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posted by martyb on Thursday November 10 2016, @12:31AM   Printer-friendly
from the it's-all-an-illusion dept.

Theoretical physicist Eric Verlinde has finally published his much anticipated article on the nature of gravity. In a 2010 New York Times article Verlinde already stated: gravity is an illusion. His theory goes beyond the concept of gravity as envisioned by both Isaac Newton and Albert Einstein. It will be very interesting to see other scientists sink their teeth into this.

Abstract of his article:

Recent theoretical progress indicates that spacetime and gravity emerge together from the entanglement structure of an underlying microscopic theory. These ideas are best understood in Anti-de Sitter space, where they rely on the area law for entanglement entropy. The extension to de Sitter space requires taking into account the entropy and temperature associated with the cosmological horizon. Using insights from string theory, black hole physics and quantum information theory we argue that the positive dark energy leads to a thermal volume law contribution to the entropy that overtakes the area law precisely at the cosmological horizon. Due to the competition between area and volume law entanglement the microscopic de Sitter states do not thermalise at sub-Hubble scales: they exhibit memory effects in the form of an entropy displacement caused by matter. The emergent laws of gravity contain an additional 'dark' gravitational force describing the 'elastic' response due to the entropy displacement. We derive an estimate of the strength of this extra force in terms of the baryonic mass, Newton's constant and the Hubble acceleration scale a0 = cH0, and provide evidence for the fact that this additional 'dark gravity force' explains the observed phenomena in galaxies and clusters currently attributed to dark matter.

Heck, I'm not even going to pretend I grok any of this: I shine shoes for a living and just hope that my understanding of gravity-as-we-know-it is sufficient to catch the coins customers drop into my weary hand.


Original Submission

 
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  • (Score: 1, Informative) by Anonymous Coward on Thursday November 10 2016, @09:05AM

    by Anonymous Coward on Thursday November 10 2016, @09:05AM (#425046)

    I always wondered about that:if a particle of light is travelling towards the sun, should it not accelerate as it approaches the sun and then decelerate as it leaves?

    No. Light doesn't gain energy by increasing its speed, but by increasing its frequency (corresppnding to E = h f).

    You are right, however, that light has to gain energy when approaching the sun, and lose it when going away; that is known as gravitational red shift. Also it is true that light is bent by the sun's gravitation, which physically is an acceleration orthogonal to the direction of movement. That is, light can change its velocity, but not its speed. So in a physical sense it can accelerate (= change its velocity), but not in the colloquial sense (= increasing its speed).

    Note that the physical meaning of acceleration encompasses the colloquial meanings of acceleration, deceleration and changing direction; only the latter thing is possible.

    Also note that the above statement is only true when measuring the speed of light locally; otherwise you will see light to (apparently) slow down when approaching the sun, due to gravitational time dilation (which is, BTW, also another way to understand the gravitational red shift). Indeed, when replacing the sun with a black hole, you'll find that, when looked at from the outside, the light comes to a halt when reaching the event horizon. An observer falling into the black hole will, however, find that the light still goes at the same speed as he falls through the horizon.

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