[George Haywood]

George Haywood  asked the Naked Scientists:
   
Dear Dr. Chris,

For many years I have been battling with the concepts of science, and have to confess to be totally mystified by many writers' references to 'time'.

According to my understanding of Einstein's Special Theory of Relativity,
both 'time' and 'space' can no longer be regarded as 'absolutes' in the
sense that Newton would have maintained, but depend rather on the velocity of the instruments doing the measuring  (as, with say, the famous 'twin paradox'). Without evoking the concept of absolute time, what possible meaning can be given to statements like 'The sun is 4 billion years old', or '1 second after the Big Bang' - concepts that are bandied around as if they might be meaningful.

Whenever I have spoken to 'physicists' regarding this problem, it seems that they are not clear as to what is meant either.  What theoretical 'clock' is doing the measuring in these instances?

I hope that I am not making some very obvious blunder, so please can you enlighten me.

George Haywood

What do you think?

[Vern]

I suspect that most physicists have a very good understanding of the effects of relativity phenomena. Not all agree upon just how the effects come about. According to adherents to the Lorentz notion, material things distort and experience the passage of time differently when they move. Einstein treats the distortion as warping of space-time. The arithmetic is easier with Einstein's view, but either view produces the same results and predictions.

[litespeed]

This is is a tough one.  First, it is true that time and distance (length) are continuously subject to the effects of relativity.  However, at human scale these things are not noticeable in daily life.  Let me try one example: the length of the meter, using physical objects. I MORE then welcome corrections!!

Assume the bureau of weighs and measures constructs a few dozen meter sticks at the same place, with the intent of installing all but one of them at railway car window level all along the the track from DC to LA. The train has a viewing port where the last stick is mounted for the purpose of comparing the moving one with those at fixed locations as the train passes.

The train is then accelarated incrementally past all the fixed locations and we compare the fixed ones to the moving one. We will notice that the faster the train goes, the shorter the stick it carries seems to be. At earthly velocities we will notice shortening on a miniscule level. In fact, the shortening is so little we need a science fiction device to do the measuring.

However, if the train goes by at about 86 percent the speed of light, the stick will actually appear to be one half the original length. In addition we also notice that the railway clock in the car is also ticking at half speed. The passengers on the train see none of this because both time and distance change in lockstep with each other.

We then gather up all the sticks and move them at road speed to LA where we compare the sticks once again. They will all be the same length.  However, we also compare the railway clock with the same type we have, and notice the one on the train seems to have lost time, which is in fact, the case.

Now here is where I get stuck. I THINK the observers on the train see our sticks as having grown in length as they passed, and our clocks seem to be running faster. I THINK this is the case because the diffences in speed come about because we applied ACCELERATION only to the train. Einstien seems to have compared induced acceleration with gravitational acceleration and having similar effects.

This seems clearer to me if we extend the passenger trip into a spaceship traveling at 86 percent the speed of light to a location exactly one light year distant. We observe the spaceship arrive about 14 months later.  However, something very stange has taken place.

First, the astronaut reports having arrived at the destination in about seven months. This is beccause at 86 percent the speed of light his time slows by half. Even stranger still, he has reported his location along the way using celestial navigation, and is astonished to report he seems to be travelling FASTER then the speed of light. Well he did after all, get there in only seven months by his own calculation.

So. How does all this relate to our sun being about 5 billion years old?  The solar system is something of a closed system in that none of the objects in it that we can see travel anywhere near the speed of light in relation to one another. So we see our year, calculated as current orbital speed, as stable over time. And we can simply infer backwards in time with such methods as red shift telling us how fast the galaxies are recedeing according to OUR time and how we measure it in light years. After all, we are the ones looking out the train window, and we will always observe the speed of light as 300,000km/s.

However, there are some flies in the ointment. The one close to home is the orbit of Planet Mucury. It is so close to the sun's massive gravity that its orbit can only be explained using relitavistic phenomena. Another possible fly is the universe is now expand faster!  Acceleative forces are being applied to us! Of course this force may be the same for all the galaxies, and does not have immediate relativistic meaning to us.

And then there are the objects most distant from us. I have read it may be possible that some objects are receding at the apparent speed of light or greater as space itself expands at relativistic speed. 

All this could be entire balderdash, based as it is on very little mathematical capacity on my part. And as for the Universe after one second? Stumps me. I HAVE heard it describe in Planck Time Units, which a measure in and of themselves. But I have a headache.

[PhysBang]

According to my understanding of Einstein's Special Theory of Relativity,
both 'time' and 'space' can no longer be regarded as 'absolutes' in the
sense that Newton would have maintained, but depend rather on the velocity of the instruments doing the measuring  (as, with say, the famous 'twin paradox').

A time coordinate must be constructed in a special way, but it is not dependent on particular instruments. Certain instruments are much easier to use in considerations using certain time coordinates, but theoretically any instrument can be used in a measurement for any system of coordinates. The velocity of any instrument can be assigned effectively arbitrarily, and this assignation is part of the construction of a time coordinate.

Without evoking the concept of absolute time, what possible meaning can be given to statements like 'The sun is 4 billion years old', or '1 second after the Big Bang' - concepts that are bandied around as if they might be meaningful.

They aren't really meaningful, because there is no such thins as "the Big Bang". People often use this term to refer to the point at which our standard physics breaks down, though.

We can identify a time coordinate that we can apply to the entire universe is a meaningful way, though. The expansion of the universe seems to take place in a well-defined time coordinate that we can assign to the position of idealized galaxies. (Actual galaxies deviate in their position from the ideal, but they average out pretty good.)

We can use this time coordinate to speak of how long it has been since the farther point back that we can meaningfully run back the expansion of the universe.

Whenever I have spoken to 'physicists' regarding this problem, it seems that they are not clear as to what is meant either.  What theoretical 'clock' is doing the measuring in these instances?

The clock is effectively the standard physical processes of the average galaxy.

[JP]

I think what you're looking for is called comoving coordinates (http://en.wikipedia.org/wiki/Comoving_distance).  You're absolutely right that the measurement of time depends on the reference frame you're using to make the measurement.  The idea of comoving coordinates is a reference frame in which the universe appears roughly isotropic (the same) in all directions.  When cosmologists give you the age of the universe, I believe they're using comoving coordinates (since they calculate it from the isotropic CMBR). 

[litespeed]

jp & George

Although time and space are relative, for us they very specifically anchored in OUR time and space.  Accordingly, we are entirely allowed to make such observations as the age of the universe in our time measurements.

Here is a way to make some sense of all this.  The earth is bombarded with various particles originating from great distances. From our point of view they may have taken billions of years to get here. However, because they travel at near the speed of light, from THEIR point of view, they may be but a few seconds old.