Time dilation

josh (Luke) · #1 15-05-2006, 02:34 PM

Is that how you spell it ?
I just read about a hipothetical space ship taking a round trip to a star 100 light years away. They are traveling a fraction under the speed of light. It said that those on board would age 2 years but 200 years would pass on earth. Everyone they knew would be dead when they got back. I just have alittle trouble understanding the hows of it . Can anyone help?
Thanks for any smarts you can all share.

josh (Luke) · #2 15-05-2006, 02:44 PM

Check out my question in general chat. Its very, very important. If we can come to a firm decision i feel it will change the world we live in for the better!

ving (David) · #3 15-05-2006, 02:48 PM

nice post count josh

while i understand how it all works explaining it is another matter... one that is boeyond me. but some bright spark will bwe able to do it

davidpretorius · #4 15-05-2006, 03:56 PM

ok, here's my spin on it,

travelling at the near the speed of light and even higher in the earths gravity will mean that time will "tick" away at a different rate. This has been proven scietifically by flying an atomic clock up up in a plane and then comparing it to a synchronized gound based one.

So, on the trip out to the stars at a CONSTANT speed, time will be different for the astronauts that the earth based guys. I can't remember who will be older though.

Now the problem is that special relativity does not allow for acceleration, and so the problem happens when you get to the star and decelerate to turn around and then accelarate to come back. This from memory is where all the comparative aging occurs

I will grab my special relativity book and quote from it tonight

robagar · #5 15-05-2006, 07:46 PM

yep, according to special relativity a moving clock appears to go slow. So from the point of view of the people on Earth, the spaceship clock ticks slowly and passengers age more slowly than themselves. But the odd thing is this: from the point of view of the the passengers, the clocks on Earth are the ones moving and running slow! (this is known as the Twin Paradox btw)

As David says, it's the acceleration that takes it out of SR's domain and breaks the symmetry between Earth and the spaceship people, so it the passengers who stay young.

fremanwarrior · #6 15-05-2006, 08:06 PM

This has levered a fantastic memory from my child-hood. This was in fact one of the very first really cool fact things I can remember discovering for myself. If my memory is correct I read about this off a super cool (well I remember it as super cool) SPACE 1999 wall poster!

http://www.space1999.net/~catacombs/...eum/index.html

drmorbius (Randall) · #7 16-05-2006, 08:28 AM

Hey Josh...

If that kind of stuff interests you, then I heartily recommend the book "The Elegant Universe" by Brian Green.

It is a laymans desciption of Superstring theory, which covers Einsteins' General and Special Theories of Relativity AND Quantum Mechanics. It's a very complicated subject, but he does a pretty good job explaining it all.

And the concept of time slowing down as you approach the speed of light is nothing compared to what gravity REALLY is.... and the theory that the fabric of the universe has 11 dimensions. I loved it!

BTW - Green also did a series that was shown on SBS... you can get the DVD at some bookstores.

robagar · #8 16-05-2006, 08:49 AM

yeah Brian Green's The Fabric of the Cosmos is a good 'un too. As it says on the back cover - "the new Hawking, but better"

drmorbius (Randall) · #9 16-05-2006, 09:12 AM

Nice title robagar...

If you don't understand his Calabi-Yau reference... you have to read the books!

robagar · #10 16-05-2006, 09:54 AM

thanks doc!

I was beginning to think it was perhaps a smidgen too nerdy

josh (Luke) · #11 16-05-2006, 10:28 AM

Thanks randall, ill give it a try.
Its good to hear your takes on it. Its a difficult topic to grasp. Brings up all sorts of question about time/reality and what it really is.... if anything.
I think i need a good book and lots of time (no pun}

AGarvin · #12 16-05-2006, 10:10 PM

Special Relativity does most folks heads when they first suss it out. Time dilation is derived through what are known as the Lorentz Transformations. Conceptually it's not that hard to understand once properly explained, just difficult to explain on a message board. A few simple diagrams can make it much easier. I'll give it crack though.

Basically, it centres around the fact that the speed of light is the same for all observers. The stationary observer will see the light travel further than the person in the ship will for the same event. If the distance is further, but the velocity is the same, then the time taken must be 'longer', or dilated.

Imagine the person in the ship, we'll call him the spaceman, is holding a mirror up to his face, and our event is a beam of light travelling from his face to the mirror, a distance of d. According to our spaceman, the light ONLY travels from his face to the mirror, distance d.

Now the 'stationary' observer sees it slightly differently. Since the spaceman is moving with uniform velocity, our spaceman has moved, according to this observer, between the time the light leaves the spacemans face and the time it reaches the mirror, a distance of d2. So according to the stationary observer, the light has actually travelled d + d2. Even though he's seeing the light travel further, the velocity of light is the same for both of them.

Using insanely oversimplistic math, (not to mention non relativistic), and plugging in a few nice round figures to illustrate the point, you have: t = d/v (t=time, d=distance, v=velocity)

According to spaceman, d=200, v=100, so time=2 (200/100).
According to observer, d=400 (d+d2), v still = 100, so time=4 (400/100).

In a classical, or non relativistic sense, the velocity of light according to the observer would be increased by the velocity of the spaceman, so even though it travels further, it would do it with a higher velocity, so time integrity would be maintained. Not so with light. All observers record it as having the same velocity. So according to our spaceman, it took 2 time units for the light to go from his face to mirror, but according to the observer, it took 4.

