Weightlessness

Sorry for the delay, but I've been off on a family holiday, touring and camping in New South Wales (which doesn't look a lot like the old version), coupled with a trip over the border into alien territory, namely Queensland.

I was on the Gold Coast, a tourist mecca and home to a number of theme parks. As the father of little-cusps, we headed to DreamWorld to be thrown about a little. When I was riding the Cyclone....
I remember a post a long time ago by my ex-Honours and MSc student, Luke Barnes (who has just returned to an Australia as a Super Science Fellow), namely what does it feel like to be weightless.

Of course, what I mean by weightless is something like this,
namely some astronauts cavorting about in orbit, floating about and just having a good old time of it.

So, what does weightlessness *feel* like? Many have seen astronauts training, in their gear, in water tanks. They can bob about and look something like this
Doesn't that look nice and peaceful? I know they have a job to do, but bobbing about in water is relaxing. Look, lots of people do it!
Gee, astronauts have it easy.

But is that is what weightlessness really feels like? Well, no.

When you are bobbing about in a pool, even in an astronaut spacesuit, you *feel* gravity, and you know what direction it points in. Gravity pulls you towards the centre of the Earth. When you are floating "head-up", your guts are pulled down towards your legs, as is your blood and other fluids. There are force gradients across your muscles, and basically you feel the direction of gravity.

If you closed your eyes and someone gently rotate you onto your side, you would sense it. Your guts would slosh over and put different pressures on your body, as would your blood flow and the other processes in your body. You would know that your body's orientation with regards to the gravitational field has changed.

However, a spaceship orbiting the Earth is in free fall, continuously accelerating towards the centre of the planet, but getting no closer.
What is this free fall like? Well, those who have been on a roller coaster, or up-and-down road, know what free fall feels like. But let's look at what Einstein told us. Basically, he encapsulated this in the equivalence principle. Essentially, what this says is that for someone free falling in a gravitational field, the effects of gravity disappear (we'll ignore the subtleties of tidal forces).

What this means is that, without a gravitational field, your body has no clues on what way is up. Your guts hang there, not pressing down or up, or side ways, but just hang there. Your gravitationally induced stresses in your muscles vanish, your blood pressure gradients change. Basically, your body has no clue to up.

What it feels like is going over the top of the roller coaster with that unpleasant feeling of your stomach in you mouth. Or more like this ride
but instead of the feeling of falling for seconds, the sensation goes on for minutes, hours, weeks and months.

No wonder some astronauts throw-up when they get into space!! Floating it is not!

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