# Time Dilation - Help me understand

#### lyndonguitar

##### I play games
[yt]G-R8LGy-OVs[/yt]

I've been trying to understand the phenomenon called Time Dilation. by reading a couple of sites and watching alot of vids including that one... I finally understood most of it, but somethings really BUGGING me.

Based on what I understand... Time is relative to the observer because it can be bent by 'light' to preserve the absolute rule that light travels at 1c per second.

Relative to your position if you observe that Light travels slower than 1c per sec, time would seem to bend in that particular place to preserve the rule of the speed of light.

EARTH PERSPECTIVE:
A spaceship travels fast going to another star, observers from earth, from their point of view, observes that the earth's time is normal, but they observe that the ship's time is going slower. and so for every 2 earth seconds = 1 ship second. (JUST AN EXAMPLE)

10 years had passed on earth and the Spaceship returns.. the astronauts on board had only aged 5 years and the spaceship timer started at the start of the journey also only counted 5 years.

SHIP PERSPECTIVE:
A spaceship travels fast going to another star, observers from the ship, from their point of view observes the ship's time as normal. but earth is going far away, therefore they are observing it as a moving object, and so from their point of view they observed that the earth's clocks are running slower. that would mean for every 2 ship seconds = 1 earth second.

10 years had passed on earth and the Spaceship returns.. the astronauts on board had aged 20 years and the spaceship timer started at the start of the journey also only counted 20 years.

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In both scenarios 10 years had passed on earth. however this is the problem, would the astronauts age 5 years, or 20 years? If so what is wrong my examples? Would this prove the existence of a multi-verse? or what?? anyone got better examples?

HELP

#### 1freedude

If I remember correctly, it has to do with speed, not direction. A satellite around earth doesn't change its distance, but time dilation still occurs.

Also, the trip towards the other planet will dilate time on that planet, too.

#### silkstone

The astronauts would have aged 5 years. Time slows down (relatively) for the object travelling at higher speeds.

I.e. The astronauts would have reached the star and come back, and it would appear to them that only 5 years has passed, but everyone on earth would be 10 years older.

#### lyndonguitar

##### I play games
The astronauts would have aged 5 years. Time slows down (relatively) for the object travelling at higher speeds.

I.e. The astronauts would have reached the star and come back, and it would appear to them that only 5 years has passed, but everyone on earth would be 10 years older.
but relative to the astronauts, the earth traveled at high speeds, wouldn't time be slowed down(in earth) in their perspective? what's what the video says http://youtu.be/G-R8LGy-OVs?t=7m17s

#### 1freedude

Relative to the speed of light, not object

Relative to the speed of light, not object
But relativity says there are no absolute reference frames, and you can't measure your own speed against the speed of light because it's always equal to c no matter how fast you're travelling.

The earth is hurtling around the sun, which is in itself hurtling around the galaxy, and yet we percieve ourselves to be at absolute rest. Would not an observer, on board a particularly energetic planet travelling at an even higher speed, still perceive himself to be at absolute rest and see us as a particularly fast-ageing species/planet when observed through his uncannily powerful theoretical telescope?

#### Jack Doph

From what I remember, it exactly comes down to the observer - thus, position is most relevant (hence "relativity").
From one point of view, time passes at one rate, from another viewpoint of the exact same event, time passes at a different rate.
They are both correct in their individual observations (they both run at the same time), relative to themselves, but different, when compared to the other's observations.
Time itself does NOT, in & of itself, slow down - this is simply how it looks compared to the observer's observed point.
Perhaps an easy example: the car you see speeding down the road, seems to travel awfully quickly, whilst the 'flying car' high above, seems to take forever to disappear from view.
HOWEVER: in the time it takes these vehicles to disappear from sight, the car may have only travelled (for argument's sake) 1 kilometre, whilst the 'flying car' overhead travelled the same distance, yet seems to have moved very little.
As the 'flying car' is clearly further away, it stands to reason that the closer the observer is to the object observed, the faster it *appears* to be going, when in actual fact, this is not the case at all.

There was once an example given to Einstein about a bus:
"if a bus travelled at the speed of light, but a passenger runs from the back to the front, would that person not travel faster than the speed of light?".
However, if you observed such motion from a different point of view, this may appear to be the case, when, in actual fact, this person only travelled as quickly as possible, within the confines of said vehicle.

Relativity is both the easiest & perhaps the most complex matter to get your head around.
Knowing when to apply this, I think, is probably the trick to work out.
I think NASA & ESA are very close though

BTW: thanks for posting this most intriguing poser ^^

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#### Inceptor

But relativity says there are no absolute reference frames, and you can't measure your own speed against the speed of light because it's always equal to c no matter how fast you're travelling.

The earth is hurtling around the sun, which is in itself hurtling around the galaxy, and yet we percieve ourselves to be at absolute rest. Would not an observer, on board a particularly energetic planet travelling at an even higher speed, still perceive himself to be at absolute rest and see us as a particularly fast-ageing species/planet when observed through his uncannily powerful theoretical telescope?

