Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: IanJamesRead on 11/10/2015 07:09:35
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I’m afraid that I am no spring chicken. I have always had an interest in science and I can’t remember a time when I didn’t watch popular science programmes on TV. Of particular importance here is one program from my youth , which attempted to describe the curvature of space-time (from Einstein’s theories of gravity) using a model. Nowadays, a model is usually represented as a slew of very slick images, generated by a computer, but back-in-the-day computers just didn’t work that way, so scientists had to use their ingenuity. This particular model consisted of a rubber sheet stretched taught over a frame, like the skin of a drum. The rubber sheet had a grid of lines marked on to its surface. We were told by the presenter that the rubber sheet represented space-time. Now, I didn’t understand what space-time was; space, I understood and time, yes that was also a familiar concept, but what was space-time? However, there was no time to think about it because a marble was now being rolled across the rubber sheet. The marble rolled across the flat rubber sheet following one of the grid lines – not very exciting. Then with the marble removed a heavy lead ball had been placed on to the centre of the rubber sheet. It sagged and stretched under the weight, the grid lines now all dipped towards the centre. Again the marble was rolled across the sheet, but this time it slowly veered away from its grid line until it started to trace a circular path around the heavy lead ball. This, the presenter explained, might represent the sun and the earth. The Heavy lead ball, we were told, was the sun and it mass causes the curvature of space-time and it is this curvature that deflects the path of the earth from a straight line into a circular orbit.
After watching the TV programme, I felt I had a vague understanding of what Einstein had been on about, but I was still confused about space-time. It wasn’t until Professor Brain Cox broached the subject on his ’Wonders of the Universe’ programme that I started to get a handle on space-time. Professor Cox was describing a black hole and its gravitational effect. He described how the enormous mass of the black hole caused the surrounding space to stretch and flow into the black hole. The speed of the flow increased as you approached the event horizon and when you did reach it the flow was travelling at the speed of light and thus nothing, not even light, could escape. In this description, space was no longer static, it flowed and to describe flow you need time. Hence, this dynamic description neatly entwined space and time and I felt that I was starting to understand what space-time meant.
However, you will notice that these two descriptions of the same theory are distinctly different, and that started to trouble me. In the second description the implication is that the effect of gravitation is due to the flow of space. Objects near to something massive are draw along in the flow of space like a floating stick is drawn towards a waterfall, but in the first description there is no flow, the rubber sheet is completely static. Now, it is true that the rubber sheet was supposed to ‘represent space-time’. We’d been told that “it’s a two dimensional model of four dimensional space-time”, which would be fair enough, if we didn’t also need time for the motion of the marble; you can’t have ‘time’ twice. If the rubber sheet model tells us anything, it’s that a curvature of space (and not just space-time) produces a gravitational effect. Here, the modelled effects of gravity are due to the shape and not to flow.
So, can we have curved space? Einstein’s theories, which we know to work very well, require space-time, and this space-time can be curved. A curvature of space on its own would simply dissipate if not maintained. To maintain the curvature we need the flow, and hence we have space-time.
Now, I know what you’re thinking; “So, what?” Well, it would appear that there are two mechanism to produce a gravitational effect; the flow of space (due to the curvature of space-time) and the curvature of space (also due to the curvature of space time). If we consider just one of these mechanisms for gravity then we would expect it be entirely responsible for producing the gravitational effects that we observe (ie. an attractive force the pulls massive objects together). However, with two mechanisms it would be the combined effect of both that would produce the observed gravitational effect. Let’s consider the consequences of this:
The weakness of the gravitational force
There are four fundamental forces: the strong nuclear force, electromagnetism, the weak nuclear force and gravity. Of these gravity is by far the weakest. It is difficult to create a theoretical framework encompassing gravity and the other forces because they are so dissimilar. If the force of gravity that we observe is the combined result of two mechanisms, then it is perfectly possible that they work in opposition; one mechanism producing an attractive force and the other a repulsive force. The weak attractive gravitation that we observe would arise because the attractive mechanism is stronger than the repulsive one, but only just.
The expansion of the universe
Einstein’s theories when applied to the whole universe do not allow it to remain static. The universe must either expand or contract. Observational evidence indicates that the universe is expanding, which poses cosmologists with a problem. Einstein’s theories describe gravity. Gravity is force which pulls massive towards one another, so, how can such an attractive force result in an expanding universe? How can massive objects end up further apart? A two component gravitation model could explain this. On a large scale where the dominant attractive component is balance (or almost balanced) because of the distribution of matter, then the weaker repulsive component can come to dominate, on average objects get pushed further apart and the universe expands.
Now, this is all hand waving waffle. When it comes to the rubber sheet experiments, I may have stretched the analogy too far (excuse the pun), but if gravity does have two competing mechanisms it would explain a lot. In any case, Einstein’s theories, which predict the expansion of the universe, are theories of gravity, gravity is concerned with massive objects and massive objects are matter. Therefore, the answer to the headline question is: yes, matter does create space!
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Hi Ian, welcome to TNS.
I’m afraid that I am no spring chicken.
If you can’t remember a time when you didn’t watch popular science programmes on TV, you still have youth on your side [:)]. Some of us can remember when there were no Pop. Sci. Progs on TV; in fact there were no TVs around for them to be on.
We probably need the more expert posters to do any sort of justice to your ideas. I would only like to mention that models are just that. Taking them too literally, or asking too much of them can lead to problems.
For example; the sheet and ball model presents the concept of a gravity well. Looks simple, but gravity is, if it is to be thought of in terms of gravity wells, an infinite number of wells, radiating in 3D from the central mass. Of course, they are not actually individual wells, they have to be seen as a single entity. Doesn't look so simple.