The h Bar

Erdel von Mises

4:25 AM

Is it sufficient to have some anti-gravity generator to make an Alcubierre drive?

John Rennie

@ErdelvonMises Hi :-)

There is no such thing as "anti-gravity".

Erdel von Mises

@JohnRennie Hi

John Rennie

Spacetime reacts to the matter and energy inside it, and there is a hypothetical form of matter called exotic matter that can curve spacetime in way that produces things like wormholes and Alcubierre drives.

But there is no "anti-gravity".

Erdel von Mises

@JohnRennie (1) We are in the hypothetical case that anti-gravity exists, is no nessesary it to be real in that situation, and (2) it may or may no be real [1].
[1] https://arxiv.org/pdf/hep-th/9205033.pdf

John Rennie

The problem with "the hypothetical case that anti-gravity exists" is that you are in effect just making something up, and if you are making up something you can make it do whatever you want.

In the real world things obey the laws of physics and that constrains what they can do. But you are basically just inventing new laws of physics so there are no constraints on what they can do.

Erdel von Mises

4:36 AM

Anyway, what is the problem of analyse a teh consequeneces of a hypothetical new physics?
And there is the contraint of no-contradiction, unless you want a hypothetical and inconsistent formal system.

John Rennie

Physicists suggest new physical principles all the time. Indeed that has become very common in the theoretical physics community since there is currently a dearth of experimental evidence to test theories.

But typically anything new is based on what we currently understand i.e. it is taking relatively small steps.

Ans as you say, the idea is to work out the consequences of the new idea and see if it produces results that look physically reasonable. Most new ideas fail this process and are quietly forgotten.

But to just make up something called "anti-gravity" doesn't fit with this. You would have to have some existing principle that could reasonably be extended or reformulated to produce your theory of anti-gravity.

This is kind of what exotic matter does. You can take general relativity, which is a well tried and tested theory, and postulate the existence of matter or energy with a negative density, and this can produce situations where we get a gravitational repulsion.

There's nothing wrong with the theory, though experimentally no such thing as exotic matter has been observed.

Erdel von Mises

@JohnRennie It will rephrase my sentence it two very exact questions.
1. Is the need of exotic mather the only physically fundamental limitation to the possibility of create a Alcubierre drive?
2. Is the effect of some hypothetical anti-gravity machine, equivalent to the ones that would have a hypothetical exotic matter?

@JohnRennie What about this paper arxiv.org/pdf/hep-th/9205033.pdf as base to the anti-gravity?

John Rennie

1. Yes
2. I can't answer unless you give me the equations that determine how your anti-gravity works.

@ErdelvonMises The paper is not talking about anti-gravity in the common sense of the word. Note that it says:

> However, in the present paper the term anti-gravity is used in the more circumscribed sense proposed by J. Scherk [2]: it stands for a theory in which the
gravitational potential between two masses at rest vanishes in the Newtonian approximation because in addition to the attractive exchange of spin-2 gravitons there
also exists a repulsive contribution coming from the exchange of spin-1 particles,
the graviphotons. Matter and anti-matter behave in the ordinary manner under
the exchange of the gravitons but, whenever appropriately charged, also couple

It is suggesting the existence of a new particle called the graviphoton. This particle is subject to the usual rules of quantum mechanics, and the authors can use these rules to calculate its behaviour.

They find that it can partially or in some case completely reduce the gravitational attraction between masses.

But this is nothing like an Alcubierre geometry.

Graviphoton

In theoretical physics, a graviphoton or gravivector is a hypothetical particle which emerges as an excitation of the metric tensor (i.e. gravitational field) in spacetime dimensions higher than four, as described in Kaluza–Klein theory. However, its crucial physical properties are analogous to a (massive) photon: it induces a "vector force", sometimes dubbed a "fifth force". The electromagnetic potential A μ {\displaystyle A_{\mu }} emerges from an extra component of the metric tensor...

Erdel von Mises

4:56 AM

I think that when they say "Strong anti-gravity is the vanishing to all orders in Newton’s constant of the net
force between two massive particles at rest" they are giving the formal definition or what would be anti-gravity in the common sense of the world.

John Rennie

Certainly not in the sense that the science fiction antigravity drives that I read about as a child exist.

Erdel von Mises

@JohnRennie It still could create a nearly propulsion less aircraft.

John Rennie

If we use quantum field theory to calculate how masses interact then we invent hypothetical particles called virtual particles to do these calculations. In 4D the only such particle is the graviton and this gives us the usual gravity we are all used to. But if you allow higher dimension spacetimes then other types of virtual particles could exist and these can modify the gravitational force.

One of these new particles that exist only if there are more than four dimensions is the graviphoton.

But even then, it takes a very particular choice of the laws of (5D) physics to make the graviphoton exactly balance out the graviton and reduce gravity to zero.

John Rennie

5:14 AM

@uhoh Hi :-) Despite the name, the room where you posted is a room created by a student to ask for help with school exam questions.

This is the main Physics SE chatroom.

uhoh

@JohnRennie got it, I'm easily confused.

5

Q: Quantum mechanical antireflection coating; what quantity would be analogous to index of refraction?

Background Quasiparticle interference (QPI) is a technique that can be used to study 2D surface state band structure; carrier reflections at boundaries or impurity sites will interfere producing standing waves which can be imaged by STM. Plotting the periods of the standing waves vary with STM ti...

Currently posted in matter modeling SE, all input welcome!

John Rennie

For a 1D Schrodinger equation about the only thing you can change is the potential.

Calculating the reflection from potential changes is pretty standard question for first year students in QM.

Erdel von Mises

@JohnRennie Also, Is possible the existence of a theory of fundamental (mircroscopic) physics, that in coherent with the experimental results but is no quantized?

John Rennie

People have been suggesting this right from the very beginnings of QM. Einstein believed that QM was not fundamental but was the result of a deeper non-quantum theory.

But in 100+ years no-one has been able to make this idea work.

So right now the idea does not seem very likely, but it has not been completely ruled out.

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