My dad is forever going on about his idea that gravity is not caused by objects pulling on each other, but by the surrounding space pushing the two objects together, via the space emitting some sort of particle. Noting that when you, say, throw pebbles at a fan, he observes that more will get through when the blades are moving versus when they are still. He then reasons that if you were to put a “gravimeter” on the ground with some sort of spinning fan or disk above it, the meter would register less gravity. He did an experiment with a very primitive apparatus (he put some bricks on a scale a few inches under a ceiling fan) and claimed that the bricks weight slightly less, but conceded that the difference was within experimental error.
I personally don’t believe his ideas, but has anyone ever done a similar experiment with proper equipment? Where would I find the results?
That has nothing to do with gravity. When the blades are moving, there is more time when they are in a position to reject the pebble, especially since they are moving much faster than you can throw a pebble. That’s because the pebbles aren’t point particles, and so they have to pass through part of the blade plane before they can won’t be kicked back by the blades. That takes a certain amount of time. The faster the blades go, the harder it will be to throw a pebble through. If the pebbles were traveling much, much faster than the speed of the blades, this would be a negligible effect.
But surely either gravitons or levitons (your Dad’s hypothetical reverse-gravity particles) can travel through ordinary matter, otherwise, people at the bottom of mine shafts would be weightless.
If a spinning fan will block levi/gravitons, why doesn’t a stationary fan? Why doesn’t a solid disk?
It sounds like he’s drawn an analogy between pressure and gravity - you know that thing about there being no such thing as ‘suck’ - when you suck soda up a straw, you’re actually not pulling it, you’re letting everything else push.
If he’s right, then there should be a maxiumum theoretical perceived force of gravity - that experienced when all levitons underneath you are blocked, and you’re only being pressed down by those coming from around and above.
Which would mean that as you approach that maxiumum, adding more and more mass results in less and less additional gravity. - Gravity exerted by a body would not be proportional to its mass - which I’m pretty sure can be demonstrated to be untrue.
The first paragraph is exactly true, except it was pulling a rock with a rope tied around it.
What I want to know is not how you would go about disproving it, but rather, if someone already has and if so, where could I find the results. That would satisfy him, I think. Regarding gravity not being proportional to its mass, he would probably come back with “Therefore there must be no observable objects which are sufficiently massive”.
Tangent, but isn’t this really just a matter of convention? Things flow from higher pressure to lower pressure, but it’s somewhat arbitrary as to whether you decide to say that it’s being sucked or blown. Generally we say “suck” if the pressure differential is created by lowering the pressure somewhere and “blow” if it’s increased somewhere. So I think there* is* such thing as “suck.” <insert Eliot Spitzer joke here>
I’m not positive your Dad’s position is entirely wrong, although his reasoning may be faulty.
I once heard it posited by a Physics professor that there is no such thing as a ‘pull’ but only a ‘push’, much like your father says.
The ‘everyday’ example he gave was that you can’t really ‘pull’ open a refrigerator door by it’s handle ; what’s happening is that your hand is pushing on the far side of the handle. (‘Pulling on a rope around a rock’ referenced earlier is a similar example; the rope is pushing on the far side of the rock.)
Do we have any General Relativity experts out ther (I think it is) who can expound on how gravity is due to mass warping space-time? (As in the old rubber-sheet example they’re always using)?
A fan is a pump.
If it’s blowing air away from you, it’ll also deflect pebbles away from you.
As the fan spins faster, the odds of a pebble being struck by the back side of a canted blade increases. Those pebbles that hit the front side of a blade will pick up a spin, and some movement tangent to the direction of the fan’s rotation. They still need to clear the space swept by the back side of the fan blades a second time in order to be ‘rejected’.
Only when the fan is blowing towards you does the rate of pebble rejection increase with fan speed.
This depends on which way the fan is turning. If the fan is blowing away from you and the the pitch speed of the blades matches the speed of the pebble, then a hit will only occur if the pebble happens to hit the leading edge of the blade. If the fan is not turning, or is blowing towards you, then the pebble is much more likely to strike a blade.
[Edit: Oops, it seems squink beat me to this explanation.]
What that has to do with gravity is beyond me, also. Depending on the scale, placing it under a cieling fan can be a pretty sure way to get an inaccurate reading, but that’s just because of air movement against the platform.
No - it’s not arbitrary, because you cannot ‘suck’ beyond a certain (quite low) value of pressure differential - you can create a vacuum, but nothing more, whereas it is possible to blow at practically unlimited pressure - if it was your sucking that was actually doing the work, why would this be so?
The liquid is not pulled up the straw under tension - it’s pushed up from underneath. There is a real-world difference.
The point of the pebbles is that particles will be deflected more by a spinning disk than one standing still. He tried the experiment with a harrow blade too, but I believe there was a problem with the apparatus. It was more than 20 years ago.
Exactly. The maximum amount of suckage a vacuum cleaner or a slurping straw or a spaceship puncture can create is the force of the air pressure on the other side. So if your spaceship gets a puncture, things aren’t being sucked out into space, the air inside the spaceship blows out and carries things with it, and the force is entirely due to the pressure difference between the inside of the spaceship and the outside.
The idea of vacuums sucking isn’t completely wrongheaded, but it’s often much easier to understand what’s happening physics-wise if you look at it from from the perspective of blowing instead of sucking.
It’s a distinct possibility, but it would be helpful if I could actually demonstrate why he is wrong. That is why I’d like to point him to a paper or something.
Here’s a simple experiment I just thought up. You could use anything as an experimental object, but something with a flat surface (like a brick) might do more to eliminate possible deflection of the gravity particles being tested for.
Set your experimental object on a scale outside on a clear day.
Set your experimental object on a scale outside on a cloudy day.
Set your experimental object on a scale inside.
If the three measurments are within instrument error, his theory is disproven.