# Why is gravity different?

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We probably think we know gravity pretty well. After all, we have more conscious experience with this fundamental force than with any of the others (electromagnetism and the weak and strong nuclear forces). But even though physicists have been studying gravity for hundreds of years, it remains a source of mystery.

In our video Why Is Gravity Different? We explore why this force is so perplexing and why it remains difficult to understand how Einsteins general theory of relativity (which covers gravity) fits together with quantum mechanics.

Gravity is extraordinarily weak and nearly impossible to study directly at the quantum level. We cannot scrutinize it using particle accelerators like we can with the other forces, so we need other ways to get at quantum gravity.

Enter black holes. In a paper in the early 1970s the late physicist Jacob Bekenstein investigated the question of what happens to entropya measure of disorder, or randomness, in a systemwhen matter succumbs to a black holes massive gravitational pull and falls through its event horizon.

Bekenstein noted that the matters entropy seems to disappear inside the black hole. Yet this would violate the second law of thermodynamics, which states two things: information cannot be destroyed, and entropy can only increase. Thus the entropy of the black hole must compensate for the loss. Bekenstein argued that this black hole entropy must not be proportional to the black holes volume, but to the area of its event horizon.

If we are describing the contents of black holes in terms of area instead of volume, we should think about laws of physics in terms of area as well. This would mean a theory of everything (gravity included) should be able to play out in fewer than three spacetime dimensions.

Now lets imagine the information that describes the state of the entire universeall stored on a single hard drive. And then throw that hard drive into a black hole. The stored information cannot be lost, so it must be contained in the surface area of the black hole (albeit scrambled).

This scenario leads to a dramatically new way of thinking in which the universe could effectively be a hologram, a seemingly 3-D object that is actually just a projection from a 2-D surface. Our ostensibly three-dimensional experience of the world would then be an illusion, convincingly generated by a fundamentally lower-dimensional reality.

Maybe were all just paper-thin cutouts drifting in gravitys cosmic breeze.

In our video Why Is Gravity Different? We explore why this force is so perplexing and why it remains difficult to understand how Einsteins general theory of relativity (which covers gravity) fits together with quantum mechanics.

Gravity is extraordinarily weak and nearly impossible to study directly at the quantum level. We cannot scrutinize it using particle accelerators like we can with the other forces, so we need other ways to get at quantum gravity.

Enter black holes. In a paper in the early 1970s the late physicist Jacob Bekenstein investigated the question of what happens to entropya measure of disorder, or randomness, in a systemwhen matter succumbs to a black holes massive gravitational pull and falls through its event horizon.

Bekenstein noted that the matters entropy seems to disappear inside the black hole. Yet this would violate the second law of thermodynamics, which states two things: information cannot be destroyed, and entropy can only increase. Thus the entropy of the black hole must compensate for the loss. Bekenstein argued that this black hole entropy must not be proportional to the black holes volume, but to the area of its event horizon.

If we are describing the contents of black holes in terms of area instead of volume, we should think about laws of physics in terms of area as well. This would mean a theory of everything (gravity included) should be able to play out in fewer than three spacetime dimensions.

Now lets imagine the information that describes the state of the entire universeall stored on a single hard drive. And then throw that hard drive into a black hole. The stored information cannot be lost, so it must be contained in the surface area of the black hole (albeit scrambled).

This scenario leads to a dramatically new way of thinking in which the universe could effectively be a hologram, a seemingly 3-D object that is actually just a projection from a 2-D surface. Our ostensibly three-dimensional experience of the world would then be an illusion, convincingly generated by a fundamentally lower-dimensional reality.

Maybe were all just paper-thin cutouts drifting in gravitys cosmic breeze.

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