kooky physics question -- gravity and vacuum energy

Factual Questions
1

So there’s this dark energy/cosmic constant/vaccum energy/zero-point energy/etc. that’s pushing the whole universe apart.

Except that it only really functions where there isn’t any matter. We don’t see the Milky Way being particularly ripped apart, nor the earth. So it’s a property of empty space.

Then you have gravity. Gravity is allegedly transmitted by allaged gravitons (yet to be observed) … somehow or other. How does this work, exactly? How many gravitons are each object sending out at any second? And won’t such objects just evaporate from all of this energy/particle loss at some point? Or are they accepting as many gravitons as they give out?

Anyway, the whole graviton thing was throwing me for a loop. Then I started to think about dark energy, and how it’s in empty space all around us, pushing in every direction, push on matter.

Is it possible that gravity is just an illusion? That it’s just the observable effect of dark energy, which forms in the quantum vacuum, pushing nearby objects together?

Consider it like macro-scale Casimir forces.

You have, say, a planet. It’s alone, and the dark energy is pushing pretty equally on it from all sides, so it doesn’t move much.

Now let’s add a small moon to the mix. Between the moon and the planet will be far less vacuum energy, since there is simply less vacuum. So the two objects will tend to be pushed towards each other. Presto! “Gravity” causes an “attraction” between the two.
Now, this is all just to get these kooky ideas out of my head so they don’t fester. I know it all has a “speed of dark” sort of flavor. Please tell me more about gravitons, how they work, and why the above scenario is wrong.

2

Anyone?

3

Two objects will be attracted to each other by gravity, which is far stronger than any purported Casimir force. I fail to see why there should be “less vacuum” between two objects separated by astronomical distance. The Casimir force, if it exists, has only been observed at very small separations.

4

Dark Energy is an extraordinarly small component of the current energy density of the universe. This is, of course, something of a mystery to scientists who, from theoretical constraints, look like they are off by some 100 orders of magnitude or so. So, there is not a very clear understanding of what this stuff entails.

A quantum theory of gravity, which is exactly what gravitons are getting at, isn’t confirmed yet either. We think that the detection of gravitational waves will give us some insights, but so far we have no such detection. While you can make some analogies between the gravitational theory and quantum mechanical treatments (which is really where the graviton comes from) it is still an unestablished theory.

So, what you’re basically asking is, “Can we explain something we aren’t sure about with something we don’t understand?” Answer: maybe.

5

I think you misunderstand what he’s saying. He’s not talking about Casimir forces, but some other repulsive force that acts like, in his words, a macro-scale Casimir force.

We currently describe gravity as a pull between masses. The OP is offering an alternative explanation that gravity is a push from empty space, and we experience a force toward other masses because they partially block the push from that direction. We think the sun is pulling on the earth because the sun is blocking the push coming from all the empty space behind it, so the sum of all the other pushes on the earth add up to a net “pull” toward the sun.

I have no good answer for the OP, but it’s a very interesting thought experiment to examine how to tell the difference between a pull from one direction compared to a push from all other directions.

6

Really? I thought current calculations made it out to be something like 75% of the universe.

Exactly what I was getting at, micco.

7

You’re missing the point of dark energy. It’s not that it’s a property of empty sapce, but rather, it’s a force whose strength increases with increasing distance–the opposite of any other force. It’s effects are negligible over distances the size of a planet, or even a galaxy, but at the scale of the size of the universe, it becomes quite strong. That’s why we don’t notice the Milky Way or the Earth being stretched apart.

Thanks for the clarification, micco.

8

I think there’s a problem with this explanation. An object blocking the push from the empty space behind it sould produce an inverse square type attraction based on the radius of the attracting object. That is, the cross-sectional area of the attracting body is what is doing the blocking. Instead we see an inverse square attraction based on mass.

Unless I misunderstand you.

9

Good point, Monkeymensch! Have to think about that…

10

Monkeymensch

That’s assuming that the “push” of empty space acts substantially like electromagnetic radiation. Perhaps it is the mass (regardless of size) that casts the shadow.

11

Whoa…

Good idea…pretty impressive, I might add.

