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synchronicity - oldbloke's mutterings
September 6th, 2012
11:03 pm
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synchronicity
James May's most recent offering mentioned something that then featured in Simon Mayo's R2 show today - he'd apparently got it from some science mag. I believe the proposal was originally made by a SF author. I may as well mention it too.

If you drill a hole right through the Earth, and can keep the air out to remove air resistance, you can get from one end to the other in 42 minutes (and a few seconds IIRC), just by jumping in (presumably in some vehicle to deal with the lack of air and the accelerations)

But, you say, you can't go through the centre, there's a solid core surrounded by molten stuff ect ect ect

No matter - if the hole is straight, it can go from any point on the surface to any other point on the surface, AND - it still works and still takes 42 minutes.

Wow! Science is wunnerful, innit?

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From:hooloovoo_42
Date:September 6th, 2012 10:29 pm (UTC)
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Yes, but how many holes would you need for all the people who want to depart and arrive in different locations?

The Stargate is so much simpler, only takes a few seconds to travel across the galaxy, but you can't have 2 on the same planet without causing problems.
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From:hooloovoo_42
Date:September 6th, 2012 10:48 pm (UTC)
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I bet you couldn't make a straight hole from my back garden to your back garden and travel between the 2 in 42 minutes.
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From:oldbloke
Date:September 6th, 2012 10:59 pm (UTC)
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Only because I can't make the hole. The physics is still valid.
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From:hairyears
Date:September 6th, 2012 11:46 pm (UTC)
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What acceleration? If you dropped someone (in a capsule, I hope) down a dead-straight airless tube through the centre of the Earth, they would experience no acceleration whatsoever.

The journey is the very definition of Free Fall.

An external observer might observe the capsule receding or approaching at an acceleration of ± 9.82 m/s² but the measured acceleration *inside* the capsule would be zero.

Assuming, of course, that there are no interactions with the Earth's magnetic field.

The underlying physics is deep, deep stuff: few people - and few physics graduates - have ever thought through the implications of knowing that gravity is not a 'force', but a distortion of spacetime associated with mass.
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From:bellinghman
Date:September 7th, 2012 07:58 am (UTC)
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The amusing thing is that one can tell the difference between gravity and acceleration by fine measurement. In the classic sealed-lift thought experiment, in a gravitational field, the top and bottom of the lift experience slightly different forces. When undergoing the supposedly equivalent acceleration, they experience the same as each other.

For a point sized body, of course, this isn't the case.
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From:hairyears
Date:September 7th, 2012 09:55 am (UTC)
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Quite. Good luck, measuring that on or in the Earth in a capsule a couple of metres long. The effect is detectable at those scales, but it needs laser interferometry, low temperatures to exclude thermal 'noise', and some ingenious lateral thinking in order to separate the measurement from the effect: light and laser interferometry exist in the same distorted spacetime as the objects under examination.

The gravity gradient is quite noticable to human senses - indeed, quite dangerous - in the vicinity of very dense, very massive objects like black holes and neutron stars.

The term 'spaghettification' has been used in fiction to describe the effect on astronauts in an intense gravity gradient: I look forward to seeing it demonstrated at WorldCon 2014 (or Eastercon 2013) in the Sience Program (assuming purplecthulhu can get hold of the neutronium).
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From:aldabra
Date:September 7th, 2012 07:13 am (UTC)
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How does getting out at the other end work? If you miss the moment do you fall back?
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From:bellinghman
Date:September 7th, 2012 07:45 am (UTC)
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Yes. But that's OK, because 84 minutes later, you can have another go. Assuming that you've not got out at the start point 42 minutes later.

If you look at the whole thing side on, it looks the same as an orbit seen from the edge. In gravitational terms, it is one dimension of an orbit: it's just that the second dimension has gone to zero.

Edited at 2012-09-07 07:54 am (UTC)
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From:bellinghman
Date:September 7th, 2012 08:08 am (UTC)
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Indeed. A straight line between any two points on the Earth's surface would carry something between them in 42 minutes using gravity alone.

There is an interesting article here on one particular implementation.
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