# Solidity and illusion

1. Jun 26, 2015

### Mike Moores

Notions of solidity are illusory but, if the Universe were seen from a sufficient distance, would it appear to be solid?

2. Jun 26, 2015

### Garth

Firstly Mike welcome to these Forums.

However you need to frame your question more rationally.

At large scales, rather than appearing solid', the universe appears to be a very good vacuum.

The average density is about 10-29 gm/cc, a far 'harder' vacuum than can be achieved in a laboratory or even in space in NEO.

It is because of the vast distances between stars and the even vaster distances between galaxies and galactic clusters that the average density is so low.

If you think about your question more you will appreciate that you cannot 'see' the universe from a "sufficient distance", light rays travel within the space-time of the universe and so you cannot 'see' the universe from outside.

You might find a basic course in cosmology helpful - try working through Ned Wright's tutorial lectures. If you cannot understand anything you can always ask about it in these Forums.

Garth

3. Jun 26, 2015

### Mike Moores

Thanks, Garth. Points taken.

4. Jun 26, 2015

### Chalnoth

The feeling of solidity comes from the electromagnetic force, which is very strong on short scales. What you feel when you press against something is the electrostatic repulsion between the electrons on your hand and the electrons in the object you're pressing.

The only force that is active on very large scales is gravity, and under the force of gravity matter is always attracted to other matter.

This is in addition to what Garth has said, which is accurate as well.

5. Jun 26, 2015

### rootone

Bear in mind too that perception of objects as being solid is a result of a lengthy evolution of the brain.
Object being apparently solid is probably of greater survival value than perceiving objects as a set of discrete particles within a largely empty space.

6. Jun 26, 2015

### Chalnoth

I really don't think that has anything to do with it. It's more about size: those particles are far too small for our eyes to make out.

7. Jun 26, 2015

### rootone

Yes I didn't explain what I meant very well.
What I meant is that having evolved eyes which respond to a limited range of light, and a brain cortex which is able to identify what is seen as an overall whole object.
This probably has survival advantage over a (hypothetical) sensory system that could directly perceive what really exists at microscopic scales
I am saying 'probably' since that's just my intuition. It isn't completely impossible that creatures on alien worlds might evolve in an environment whereby direct sensing of atoms and etc could confer an advantage.

8. Jun 26, 2015

### Chalnoth

I'm not sure that it could ever be possible for macroscopic organisms to evolve atom-sensing organs. The problem is that in order to view atoms, you need something at around the energy scale of x-rays*. But x-rays, being ionizing radiation, are highly destructive to organic molecules. And as x-rays and other high-energy radiation aren't abundant in nature, the organism would also have to evolve an emitter, which is even more unlikely than a detector that doesn't break down rapidly. I just don't think there's any pathway that could lead to that sort of thing.

* Well, you can also use electron tunneling for the same purpose, provided you're only interested in surface features, but the design requirements of using electron tunneling are way too precise for a biological organism, plus there's no plausible evolutionary path as it requires both extremely short range interactions and induction of an electrostatic potential between the target and the sensing organ.