How does the Twins Paradox challenge our understanding of ageing?

In summary, a biology teacher is discussing the effects of traveling at speed on ageing with physics teachers. They explain that time passes more slowly for objects traveling at speed and that speed and time are relative to the frame of reference of the observer. However, the biology teacher still struggles to understand how the twins paradox results in one twin aging more than the other due to traveling at the speed of light. The physics teachers simplify the concept by comparing it to tossing a ball in a moving car and emphasize the importance of understanding simultaneity.
  • #1
adw73uk
3
0
Hi, I'm a Biology teacher, constantly getting into discussions with the physics teachers at my school regarding the effect of traveling at speed on ageing.

They have explained to me (countless times) all the examples and experiments that show that time passes more slowly for objects traveling at speed (atomic clocks on planes, flashing light at one second intervals, looking at a stationary clock while traveling at the speed of light).

I have got my head around the fact that speed and time (I think) is relative (except the speed of light) to the frame of reference of the observer.

What I can't get my head around is how in the twins paradox one twin has physically aged more than the other. How does traveling at the speed of light affect the chemical reactions involved in the ageing of (biological) cells?

I have tried to get my head around Einsteins publications, but are a bit over my head so you'll need to bring any explanation down to a low level!

Any help would be appreciated before I finally fry my brain pondering this one!

Regards
 
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  • #2
I quess the simpliest thing I could tell you is to remember that all chenical reactions take place by electromagnetic interaction.
 
  • #3
How would a clock be affected, but not biological processes?
 
  • #4
adw73uk said:
What I can't get my head around is how in the twins paradox one twin has physically aged more than the other. How does traveling at the speed of light affect the chemical reactions involved in the ageing of (biological) cells?
Even simpler – it doesn’t! Why would a chemical reaction change because your moving any more than juggling a ball on a jet flight would change. You don’t toss a ball up and have it slam into the back of the plane because you and the plane traveled away from it, the ball stays with you. Just like the traveling chemical reactions moving along with the Twin with his ‘official time wrist watch’. When a one hour chemical reaction is complete and reports to the twin, the clock on the wall also reports to the twin that one hour is up, and sure enough when the twin checks the ‘official time wrist watch’ they are right on time. I.E. Chemical reactions don’t change, time is always normal within a common reference frame!

BUT to report this info back to the #2 Twin back home that’s a different issue. Let's relay the info to a station #1 twin is passing by – that station being in the SAME FRAME as #2 back home. Only now can he can see the time change on the station clock is already reading two hours. This information sent back to #2 will confirm traveler #1 is younger than #2. It will take awhile to get the info back there even sent by light speed radio, even longer for a reply, all the while getting father apart .

But wait – after waiting a while the same kind of info is coming back ahead of schedule! Not a reply but twin #2, is relaying the same kind of info via a ship passing twin #2 at home using the same exact traveling frame as twin #1 including a perfectly synchronized clock. The news -- #2 is claiming to be the younger and can see #1 is aging faster!

Only one way to see who is right, that’s to put them back together and resolve it once and for all by looking at both wrist watches at the same time in the same place. NOW is when you get to the point that makes all the difference.
Who’s room do we go to??
They are both perfectly happy AND STATIONARY in their own current room since separating.
Once they are together to settle it the thing that makes THE a difference is where is the point of meeting in comparison to the point separating.

IF it’s where twin #1 has been stationary all this time in his traveling frame his room is the departure point, with twin #2 moving away. So, for the meeting to occur the non-traveling twin #2 must now move and travel at twice the speed to catch up. Now even though #1 was traveling all the time since that frame has both the start and end from the relative view of that frame #1 wasn’t traveling at all so yes the twin #2 will be younger.

But how about meeting back at Earth – that means the traveler #1 must go back leaving his original “departure point/room” to continue on. And must also leave that point at Twice the original speed in order to approach home at the same speed a the original departure speed. Now when they do meet the start and end point will be the same only in the home ref frame and Twin #1 will be the younger.

