# Space & older

1. Feb 4, 2006

space & "older"

hi
i heard once if you go out into space you get older faster than if you were on earth... any truth in this? if so...why?

thanks

2. Feb 4, 2006

### mijoon

Actually, the opposite occurs. Generally speaking, an astronaut ages more SLOWLY than those of us on Earth. This has nothing to do with being in space, but rather it is because they are moving with respect to us. The same effect is seen in particles accelerated in a physics lab. This effect is described in Einstein's Special Theory of Relativity. Time dilation also occurs due to gravitational fields, as described in Einstein's General Relativity Theory.

3. Feb 4, 2006

### PatPwnt

It has nothing to do with being in space. It has to do with relative velocities. The faster you go, the slower you age relative to someone standing at rest. Also, someone at rest will appear to slow down in age if you are moving by them at a velocity. This is at any velocity. The faster you go the more apparent it is. Speed is a time machine pretty much, but you can only go into the future with it.

4. Feb 4, 2006

### prochatz

That's false. Being on space doesn't effect time. Velocity is responsible for what you 've heard. For example, imagine that you have a twin brother (if you already don't have one). If he travels around our solar system(!!!) for some years(!!!) and return back to Earth, he will be younger than you because of the spaceship's velocity. The faster you move, the longer dilation of time occurs. This also happens in ordinary life. But the results aren't obvious because of the very slow velocities we move around. In order to have obvious results we must observe objects (e.g. particles) which have velocities near the speed of light in the vacuum.

5. Feb 4, 2006

### Mk

Yes, the difference in relative velocity causes time to move slower for the faster moving object.

6. Feb 5, 2006

### rcgldr

It's how the velocity is acheived, since velocity is relative to a frame of reference. It's the object that is accelerated that experiences a change in the rate of time.

It's also possible that the reference to astonauts getting older in space is due to radiation, since there is no atmosphere to protect them.

7. Feb 5, 2006

### Pengwuino

Radiation does not make people older in the sense that "older" means moving forward through time. They suffer health related complications.... and i guess you can call that "getting older" in a sense hehe.

8. Feb 5, 2006

i still don't understand. so if i had a really fast car/train/etc, that went almost at the speed to light, and i went in it for 2 years, how old would i be when i come out? (approx). if i had an identical twin, and i went in the really fast car (or went into space in a rocket or whatever), if i went for enough time, would i look older than my twin if he just stayed on earth

thanks

9. Feb 5, 2006

### Pengwuino

Well a key thing in relativity is who holds the "proper time". If you're in the spaceship and you have a watch, you would have "proper time" so we can say it is 2 years for you.

$$T' = \gamma T_0$$
$$\gamma = \frac{1}{{\sqrt {1 - \frac{{v^2 }}{{c^2 }}} }}$$
$$T' = \frac{{T_0 }}{{\sqrt {1 - \frac{{v^2 }}{{c^2 }}} }}$$

To = proper time
T' = relative time
V = speed of spaceship
C = speed of light ( ~ 3.0 * 10^8 m/s )

For you, the time would be two years. For your twin who is on earth, more time will have passed. The person who holds the "proper time" is always going to be "younger"; that is, he has the smallest time change.

10. Feb 5, 2006

### rcgldr

See my next post below for why an astronaut would age quicker. This post explains why they typically age slower.

If I remember correctly, if a rocket could sustain 1G of acceleration, then in about 1 year, it ends up around 1/2 C (as viewed from an earth based observer). So on a 4 year round trip, 1 year to speed up, 1 year to slow down and turn around, another year to speed up on the return path, and the final year to slow back down to earth speed, the person who underwent the acceleration would be younger than the person who remained on earth. I don't know the math to calculate the exact amount.

There's also a gravitational effect on time, but assuming that the rocket doesn't spend much time in strong gravitational fields, this aspect can be ignored.

Last edited: Feb 5, 2006
11. Feb 5, 2006

### ZapperZ

Staff Emeritus
Still, this time dilation effects make the person in the rocket YOUNGER than the non-travelling observer, not older from the way the OP asked and what you claimed in your post.

Zz.

12. Feb 5, 2006

### rcgldr

Time slows down when a person is in a strong gravitational field. As an astronaut leaves the earth, the gravitational field weakens, and the astronaut ages quicker. However, this is offset by how fast the astronaut ends up traveling. If the astronaut accelerates to gain enough velocity, the velocity related time dialation will be more than the gravational field change effect and the astronaut will age slower.

