# Gravity and Matter

1. Apr 15, 2015

### Zwarmapapa

Let me start of with this first: I'm not a physicist, I'm a software engineer, so my knowledge about this is pretty much nihil.

Anyway, my question is, why is it assumed that matter causes gravity? Is there any proof or logic behind it?

What if matter and gravity are two separate things that both attract each other?

2. Apr 15, 2015

### Staff: Mentor

Welcome to PF!

Its starts with the observation that the moon revolves around the Earth and the Earth and other planets revolve around the sun. In explaining the behavior, we postulate gravity. This was how Newton formulated his gravitational theory and came up with the equations of motion for the system that agreed well with measurements.

Later Einstein extended the idea using geometry as the basis with matter telling space how to bend and space telling matter how to move.

http://en.wikipedia.org/wiki/General_relativity

3. Apr 15, 2015

### Staff: Mentor

Physics is an empirical science, meaning that we depend on observations to confirm or falsify our hypotheses.

Every time that we have ever observed gravity, there has been matter nearby, and Newton's $F=Gm_1m_2/r^2$ is consistent with the mass of the matter. Conversely, every time that we have been able to measure the gravity around some matter, we've found that the gravity is there and again Newton's law accurately predicts its strength.
Thus, we advance the theory that matter causes gravity and that Newton's law is a good description of how the two are related. After better than three centuries of testing this theory, it's looking pretty good. It doesn't have to be right, but then again the sun rising in the east tomorrow isn't a sure thing either... you and I likely wouldn't bet against either of these propositions..

Now consider your alternative hypothesis, that matter and gravity are two separate things that attract each other. One solid and reproducible observation of matter without gravity, or gravity without matter, would suffice to show that they're two separate things. But without that observation there's no reason to take that hypothesis seriously, just as there is no reason to take the hypothesis that some unknown astronomical phenomenon will cause the sun to rise in the west tomorrow.

(Although it's a digression here, it is worth noting that general relativity has replaced the statement "matter causes gravity" with "matter and energy cause gravity". Thus general relativity can be considered an extension of Newton's theory; this doesn't change the basic point about basing the theory on obserbvations).

4. Apr 15, 2015

### houlahound

I will add if there was something else causing it, then because it acts in the precise same way as what we expect then it does not matter if we have it wrong as there is no way to tell the difference between our wrong view as we understand it and the right view which nobody is aware of.

we could make up a hypothetical cause eg green pixies under my carpet that are actually the real reason for gravity but there is no motivation to do so.

5. Apr 16, 2015

### Zwarmapapa

But aren't photons matter without mass/gravity? And isn't dark matter mass/gravity without matter?

6. Apr 16, 2015

### Staff: Mentor

Photons aren't matter, dark matter is matter, and stuff like this is the reason for the "although it's a digression..." paragraph at the bottom of my last post.

7. Apr 16, 2015

### ZapperZ

Staff Emeritus

There are a few "cultural lessons" that need to be taught here.

In physics, and in science in general, nothing is accepted until there is a convincing set of empirical evidence! This is important for you to understand because you seem to think that "matter causes gravity" is an "assumption" in physics! This is FALSE!

We do not build spacecrafts, spend billions of dollars, or risk the lives of people, based on an assumption! We know it well enough to make it work! That's a very important aspect that somehow has eluded you.

Yes, so what IF they are two separate things that both attract each other? This is where you need to learn a bit how to think like a scientist. If you have a hypothesis, then ask yourself "If this is true, then....." In other words, you need to carry it out and figure out the consequences of that hypothesis. THEN, compare it with what we have already observed. Do they match? Do they agree qualitatively and quantitatively?

Unfortunately, in your case, you ask if something is true, but then expect the rest of us to do the work for you, i.e. to figure out the consequences if it were true, and to either verify or falsify them. This is lazy because you are simply throwing out stuff and expect the rest of us to do the hard, tedious, dirty work for you. The burden here is on YOU to show argument on why you think your idea is worth thinking about, which means you need to back it up with solid evidence. If you can't, then you have zero basis on bringing it up because you might as well say that your idea came to you in a dream with no rational reason for it.

The way it works in science (and the way *I* believe it should work elsewhere in society) is that the person who proposes something must be the one who provides the necessary evidence to argue for its validity. You simply cannot throw something out and expect the rest of us to falsify it.

Zz.