Question Relativity and the past and future

[asprung]

I have been told that according to the theory of relativity if a man leaves Earth in a spaceship, and travels at a speed approaching I the speed of light his time passes slower than time on earth.

 

If this is so, when the man returns to earth his calendar date and his time would be behind earths; he would be in earths past, and earth would be in his future.

 

How could this be?

[iNow]

I have been told that according to the theory of relativity if a man leaves Earth in a spaceship, and travels at a speed approaching I the speed of light his time passes slower than time on earth.

 

If this is so, when the man returns to earth his calendar date and his time would be behind earths; he would be in earths past, and earth would be in his future.

 

How could this be?

 

There seems to be a slight confusion above.

 

If he left earth, no matter how fast he travels, he cannot return before he actually left.

 

This would only be possible if he went faster than the speed of light, which is considered impossible. Also, since he (and presumably his spaceship) have mass, then they could not even go at the speed of light itself (let alone faster than the speed of light).

 

As one gets closer and closer to the speed of light, the amount of energy it takes to accelerate further increases. So, the energy it takes to go from 80% the speed of light to 90% the speed of light is actually LESS than the energy it takes to go from 98% the speed of light to 99% the speed of light.

 

Either way, the only way the traveller could return before he left would be to travel faster than light (or, perhaps to through a hypothetical tunnel known as a wormhole). Even if he travelled AT the speed of light, and time on Earth passed more quickly than it did for him, he would still return at some point after his original departure (although, he may have only aged 1 year while earth several thousand).

 

It doesn't seem very intuitive, and often doesn't appear to make sense, but that's how it happens. The truth is sometimes stranger than fiction.

[theCPE]

I have been told that according to the theory of relativity if a man leaves Earth in a spaceship, and travels at a speed approaching I the speed of light his time passes slower than time on earth.

 

If this is so, when the man returns to earth his calendar date and his time would be behind earths; he would be in earths past, and earth would be in his future.

 

How could this be?

 

Its a thought experiment, that usually goes as follows:

 

If you have twins and one leaves earth in a spaceship at near the speed of light, when he returns to earth he is much younger than his earth bound twin.

 

It is due to a concept called time dilation, which was verified using tau particles and a particle accelerator.

 

Relativity is very interesting to read about and you can find many descriptive explanations using google for general information about relativity.

[ajb]

Even more loosely, everything travels in 4-d space-time. They only thing anybody agrees on is the length of any two journeys in space-time that start and end at the same points.

 

Thus, for this to be true any observers on different paths must experience different "amounts" of space and time in order for the total to be the same.

 

This is the origin of Lorentz invariance and all the other weird effects.

[asprung]

If the space Twins' time passes slower than that of his earth brother his clock and calendar would be behind the earth's, he would thus be in the earth's past.He would not arrive at earth before he started only at an earlier date than his brothers had then reached. How could the two get together

[iNow]

If the twin on earth were still alive when the travelling twin returned, the twin on earth would just be older. The travelling twin would not be in the past of the other. They would both be in the present, but both would have aged at a different rate.

[thedarkshade]

Heard of twin paradox?

[asprung]

What happens to the space twins clock and calender ? Do they just jump ahead? What happens to acts of the space twin that can effect the earth twins present are they just rewritten in his history?

[swansont]

What happens to the space twins clock and calender ? Do they just jump ahead? What happens to acts of the space twin that can effect the earth twins present are they just rewritten in his history?

 

The two flip through their calendars at different rates — they will disagree on the date and time. Nothing to do with rewriting history — they will agree on what happened, just not when it did.

[Dr.CWho]

The faster you go,

The shorter you are.

- Albert Einstein

[john5746]

What happens to the space twins clock and calender ? Do they just jump ahead? What happens to acts of the space twin that can effect the earth twins present are they just rewritten in his history?

 

When the space twin returns to earth, his clock and calendar would be "wrong". He would need to change it to match the earth bound clocks. Time is not absolute, so time slowing down for the space twin has no effect on the earth twin. They meet again in the same present, just with a different passage in time.

[asprung]

If the earth twin moves ahead intime where does it leave the space twin other than in his past? If they get together at the earths time the space twin must have jumped a time gap.At what point and where does this occur.

[timo]

Combining ajb's post #4 above with the information that in the framework of relativity the length of the path is the time experienced by the traveler on that path, http://www.scienceforums.net/forum/showpost.php?p=336818&postcount=4 might be helpful.

[iNow]

If the earth twin moves ahead intime where does it leave the space twin other than in his past? If they get together at the earths time the space twin must have jumped a time gap.At what point and where does this occur.

 

There are no gaps jumped. It's not intuitive, so I can appreciate why you are struggling with this.

 

 

The travelling brother experiences time more slowly than the earth brother.

The earth brother experiences time more quickly than the travelling brother.

 

When they meet, it is always the present, regardless of their personal experience of time.

