Why is gravity so weak? The answer may lie in the nature of spacetime

Why is gravity so weak compared to the other four fundamental forces?

Even if it were a billion times stronger, it would still be the weakest force – by a factor of a billion billion. The strange weakness of gravity stands out, and almost requires an answer.

Strangely enough, the solution to weak gravity may lie not in gravity itself but in the mechanics of the Higgs boson and the nature of space-time itself.

hierarchy problem

Lift a piece of paper. Congratulations, you have successfully overcome the combined gravitational force of the entire planet.

It didn’t take much effort because gravity He is, by far, the weakest Four fundamental forces of nature. By one measure, gravity is a thousand billion billion times weaker than strong nuclear powerthe most powerful forces.

Here’s another way to visualize the true magnitude of double gravity. There is a limit to the smallest black hole you can form, and it’s called Planck mass. You can calculate it by taking the square root of Planck’s reduced constant multiplied by the speed of light divided by NewtonThis mass is about 10^-8 kilograms. If gravity is strong – if Newton’s G is bigger – then you can form smaller and lighter black holes.

By comparison, the W and Z bosons – the force carriers of the weak nuclear force – are about 10 quadrillion times lighter than the Planck mass. So the weak nuclear force, the second strongest after gravity, is billions of times stronger than gravity.

The “hierarchy problem” seems strange to most physicists. Sure, it could be just as in the universe, with no explanation required, but that’s not very satisfying. Instead, it seems like a chance to dig deeper into the physics of fundamental forces and see if there’s anything new we can learn.

What’s going on with the Higgs?

Let’s leave aside electromagnetism and the strong nuclear force and compare gravity to its closest competitor, the weak nuclear force. Perhaps if we can answer why the weak nuclear force is so impressively stronger than gravity, we can get the whole picture.

We have no idea why it has the force of gravity. There is nothing that appears in any physical theory to explain its power. But there seems to be something that explains the properties of the weak nuclear force, and that is Higgs boson.

The Higgs boson is the field that soaks all of Spare time It forces many other particles, such as electrons, to interact with it. This interaction causes these Electrons to gain mass. The more something interacts with Higgs, the more mass it has.

Among the many particles that interact with the Higgs boson are the W and Z bosons, and through this interaction they gain mass. and he The mass of the W and Z bosons that determine the properties of the weak nuclear force, because it is those particles that do the work.

And what determines the mass of all the particles that interact with the Higgs? Why, it is none other than the Higgs mass itself. If it had a different mass, all other particles, including the W and Z bosons, would change.

A computerized image of the collision of two particles and the decay of the Higgs boson during a test event at CERN. Photons are represented by yellow lines and green constellations radiating from the collision.

A visual representation of the 2012 event in CERN’s CERN detector shows the decay characteristic of the Higgs boson into a pair of photons (yellow dashed lines and green turrets). (Image credit: CERN)

Now it’s time to point out that the Higgs mass is very strange. It’s big – about 250 gigaelectronvolts, which is big the further the particles move – but it’s not huge. Nor is it small. Indeed, a naive quantum mechanical understanding of how the Higgs works predicts that all the interactions in which it participates are constantly – and are Much – Either they cancel each other out completely, sending their mass to zero, or they reinforce each other, inflating their mass somewhere near infinity.

Something is causing the Higgs boson to be fine-tuned to an “acceptable” range that keeps everything safe. But the Higgs boson limits the W and Z bosons to their small values, allowing the weak nuclear force to be much stronger than gravity.

In other words, gravity is the weakest force in the universe not because something is wrong with gravity, but because the weak force is a “bluff”.

Simple distortion of spacetime

There is no acceptable solution to the abnormal state of the Higgs mass, and therefore no acceptable solution to the problem of hierarchy and the strange weakness of gravity.

But all this discussion assumes that we are calculating everything correctly – Higgs mass, Planck mass and so on. Perhaps we are missing something fundamental in the universe.

Among the many potential solutions, some ideas call into question our understanding of the very fabric of spacetime. string theory He’s already primed the pump for such ideas, requiring new, compressed spatial dimensions to get the mathematics of the theory to pop up just right.

Circular blue and violet swirls of light distorted as they expand from the bright center to illustrate string theory conceptually.

A conceptual technical illustration of string theory. (Image source: Getty Images)

But in string theory, these Extra dimensions She is super deceptively petite, curled up into tiny, narrow shapes no bigger than a plank’s length.

However, it is possible that some of these additional dimensions are slightly larger. These theories are generally called “large extra dimensions,” but these extra dimensions aren’t as big as you might think – just a millimeter or so.

In these theories, the other three forces of nature are limited to our ordinary three-dimensional universe, which is sometimes called the “membrane”. However, gravity extends its reach across all dimensions called “mass”. From this point of view, gravity is just as strong – if not stronger! – compared to other forces, but it is forced to spread to more dimensions than anyone else. So it seems weaker than our 3D experiments.

We’ve tested gravity to incredible levels of accuracy, but not necessarily on such small scales. If our universe had extra “large” spatial dimensions, we’d start to see funky things happening at distances of less than a millimeter.

For example, we might see the effect of gravity stronger than expected at small distances, because there was no chance for it to “leak” into the extra dimensions. Or we may start designing small fashions black holes In our particle colliders, because at such small scales, building a black hole would be easier than we thought.

So far, no experiment to date has found any evidence of additional dimensions. Gravity remains frustratingly weak.

Learn more by listening to the Ask an Astronaut podcast, available at Itunes (Opens in a new tab) And the askaspaceman.com (Opens in a new tab). Ask your own question on Twitter using #AskASpaceman or by following Paul Tweet embed (Opens in a new tab) And the facebook.com/PaulMattSutter (Opens in a new tab).

Additional Resources

For more information on gravity check out “The Rise of Gravity: The Quest to Understand the Force That Explains It All“ (Opens in a new tab) by Marcus Chun and “Reality Is Not What It Seems: The Journey to Quantum Gravity“ (Opens in a new tab) by Carlo Rovelli.

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Shachar Hood”Evidence of weak gravity (Opens in a new tab)International Journal of Modern Physics D, Volume 26, June 2017.
CERN,”Higgs boson (Opens in a new tab)‘, accessed in June 2022.
CERN,”Z . boson (Opens in a new tab)‘, accessed in June 2022.
CERN,”W boson: the brightness of the sun and stardust (Opens in a new tab)‘, accessed in June 2022.
National Space AssociationWhat is gravity (Opens in a new tab)‘, accessed in June 2022.