Exactly. In the story, many pairs of twins fell out of touch forever when their ships approached a measurable percentage of lightspeed.
Metabolism is not the same thing as ageing.
Why would there be such a trigger as humans have not evolved in that way.
How can we know that humans (life on Earth, for that matter) have not evolved a trait, if we have not experienced the conditions which would activate it (travel at light speed)?
As far as why this would be a trait, perhaps it’s fundamental to life.
ooops, there is no way for the body to know it’s moving at relativistic speeds. That’s one of the postulates of relativity – you don’t know your velocity. There is no trigger that could ever ever be in any way detected by the body. End of story.
Why even bring up the question of how we would have evolved in response to a condition that we never experience? I won’t, that’s for sure! 
As far as I can tell, there is no organism that was selected for because of some hypothetical increase in metabolism when traveling at some hypothetical speed that is close to c since that situation had never arisen.
I fail to see why it is better to have accelerated ageing. In fact, that looks like something to be selected against
oooops, nothing with a rest mass above zero can travel at light speed. It can assymptotically approach the speed of light but will never get there. Furthermore, in its frame of reference, it will still see light travelling at 3 * 10^5 km/sec faster than it (even if it is going 99.999999999% the speed of light wrt Earth). So, putting you on the rocketship, you will still be at rest, aging perfectly normally, clocks ticking perfectly correctly, heartrate going as usual, and aging just as you always had. If you look out your window behind you, you will note Earth zipping away at 99.999999999% the speed of light while you sit in a rocketship that is perfectly stationary. Then your friend next to you will tap you on the shoulder and ask, “Do you think that the life on Earth has evolved a trait that allows them to change their metabolic conditions now that they are travelling near-light speed?”
To which you will graciously reply, “Back when we were there, I read this thread on the SDMB…”
Urban Ranger, indeed the situation as you described it in your last post can never arise. Not ever.
In the rest frame of the organism, you are as close to the speed of light as you’re ever going to get… unless you convert your entire rest-mass into photons.
OK I gotta say, that while I’m strongly inclined to disagree with both Paul and “Ooops” (I suspect they are one and the same, by the way), I do think there could be some fresh input on this question if it was weighed in by someone with a biology or biophysics background. It seems as if we’ve exhausted all the points as they relate to relativity & physics.
now I have no expertise whatsoever on evolution, but I know humans have evolved so that they all have teeth that get big cavities and fall out, so it doesn’t seem to be reasonable that a negative trait (in this case, aging at earth’s timeclock rate) could not exist because it isn’t a desirable trait. Not to get into evolution, but selection is arbitrary. John Lennon, as much as people liked him, as talented as he was, got shot. He only managed to have two kids. Willie Kapinski has been unemployed and living on welfare on my block for the last 22 years, and what’s more he keeps stealing my newspaper. He has 8 kids. That is how selection works. There’s a lot of luck involved. But like I said, let’s please not go there. It definitely won’t help us. By the way, Willie evolved in such a way that he got big cavities and lost two teeth right in the front. Perhaps one day we will all be like him.
The fact remains that the body doesn’t know at what speed it is going. You can determine accelleration, and have responses based on that (see inner ear for example), but there is no way to determine your velocity. So how can any response occur when there is no stimulus?
muttrox hit the nail on the head. It’s impossible to know that your timeclock is different from Earth’s. When I say impossible I mean impossible. End of story. Do not pass Go. Do not collect $200.00.
Biophysics does not come into play because there is no difference in physical processes in two different frames of reference. Their hands are tied.
On the other hand, it is possible that some of the other aspects of the space environment (different radiation levels, reduced gravity, etc.) might have some effect on human physiology. This would be the same in a parking orbit around the Sun or in a starship travelling at relativistic speed, though. This can also be overcome on board your ship: You can spin the ship or continually run the engines to simulate gravity, you can build in more radiation shielding, etc. If you do this, then you should age at the rate your clock tells you. Again, on board your ship, you can’t tell if you’re moving or not. Your metabolism will always match up to the clock in the same room in the same way.
As for the book recommendation: I’ll usually recommend Heinlein myself, but he has a poor grasp of special relativity. You can only make unambiguous time comparisons after one or both twins turns around and heads back: While travel is still outbound, the twins will not agree on who’s aging faster. The way it’s presented in Time for the Stars would only work if there were an absolute frame of reference, which relativity postulates to not be the case.
One thing you might want to remember, oooops is that all of us, you, me, and maybe even Cecil, are experiencing time dilation right this moment. The earth is spinning around at up to 1000mph, it is revolving around the sun, the solar system is moving in relation to nearby stars, and the whole galaxy is spinning around as well as heading toward the Great Attractor. That results in a slight slowdown from time in an imaginary stock still frame of reference.