12

The pressure shadow idea is an old one, and it is rather clever, but unfortunately, it doesn’t work. The problem is that you can’t reconcile it with relativity: No matter what form you give to the “push-field”, you can’t make gravity work in all reference frames. That is to say: If a binary star system were to fly past us at high speed, the stars of the system would still orbit each other as though their center of mass were at rest. Because, of course, in their reference frame (which is just as good as ours), their center of mass is at rest. This is what is predicted by conventional (Newtonian and General Relativity) models of gravity, and it is what’s observed, but this is not consistent with the pressure-shadow model. Feynman goes into some detail on this in one of his books (I think it’s Six Easy Pieces).

As for forces being carried by virtual particles, we don’t yet have a quantum theory of gravity, but we do have a very good quantum theory of electromagnetism, and a quantum theory of gravity, when we find it, it expected to be similar to quantum electrodynamics. At least, similar enough that the vague classical analogies used to describe QED in laymen’s terms shouldn’t be much worse at describing QG. So what are those analogies? The simplest to describe is a repulsive force. If you and a friend are both standing on skateboards, and throw a basketball back and forth between you, then you’re going to be pushed apart. But what about an attractive force? How would you attract each other by throwing a basketball between you? You need a basketball with negative mass and momentum. Now, classically, this is absurd, but quantum mechanically, for a virtual particle (that is to say, a particle that’s just being passed back and forth) this is possible. This is also why an object doesn’t lose mass from all of these virtual particles: Some have negative mass, some have positive, and it all balances out.

Now, suppose that while you and your friend are tossing this basketball around, someone else comes along and adds energy to the system, say, by tackling one of you. What’s going to happen? A particle (the basketball) is going to be emitted. But now, it’s going on it’s merry way, not just being passed back and forth, so it’s a real particle, not a virtual particle. That means that it’s now somewhat less weird, and needs to have positive mass and momentum, just like “ordinary” objects.

13

IANATP, but I didn’t think the shadow idea could be original.

However, I’m still not sure that I understand what’s wrong with it. Posit:

  1. “Empty” space is constantly emitting some Particle (X).
  2. Direction of emission is random.
  3. X moves relativistically.
  4. X is capable of exerting a “push” on matter, but is destroyed/absorbed/rendered not X in the process.

So, instead of two people throwing a basketball back and forth, we have two people getting pelted from all directions. However, since Person A naturally blocks some of the basketballs hitting Person B (and vice versa), they get pushed together. In this case, the basketballs don’t need to be virtual at all (although they are currently undetectable).

Am I completely out in left field or is there evidence that one or more of my posits can’t be true?

14

Hey now! Describing me is MUCH more difficult than describing quantum gravity, I’ll have you know.

15

I would need further clarification but ltes imagine that we have two parallel plates hanging out in “empty” space. Altough they are getting barraged on the outside by these X particles, they are also being barraged by the X particles being generated in the empty space between them. If the number of X particles being created is constant for all of space and they all travel at the speed of light, then I think that the “pressure” created inbetween the plates would perfectly offest the “pressure” on the outside. This would lead to zero net force, not the two plates being pushed together. I however might not understant your postulates.

16

It’s very old. As far as I know, the first person to propose it was Fatio de Duillier in the late 1600s. The suggestion sort of engaged Newton as an explanation of gravity for a bit, but he was never convinced and quickly lost interest in the idea.
As for the Feynman reference, it’s chapter 2 of The Character of Physical Law.

Chronos’ argument from relativity is a good one; Feynman’s version of that is an analogy with running in the rain. However, it might be simpler to think about three bodies, rather than just two. What happens if you put Person C between A and B? In Newtonian gravity the effects of A and C on B add together. C’s pull on B doesn’t depend on whether A is there or not. Here this can’t quite work; A is already shielding from both C and B some of the basketballs that C is meant to produce its effect from by shielding from B.

It’s been mentioned that the gravitons involved are virtual particles. Asking how many virtual particles are involved in a situation is one of those nasty, ill-defined questions that quantum mechanics refuses to answer. Sorry …