- #1
Robert100
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How can "empty space" expand? (Reality behind the GR equations.)
I have a question - with some concerns over how it may be answered. On other websites, and in other print publications, I've seen Ph.D. physicists and cosmologists misunderstand the question: What does it mean when we say that empty "space" is expanding?
Yes, I know that "space is expanding". We see this when we look at the motion of distant galaxies. We see this expansion modeled - predicted, even - in General Relativity (GR). Expansion of "empty space" is the basis of the Big Bang theory. GR mathematically models the expansion of empty space.
Yes, I know that only the space between distance galaxies is expanding, not the size of the galaxies themselves. The size of stars and atoms is constant. The expansion of the universe only involves the space between very distant objects, and not the objects themselves.
But what is the ontological reality that causes this to occur?
Most books and websites dodge the issue. They simply state that space is described as a metric, and a metric can expand without objects in it. Yet that isn't an answer. The metric is not space itself! The metric is only a coordinate system that we use. The "metric" is no more real than "F = ma". It is a relationship and a mathematical tool.
The question I'm after is this: What is the physical reality that causes this?![Confused :confused: :confused:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Correct me if I am mistaken, but Einstein's theory of general relativity (hence, GR) doesn't address this issue. His equations predict the relationship between matter, energy and the path that matter will follow in a curved space. The equations create a coordinate system that is "stretchy". From a conceptual point of view, it is a tremendous advance over the Newtonian conception of space and gravity, and it actually answers the question "How does gravity work?" Wonderful!![Egg on Face o:) o:)](/styles/physicsforums/xenforo/smilies/eggonface.png)
But we now should be allowed to ask a deeper question: What is it about space that is expanding? How can empty space "expand", or have any properties at all? This isn't answered by GR; it only described.
So what is the physical reality that lies behind the statement that the metric of space itself is expanding? I know that we don't have firm answers yet, but I'd like to know the current range of ideas. (If I understand correctly, the properties of space described by GR may actually be caused by various quantum effects, the foaming sea of virtual partices that exists even in empty space.)
Any responses would be much appreciated! If this specific issue is addressed in some previous thread, then by all means please let me know the appropriate link.
Thanks,
Robert
I have a question - with some concerns over how it may be answered. On other websites, and in other print publications, I've seen Ph.D. physicists and cosmologists misunderstand the question: What does it mean when we say that empty "space" is expanding?
Yes, I know that "space is expanding". We see this when we look at the motion of distant galaxies. We see this expansion modeled - predicted, even - in General Relativity (GR). Expansion of "empty space" is the basis of the Big Bang theory. GR mathematically models the expansion of empty space.
Yes, I know that only the space between distance galaxies is expanding, not the size of the galaxies themselves. The size of stars and atoms is constant. The expansion of the universe only involves the space between very distant objects, and not the objects themselves.
But what is the ontological reality that causes this to occur?
Most books and websites dodge the issue. They simply state that space is described as a metric, and a metric can expand without objects in it. Yet that isn't an answer. The metric is not space itself! The metric is only a coordinate system that we use. The "metric" is no more real than "F = ma". It is a relationship and a mathematical tool.
The question I'm after is this: What is the physical reality that causes this?
Correct me if I am mistaken, but Einstein's theory of general relativity (hence, GR) doesn't address this issue. His equations predict the relationship between matter, energy and the path that matter will follow in a curved space. The equations create a coordinate system that is "stretchy". From a conceptual point of view, it is a tremendous advance over the Newtonian conception of space and gravity, and it actually answers the question "How does gravity work?" Wonderful!
![Egg on Face o:) o:)](/styles/physicsforums/xenforo/smilies/eggonface.png)
But we now should be allowed to ask a deeper question: What is it about space that is expanding? How can empty space "expand", or have any properties at all? This isn't answered by GR; it only described.
So what is the physical reality that lies behind the statement that the metric of space itself is expanding? I know that we don't have firm answers yet, but I'd like to know the current range of ideas. (If I understand correctly, the properties of space described by GR may actually be caused by various quantum effects, the foaming sea of virtual partices that exists even in empty space.)
Any responses would be much appreciated! If this specific issue is addressed in some previous thread, then by all means please let me know the appropriate link.
Thanks,
Robert