[Hessu (OP)]

We have learned that my time gets slower when I move faster.

An example has been that, after a long and very fast journey, I would be younger than my twin brother who didn't move at all. In relative terms.

Do we know if our bodies would know the difference? Is aging happening on relative or absolute time?

[chris]

We've looked at this topic from a number of perspectives over the years; here are some links to those discussions; if anyone spots any more, please do update my list:

https://www.thenakedscientists.com/forum/index.php?topic=39375.0

https://www.thenakedscientists.com/forum/index.php?topic=70057.0

https://www.thenakedscientists.com/forum/index.php?topic=70084.0

https://www.thenakedscientists.com/forum/index.php?topic=70038

[Colin2B]

Is aging happening on relative or absolute time?

For you the aging is absolute, your wristwatch will show less time - if accurate enough.

[Hessu (OP)]

Thanks, interesting discussions and theories about time and maybe the answer is somewhere well hidden behind, but I cannot see it.

Was it yes or no or "it depends"?

I can agree that we can carry watches with us and see the difference. They are made to measure time so that's what they do. I don't have any problems there.

But my blood cell does not think in terms of relativity (I think). If my watch goes slower (because I speed up), does it mean that my blood cell divides slower?

[Colin2B]

But my blood cell does not think in terms of relativity (I think). If my watch goes slower .... does it mean that my blood cell divides slower?

Yes, the time difference is absolute as i said.
Your blood cells do not have to think. All processes either mechanical, atomic, chemical or biological are effectively clocks, they proceed at a set rate so if a clock slows down so do all processes eg atomic decay.

[alancalverd]

Slower than what?

The earth is spinning at about 1000 mph at the equator and travelling round the sun at 67,000 mph, whilst the sun is orbiting the center of the galaxy at 483,000 mph.

Fact is that if you move relative to another observer, your watch and every other time-dependent process in your vehicle, will appear to be going slower as observed by him. But not by you. So you will age at exactly the same rate, or not, depending on what you mean by "same".

[jeffreyH]

Time dilation is determined by comparison. So when Alan says 'Slower than what?' It is a very serious question.

[Janus]

Thanks, interesting discussions and theories about time and maybe the answer is somewhere well hidden behind, but I cannot see it.

Was it yes or no or "it depends"?

I can agree that we can carry watches with us and see the difference. They are made to measure time so that's what they do. I don't have any problems there.

But my blood cell does not think in terms of relativity (I think). If my watch goes slower (because I speed up), does it mean that my blood cell divides slower?

Why would clocks behave differently than the metabolic rate of cells.  When clocks "measure" time, all they are doing is counting events that we've learned occur at known regular periods.  You could do the same same with cell division. Once you determine the average rate, you can start with a known number of cells and measure time by counting how many cells there are.  Probably not extremely accurate, but workable.
But this is kind of a moot point, because the clocks or cells don't have to "know" anything. They are just running and dividing like they normally are.   As pointed out by others, time dilation is a comparative thing.  It what you would see happening to the clock moving with respect to you as compared to an identical clock at rest with respect to you.  You would see it running slow, along with any physical processes of anything moving with it.    Someone with that other clock would conversely see your clock ticking slower than his clock.

So about now you are wondering, "But if I see him aging slower, and he sees me aging slower, then how can one of us be younger than the other if we come back together again?  What decides who really is aging slower?"

The answer is in the difference between "time dilation" and "total accumulated time".
Time dilation is the comparison of time rate between moving clocks while they are in motion with respect with each other, But total accumulated time relies on more than just time dilation.

To illustrate, I'm going to use an analogy. 

Imagine you have two men that start walking from the same point with the same stride but at an angle to each other.
After a  while they check on each others progress.  But each man measures progress as distance measured along the line in the same direction as he is walking. Thus each man will note that the other man is "behind" him and making less "progress" than he his. This is the equivalent of "time dilation", where each clock measures the other as running slow.

