[Bill S (OP)]

Sean Carroll asks the question: What would it be like if time passed more quickly, or more slowly?  His answer is interesting.

“The crucial question there was: Compared to what? The idea that “time suddenly moves more quickly for everyone in the world” isn’t operationally meaningful; we measure time by synchronized repetition, and as long as clocks of all sorts (including biological clocks and clocks defined by subatomic processes) remain properly synchronized, there’s no way you could tell that the “rate of time” was in any way different. It’s only if some particular clock speeds up or slows down compared to other clocks that the concept makes any sense.”

This raises the question: what do we really know about biological clocks in terms of time dilation?  We are told that the astronauts who have spent the most time in Earth orbit are minuscule fractions of a second younger than they would be if they had remained on Earth; but what does that actually mean? 

People who suffer from any of the forms of progeria appear to age more quickly than the vast majority of people. Is this due to their biological clocks running at a different rate? Are their bodies "experiencing" time at a rate that is different from that experienced by their minds?   If biological clocks can vary in this way, what evidence do we have to indicate that they will be influenced in the same way as mechanical or atomic clocks by time dilation?

[jeffreyH]

The lifetime of cells in the body is a complex subject. In some cases there is a pre programmed cell death. It doies not mean that time is running at a different rate. Simply that the processes that mediate the life cycle are different to the norm.

https://en.m.wikipedia.org/wiki/Programmed_cell_death

[Bill S (OP)]

Do we know what effect time dilation would have on those processes?

[evan_au]

Progeria is caused by a mutation that interferes with DNA copying during cell division.
https://en.wikipedia.org/wiki/Progeria#Cause

Some laboratories suggest that they can measure your DNA age by measuring the length of your Telomeres. Telomeres get "used up" each time the cell divides; when the Telomeres "run out", this interferes with DNA copying during cell division. They suggest that this gives clues about your remaining life expectancy.
https://en.wikipedia.org/wiki/Telomere#Measurement

All of this suggests that biological clocks are a poor form of time measurement. The differences between individuals will swamp the differences due to spending 6 months on the ISS (but a twin study is now underway with astronaut Scott Kelly to study this and other aspects of long duration space flight).

I recommend a Cesium atomic clock. These are known to be able to measure time dilation on Earth and in its vicinity (the GPS satellites have pre-distorted atomic clocks, so they run "correctly" when they are on orbit, as seen by an observer on Earth).

[Alan McDougall]

YES THEY ARE!

[PmbPhy]

This raises the question: what do we really know about biological clocks in terms of time dilation? 

There is absolutely no difference between a biological clock and any other clock. Think about it for a moment. What is a biological  clock? Simply put it operates under the principles of classical physics, i.e. it's a machine. This is something that is kept in mind when discussing the Twin Paradox. See:
https://en.wikipedia.org/wiki/Twin_paradox#The_equivalence_of_biological_aging_and_clock_time-keeping

The equivalence of biological aging and clock time-keeping

All processes—chemical, biological, measuring apparatus functioning, human perception involving the eye and brain, the communication of force—everything, is constrained by the speed of light. There is clock functioning at every level, dependent on light speed and the inherent delay at even the atomic level. Biological aging, therefore, is in no way different from clock time-keeping.

So, in principle, and as far as the physics is concerned, there's absolutely no difference between a biological clock and any other kind of clock. It's only humans that perceive a difference, not nature.

It's implicit in the derivation of the expression for time dilation that the clock used what we refer to as an ideal clock also known as a standard clock. Such a clock is defined as a one that is completely unaffected by acceleration. This is sometimes known as the clock hypothesis.  See:
https://en.wikipedia.org/wiki/Clock_hypothesis

For more on biological and other kinds of clocks please see:
http://newt.phys.unsw.edu.au/einsteinlight/jw/module4_time_dilation.htm
(underline is mine)

What about other clocks? The choice of this rather peculiar clock was made only because it is one that so clearly depends on a simple electromagnetic phenomena. Other clocks (quartz crystals, springs, even Zoe's biological clock) depend on complicated combinations of electromagnetic phenomena such as the forces between atoms and molecules, and on Newton's laws. If they didn't differ from Jasper's clocks by the same factor of γ, then we would conclude that the laws of mechanics and/or electromagnetism are different between the two frames, contrary to the principle of relativity. So yes, time dilation would affect biological clocks as well, and Jasper thinks that Zoe is getting older more slowly than he is.

I hope this was useful, Bill.

[Alan McDougall]

To simplify what about a beating heart , if you are standing stationary relative to some one approaching you at say 40c, your heart beat would be faster than his, and his heartbeat would be slower than yours, relative to each other, because of time dilation, albeit by a tiny, but measurable amount.

Alan

[Alan McDougall]

So, in principle, and as far as the physics is concerned, there's absolutely no difference between a biological clock and any other kind of clock. It's only humans that perceive a difference, not nature.

I hope this was useful, Bill.

"Nice to see you smile and lighten up a bit?  Of course you are absolutely correct in that there in no difference between a biological clock, or a cosmic clock like a neutron star, or an oscillating atom or the heart- beat of a human being. All are subject to special relativity!

Alan"