gravity 101

[sjastro]

Occasionally it becomes very important to remember that the interpretation attaching to the indirect things is not necessarily real, just useful. A good example of this is the idea of the electromagnetic field. Very important in classical EM, but starts to look distinctly less real in the Feynmann approach (glossing over a lot of technical stuff).

Very true.
The electromagnetic interaction between a positron and electron via a Feynmann diagram is most intriguing as it suggests that a positron is nothing more than an electron travelling backwards in time.

Steven

[sjastro]

Originally Posted by yusufcam

thanks, thats quite informative...

it does open a different door though, is space/time considered a void or an object.
if its a vaccuum how does an object effect it?

its curious stuff.

If an object is defined by possessing certain properties than space-time is most definitely an "object".
The geometry of space-time is a property that can be determined by a fundamental property of light. Light will always travel along the shortest pathway. If space-time is flat, light will travel in a straight line, if the geometry of space-time is curved like on the surface of a ball, light will travel along an arc.

We can observe the bending of light as it travels through pockets of space time that are not flat, such as gravitational arcs observed in galaxy clusters or the position of stars near the limb of a solar eclipse that are slightly displaced when compared to their positions in the night sky.

Vacuum has a somewhat different connotation. The electric field around a charged particle possesses quantum properties like the particle. The energy levels of a field can be quantized like the energy levels of electrons in an atom.
A vacuum in this case is a field with the lowest energy level.

There a different types of vacuums as well. Apart from an electromagnetic vacuum there is also vacuum relating to the strong nuclear force.

Steven

[Eratosthenes (Peter)]

Originally Posted by bojan

Back to analogies..

When thinking about counter-intuitive concepts of residual and/or "emerging" forces, aerodinamic lift comes to my mind.
I bet that 90% of public do not realise that the force that keeps the aircraft in the air is a result of difference of speeds of airflow below and above the wing.. resulting in difference of static pressures between upper and lower side.. and hence the lift.

This is another interesting read on gravity as emerging force:
http://en.wikipedia.org/wiki/Entropic_gravity
http://arstechnica.com/science/2011/...hermodynamics/

good example bojan,

The Bernoulli effect is counter intuitive. If you hang two ping pong balls from the ceiling with pieces of string, separated by a small gap and then blow air into the gap between the ping pong balls, one's intuition says that the balls should fly apart - ie the air should push the balls apart. And yet the Bernoulli effect causes the balls to comes close together and even touch.

[yusufcam (Colin)]

well, its great feedback for my question, and thanks. now to digest it .

Originally Posted by sjastro

If an object is defined by possessing certain properties than space-time is most definitely an "object".
The geometry of space-time is a property that can be determined by a fundamental property of light. Light will always travel along the shortest pathway. If space-time is flat, light will travel in a straight line, if the geometry of space-time is curved like on the surface of a ball, light will travel along an arc.

We can observe the bending of light as it travels through pockets of space time that are not flat, such as gravitational arcs observed in galaxy clusters or the position of stars near the limb of a solar eclipse that are slightly displaced when compared to their positions in the night sky.

Vacuum has a somewhat different connotation. The electric field around a charged particle possesses quantum properties like the particle. The energy levels of a field can be quantized like the energy levels of electrons in an atom.
A vacuum in this case is a field with the lowest energy level.

There a different types of vacuums as well. Apart from an electromagnetic vacuum there is also vacuum relating to the strong nuclear force.

Steven

[Eratosthenes (Peter)]

Originally Posted by xelasnave

Hi Peter
I just thought you may have had a speculative view on the matter.

which matter is that exactly Alex - I don't quite follow you?

(remember Alex, pure mathematical notions are abstract constructs - even mathematical axioms which everything in the field henceforth relies upon. Many mathematical philosophers are adamant that absolute reality and truth (whatever these concepts are) can only be described by mathematical laws and their proofs. Would that imply that reality/truth is abstract by definition? I have a friend who is a research mathematician in academia, who has often argued with me over this very point - he stance is basically "truth can only be mathematically described". I have always wondered why there is no mathematical category in the Nobel Prize medals. They have the Fields medal to give out every 4 years. That Pythagoras has a lot to answer for with his first proof)

[xelasnave]

Peter don't worry about my question I have my answer.

[g__day (Matthew)]

Gravity is the topology of 4D space-time caused by the distrbution of energy and mass within it!

[Eratosthenes (Peter)]

....in other words gravity emerges from the lowest vibration of a closed string

[bojan]

Links to a documents that discuss this aspect:
http://arxiv.org/pdf/1001.0785v1.pdf
http://www.researchgate.net/profile/...0d4b000000.pdf

[Eratosthenes (Peter)]

Originally Posted by bojan

Links to a documents that discuss this aspect:
http://arxiv.org/pdf/1001.0785v1.pdf
http://www.researchgate.net/profile/...0d4b000000.pdf

good sources bojan....

.....what is your View on Sheldon Gao's counter arguments that try to refute Verlinde's notion that Gravity is an entropic force?