Now let's say that one of the men changes direction, so that his new path will cross the other man's path. As he turns towards the other man's path, the other man's position relative to the direction the first man is facing changes, he goes from being behind, to being in front.   
As or man who changed directions his perception of "progress" also changes with him. It is now defined by the new direction he is facing.  When he compares his progress to the other man, he finds that though the other man is "ahead" of him, he is gaining ground as the other man is still making progress slower.  But by the time his crosses the other path, the other man is still ahead of him, and if he turns to follow him, the other man will remain ahead.  Now the distance between the two men and the starting point can be made in a way that both men will agree on the results that one of the men and made less total progress than the other.  This is the equivalent of "total elapsed time" with clocks.

Now with our analogy above, the two men were walking along two different paths over a two dimensional plane, and as a result, one ended up further from where they started than the other.  Neither man altered his pace during this time (we can assume that the change of direction made by the one man resulted in no change in pace.).

With Relativity, instead of a 2-D plane, we are dealing with 4-D space-time.  Two clocks that are separated at some relative speed and then brought together again, follow different paths through space-time, and it is the difference in those paths that results in the total accumulated difference between the clocks when they meet up.  And by "clocks", I don't mean just devices specifically made to mark off time, but also every physical, chemical, biological, etc. process also.

There would be no divergence between the clock operation and any other process that is co-moving with the clock.

[Bill S]

I empathise with Hessu here. I struggled for a long time with this.  I think I reached a point where I could say maybe I have a handle on that, but I still find myself having moments of doubt. 

Possibly a reasonable way to think about it is that, because all motion/change is relative to something, it is change that is influenced by variations in relative velocity.  Time is simply the measure of this change, so we have to adjust our concept of time to accommodate this.  It may have little scientific veracity, but it can be a jumping off point, if nothing else.

I’m fairly sure I posted this link before, but I found it helpful, so for any struggling hitch-hikers who might have missed it, here it is again.

  (http://home.earthlink.net/~owl232/twinparadox.pdf)

[Hessu (OP)]

As I said, I don't have a problem with time. We can define relation between
time and speed so that one will affect the other.

I think one poor term here is aging. I was thinking aging as the process on
cells dividing more and tissue getting bad and eventually dying, not just old
specified by time. From now on, I will replase the word aging with rotting
(because Einstein probably didn't use it) - and let's try not to define rotting as
function of time, just in this case.

Now, somehow I like the idea that my blood cell (and others) rots at absolute speed
in similar environment, in absolute terms.

So if I sit in one closed room and my twin brother in another one, both of which
have the same temperature, humidity, etc., our blood cells divide (rot) at same
speed.

If my room is put in a truck (or something) and starts to move, my cell doesn't
notice it and it keeps dividing (rotting) the same speed in absolute terms.
After we have taken a round around the block and come back to the same place, it
makes sense that my cell has divided (rotted) as many times as my brother's so we
would look just as rotten, even if I would be younger when measured by time
(because that's how time is defined).

What do you think?

[Janus]

As I said, I don't have a problem with time. We can define relation between
time and speed so that one will affect the other.

I think one poor term here is aging. I was thinking aging as the process on
cells dividing more and tissue getting bad and eventually dying, not just old
specified by time. From now on, I will replase the word aging with rotting
(because Einstein probably didn't use it) - and let's try not to define rotting as
function of time, just in this case.

Now, somehow I like the idea that my blood cell (and others) rots at absolute speed
in similar environment, in absolute terms.

So if I sit in one closed room and my twin brother in another one, both of which
have the same temperature, humidity, etc., our blood cells divide (rot) at same
speed.

If my room is put in a truck (or something) and starts to move, my cell doesn't
notice it and it keeps dividing (rotting) the same speed in absolute terms.
After we have taken a round around the block and come back to the same place, it
makes sense that my cell has divided (rotted) as many times as my brother's so we
would look just as rotten, even if I would be younger when measured by time
(because that's how time is defined).

What do you think?