But what if they both decide to move to reconnect – how do we get the start and end point to be the same place in just one frame. Well there is always one place in some frame where any two events occur at that one location. In this case it’s in the frame that follows the behind #1 at 1/2 speed. (remember you can not get half by dividing by 2 in this case). This point will always be exactly half way between the twins. Thus this time when they compare their ages, they will be exactly the same. As if they had both left Earth in different directions and returned.

So for chemical reactions – it’s like tossing a ball in a moving car – simple.
Understanding the twins,
best make sure the physics teachers get you to understand all three examples above.
Have them slow down a little when they start talking about “simultaneity” or you will get a head ache.

RB
 
  • #5
Thanks for the answers guys - Though I have a headache now thanks RandallB! I suppose the answer I was looking for was the connection with electromagetism. Good point Russ, I guess I had accepted the clock theory as it has been experimentally tested and didn't stop to think why the clock runs more slowly. Being a biologist I more used to considering the 'concrete' - or maybe I'm a little thick!

I've posed the same question to some of my students who are also studying physics. I’ll post any of the explanations that they come up with.
 
  • #6
adw73uk said:
... I guess I had accepted the clock theory as it has been experimentally tested and didn't stop to think why the clock runs more slowly. Being a biologist I more used to considering the 'concrete' - or maybe I'm a little thick!

No you aren't thick, the theory is very unintuitive. However, I think it would be very odd if biological processes were exempt from time dilation. It seems equally unlikely that we could experience time outside our biological constraints from an ivory tower. This basically means we could only experience time dilation by comparing clocks or watching other objects undergoing it differently from us.

As an illustration, I sometimes imagine a small black hole appearing in my front room. On this occasion, instead of getting crushed to a paste and sucked in, I imagine that I can experience the time dilation alone, while outside the world goes on as normal. My time has slowed right down (or suppose - basically completely frozen) but because my brain is frozen I don't notice anything. Everything stops. When the black hole disappears two minutes later (measured from outside) I unfreeze. Everything restarts from where it got to before. I notice two things. My clocks are behind and the man I was watching run round the field outside my window has jumped into the future. I will probably never be able to measure the small change to my biological age.

I'm sure this illustration isn't water-tight, it helps me sometimes.
 
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  • #7
Thanks for all your help. It is becoming clearer in my mind, just the act of 'discussing' it is helping. Cheers!
 
  • #8
If it is all relative - who is to say that one twin is going faster than the other - they both are traveling at the same speed relative to each other. Why would one age and not the other, or conversely, what decides which clock goes faster: they both are traveling the same speed relatively.
 
  • #9
croghan27 said:
If it is all relative - who is to say that one twin is going faster than the other - they both are traveling at the same speed relative to each other. Why would one age and not the other, or conversely, what decides which clock goes faster: they both are traveling the same speed relatively.

Because one of them accelerated and the other didn't. That breaks the symmetry of relativity.
 
  • #10
Does the fact that inertia of the travelers as seen by the stay behind folks increases by a factor of gamma [(1/sqrt(1-(v/c)^2))] influence our thinking in any way? After all if F=ma and we increase m does not a decrease?
 
  • #11
edpell said:
Does the fact that inertia of the travelers as seen by the stay behind folks increases by a factor of gamma [(1/sqrt(1-(v/c)^2))] influence our thinking in any way? After all if F=ma and we increase m does not a decrease?

Well, yes. That's one of the ways of rationalizing why the ship can never reach c. As its velocity approaches c, so does its mass climb without limit, so does its acceleration approach zero.
 
  • #12
I was thinking about the biological aging processes that are going on in the ship. The forces are not effected by the velocity, but the inertia is, so the rates of chemical reactions are slowed.(?)
 
  • #13
DaveC426913 said:
Because one of them accelerated and the other didn't. That breaks the symmetry of relativity.

Yet at the time they pass the imagined railroad station the acceleration stage is over (Although speaking of past/present/future in this instance is fraught with danger) ... I am sure you are correct - and the confusion lies with me ... is there a explanation of this you could direct me to?

