3 ms·

Since reading Brian Greene’s books I have a question that I can’t seem to get an answer to. If there really are extra curled up dimensions and the graviton is n

Since reading Brian Greene’s books I have a question that I can’t seem to get an answer to. If there really are extra curled up dimensions and the graviton is not connected to the brain of the 3 extended dimensions in our Universe and is therefore free to travel into and out of the curled up dimensions then why does gravity obey the inverse square law? Shouldn’t we be able to detect the extra dimensions by the effect of gravity diminishing faster than inverse square as the gravitons go into and out of the extra dimensions? The inverse square law of gravitational propagation indicates that there either are no extra dimensions or that the graviton is also connected to a brain that prevents it from entering and exiting these extra dimensions. Since the other 3 forces also obey the inverse square law the extra dimensions might as well not exist making the conclusions of string theory invalid. What am I missing?

simonh 6y ago
This experiment has actually been done, measuring the precise rate of diminishment of gravity over very short distances. So far as we can tell gravity does follow an inverse square law, and the precision of the experiment means that if it does deviate from inverse square, the extra dimensions can be no bigger than around half the width of a human hair. Any bigger than that and the experiment would have detected the deviation. https://www.scientificamerican.com/article/string-theorys-extra-dime/ https://www.scientificamerican.com/article/string-theorys-ex...
JimBlackwood 6y ago
No, what you’re saying is correct. If the inverse square law was obeyed perfectly, space would be restricted to 3-spatial dimensions. The extra spatial dimensions are usually theorized to be extremely compact so the effects of a higher power law (the inverse square law can be generalised to 1/r^(n-1) with n as your amount of dimensions.) can be neglected. Of course, to prove it you’d have to at some point measure these effects, and there is some evidence that the inverse square law breaks down. (https://arxiv.org/pdf/hep-ph/0307284.pdf https://arxiv.org/pdf/hep-ph/0307284.pdf)
- jscipione 6y ago
  I would think that over cosmological distances even if the effect of extra dimensions on the propagation of gravity was tiny, Less than half a human hair, the net effect would be measurable especially if there are not just 1 but 8 extra curled dimensions as M-theory predicts. Well thanks for confirming that what I was thinking was an accurate criticism and I hope that physicists can devise an experiment to prove or disprove the extra dimensions some day soon.
d_tr 6y ago
I think (because you talk about exiting from and entering into dimensions) you are referring to the conjecture that our universe is embedded in a hyperspace and that gravitational pull from objects inside our universe could be detected by objects somewhere in that hyperspace (perhaps by objects living in other universes embedded in the same hyperspace). This is something entirely different from the conjecture that our universe could have additional, compact (i.e. "curled up") dimensions. Maybe you have mixed these two up? Regarding the first conjecture, the fact that the inverse square law holds over large distances might be due to the fact that we do not extend into the hyperspace, so we only "feel" the dispersion along two dimensions (hence inverse square law). Regarding the second one, the extra compact dimensions are supposed to be so extremely "small", that you would need to probe extremely small distances to find any anomalies. I do not think you can say that anything goes into and out of these extra compact dimensions any more than you can say that anything goes into and out of any of the three "large" dimensions. We are just moving along all these dimensions at the same time, all the time.
- jscipione 6y ago
  The two dimensions instead of three represented by the square part of the inverse square law is due to the fact that the surface of a sphere is 2 dimensional, not three dimensional. I don’t know physics well enough to dispute anything else you said but as I understand it only gravity interacts with the extra curled up dimensions, not magnetism or the strong and weak nuclear forces. How do physicists “know” this? No clue but as the article says, gravity behaves differently than the other three forces so there should be an experiment that can show the difference by measuring one thing for gravity and another for the other three forces. I understand that the experiment would have to be conducted over currently impossibly small distances to actually see the graviton disappear as it leaves our three extended dimensions and reappear as it comes back. I don’t understand why the effect wouldn’t be clear to see over cosmological distances but that most likely stems from my ignorance more than theirs.
Zenst 6y ago
Maybe like a staple in a pile of two dimensional papers, whilst the individual papers will feel the effect of the staple and understand it from one dimension, they wouldn't know about the other bits of paper, they can infer them from the effect of the staple perhaps, but unable to directly or indirectly know for sure. Maybe that will change, but until then, it's a nice way to visualise things.
pitaj 6y ago
Gravity obeys the inverse square law over large scales. The extra-dimensional curls only apply over very small distances. We haven't yet measured any evidence for gravity behaving differently at those scales, though.
- jscipione 6y ago
  Why isn’t the net effect of gravitons going into and out of curled up dimensions as they radiate out detectable over cosmological distances?
  - jscipione 6y ago
    To answer my own question, perhaps gravitons accelerate as they travel through the curled up dimensions so that the acceleration very closely cancels out the extra distance that the graviton travels through the extra dimension. That would explain it, and would also jive with why gravity is 10^24 weaker than the the weak nuclear force when measured only while the graviton travels through the three extended dimensions we can currently detect.
wnevets 6y ago
PBS Spacetime had a good episode about this IMO. https://www.youtube.com/watch?v=z91oGI5aP0A https://www.youtube.com/watch?v=z91oGI5aP0A

