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About Philip A. Stahl
Expertise
I specialize in stellar and solar astrophysics. Can answer any questions pertaining to these areas, the spectroscopic analysis of stars – as well as the magneto-hydrodynamics of sunspots and solar flares. Sorry – No homework problems done or research projects! I will provide hints on solutions.

Experience
Have published papers on the relationship between sunspot morphology and solar flares; discovery of SID flares related to this, constructed computerized stellar models; MHD research.

Organizations
American Astronomical Society (Solar physics and Dynamical astronomy divisions), American Geophysical Union, American Mathematical Society, Intertel.

Publications
Solar Physics, Journal of the Royal Astronomical Society of Canada, Journal of the Barbados Astronomical Society, Meudon Solar Flare Proceedings (Meudon, France)

Education/Credentials
B.A. degree in Astronomy; M.Phil. degree in Physics - specializing in solar physics.

Awards and Honors
Postgraduate research award- Barbados government; Studentship Award in Solar Physics - American Astronomical Society

You are here: Experts > Science > Physics > Astrophysics > Relative and Absolute Time

Astrophysics - Relative and Absolute Time

Expert: Philip A. Stahl - 11/3/2009

Question
Dear Professor

I have no training in astrophysics and very little knowledge about it. However, I am an interested layman, and here is a problem that I am curious about. Please correct any wrong assumptions I might be making.

If I understand rightly, Doppler effect causes light to shift to the red end of the spectrum, so we can deduce that any source emitting red-shifted light in our direction is in motion away from us. Or alternatively that we are in motion away from it. From these, and related observations we infer that the universe is in constant expansion, which must have been set in motion by what we call the Big Bang. We can also estimate the date of the Big Bang, which I believe is taken to be at about 4 billion years ago.

Now, if this argument is correct, then it must apply to any point in the universe. That is, from any point in the universe, we could apply the same equations and arrive at the same date for the Big Bang event. But it seems to me that this forces us to accept the notion of absolute time.

However, (and again please correct me if I’m wrong) the notion of absolute time is rejected. Einstein imagined himself riding on a light beam reflecting from the town hall clock face at 12 o’clock. As long as he travelled at the speed of light, abreast of the light beam, then the time on earth would always be 12 o’clock. This idea is behind the science fiction story of the space traveller who, travelling at, or close to, the speed of light, returns home to find his family grown up, or dead, while he himself has hardly aged. I think the notion of relative time is accepted by modern
astrophysicians.

It seems to me that the Big Bang forces us to accept the notion of a fixed point in space /time. The space travel example is intended to show that the measurement of time is
a function of the point of observation and the velocities of points relative to each other.
But there is no sense in the idea that the point of the Big Bang had a relative velocity at all. Relative to what?

Let’s imagine Captian Kirk goes off round the universe leaving his wife at home to look after the kids. Travelling at or near to the speed of light, he passes through many different points. From each of these points, it is possible to apply the relevant equations and fix the date of the Big Bang at the same moment. In other words, wherever Kirk goes in the universe the time elapsed between the Big Bang and the present moment will always be the same. But Captain Kirk is also himself a point in the universe. And when he gets back home he will be at the same point as his wife. If Kirk has aged two years, but his wife has aged 50 years, then we would have to say that if the time of the Big Bang measured from Kirk is x, the time measured from Kirk’s wife will be x + 50. This is not possible if they are both at the same point.

If we insist of the notion of time as relative, then arguably we would also have to allow that there might be some point in the universe from which we could fix the Big Bang event at some other competely different moment. May 2 billion years ago,, maybe just two years ago, or maybe even yesterday.

So my problem is, how do astrophysicians reconcile these two notions of time?

Many thanks,

Alan Reeves.

Answer
Hello,

Okay, you have really two separate issues and I don't want to get them confused - so will stick with just the question to do with the expansion of the universe.

Basically, there are two theories of importance: the special theory of relativity and the general theory of relativity.

In the special theory, Einstein showed that there is no such thing as an "absolute space or time". Thus, while to us on Earth such concepts as time, mass, length etc. of an object seem pre-determined and fixed - they aren't for someone traveling at close to the speed of light.

In that case, as Einstein showed, the length of an object contracts, its mass increases, and any clocks it has slow down - the closer one approaches, c ( = 300,000 km/second) the speed of light.

According to Einstein's Principle of Relativity: "All the laws of physics are the same in all inertial reference frames."

Now, the important point here is that the special theory wherein times are compared(using stationary and moving clocks(, is used for *non-accelerated* frames. It is not applicable for accelerated frames such as apply to the expansion of the cosmos (we now have the evidence that the cosmos is accelerating, and for more details on that I suggest you may want to read: 'Supernovae, Dark Energy and the Accelerating Universe', by Saul Perlmutter, in Physics Today, April, 2003, p. 53)

Also, what we are in fact measuring with expansion is an *approximation* of the AGE of the universe, and I emphasize it is only an approximation! The reason is that the so-called "Hubble constant" used to obtain it can itself vary in time. Currently, we estimate H ~ 70 km/ sec/Mpc
(Where Mpc denotes ‘megaparsec’ – e.g. 3.26 light years is one parsec)

We know that all *external galaxy clusters and objects* are receding from us with some velocity v, that increases with distance R:

v = H R

where v is the recessional velocity, and H the Hubble constant, with the age of the universe (in seconds) related to the Hubble constant by:

t = 1/ H

Since not too long ago H was used as 100 km/ sec/Mpc, it follows that t can vary contingent on observations to that time.

Again, your worries about "absolute" times are not germane here, since we are looking at accelerated frames in the expansion.

Please read the article I cited earlier, and - if you are still unclear, get back to me. In addition, I suggest trying to get hold of the excellent little book, 'The ABC of Relativity' which will make a lot of this clear at a basic level.

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