I have this website: http://www.einstein-online.info/en/spotlights/sr/index.html [Broken]

Hope I am not disrupting a discussion that has gone beyond my simple knowledge.
 
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  • #14
All clocks are time dilated by relative motion. Biological processes could be used as crude clocks. For example the time it takes a yeast culture to increase its population one thousand times or the time it takes a young seedling to double in height, could be used as a form of clock.
 
  • #15
kev said:
All clocks are time dilated by relative motion. Biological processes could be used as crude clocks. For example the time it takes a yeast culture to increase its population one thousand times or the time it takes a young seedling to double in height, could be used as a form of clock.

So I understand, kev, but thank you. My question is what mechanism governs which biological process is the one to be affected. (Even if I suspect that is not the word.) I thought it was related to speed - as in how close it approaches that of light, but now I find that it is a previous happening - the acceleration, not the % of the SOL, is the culprit here.

Is it possible I am not understanding acceleration - I took it to mean the achieving of a difference in speed between what it was when the twin was stand with the other twin and what was eventually arrived at. Once there, the 'acceleration' ceases, and a constant rate of movement is in place.

Is it that 'acceleration' imparts a potential, an inertia to one that is not given to the other?
 
  • #16
croghan27 said:
So I understand, kev, but thank you. My question is what mechanism governs which biological process is the one to be affected. (Even if I suspect that is not the word.) I thought it was related to speed - as in how close it approaches that of light, but now I find that it is a previous happening - the acceleration, not the % of the SOL, is the culprit here.

Is it possible I am not understanding acceleration - I took it to mean the achieving of a difference in speed between what it was when the twin was stand with the other twin and what was eventually arrived at. Once there, the 'acceleration' ceases, and a constant rate of movement is in place.

Is it that 'acceleration' imparts a potential, an inertia to one that is not given to the other?

In other threads Dr Greg and I have shown it is possible to set up a version of the twin's paradox that elliminates the acceleration as a cause of differential ageing. Basically both twins accelerate away from the Earth with identical acceleration. They cruise at constant velocity for a while and then twin 1 turns around and cruises back to the Earth where he comes to stop. Twin 2 continues to cruise for a while and then using an identical acceleration pattern to twin 1, he turns around and cruises back and then stops. When they reunite, twin 2 has aged less than twin 1 despite the fact they have both experienced identical acceleration patterns. So you see it is not simply differences in acceleration that causes differential ageing. To be more technical it the path lengths through spacetime that causes the differential. The twin that takes the longest path through spacetime experiences the least proper time. Another analogy I once read is wind chill factor. If you are running into the wind, the chill factor might be greater than the chill factor when you are standing still. In order to go from standing to running you have to accelerate, but it is not the acceleration per se that causes wind chill.
 
  • #17
Like any good explanation, kev, that opens all manner of possibilities for further inquirery. I fear they would stray beyond the parameters of this thread, which was indeed begun by a biology teacher - so thank you for that.
 
  • #18
croghan27 said:
My question is what mechanism governs which biological process is the one to be affected.
This leads me to believe you are still thinking there's some sort of discrete effect on particular aspects of the moving frame.

It affects everything. There is no experiment you can do within your frame of reference - even in principle - that might show a discrepancy in the passage of time between one mechanism and another. You might as well look at it as time itself has been dilated.
 
  • #19
In space/time every thing is always traveling at the speed of light. That is

[tex]
(vt)^2 + (ct)^2 = c^2
[/tex]

It is only a question of how much of the traveling is done in the spatial direction and how much is done in the time direction.
 
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  • #20
Kev, this all seems very Machian to me. There seems to be something special about the stay behind folks they age the most. How? Given that velocity is "relative". How do account for the asymmetry? The folks left behind are moving at exactly the same speed as the folks in the ship (when seen from the ship) (I say speed not velocity, the velocities are equal and opposite).
 
  • #21
edpell said:
Kev, this all seems very Machian to me. There seems to be something special about the stay behind folks they age the most. How? Given that velocity is "relative". How do account for the asymmetry? The folks left behind are moving at exactly the same speed as the folks in the ship (when seen from the ship) (I say speed not velocity, the velocities are equal and opposite).