simonh 6y ago

This experiment has actually been done, measuring the precise rate of diminishment of gravity over very short distances. So far as we can tell gravity does follow an inverse square law, and the precision of the experiment means that if it does deviate from inverse square, the extra dimensions can be no bigger than around half the width of a human hair. Any bigger than that and the experiment would have detected the deviation. https://www.scientificamerican.com/article/string-theorys-extra-dime/ https://www.scientificamerican.com/article/string-theorys-ex...

JimBlackwood 6y ago

No, what you’re saying is correct. If the inverse square law was obeyed perfectly, space would be restricted to 3-spatial dimensions. The extra spatial dimensions are usually theorized to be extremely compact so the effects of a higher power law (the inverse square law can be generalised to 1/r^(n-1) with n as your amount of dimensions.) can be neglected. Of course, to prove it you’d have to at some point measure these effects, and there is some evidence that the inverse square law breaks down. (https://arxiv.org/pdf/hep-ph/0307284.pdf https://arxiv.org/pdf/hep-ph/0307284.pdf)

jscipione 6y ago
I would think that over cosmological distances even if the effect of extra dimensions on the propagation of gravity was tiny, Less than half a human hair, the net effect would be measurable especially if there are not just 1 but 8 extra curled dimensions as M-theory predicts. Well thanks for confirming that what I was thinking was an accurate criticism and I hope that physicists can devise an experiment to prove or disprove the extra dimensions some day soon.

d_tr 6y ago

I think (because you talk about exiting from and entering into dimensions) you are referring to the conjecture that our universe is embedded in a hyperspace and that gravitational pull from objects inside our universe could be detected by objects somewhere in that hyperspace (perhaps by objects living in other universes embedded in the same hyperspace). This is something entirely different from the conjecture that our universe could have additional, compact (i.e. "curled up") dimensions. Maybe you have mixed these two up? Regarding the first conjecture, the fact that the inverse square law holds over large distances might be due to the fact that we do not extend into the hyperspace, so we only "feel" the dispersion along two dimensions (hence inverse square law). Regarding the second one, the extra compact dimensions are supposed to be so extremely "small", that you would need to probe extremely small distances to find any anomalies. I do not think you can say that anything goes into and out of these extra compact dimensions any more than you can say that anything goes into and out of any of the three "large" dimensions. We are just moving along all these dimensions at the same time, all the time.

jscipione 6y ago
The two dimensions instead of three represented by the square part of the inverse square law is due to the fact that the surface of a sphere is 2 dimensional, not three dimensional. I don’t know physics well enough to dispute anything else you said but as I understand it only gravity interacts with the extra curled up dimensions, not magnetism or the strong and weak nuclear forces. How do physicists “know” this? No clue but as the article says, gravity behaves differently than the other three forces so there should be an experiment that can show the difference by measuring one thing for gravity and another for the other three forces. I understand that the experiment would have to be conducted over currently impossibly small distances to actually see the graviton disappear as it leaves our three extended dimensions and reappear as it comes back. I don’t understand why the effect wouldn’t be clear to see over cosmological distances but that most likely stems from my ignorance more than theirs.

Zenst 6y ago

Maybe like a staple in a pile of two dimensional papers, whilst the individual papers will feel the effect of the staple and understand it from one dimension, they wouldn't know about the other bits of paper, they can infer them from the effect of the staple perhaps, but unable to directly or indirectly know for sure. Maybe that will change, but until then, it's a nice way to visualise things.

pitaj 6y ago

Gravity obeys the inverse square law over large scales. The extra-dimensional curls only apply over very small distances. We haven't yet measured any evidence for gravity behaving differently at those scales, though.

jscipione 6y ago
Why isn’t the net effect of gravitons going into and out of curled up dimensions as they radiate out detectable over cosmological distances?
- jscipione 6y ago
  To answer my own question, perhaps gravitons accelerate as they travel through the curled up dimensions so that the acceleration very closely cancels out the extra distance that the graviton travels through the extra dimension. That would explain it, and would also jive with why gravity is 10^24 weaker than the the weak nuclear force when measured only while the graviton travels through the three extended dimensions we can currently detect.

wnevets 6y ago

PBS Spacetime had a good episode about this IMO. https://www.youtube.com/watch?v=z91oGI5aP0A https://www.youtube.com/watch?v=z91oGI5aP0A