[bojan]

Still reading and trying to digest the first paper
So, no comment... yet

Originally Posted by Eratosthenes

good sources bojan....

.....what is your View on Sheldon Gao's counter arguments that try to refute Verlinde's notion that Gravity is an entropic force?

BTW.. there are also counterarguments on counterarguments available.... watch this space
http://www.researchgate.net/publicat...Laws_of_Newton

[sjastro]

Originally Posted by Eratosthenes

....in other words gravity emerges from the lowest vibration of a closed string

Wrong conclusion to Matthew's post.
When does a string exist in (3+1) space-time?

[sjastro]

Originally Posted by bojan

Links to a documents that discuss this aspect:
http://arxiv.org/pdf/1001.0785v1.pdf
http://www.researchgate.net/profile/...0d4b000000.pdf

Hmmm.

One argument against entropic gravity is the humble double slit experiment.
If you shot neutrons at the slits the quantum wavefunction of the neutron before the measurement is made is a linear combination of the Quantum states Ia> + Ib>.
Here Ia> could mean the neutron passes through the left hand slit and Ib> the neutron passes through the right hand slit.

If you rotate the slits 90 degrees so one slit is on top of the other, the slits exist in two different gravitational potentials.
Now according to entropic gravity if the holographic screen is at some distance below the bottom slit, the number of possible microstates which is a measurement of entropy increases as one gets further away from the screen.
Hence the top slit has many more microstates than the bottom slit even if the slits are separated by only the Compton wavelength of the neutron.

The problem is these microstates need to be considered in the quantum wavefunction. The quantum wavefunction is no longer simply Ia> + Ib>.

The difference in the number of microstates between the top and bottom slits would destroy the observed interference pattern as observed in the slit experiment as the probabilities of where the neutron hits the detector behind the slits have been changed.

Steven

[bojan]

Interesting..
But... In the real experiment, aren't the diffraction patters for both cases the same? or different and by how much?
I presume you used neutron example because of it's rest mass and because it is not influenced by electrical fields?

[sjastro]

Originally Posted by bojan

Interesting..
But... In the real experiment, aren't the diffraction patters for both cases the same? or different and by how much?
I presume you used neutron example because of it's rest mass and because it is not influenced by electrical fields?

Experiments show the orientation of the slits doesn't effect the observed interference pattern.
Before the observation is made the neutron is in a superimposed quantum state Ia>+Ib>.
Mathematically each state Ia> and Ib> has the same dimension or contains the same number of terms for superimposition to occur.
In the case of entropic gravity the neutron passing through the higher slit has many more terms describing it's quantum state when compared to when it passes through the lower slit.
Mathematically it is not possible to form a superimposed quantum state.

The lack of a superimposed state means that quantum interference cannot occur and one would not expect to see an interference pattern if the model of entropic gravity is correct.

One can use protons, electrons, even buckyballs instead of neutrons.

Steven

[xelasnave]

Now you have let the cat out of the box Steven.
I have spent some time trying to understand this via a google and I miss the significance of why the difference in states matters..I am frustrated that I can't understand.
Nevertheless thank for making me aware of it..sorry I no longer have the mental power to digest even what quantum physics is trying to say.
I bet you are a good poker player or would be.

[Eratosthenes (Peter)]

Originally Posted by xelasnave

......sorry I no longer have the mental power to digest even what quantum physics is trying to say.
.

"I think I can safely say that nobody understands Quantum Mechanics"
"Anyone who claims to understand Quantum Theory is either lying or crazy"

(Richard Feynman)

[xelasnave]

But he was only a drummer.

[Eratosthenes (Peter)]

...mainly bongos

[sjastro]

Originally Posted by xelasnave

Now you have let the cat out of the box Steven.
I have spent some time trying to understand this via a google and I miss the significance of why the difference in states matters..I am frustrated that I can't understand.
Nevertheless thank for making me aware of it..sorry I no longer have the mental power to digest even what quantum physics is trying to say.
I bet you are a good poker player or would be.

Alex,

Quantum states are described using Dirac's Bra-Ket notation.

The Quantum states are mathematically handled as single column or row matrices.
A superimposed quantum state such as a neutron before it hits the detector in the double slit experiment is in the form Ia>+ Ib>.
This involves matrix addition.
For matrix addition to occur the wavefunctions Ia> and Ib> must contain the same number of matrix elements.

However as has been pointed out if entropic gravity is correct the upper slit has many more microstates than the lower slit in which case Ia> and Ib> now longer contain the same number of elements.
Matrix addition is impossible and a superimposed quantum state cannot be formed.
The familiar interference pattern observed on the detector screen caused by the collapse of the superimposed quantum state should not be observed simply because there is no superimposed state to start with.

This a mathematical argument against entropic gravity.
The issue is on the validity of whether the thermodynamic microstates themselves can form the basis of the wavefunctions.
Another argument purely from a physics perspective is the different gravitational "environments" of the lower and upper slits results in quantum decoherence which would destroy the observed interference pattern.

Steven