Special Relativity applies in inertial frames of reference; it does not apply to non-inertial frames of reference. The Earth and its inhabitants stay in an inertial frame of reference (by not accelerating), whereas the occupants of the spaceship change to an accelerating frame of reference. Thus it is no longer "all relative".
 
  • #22
edpell said:
Kev, this all seems very Machian to me. There seems to be something special about the stay behind folks they age the most. How? Given that velocity is "relative". How do account for the asymmetry? The folks left behind are moving at exactly the same speed as the folks in the ship (when seen from the ship) (I say speed not velocity, the velocities are equal and opposite).

It is easy to account for the asymmetry. Have a look at the attached space time diagrams. The first is the point of view of an observer that stays in the Earth frame. (The stay at home twin that remains on the Earth only goes forward in time as represented by the red line A,C.) The second is the point of view of an observer that is at rest with the frame that represents the outward journey of the traveling twin (The green line labelled A,B). The third is the point of view of an observer that remains at rest in the frame that represents the traveling twin's return journey (The green line labelled B,C). In every case every observer measures the traveling twin's path through spacetime (path A,B,C) to be longer than the stay at home twins path through spacetime (path A,C). When looked at from the spacetime path point of view there is no symmetry.
 

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  • #23
croghan27 said:
So I understand, kev, but thank you. My question is what mechanism governs which biological process is the one to be affected.
The assumption in the twins paradox is that biological processes are completely unaffected. The reason the traveling twin ages less is simply because less time elapsed for him than the Earth twin, not because some mechanism caused a change in some biological process.

The differential aging predicted is a result of assuming that a person will age less in 10 years than in 20 years, for example. The point of the twins paradox is that less time elapses for the ship twin, not that some mechanism slowed his aging process.
 
  • #24
edpell said:
The folks left behind are moving at exactly the same speed as the folks in the ship (when seen from the ship) (I say speed not velocity, the velocities are equal and opposite).
This is simply not true. The coordinate velocity of Earth in the ship's frame doesn't match the velocity of the ship in Earth's frame for the entire trip.

In the ship's frame(s), Earth's coordinate distance from the ship increases at a constant rate, then during the turnaround goes from the length contracted distance to the proper distance and back again in a very short time, then decreases at a constant rate back to zero.

This is very different from the ship's coordinate distance from Earth in Earth's frame, which just increases at a constant rate then decreases at a constant rate.

Maybe someone could graph the coordinate distance between the Earth and ship against each twin's clock reading to show how different they are.
 
  • #25
Kev, your diagram is very helpful. In others words a straight line is the shortest path between two points. The clock/observer who never alters their path direction in spacetime will have the shortest path length. The clock/observer who takes a turn in their spacetime path will have a longer path. I think this is the whole story of the twins.

But I still think we need better wording (maybe more careful wording) to talk about "time".
 
  • #26
Kev, in your first diagram you show time in the Earth frame only. That is at the start before takeoff when they are both in the same inertial frame they both see 0 years on the Earth clock. And when the ship returns from its round trip and stops they both see 20 years on the Earth clock. And they both see 2 years (assuming gamma of 10) on the ship clock. To say that the ship board observer went only 2 years in time but ended up at the 20 year mark by the Earth clock needs some careful wording. It gives the appearance that things went slower on the ship?
 
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  • #27
It is almost like the traveler has a complex time vector with a "real" part and an "imaginary" part. Where the total time vector length is 20 in the Earth frame (and all real) but the real component is 2 in the ship frame and the orthogonal part (imaginary) is 19.899 in the ship frame. and physical aging only takes place with movement along real time axis. I guess I would have to say the real time axis in the moving frame does not point in the same direction as the real time axis in the Earth frame. I have no idea what that statement means "the real time in frame1 points in a different direction than real time in frame2".(?) Is there always a local (frame) time and when you transform the local (frame) time to another frame you get a real and imaginary part in the other frames point of view? That is Earth sees 20 years of time vector for itself and 2 years of traveler time vector that projects on the real time in the Earth frame and 19.899 that projects on the imaginary time axis. Help I am lost...
 
