# time dilation + time dilation = time travel?

say you are going in a ship at 99% of the speed of light, and you fly by a guy going 99% of the speed of light in the opposite direction

your speed relative to him is 198% of the speed of light! oh my god!

what now?

Now you get to see the big bang.

dammit uniqor

please, the one good physicist member of this forum, please answer me

Drag Albert out of his grave and you will get nothing.

the speed of light in any direction aaway from any object is always c (300000 m/s). the fastest any two objects can seperate, by that rule, is c. that’s the whol concept of (special?) relaticity. so, you would not have time reverse. you would only be moving apart at 99% the speed of light. it’s tough to grasp without understanding the theory of relativity…and i don’t feel like explaning it now.

well i already understand it luckily, ill do that part for you

you watch a train go by in front of you at 10 mph and a guy on the train throws a ball in the direction that train is moving at 5 miles an hour. you see the ball go by at 15mph. then the guy turns on a flash light, he sees it go the speed of light and so do you, not at c+10 like youd think.

and thats because time slows down while hes on that train, relative to the guy next to him on the ground who is ‘not moving’. so when that guy on the train measures time, things happen slowly for him, including his perception of the slow outside world. hell see you on the ground move past him faster than 10mph? since his brain is slow?

well then the 99%c ships should also see their counterpart moving by at plenty faster than 99%c due to the brain slowing effects of the time dilation effects of going fast relative to the other guy?

Personally, I don’t agrree with Einsteins ‘You can’t travel at the speed of light or you will become light’ because frankly, if you run as fast as a car, you dont become a car.

you just answered yoru own question…it’s the same situation, just a different numbers.

Your mass also increases (in theory) as you approach the speed of light. the only thing that can travel at the speed of light without having an infinite mass is light. therefore, if you are traveling at the speed of light, then you’ve become light.

Actually, cars do not have a defined speed, light does.
How did I answer my question? Please let me know what I told myself.

Think of passing buy a tree in the distance, you say relative, so how close is the ship?

It could be that the magnetic force between you and the ship would tear space/time continuem creating a slip through time ( alittle scifi there)

Or it could be that the ship is a lightyear beside you in which case it doesnt matter.

And according to einstein, if your not going the speed of light, your fine, it and the same principal would occur if you were travelling 30 mph as 99% the speed light.

i think your confusing the statement “only light goes at light speed” with “youll turn into light when you go fast”

you keep getting more massive as you get up to a considerable fraction of the speed of light, like hundreds of thousands of miles per second? and the graph of the force of your spaceship’s engine vs your weight considering the effect that speed has where it makes you heavier as you get up there, that graph will be an asymptote so that no matter how forcefull our engines get, some decimal value will either keep adding .9’s or .0’s

and for some reason light doesnt have this problem, it just always goes speed of light. if we want to physically go at the speed of light, we would have to somehow turn things into light and back again, not just expect the universe to do it for us.

what does that mean

refer to whats called the ‘light clock’ metaphor einstein uses and imagine that everything in your body goes at exactly the speed of light, no matter what relative speed an observer is viewing you from. according to that observer, he sees how much of a distance your bodys particles have crossed going light speed, combined with the fact that he sees them cross his vision at exactly light speed regardless of how fast the spaceship is going, he says that you are going slower than you think you are.

you measure how fast your light speed particles in your body are moving by also taking into account how fast your ship is going. they look to you like they are moving at light speed, and you are also inside a spaceship going real fast, so add them up, multiply by distance and thats how long it took you. the observer on the ground will not take into account how fast your ship is going because he sees your particles inside your body moving at exactly the speed of light, not s.o.l. plus speed of spaceship like you do

so his view of light is (speed of light)(the distance he watches you travel across the sky)=(ambiguous time value) while yours is (speed of light)+(the speed the spaceship is going)(the distance you observe yourself travel, same value as before)=(the same time value as before)

i guess you could say that the mechanism is a space time rip that causes all observers to observe particles moving at the speed of light to always be moving at the speed of light, even if the spaceship should make it look faster

uh something like that

actually, your train cars have a defined speed of 5 and 10 mph. The point is it doesn’t matter what that number is…it could be 299999 m/s anmd it’s the same situation that you described.

As for your other statement, i know full well the difference. If you read between the lines, however, you’ll see that it implies the only way you can attain the speed of light is to be light.

Light can go at the speed it does because it has an extremely tiny mass, or at least M-theory describes it that way. to have a finite momentum, it must travel at the speed of light.

Ntoice that I said “momentum” not “energy”. photon energy varies with the photon equivalent of angular momentum, i.e. the frequency.

so wait tminion, i never understood what will happen

say you are on the ground watching the two ships go past eachother both at 99% of c.

surely, as you watch them, you calculate their relative speed to be well over the speed of light.

math…hard! rah!

well, yes…You see 2 ships moving at 99% of c, but you can’t judge their relative speed from where you are. You can only judge relative speed between 2 objects if you are (or are in the same reference frame as) one of the objects. If you were on one of the ships, then time dialates at 99% of c, and you would only see (in ship time) the other ship moving away at very close to c.

Now, since they are moving apart in opposite directions, then in the time frame on earth, the distance between them is increasing at a rate of 2*c. Do you see the difference?

yeah, the distance changes at 2c

if im on the ground, not moving, and i see one ship go by at 99%c, and i see another ship going at 50%c in front of him, so that ship 1 is appraoching ship 2 at 49% c, then i can calculate the time dilation effects of 49% c, and i can guess that thats time dilation that will occur for them.

that would be a slightly different math problem, but it worked. whats the difference between that one and the one if they are going in opposite directions?

when your on the light speed ship, time is slow for you, so you see things out the window go super fast. anything going anywhere near the speed of light, and possibly even things that dont are going to appear to go much faster, just because youre in your hypersleep deal where youll look like your frozen to outsiders.

so where does the measurement take place that determines what happens here? i am like a whole step of abstraction away from understanding this i think. i do have a full book reading experience with the theory though. im just not clear on how the light clock example makes the phenomenon happen.

you’re misunderstanding the nature of the time dilation. You aren’t in side a little bubble of slow time, so that youc an see normal time outside the windows going super-fast. time, in every way you perceive it, slows down. The other ship, which is travelling at 99%c compared to a “stationary” reference, is really travelling at 99% + a bit of c away.

Imagine it this way: if the other ship emits a pulse of light for every 300000 meters travelled, then the stationary reference point will detect pulses once every second or so. however, your ship is moving away from those pulses at 99% of c, so you detect them at a much greater time interval due to the doppler effect. Therefore, the speed you calculate from that information is not 300000m per 1 second, but 30000m per x seconds, which is much slower. In that way, time dilation makes the relative speed between your ships always less than c.

In your math problem, if you factor in time dilation effects, then the result will give you the same thing as if you measured it directly from one of the ships. I’ll show you exactly how when I get home from work about 3.5 hrs. from now.

[contented edited by ILP]

yea, exactlky…saved me the trouble there…thanks

If the theory is that in order to “travel” through time, we must go at/faster than the speed of light, then why not slow down the speed of light in an isolated chamber to make going faster an easier task? If I am correct, doing a google search turned up an article on scientists slowing down light to 38mph, although on atomic scale (nytimes.com/library/national … light.html). The latest theory is that slowing light is the key to time travel, and in my own opinion, I feel that it might have some merit.