the travellers would have to go in order for them to think they were travelling at the speed of light. The apparent velocity would be v/[1-(v/c)2]1/2. Setting this equal to c, we get v=[.618033988]1/2 c=.78615 c. At this velocity relative to us, the travellers would think they were travelling at the speed of light because of their time dilation. So we can actually travel at any apparent speed we desire, even many times the speed of light, but this would only be because of the time dilation involved, and the light they receive will still appear to travel at c to them. In order for the travellers to accelerate to the speed of light from our perspective on Earth, however, they would then have to accelerate to an infinite speed according to their own frame of reference. But in order to do this, they would have to expel an infinite amount of energy.

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You cannot construct a frame for soemthing travelling alone a null worldine in relativity. End of.

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