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  • #28
edpell said:
major snip action here ...
Help I am lost...

Thanks for that edpell - methinks that herein we are not only http://www.qunl.com/rees0008.html" [Broken] (and physics) - we are also dancing about the limits of the language. English tenses were never meant to be treated so. "I am" works fine in my own temporal plane, - but "you are" is something temporally different for thee and for me. I shudder to consider "s/he is". :confused:
 
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  • #29
With regard to the comment by kev by response is:

Obfuscation. Make the problem so complex that you can find your own mistake anymore.
It is the same mistake that evey other solution includes which does not take acceleration into account.
 
  • #30
Al68 said:
The assumption in the twins paradox is that biological processes are completely unaffected. The reason the traveling twin ages less is simply because less time elapsed for him than the Earth twin, not because some mechanism caused a change in some biological process.

The differential aging predicted is a result of assuming that a person will age less in 10 years than in 20 years, for example. The point of the twins paradox is that less time elapses for the ship twin, not that some mechanism slowed his aging process.
I disagree with this. Acceleration affects the periods of oscillators. If the twins refer to a common third clock (say, revolutions of the Earth around the sun), then they will both note the same elapsed time for the traveling twin's trip. But, the traveller will have aged less, and the traveller's clock will have counted fewer oscillations than his earthbound twin's clock.
 
  • #31
croghan27, yes I agree the English language is based on Newtonian time and has no direct words to deal with my time versus your time versus their time. In Newtonian English when we say the Earth observer goes 20 years in time and the ship observer goes 2 years in time and they both meet at 2030 (say it starts in the year 2010 and gamma is 10). This does not make any sense in Newtonian English. I have no disagreement with SR or the results I just think we have to come up with a better vocabulary for SR time.
 
  • #32
ThomasT said:
I disagree with this. Acceleration affects the periods of oscillators.
If a clock is affected by acceleration, then it is simply not a valid clock in SR. This is the "clock hypothesis", that a clock's rate is unaffected by acceleration. Of course a real clock may be affected by acceleration, but the predictions of SR are not valid for such a clock.
If the twins refer to a common third clock (say, revolutions of the Earth around the sun), then they will both note the same elapsed time for the traveling twin's trip. But, the traveller will have aged less, and the traveller's clock will have counted fewer oscillations than his earthbound twin's clock.
I think you must have misread my post. My point was that the traveler's clock is predicted to show a lower reading and the ship twin is predicted to age less for a common underlying reason: Less elapsed time passes.
 
  • #33
ThomasT said:
If the twins refer to a common third clock

What velocity does the third clock have with respect to the Earth clock? What velocity with respect to the ship clock?
 
  • #34
Somewhere between:

"Acceleration affects the periods of oscillators."
and
"What velocity does the third clock have with respect to the Earth clock? What velocity with respect to the ship clock?"

I have become lost again. :cry: While velocity and acceleration are connected, they are not, in my philosophy, Horatio, the same thing. One involves achieving and is a change of velocity; while the other is a constant.

One must accelerate to achieve a certain velocity, or conversely decelerate, but the latter is the result of the former. What is the connection here and which results in the ageing or less ageing of the twins?

(At the risk of being a drag, there is another question in the background of this.)
 
  • #35
croghan27 said:
Somewhere between:

"Acceleration affects the periods of oscillators."
and
"What velocity does the third clock have with respect to the Earth clock? What velocity with respect to the ship clock?"

I have become lost again. :cry: While velocity and acceleration are connected, they are not, in my philosophy, Horatio, the same thing. One involves achieving and is a change of velocity; while the other is a constant.

One must accelerate to achieve a certain velocity, or conversely decelerate, but the latter is the result of the former. What is the connection here and which results in the ageing or less ageing of the twins?

(At the risk of being a drag, there is another question in the background of this.)
The differential aging is the result of less elapsed time for the ship's twin, which is a function of velocity. Acceleration is relevant as the time derivative of velocity.
 

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