 

The one who experienced time more quickly will have aged more than the one who experienced time more slowly.

They are both in the present, but they have aged at different rates... at different speeds.

 

 

 

There is no such thing as past and future. It's always right now.

[Janus]

If the space Twins' time passes slower than that of his earth brother his clock and calendar would be behind the earth's, he would thus be in the earth's past.He would not arrive at earth before he started only at an earlier date than his brothers had then reached. How could the two get together

 

The problem is in how you are visualizing time.

 

Maybe an analogy will help.

 

Here's the analogy of how you are looking at time.

 

You have two men starting at the same point and walking in the same direction at different speeds (the two twins aging at different rates.) In this scenerio, one man is always ahead of the other, and if "the future" lies in the direction they are walking, then the other man is always in his "past". (thus your confusion of how they could meet again.)

 

In this view "Time" is a absolute reference and everyones movement through time is measured with respect to this reference.

 

But this is not how "Time" works. Instead it works more like this:

 

The two men start off at the same point, but don't walk in the same direction, but at an angle to each other. Again, one man walks faster than the other. At some point, the faster walking man changes direction so that he will now intercept the slower man. After the two men meet again they will have walked different distances, with the faster man having traveled the longer distance. (This is like one twin being older when they meet up again.)

 

In this case, "Time" is not a universal absolute measurement, but a relative one. Just as as the two men can walk different distances starting from the same point and still meet up, The two twins can age differently and not end up in each other's past or future.

[asprung]

The man walking in the longer direction is in the year 3000. The man waking in the shorter direction is in the year 2008 . At what point does he jump to the year 3000.

[insane_alien]

there is no jump. you're still thinking of time as an absolute.

[Janus]

The man walking in the longer direction is in the year 3000. The man waking in the shorter direction is in the year 2008 . At what point does he jump to the year 3000.

 

How do you get that from the second analogy? Both men end up next to each other at the end. Each man just took a different route to get there, with one of them taking a longer path.

 

In the analogy, the "time" it takes to get to that final point as measured by each man is equal to the number of steps he had to take.

 

They start at the same point and end at the same point, but each take a different number of steps(time) to get there. There is no need for either to "jump forward or back".

 

Again, time is not the absolute progression you keep wanting to make it.

[Norman Albers]

OK. Many of us know the result. What distinguishes the younger one?

[asprung]

If the twins end up at the same point why are they not the same age?

[swansont]

If the twins end up at the same point why are they not the same age?

 

They took different sized/number of steps in time (to use Janus's analogy)

[iNow]

If the twins end up at the same point why are they not the same age?

 

It's not intuitive, but they are not the same age because one was moving faster than the other.

 

Have you read about the hafele keating experiment where they had four insanely accurate atomic clocks? They synchronized the clocks. Two stayed on the ground, the other two were put into an very fast jet and flown around the world. When the plane landed, the clocks that they took around the world had "ticked" fewer times than the clocks on the ground. When they compared the clocks side by side, they were both in the present, despite the fact that one had ticked less.

 

Same with the twins.

 

 

Check out this video:

[swansont]

Have you read about the hafele keating experiment where they had four insanely accurate atomic clocks? They synchronized the clocks. Two stayed on the ground, the other two were put into an very fast jet and flown around the world. When the plane landed, the clocks that they took around the world had "ticked" fewer times than the clocks on the ground. When they compared the clocks side by side, they were both in the present, despite the fact that one had ticked less.

 

I believe they flew four clocks, and compared them with the ensemble remaining at the Naval Observatory. While they were Insanely Accurate™ for the era, current clocks are better, some by a few orders of magnitude.

 

Better still for the experiment, they flew the clocks again in the other direction, and again confirmed relativity; the fact that the earth is rotating means the planes' speed with respect to an inertial frame is different if you go east vs going west (the rotation of the earth means it is not in an inertial frame). Because the clocks going west were going slower with respect to an inertial observer, those clocks actually ran faster than the ground station clocks.

[iNow]

That makes sense. If the earth is spinning (rotating), and a clock is left on the earth relative to one in a plane, then you must account for the fact that the clock left on earth (not brought on the plane) was spinning (rotating) with the earth.

 

Also, thanks for the clarification above. When I typed my post, I was doing it from memory and thought that one clock was on the plane and one clock was on the ground, but saw the reference to four clocks in the wiki link so adjusted my words.

 

So, with the currently even more accurate clocks, my guess is that they can confirm the predictions on even shorter flights (aka... not complete circumnavigation of the globe).

 

 

Seriously. How could anybody waste their time with creationism when reality is so wonderous and awe inspiring.

[Norman Albers]

What is the essence of "inertial frame" here? I find confusions. It seems we are saying that rotation of the planet is an upset of that. By themselves, velocities are relative, so I think this is a subtle and cool point. It relates to my previous question.