Flamsteed AS Dr Stuart Clark A new theory of gravity?

Flamsteed Astronomy Society

‘Do we need a new theory of gravity?’ by Dr Stuart Clark — January 5, 2009

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So, with the help of Albert, is all well with Sir Isaac now? Sadly, no. There are at least two areas of observation where Newton and Einstein fail to account for what we can see.

The first is on the scale of the very large. Both in the rotation of individual galaxies, and in the movement of galaxies within clusters, Newton doesn’t come up with the right answer — stars within galaxies, and galaxies within clusters, are moving much faster than they should according to Newton. Much more mass than we can see, is needed, and distributed differently than that observed, for Newton’s laws to work here. Astronomers have come up with a ‘Vulcan’-type idea to try and account for this ‘missing mass’ — it’s called dark matter. It’s mass that must be there according to Sir Isaac, but can’t be seen. Dark Matter turns out also to be just what the particle physicists ordered — the physics of the very large and the very small suddenly converge and all sorts of research funding comes tumbling out. The latest hypotheses of sub-atomic structure propose a family of so-far-undetected particles some of which could be the astronomers’ dark matter — they don’t interact with light (can’t be seen or detected via electromagnetic radiation effects), but they exercise gravitational attraction. Oodles of research time and dollars are being poured into the search for these particles and also into calculating and looking for large-scale effects from gravitational lensing, to clumping and clustering during star and galaxy development. So far the search for direct detection of these particles has come up empty. A US experiment recently logged two events which may be dark matter effects, but the odds are no better than 1 in 5. As for the large-scale effects, maybe there are other explanations — maybe Newton’s laws don’t work in regions of very low acceleration.

Some physicists have proposed small modifications to how Newton’s laws might operate in regions of very low acceleration — these theories are called MOND (modified Newtonian dynamics), TeVeS (tensor vector scalar theory), and MOG or STVG (modified gravity). These approaches have enjoyed some success in explaining the large-scale anomalies, but they are criticised for being empirical. That is, the equations have been tweaked ‘after the event’ to fit the observations — there is no underlying hypothesis from which the equations can be derived. It has to be said that the same criticism can also be directed at the dark matter hypothesis. In the absence of any direct detection of dark matter particles, it is just as likely that Newton was a bit wrong. Maybe we should just go on what we can see. Dark Matter (never mind Dark Energy) seems to share some characteristics with the infamous and unlamented luminiferous aether. Physics seems still to need a defining test to decide between dark matter and MOND, much as the cosmic microwave background emerged finally to kill off Steady State Theory in favour of the Big Bang.

Are these large-scale effects the only rain on Sir Isaac’s parade? Not quite. There is the Pioneer Anomaly. The Pioneer probes launched in the early 1970s, are now on the very edge of the Solar System almost in deep space, and they’re not where Sir Isaac says they should be. Pioneer 10 is 400,000 km off course according to Newton. Why so? In truth there could be many mundane explanations — a leak of fuel perhaps, but the possibility of finding anomalies to Newton right here in our own backyard are sufficiently intriguing to warrant a very careful re-analysis of the Pioneer telemetry — all 95,000 measures of it. The jury is still out. Another line of enquiry involves efforts to design experiments that can take place in regions and times of very low accelerations right here on Earth.

So do we need a new theory of gravity? One way or another, the answer seems to be ‘yes!’. Either Newton needs to be modified, or the existence of dark matter will be proven, in any case opening up all sorts of new possibilities. Stuart thinks that within 5 to 10 years we will be staring at an entirely new picture.

MRD