# Gravitational waves in other dimensions

I know this question is purely speculative, as we don't know if more dimensions do exist and also we do not know if gravity is indeed stronger in other dimensions (if they were to exist). But, one of the possible explanations of why gravity is so weak compared to other forces is that it exerts its strength in other dimensions, which are too small for us to detect them. However, if that were true, wouldn't the gravitational waves on those dimensions be stronger and cause larger stretching and therefore, in some cases, allow us to detect those extra dimensions? Are there any experiment that look at this case scenario?

• laura howden has done mathematical reseach, does that count? Feb 15, 2016 at 5:19
• @chaz327 can you point to links for that research?
– user
Nov 16, 2017 at 1:55
• it didn't help that i misspelled laura's name. en.wikipedia.org/wiki/Laura_Mersini-Houghton. my info came from 'through the wormhole' s02e02, 2011. other references to parallel universes or dimensions in s02e04 and s02e06; available on youtube. Nov 21, 2017 at 3:00
• i have been thinking about for some time and will express my opinion in the hopes that it will help. first i personally do not think gravity is force, or weak, it is a curvature of space. the hypothesis that works for me is that something(s) in other dimensions are curving space in our universe. this would explain some of our observations. experiment; i think this would involve placing something like LIGO near unexplained distortions in space. Aug 9, 2018 at 12:42

The other dimensions in those examples are quite small.

Imagine a long very unwide piece of paper. Now attached the long ends together so you get a tall narrow cylinder. It's like you have one direction where if you go that way you very quickly end up back where you started and one direction where you can walk very far and end up someplace different.

That's what those extra directions are like. What we'd call a wave is a wave going in that long direction. Something going in the orthogonal direction would just circle around and at some point be right where it started, it wouldn't end up going anywhere.

I think the idea is that you fill up in all the directions in a $1/r^3$ and get weaker and weaker until the other directions curl around at a distance $R$ and you stop getting weaker at $1/r^3$ and start getting weaker at $1/(r^2R)$ since then an expansion in the additional small directions is now coming back.

So a wave could expand and get weak but then when it's expanded through that other dimension it now just starts affecting itself.

It's my understanding that this theory with the extra dimension was already falsified.

• Falsified? Wouldn't it be more accurate to say an upper limit has been placed on the size of the rolled-up dimensions (by testing the inverse-square law for gravity down to a certain distance)? Feb 22, 2016 at 4:47
• @HughAllen My understanding is the size of the other dimension was specifically tailored to a very narrow range in order to achieve the relative weakness of the gravitational interaction. And that this made specific predictions that weren't seen. And this seemed like the exact specific hypothesis explicitly referred to by the OP. Feb 22, 2016 at 4:50
• I notice you used the singular. Was that theory predicting just one compactified spatial dimension? If there were more of them (see string theory) then the size range would be quite different. Feb 22, 2016 at 5:09

But, one of the possible explanations of why gravity is so weak compared to other forces is that it exerts its strength in other dimensions, which are too small for us to detect them.

why do you say that "gravity is so weak"? if you say that because the repulsive force due to EM between two protons greatly exceeds the attractive force due to gravity, the reason for that is because the charge on the protons is approximately a natural (Planck) unit of charge (an order of magnitude) while the mass of the protons is far, far less than the natural (Planck) unit of mass.

We see that the question [posed] is not, "Why is gravity so feeble?" but rather, "Why is the proton's mass so small?" For in natural (Planck) units, the strength of gravity simply is what it is, a primary quantity, while the proton's mass is the tiny number [1/(13 quintillion].) Frank Wilcxek in June 2001 Physics Today

I know this question is purely speculative, as we don't know if more dimensions do exist and also we do not know if gravity is indeed stronger in other dimensions (if they were to exist).

We have no evidence whatsoever of any extra dimensions.

But, one of the possible explanations of why gravity is so weak compared to other forces is that it exerts its strength in other dimensions, which are too small for us to detect them.

It's brane-theory pseudoscience I'm afraid. Gravity is weak because it's a residual force. The "electric" force between charged particles is pretty titanic. See Rod Nave's hyperphysics:

When you contrive those charged particles into a current in a wire, the residual "magnetic" force between two wires is pretty weak. Again see hyperphysics. Then when you stop the current, the force is weaker still. Only we don't call it electric force or magnetic force any more.

However, if that were true, wouldn't the gravitational waves on those dimensions be stronger and cause larger stretching and therefore, in some cases, allow us to detect those extra dimensions?

No. Gravitational waves are extremely weak, much much weaker than the force of gravity at the surface of the Earth. They offer evidence in support of general relativity, but not for string theory or M-theory, which has been bereft of supporting evidence for fifty years. Compare and contrast with the evidence for general relativity.

Are there any experiments that look at this case scenario?

No.