And you know what? We can’t tell. For us and for everything that we can imagine using, time flows at the same rate. As JS Princeton properly said, time flows at the rate of one second per second in all events, in all places, under all conditions, for those inside those conditions. You cannot, even theoretically, come up with a way to tell the difference. There is no such thing as aging outside of time, no way for the body to notice that anything unusual is happening – because to the body nothing unusual is happening. Time is not flowing at a different rate, not to the body.
I know I’m just saying the same thing others have said. I jut keeping hoping that if enough of us say it in enough different ways, one of us will get through.
I’m grateful for the response this has generated, but I’m wondering if something a little more relevant than a 1950’s science fiction novel has been published that deals with the biological effects? For example, something like the studies measuring the how astronauts are affected by (presumably) the gravity in space?
The time dilation on Earth has been relatively the same since the beginning (I don’t mean literally) of time, so why is it impossible that biology would respond differently to a different dilation? It may or may not be the case, but I don’t see how you can claim it’s impossible.
oooops, indeed basically all the ISS is doing right now is studying the long term impacts of exposure to microgravity environments of space. The prolonged exposure to radiation has also been a concern. This is not necessarily a concern of ours, as we could send our rocketship on a 1 g acceleration with some sort of radiation shield that would allow the environment of the rocketship cabin to be the same as the environment of earth (at least as far as those two stimuli, which are actual stimuli go).
The last paragraph you wrote really concerns me. It just seems to me you need to pick up an intro relativity text because you obviously aren’t understanding what people are trying to say in this thread. There is no such thing as a “different dilation” unless you try to compare the clocks of two different reference frames.
Let me try to get at this another way.
You seem to think that there are all sorts of reference frames. In some reference frames clocks move at speed a, in some reference frames clocks move at speed b, and that these two frames are somehow different.
They are not different. There is no way to tell that you are going at speed a or speed b unless you cross paths with the other reference frame and compare notes. Otherwise you are happily going along and the laws of physics (and chemistry and biology) work the same in your reference frame.
So, it doesn’t make sense to say that the “Earth has been relatively the same since the beginning (I don’t mean literally) of time, so why is it impossible that biology would respond differently to a different dilation?” There’s no such thing as a “different dilation”. Each reference frame is not unique, each reference frame is indistinguishable. By indistinguishable I mean you cannot tell the difference. Period. The reason you cannot is because if you did, then you could tell that you were moving and in some frame of reference then the speed of light would not be constant. You could say, “oh, I’m moving because my biology is responding differently”. Then you could go change your speeds a bit and graph the changes in your “biology” and deduce what speed you were going at. Then for those slower speeds light would have to be travelling faster than for those higher speeds. Then you have reached a violation of one of the fundamental rules of relativity, that is the speed of light is constant.
A way to put the previous idea into three words is to invoke the Einstein Equivalence Principle. It applies to every possible observation you can possibly think of. That’s every possible one.
Capice?
Isn’t it true that we are all about 12 billion years old?..since all of the atoms in our bodies have been around since the Big Bang?
It’s called the twin paradox. In the original version there are two twins, one travels while the other stays on earth. I used to think that the paradox was “The astronaut twin will be younger when he gets home - Weird, huh?”
But that’s not the paradox. This is the thing that really bakes my noodle: Why doesn’t 5 years pass for the twin on earth while 40 years passes for the astronaut? After all, from the spaceship’s reference frame, it’s the earth that is moving away and then returning.
I thought that all of us are about 12 billion years old as all the atoms in our body originated at the time of the Big Bang. Am I wrong?
Please disregard the 2nd posting of mine…One of the disadvantages of being so old.
muttrox has said it.
JS Princeton has said it.
I sure tried my best to say it.
There is no different dilation that the body can feel. Time passes completely normally to the body. There is nothing to select for or against. The time dilation effect has been measured to agree perfectly with theory in all acclerator experiments.
oooops, you have no case. There are no ifs or whatevers. Aging is not some mystical notion out of D&D or bad comic books. A body that feels five years pass will age five years. Period.
This is why the Twin Paradox is not about Special Relativity. It’s about General Relativity.
The reason the spaceship twin is younger when he returns is that his ship had to accelerate a few times on his trip. It had to speed up to some significant fraction of c, turn around, and decelerate on return. So the spaceship twin’s reference frame was not always inertial, and special reletivity doesn’t apply. Because of the acceleration, you can distinguish the frames.
Most of the discussion on this thread has been about comparing the frames when the ship is going along at speed, and not about what happens when the velocity is changing. If you look at the twins when the spaceship is not accelerating, there is no way to distinguish the frames. But if the ship never accelerated, it wouldn’t be possible for the twins to be together in the same place, either before or after the trip.
I hope I haven’t made this discussion worse. Nothing I’ve said changes the fact that the spaceship twin’s biological processes would indeed have passed through less time, thus making him younger.