For the purists, the actual Lorentz formula is:

t0 = t1/sqrt(1-(v/c)^2)

where t0 is time in your reference frame lasting t1 seconds in a frame moving at v.

Hope I havent rambled on to much (and all of this makes sense).

Cheers,
Andrew.

josh (Luke) · #13 17-05-2006, 10:38 AM

Thanks Andrew.
It seems the harder i look , the more questions appear. I was never that good with maths so its pretty hard for me to grasp, but it think im getting there.
Ill go away and think, and if my mind is still working ill come back with some more questions.
Here goes

xelasnave · #14 29-05-2006, 02:37 PM

Andrew if you are still there???
Is the effect observational or real in your view? I imagine yet another observer "standing" elsewhere may see things differently?
alex

AGarvin · #15 30-05-2006, 09:18 PM

Hi Alex,

As far as I know it's very real and very observable and has been proved using such tools as satellites and atomic clocks, and I think more recently by somehow measuring lasers via spectroscopy. Don't ask me about the actual experiments though as I don't know much about the details. I'm guessing there's been a number of experiments over the years.

And yep, each observer will see things relative to his frame of reference.

Cheers,
Andrew.

xelasnave · #16 05-06-2006, 10:22 PM

I have been trying to find something but I will go on without it. Does not the time remain the same but because of the speed of light each will "see it" differently...and yet another observer will see the times as different again... but in the real world has anything changed? I think the atomic clock observations are dubious. As you say you are not sure about the experiments, neither am I but I have not been able to find anything conclusive.
alex

Starcrazzy · #17 05-06-2006, 11:45 PM

Hey giys..time dilation is there alright and a very real consequance of the c constant...If you imagine a guy bouncing a ball up and down on a train, according to him the ball is travelling down to the florr and bouncing back up to his hand , all this takes a certain amount of TIME..and the ball has traveld a certain distance say 2 meters..now...an observer standing on the side of the track and looking through the glass wall (hehe) would see something completely different...( i forgot to mention the train is going along at 100 km/h) he sees the ball travell forward around 10 meters, hit the floor and rise up and travel forward another 10 meters as is goes back into the blokes hand...now...the two observers agree on the TIME it has taken, but not the distance...and seeing as the SPEED is distance over time, they can not agree on its speed, and the kicker is, neither has a privaliged point of view...while the train is travelling at a constant speed the ball still obeys all the laws of motion(on the train) .....there ya have it...time dilation on a train...lol...its a real and measurable quantity, they teach it to year 12 physics students and test there use of the formula...The effects are also quantised for the use of gps sateelites and so-on, they require an absolutely accurate measure of time to give realiable readings and have a dilation factor built into them becasue of there speed...hawking alex...you NEED him in your life...if you pm me your adress ill send you a couple of his books....

ballaratdragons (Ken) · #18 06-06-2006, 01:02 AM

OK, you are all talking about a Spaceman leaving Earth and travelling at the speed of light and does he age or slow his own time compared with Earthbound folk.

What if the scenario was reversed. A spaceman coming from a distant star system at the speed of light! Is he coming from our future but his normal time or from the VERY distant past just like starlight?

I love throwing spanners into the works, but try sort that one out.

xelasnave · #19 08-06-2006, 08:54 PM

I have read a fair bit of Hawking its just that I dont see things the way others see things I guess. A mental impass or a belief that there is something not quiet right. That makes me a laughing stock but that is of little concern.
The guy on the train has the same opportunity of making the same calculation as to the speed of the ball if he simply inputs all the information available to him. He in not unawatre of the speed of the train and how its speed will contribute to the speed of the ball... ok its different at the speed of light.
I found somehting today (that I left at home) in a mag re a star that is exhibiting time dialition and doppler effect simultaneously. I thought that would be a real life example to walk us through as to how they see time dialation .. but effectively they say because of the speed of the outflowing gas the effect is present..I admit a could not understand it on what was presented but as I said they claimed it was a "live" example so I will follow it up
alex

xelasnave · #20 08-06-2006, 09:14 PM

In the case of the different ageing of the person on Earth compared to the person travelling at C I cant see how the perceived differences translate to a physical difference in their ageing. The senerio is impossible to test in real life but I think people lose site of the fact we refer to "observations" of time change but surely to some degree all the observations made in these cases are personal to that observer and in the total picture are somewhat incorrect (except from the point of view of that observer). I can not see how observation changes anything in real terms.
I know there is so much I dont know that if I did know I would not say this.. but I do know a fair bit and have read outside Hawking to the point where I think I grasp a lot of this stuff. I think there is a slip between what can be "incorrectly observed" and the facts, which allows our minds to be carried in a dirrection that entertains things which are impossible to accept (time travell of a sort). The fundamental of frame references seems to me to give a message that the further one stands back the more likely one can make a fair observation but nevertheless that frame also is subject to the same scrutiny from elsewhere which may present yet a further varied picture to that observer.
But does all this whatching really change anything... each frame is relevant only to that observer, that is not big deal but this mental exercise (and others) used to explain the principles of relativity cause those involving themselves in explanations curiously entangled in a non real explanation of the facts... the only place to obtain those facts is from a distant observer who can point out to those on the train and those on the platform that all is not as they see it unto themselves.
alex