h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Centrefugal force is nothing more than acceleration effects due to changes in direction (like turning a corner). It doesn't require a central reference point. It's just that with a local reference point (like a wheel) and with a rotating frame of reference the momentum causes particular effects.

Ouch, deja vu. Just in case anyone is thinking of debating Centrifugal force, can you take it here: F70225?thread=120615 so we can keep this thread close to on topic...

H.
Curious about that time dilation thing...

I read that article back when it came out hooloovoo. I just feel that you can't really have centrifugal(or centripetal or whichever) without some sort of outside reference.

The outside reference is Newtons laws, written for inertial systems. If you discover centripetal or corriolis forces, it is proof of being in a non-inertial rotating system. From measuring those forces at different points the axis of rotation and the frequency can be found.

Ok, I'm a non-physics type of person, so feel free to berate me at any point .

As I understand it you wouldn't need an outside frame of reference, that, surely, is the whole point of relativity. That the two clocks are different relative to each other, they're each others frame of reference. Right?



Ste

I can answer that one...

You're right, Xanatic, that's the problem with the so-called 'twin paradox'.

Two twins, one stays on earth, the other flies in a really fast ship for a couple of years, and comes back.

And, as you point out, the twin in the space ship was 'moving faster', and so ages less, and is younger.

As you also point out, there's no right position to measure speeds from, and so it's wrong to say that the space ship was moving faster.

The answer is: because the space ship was accelerating, different rules apply to it. On earth we're staying still (near enough), so this is a valid position to measure speeds from, and time dilation applies as you said.

The space ship, on the other hand, is accelerating, and hence is not a valid position to measure speeds from. It's not in an 'intertial frame of reference'.

Okay, so how do you measure acceleration without any external reference? I mean if you sit in one spaceship and look at the other, how do you tell which one is accelerating?

In the twin brothers case, one of them stays at Earth, and stays all the time practically at rest compared to the whole universe (the few km/sec movements of Earth around the sun and the sun around the milkyway and so on can be ignored against speeds near lightspeed).

So the other one accelerates compared to the whole universe, and that makes the difference why only his clock shows relativistic effects.

"Okay, so how do you measure acceleration without any external reference? I mean if you sit in one spaceship and look at the other, how do you tell which one is accelerating?"

Force = mass x acceleration. If you're applying a force to your mass, you're accelerating.

Here on earth you're under a more or less constant acceleration of 9.81m/s^2, the force in this case being gravity. (not absolutely accurate, because of course you and the rest of the planet are under an acceleration inwards towards the sun which keeps us in orbit about it once a year, and another acceleration inwards towards the centre of the galaxy which keeps the sun in orbit about THAT every 200 million years, and possibly some other acclerations which are even more mindshatteringly tiny, insignificant and tedious...)

In a ship in flight between here and the moon, say, you're under acceleration as long as the engine's on. When the engine shuts off, you're not accelerating any more (well, except for the acceleration imparted by the moon's gravity... OK, bad example, next paragraph, move along, nothing to see here)

In a ship (call it "The h2g2") out in nice, flat interstellar space, say, you're accelerating if your engine is on. If you're ON "The h2g2", you'll know it's accelerating (even if you've no instruments or anything else (e.g. noise, vibration etc.) to tell you what the engine is doing) because you'll experience the direction of thrust as local "down". If the acceleration is very low (say a solar sail or an ion drive), this might not be directly perceptible - it might take a pen an hour or a day to "fall" to the floor. If you've got a beefier drive - some form of functioning ramscoop A600436, say, you might manage an acceleration in the range of 10m/s^2 - in which case the impression of gravity will be so good you won't have any way of telling the difference between shipboard "gravity" and gravity on earth - no pesky Coriolis effects A593273 like you get on rotating ships like the Discovery in 2001 A533413 or in Babylon 5. (Hideo's excuse for catching Maelcum's hand with an arrow near the conclusion of "Neuromancer" is his employment of a slow moving projectile in a rotational frame of reference... read the book!)

If you're *watching* "The h2g2", from any external frame of reference be it in uniform motion or accelerating, it will be obvious that it's accelerating because (a) it's engine will be on, and barring some seriously bizarre technology you'll be able to tell that just by looking in the vicinity of the ship and (b) the rate at which it's moving along its own velocity vector will be changing.

No need for external references in either case.

A simple comparison is you KNOW when a car's accelerating because you're pressed into your seat. You'd be able to feel that even if you couldn't hear the engine or see out of the windows.

Does that help?

H.

Well put, Hooloovoo...

I'm afraid 'compared to the whole universe' doesn't work, though, Marjin... one of the basic postulates of relativity is that the laws of physics are the same in all inertial frames of reference - there simply isn't one frame that's 'right'.

An inertial frame is one which is not accelerating, and the earth is for relativistic purposes basically not accelerating. Even if it was moving across the milkyway at speeds (in the milkyway frame) approaching the speed of light, as long as it was not accelerating the twins experiment would work in the same way.

As Hooloovoo points out, acceleration always has a noticable effect (although it can be confused with gravity). Therefore it's fairly easy to tell if you're in an inertial frame; if you are, you can measure speeds relative to your position and use those for relativistic calculations.

If you are accelerating, you have to go into general relativity, and I can't help you there 'cause I don't know any - just special relativity

Two points...

First, the thread is misleadingly titled. In the context of gyroscopic propultion, a solid fuel rocket is just as much an "anti-gravity" machine! When we talk of Laithwaite's experiments, we're discussing an inertial propultion system, providing a force in a given direction. This can be used to pull against the direction of prevailing gravity, making the attached object easier to lift, or equally it could be applied in the other direction, for example acting as an axle weight on a racing car at bends helping to keep it on the ground. Gravity is irrelevant; it's just a force generator...

Secondly, Queex, re post #24, Imagine if you have two kinds of matter; those kinds which produce the gravity effect and those which produce a repulsive, anti-gravity effect. Doesn't it make sense for all the gravity-producing matter to clump together in one place, while the (hypothetical) other stuff dissipates as quickly and efficiently as possible?

Nick Cook travelled the globe collecting evidence of trials in various anti-gravity experimentation throughout the C20th in his book "The Search for Zero Point," as well as meeting (where possible, friend/colleagues where not) notables already mentioned, research conducted by the Nazis and major players the US Defence Agencies and associated companies.

"Secondly, Queex, re post #24, Imagine if you have two kinds of matter; those kinds which produce the gravity effect and those which produce a repulsive, anti-gravity effect. Doesn't it make sense for all the gravity-producing matter to clump together in one place, while the (hypothetical) other stuff dissipates as quickly and efficiently as possible?"

Yeeees. I guess. Although 'negative gravity' matter sounds highly exotic. Without a theoretical basis there's no telling what it might do. Plus what you got out of a system with these two types of matter would depend on what you started with. For example, if you had a core of negative-gravity matter and a shell of ordinary matter the system can be stable.

Maybe it would be similar to particles and anti-particles. We could have anti-gravity which would be annihilated on contact with gravity. Probably a load of old cobblers but that's my bit.

There's gravitinos (I mentioned them earlier in the thread) that should exist if Grand Unified Theory works.