David Miller
Hon Res Fellow, University of Glasgow and History & Archives, Physiological Society

https://doi.org/10.36866/pn.99.6a

The fascinating article by Harridge et al. ‘Space Flight and ageing’ in PN98 (pp26-29) deployed the subtitle (or standfirst) ‘Zero gravity mimics some of the effects of ageing’, but in the text itself referred to microgravity: these terms refer (in subtly different ways) to the status of those a few hundred km ‘up’ in orbit around our planet. But we struggle to find a term other than ‘free fall’ that covers the physics without risking a serious muddle of comprehension for the unwary: every utterance of ‘zero gravity’ will have Newton and Einstein spinning in their graves.

OK, so we all know that gravity is everywhere; that its magnitude falls with the inverse square of the distance between massive objects, etc. So how come that, a mere stone’s throw above the Earth in space-travel terms, we can talk, even loosely, of ‘zero gravity’? Well, you might argue, everyone understands ‘zero-gravity’ as shorthand for free-fall and thus the appearance of weightlessness generated in sustained orbit … right? Wrong! In a MCQ-style poll of my 400-strong physiology Year 2 class ten years ago, nearly half believed that humans would be weightless on the Moon, never mind in orbit. Should we blame the press, or the schools? (See e.g. Gürel & Acer, 2003 for a full discussion of the educational issues). In my classes, only a minority chose the correct answer for Moon gravity (i.e. one eighth that on Earth … and NO, that’s not ‘eight times less’ … don’t get me started).

Recognising that gravity very much acts on people in Earth orbit was dinted into me by my late colleague in Glasgow, Tris Roberts (see e.g. Roberts, 1983). He sought to understand the special case for proprioceptive and central sensory physiology in which ‘free-fall’ astronauts are actually operating. How do we know what way is down? Could astronauts perhaps perceive that gravity (…the acceleration due to…) is still there, together with a constant velocity orthogonally to it? Is it principally the ‘contact force’ that we perceive through our feet (when standing) plus the relevant proprioception (within postural muscles opposing gravity’s pull) that delivers the relevant sensation?

Finally, the authors of the PN98 article were following space science convention in referring to the ‘microgravity (µG) environment’ within their text. (Confusingly for those used to microbalances, microgravity is more routinely symbolised as ‘µg’). Strictly, µg or µG refer to the truly micro-gravitational forces generated when, for example, astronauts move around relative to the centre of gravity of an orbiting vehicle. In free-fall, these tiny forces become of potential physiological significance. But even in orbit, ‘Big-G’ is still there: we really do need new words.

Gürel, Z & Acer, H (2003). Astronomical Education Review 2 (3) 2003

Roberts, TDM (1983). Nature 306, 742-742

Editor’s note: The zero was inserted by the Editor, not by the authors.