What I think is that you didn't get the gist of my post.  It doesn't matter what process you are dealing with,( a ticking clock or an aging body) they are both equally affected.  You will be physically younger than your brother by any means you want to define age, biologically or by clock.  Of course for a trip around the block at typical truck speeds, this difference is for all practical purposes too small to measure.  This is one of the problems people deal with when it comes to Relativity, the effects we are talking about are so immeasurably small at the scale of normal day life that they aren't noticed.  The difference in you and your brother's biological age after the trip around the block is so tiny, that no one could notice it.  A non-noticeable difference in our minds becomes no difference and we just assume that you and your brother aged the same amount and there was no difference when you met back up again.  We then just assume this would be true in all cases.

But that is just not how the universe is put together, there is a difference in total age, and this difference will become more measurable as the relative velocities increase.

Let's try another tack:
You are on a spaceship traveling at 0.866 c relative to the Earth, to a planet 8.66 light years from the Earth as measured from the Earth. Once you get there, you instantly turn around and head back to Earth.
Your brother stays home. Everything is done to ensure that conditions you both are exposed to are the exact same.
For your brother, it will take 20 years for you to make the round trip. He will measure that many days, have 20 20 years worth of sleeps, eat twenty years worth of meals. etc. He will, in fact age twenty years biologically.
For you however, there is a difference. The relative speed between you and the Earth-distant planet pair is 0.866c.  For you it is them that are zipping by at that great speed, and because they are, they are length contracted, and this length contraction includes the distance between them. In other words, according to you, they are only 4.33 light years apart. And thus the time between the Earth being next to you and the planet being next to you is 5 years (at 0.866c), and then it will take another 5 years for the Earth to return to being next to you. Total time between being separated from and reuniting with your brother, 10 years, you will measure that many days, have 10 years worth of sleeps, eat 10 years worth of meals and biologically age 10 years.  In every sense of the word, you will have aged 10 fewer years than your brother.

As odd as this may seem and as contrary as it is to everyday experience as we live it, this is how the universe is constructed to work.

[syhprum]

You are traveling at .866 c and instantly turn around ! have you considered how much energy this involves ? even if you do a Hyperbolic loop around some super dense object you will be subjected to immense gravitational forces.
All this must be included in twin paradox discussions.

[geordief]

You are on a spaceship traveling at 0.866 c relative to the Earth, to a planet 0.866 light years from the Earth as measured from the Earth. Once you get there, you instantly turn around and head back to Earth.

What is the distance between Earth and the planet ,Janus? Not 0.866 light years surely? A typo?

Is it 10 light years?

Anyway I love your explanations even if they  can be hard to follow ;-)

[Janus]

You are on a spaceship traveling at 0.866 c relative to the Earth, to a planet 0.866 light years from the Earth as measured from the Earth. Once you get there, you instantly turn around and head back to Earth.

What is the distance between Earth and the planet ,Janus? Not 0.866 light years surely? A typo?

Is it 10 light years?

Anyway I love your explanations even if they  can be hard to follow ;-)

I meant 8.66 light years, misplacement of the decimal point. I went back and edited it. Thanks for catching that.

[Janus]

You are traveling at .866 c and instantly turn around ! have you considered how much energy this involves ? even if you do a Hyperbolic loop around some super dense object you will be subjected to immense gravitational forces.
All this must be included in twin paradox discussions.

Sure, in the real world, you are not going to do that instantaneous turn around, but for the purposes of the discussion, this makes no difference.  You could make it so that he accelerate at 1 g ( as experienced by him) for half the trip, then reverse the direction of the engines until he reaches the planet and then returns half-way back, and finally flip again until you are back at rest with respect to the Earth again.

In such a case, the total trip time for the Earth would be t = 2 sqrt((d/2c)^2+d/a) for the Earth twin.
with d being the distance between the Earth and planet
For the ship it would be T = 2c/a cosh(ad/2c +1)

Factoring acceleration into the problem just adds an unneeded complication.  All it really means is that the velocity is not constant as far as the Earth twin is concerned an thus the time dilation (which depends on the instantaneous relative velocity alone) is constantly changing, which means you have to integrate to get the total time difference.
The acceleration has no effect other than by changing the relative velocity.