Motion is relative to our frame of reference. In the example of the Astronaut(s), their frame of reference is 'anchored' to the Earth -- so, within that reference frame, the Earth is virtually motionless. There is no universal reference frame, it's all relative motion, there's no use in trying to think of some kind of universal reference frame. If you want to imagine yourself as being in motion at c velocity, then everything in your reference frame would be in motion at the speed of light, everything would have to be in order to preserve causality, so in effect, you and your reference frame are all travelling at c, preserving causality, so that everything works as it should...meaning you and the entire observable universe...then you aren't moving... because everything is moving with you. It all cancels out, figuratively speaking. Stop thinking of universal frames of reference.

#### silkstone

but relative to the astronauts, the earth traveled at high speeds, wouldn't time be slowed down(in earth) in their perspective? what's what the video says http://youtu.be/G-R8LGy-OVs?t=7m17s
If i remember correctly, If it were possible for the people in the spacecraft to view people on earth, they would appear to be moving in slow-motion and vice-versa.

#### Huddo93

##### New Member
I see your problem, and with the basic idea of time dilation there is no explanation, it is simply a Paradox. Look up "The Twin Paradox" and you can find a way they "solved" the issue. In answer to your question: "When they meet up on earth after the trip, the ages of the space traveler moving close to speed of light and the stationary observer, the space traveler will be seen to be 5 years younger that the observer on earth.

In a little bit of self investigation (wikipedia ), the difference in time elapsed for the observer and the space traveler is due to the fact that the space traveler goes through a "U turn" acceleration from when he heads away from earth and then turns around the head back. At the point where the space ship comes to rest, before starting to move back towards earth, it must re asses its relative frame to earth and thats where the actual time difference occurs, making the space traveler physically younger.

You must remember when looking into things like Special Relativity that it isn't the whole story. A lot of newer discoveries in string theory, much better fit the laws of general relativity more than that of special relativity (don't be fooled by the names either, general relativity isn't "general" but rather very complex).

Good luck on your journey for knowledge, if your after something else to get your head around, try reading introductory books on string theory and the like. The one I just read about 4 months ago is a small book (around 200 pages) called "The Little Book of String Theory by Steven S. Gubser" and its a good way to start off giving a basic understanding of how the basis of string theory and general/special relativity do and don't fit string theory.

#### lyndonguitar

##### I play games
If i remember correctly, If it were possible for the people in the spacecraft to view people on earth, they would appear to be moving in slow-motion and vice-versa.
That's my problem, from each perspectives, they view the other one as going slow motion = meaning they would both age faster in their perspective

I see your problem, and with the basic idea of time dilation there is no explanation, it is simply a Paradox. Look up "The Twin Paradox" and you can find a way they "solved" the issue. In answer to your question: "When they meet up on earth after the trip, the ages of the space traveler moving close to speed of light and the stationary observer, the space traveler will be seen to be 5 years younger that the observer on earth.

In a little bit of self investigation (wikipedia ), the difference in time elapsed for the observer and the space traveler is due to the fact that the space traveler goes through a "U turn" acceleration from when he heads away from earth and then turns around the head back. At the point where the space ship comes to rest, before starting to move back towards earth, it must re asses its relative frame to earth and thats where the actual time difference occurs, making the space traveler physically younger.

You must remember when looking into things like Special Relativity that it isn't the whole story. A lot of newer discoveries in string theory, much better fit the laws of general relativity more than that of special relativity (don't be fooled by the names either, general relativity isn't "general" but rather very complex).

Good luck on your journey for knowledge, if your after something else to get your head around, try reading introductory books on string theory and the like. The one I just read about 4 months ago is a small book (around 200 pages) called "The Little Book of String Theory by Steven S. Gubser" and its a good way to start off giving a basic understanding of how the basis of string theory and general/special relativity do and don't fit string theory.
Thanks for the explanation!, now I understand, SO it is called the Twin Paradox!!

#### DannibusX

[yt]-CIs3jOnfiM[/yt]

#### lilhasselhoffer

SHIP PERSPECTIVE:
A spaceship travels fast going to another star, observers from the ship, from their point of view observes the ship's time as normal. but earth is going far away, therefore they are observing it as a moving object, and so from their point of view they observed that the earth's clocks are running slower. that would mean for every 2 ship seconds = 1 earth second.

10 years had passed on earth and the Spaceship returns.. the astronauts on board had aged 20 years and the spaceship timer started at the start of the journey also only counted 20 years.

---------------------------------------------

In both scenarios 10 years had passed on earth. however this is the problem, would the astronauts age 5 years, or 20 years? If so what is wrong my examples? Would this prove the existence of a multi-verse? or what?? anyone got better examples?

HELP
No. You have not set a perspective, and you have gotten the statement in reverse.

Case 1: Earth Reference
Earth Relative Velocity=0
Ship Relative Velocity=V
Earth observer has 10 years of time pass. Traveler has 5 years of time pass.

To an observer on the ship 2 seconds on earth will be observed as 1 second. Earth will appear as if being fast forwarded, while the ship will appear to be moving at normal speed.

Edit:
Time dilation is rarely accounted for in real life. Rarely do bodies on the surface of earth move fast enough to experience appreciable dilation. One place where this is accounted for though is GPS. The observer is stationary, but each satellite is orbiting the earth at a great speed. As such, the triangulation algorithms need to account for the delay due to dilation. If the dilation did not get accounted for you wouldn't be able to use gps to get any more accuracy than a few dozen meters.

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#### silkstone

Another interesting aspect of travel at relativistic speeds is contraction. I forget the correct term for it, but it's when things get smaller the faster they travel.