First real evidence of a strange quantum distortion in empty space • Hacker News Board

Re: First real evidence of a strange quantum distortion in empty space

#31

Post by randomguy1254 » Fri, Dec 02, 2016, 10:15 PM UTC

aroberge wrote: post #30

randomguy1254 wrote: post #16

Earlier quoted context omitted.

Can you give me some hints on what virtual particles actually are if its not too complicated? I keep reading that they are not actually real particles, only useful for calculations/as a simple way to understand what is going on. How does something like Hawking radiation work if they are not real particles?

(Apologies to Quantum Field Theorist for the gross simplification below.) I'm going to assume you know about basic calculus and Taylor series; if not, I'll explain what these are in a subsequent answer if you are interested. In Quantum Field Theory, we cannot do exact calculations ... only (better and better) approximations. The way we do these calculations is very much similar to doing a Taylor series in Calculus. H…

Thanks! That's an interesting fact about Feynman diagrams. If we had a hypothetical machine which could perform these calculations with infinite precision, would the "virtual particles" disappear from the diagram, or is it impossible to know this?

Re: First real evidence of a strange quantum distortion in empty space

#32

Post by aroberge » Fri, Dec 02, 2016, 10:39 PM UTC

randomguy1254 wrote: post #31

aroberge wrote: post #30

Earlier quoted context omitted.

(Apologies to Quantum Field Theorist for the gross simplification below.) I'm going to assume you know about basic calculus and Taylor series; if not, I'll explain what these are in a subsequent answer if you are interested. In Quantum Field Theory, we cannot do exact calculations ... only (better and better) approximations. The way we do these calculations is very much similar to doing a Taylor series in Calculus. H…

Thanks! That's an interesting fact about Feynman diagrams. If we had a hypothetical machine which could perform these calculations with infinite precision, would the "virtual particles" disappear from the diagram, or is it impossible to know this?

Let me answer your question first very obliquely. We know that we can represent the sine function near x=0 as the following Taylor series:

sin(x) ≈ x - x^3 / 6 + x^5 / 120 - ....

With more and more terms, we can can calculate it more and more precisely. Does this means that this function really contains all of these powers of x?

Well, suppose we want to calculate sin(pi/2) (angle in radians, which is required for the Taylor series). We know that it is exactly 1. There are no powers of (pi/2) appearing in "1". The Taylor series gives us a convenient way to calculate the value of the function (technically: inside a radius of convergence), but it is not the same as the function itself.

Now, to give an example from Quantum Field theory: let me quote from a wikipedia page [1]:

_In a perturbative approach to quantum field theory, such interactions may require the calculation of hundreds of Feynman diagrams. In contrast, twistor theory provides an approach in which scattering amplitudes can be computed in a way that yields much simpler expressions._

So, these hundreds of Feynman diagrams would lead us to think of many virtual particles describing an interaction. The Amplituhedron type of calculation is done completely differently, in a way that does not lead one to think of interactions through virtual particles.

Now to answer your question: I do believe that, if we were smart enough to find a way to compute things exactly, "virtual particles" would not appear as intermediate steps of computations. But, then again, to be perfectly honest, the same would likely be of "real" particles which are just a convenient way of thinking of excitations in quantum fields ...

[1] https://en.wikipedia.org/wiki/Amplituhedron

Re: First real evidence of a strange quantum distortion in empty space

#33

Post by fspeech » Sat, Dec 03, 2016, 1:14 AM UTC

tfgg wrote: post #7

fspeech wrote: post #3

I see by "first" they meant the first evidence of vacuum birefringence. Quantum vacuum fluctuation is by itself well established. E.g., Casimir effect.

The Casimir effect, contrary to popular understanding, doesn't require reference to vacuum zero point energy, it's a relativistic quantum effect: http://journals.aps.org/prd/abstract/10.1103/PhysRevD.72.021...

Thanks for the reference. Just to be clear, his point is made with regard to the reality of vacuum zero point energy, esp. whether it should affect the cosmological constant. However quantum vacuum fluctuation (which I read as meaning the virtual particles) is essential to QED whether zero point energy is real or not. As far as I can tell, quantum birefringence also does not refer to the zero point energy.

Re: First real evidence of a strange quantum distortion in empty space

#34

Post by joantune » Sat, Dec 03, 2016, 4:38 PM UTC

evanb wrote: post #9

joantune wrote: post #2

"In the classical physics of Newton and Einstein, the vacuum of space is entirely empty, but the theory of quantum mechanics assumes something very different. According to quantum electrodynamics (QED) - a quantum theory that describes how light and matter interact - it’s predicted that space is actually full of 'virtual particles' that pop in and out of existence and mess with the activity of light particles (photon…

No. Vacuum birefringence is understandable from a perfectly conventional understanding of quantum field theory, and does not remotely require anything about the pilot wave idea to be true. Every quantum field theory, including QED, has a roiling vacuum of virtual particles, whether you hold an orthodox view or not. Source: am physicist ;P

Got it, thanks!

The only thing it contradicts was already contradicted by Quantum theories which were some of the more classical Newton and Einstein's theories.

So this is exciting 'only' because it is a larger version of a man made in lab phenomenon, and seen occurring naturally.

Again, thanks

Re: First real evidence of a strange quantum distortion in empty space

#35

Post by randomguy1254 » Mon, Dec 05, 2016, 6:34 PM UTC

DavidPlumpton wrote: post #27

randomguy1254 wrote: post #16

Earlier quoted context omitted.

Can you give me some hints on what virtual particles actually are if its not too complicated? I keep reading that they are not actually real particles, only useful for calculations/as a simple way to understand what is going on. How does something like Hawking radiation work if they are not real particles?

In Many Worlds virtual particles are simply other universes (other parts of Hilbert space) that have electrons in different positions. They are nearby enough to contribute some amplitude to have an observable effect. Edit a word.

Interesting, thanks!

Re: First real evidence of a strange quantum distortion in empty space

#36

Post by randomguy1254 » Mon, Dec 05, 2016, 6:36 PM UTC

aroberge wrote: post #32

randomguy1254 wrote: post #31

Earlier quoted context omitted.

Thanks! That's an interesting fact about Feynman diagrams. If we had a hypothetical machine which could perform these calculations with infinite precision, would the "virtual particles" disappear from the diagram, or is it impossible to know this?

Let me answer your question first very obliquely. We know that we can represent the sine function near x=0 as the following Taylor series: sin(x) ≈ x - x^3 / 6 + x^5 / 120 - .... With more and more terms, we can can calculate it more and more precisely. Does this means that this function really contains all of these powers of x? Well, suppose we want to calculate sin(pi/2) (angle in radians, which is required for the…

Very interesting, good analogies and information for me to think about.