So the astronaut would only age quicker if the velocity were too slow to compensate for the gravitational effect.

In the train example, you've eliminated the gravitational effect, so the traveler would be younger in this case.

Here's a link to the gravitational effect:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/gratim.html

Last edited: Feb 5, 2006
13. Feb 5, 2006

### rcgldr

I don't understand, in my post I state that it's the traveler that ends up younger, not the non-traveler.

Regarding the OP, I also added a post explaining that gravity's effect on time could cause an astronaut to age quicker, if the velocity involved isn't fast enough to make up for the gravitational effect.

Last edited: Feb 5, 2006
14. Feb 5, 2006

### Son Goku

I don't want to post beyond my ken and I may have missed something but here is the way I see it:

General Relativity causes those further out from the gravitational field to have their clocks tick faster.
Obviously, special relativistic time dilation causes those moving faster to have their clocks slowed.

Whether your clock will tick faster or slower in space, with respect to Earth, will depend on which effect dominates, the GR effect or the SR effect.

For a GPS satellite, the GR effect is six times more dominant. It speeds up the GPS clock by 30 ns, for every Earth second. Where as the time dilation caused by its velocity only causes it to slow by 5 ns.
So the clock is running fast by 25 ns from Earth's perspective.

However if you were to achieve a high enough velocity the SR effects would over take the GR effects.

So there are two variables, height in the Earth's gravitational field and velocity with respect to Earth. Depending on the values of these two variables the clock will either run slow or fast.

15. Feb 5, 2006

### pervect

Staff Emeritus
There is some truth to this. This is due to gravitational time dilation. The effect is extremely small however.

See for instance

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/gratim.html

The other posters who have remarked that travelling at a high velocity makes clocks go slower are correct as far as they go, but the effect of gravitational time dilation is an additional effect that makes higher clocks tick faster (and clocks deep in a gravity well tick slower).

Last edited: Feb 5, 2006
16. Feb 6, 2006

I still don't get why this happens...

Surely by the most basic logic, if two identical twins are born (ignoring the fact that they will develop differently), then if one went to space (and ignoring the fact that it may be impossible to notice any change, so lets say the twins live till they are 10,000,000, and all of that time for one twin is in space), travelling fast in space, you'd still think they would be the same age, and both look as old as each other

17. Feb 6, 2006

### Pengwuino

And that's where relativity comes into place. Yes, it sounds logical that such a thing might happen but Einstein proved that this was not the case if you travel exceedingly fast nearing the speed of light. The twins are in fact, just a replacement for a "clock". If you watch a clock that accelerates away from you at say, 1/2 the speed of light, that clock will tick slower then a clock you have in your hand!

This all came out of the idea that Maxwell's equations do not require any reference frame thus making the speed of light equal to ~3x10^8 m/s. The only way for such a thing to be possible is if time and space were connected in a closer relationship then previously thought. etc etc, someone else here will be able to explain it better and do the proof. I can't, im devouring some nice chemistry work.... bleh.

Last edited: Feb 6, 2006
18. Feb 6, 2006

### prochatz

Yes, but relativity proved the opposite. When you move so fast, let's say near the speed of light, a time dilation occurs. As you could see from the previous posters, time is retired by the γ(gamma) factor. Relativity takes place in every move that you do or you observe, but for small velocities the γ(gamma) factor is almost equal to 1. So in our example, the travelling twin (when he returns back) will be younger than the other one. But be carefull, when we say younger we don't mean the time will go back! We mean that if the twin on Earth ages 5 years, the other twin will age less than 5 years.

19. Feb 6, 2006

### pervect

Staff Emeritus
If one twin is travelling _fast_ in space, he may well be younger. It is only the twin who is far away from a gravity field and NOT moving fast who will definitely be (very slightly!) older.

I'm not sure how to answer your "why" question. Einstein came to the conclusion that clocks must tick faster the higher they are in a gravitational field due to an enregy conservation argument. If we look at the light from a laser, it must redshift when it goes up and blueshift when it goes down because of Einstein's energy argument.

If we imagine two hydrogen maser clocks, one higher than the other, the higher clock must tick faster, because its light will be blueshifted when it falls "down" to the lower clock.

We cannot explain the apparent frequency difference between the two clocks by changes in the length of the trip time, because the geometry of the situation is static - the only explanation we can offer is that the higher clock must actually be "ticking" faster

20. Feb 6, 2006