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Virtual Particles and Gravity

by barbacamanitu
Tags: gravity, particles, virtual
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barbacamanitu
#1
Jan17-13, 01:40 PM
P: 29
Hey guys, first post here. I'm just starting to learn physics at the college level, so keep that in mind.

While doing some reading on dark energy and dark matter, something occurred to me that I'd never had answered before. Do virtual particles exist long enough to have even a slight gravitational effect on other things? If so, then couldn't the acceleration of galaxies be due to the fact that there is much more empty space on the "outside" of them then there is matter on the "inside"? This empty space would contain countless virtual particles, and if they had any gravitational effect at all, then they would eventually have more of an effect than the matter trying to bring everything together.

Does this sound valid at all, or am I totally missing something?
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Mordred
#2
Jan17-13, 01:56 PM
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P: 1,857
The term Virtual particle is oft misleading its probaby best explianed in this article. There are better articles but this one has several key principles.

http://profmattstrassler.com/article...what-are-they/
LastOneStanding
#3
Jan17-13, 01:56 PM
P: 718
When talking about actual matter, this is something that already gets taken account. For instance, virtual particles being exchanged between quarks contribute to the mass of the proton. This has been confirmed by some very precise calculations and measurements. General relativity tells us that inertial mass and gravitational mass are precisely the same thing, so this does indeed contribute to the gravitational field of a proton.

The vacuum is a different matter. What you're describing is actually one of the leading ideas people have thrown around as a possible explanation for dark energy. The gist of the idea is that quantum field theory predicts that the vacuum has a minimum energy greater than zero (and what you've described is one of the ways of thinking about this zero point energy) and relativity says this energy should couple to gravity. The issue is that if you actually do the quantum field theoretic calculation, it predicts a value for the vacuum energy that is larger than can fit with observational data about the universe's expansion. In fact, it's too big by a factor of about ##10^{107}## and has, accordingly, been called the worst prediction in the history of physics.

barbacamanitu
#4
Jan17-13, 02:01 PM
P: 29
Virtual Particles and Gravity

Great, I have made the worse prediction in the history of physics. Awesome. Haha, thank you for making sense of that for me.
Mordred
#5
Jan17-13, 02:09 PM
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P: 1,857
Roflmao
LastOneStanding
#6
Jan17-13, 02:09 PM
P: 718
Oh, not at all—the calculation is the worst prediction, the idea itself is a nice one! It's all the more troubling since we can actually directly detect vacuum energy through something called the Casimir effect. In any case, this whole fiasco is called the vacuum catastrophe if you want to read more.
Bill_K
#7
Jan17-13, 02:15 PM
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P: 4,160
The issue is that if you actually do the quantum field theoretic calculation, it predicts a value for the vacuum energy that is larger than can fit with observational data about the universe's expansion. In fact, it's too big by a factor of about 10107
This always makes a nice story, but in fact the "calculation" was nothing more than a wild and silly guess: one Planck energy per cubic Planck length.
LastOneStanding
#8
Jan17-13, 02:35 PM
P: 718
Quote Quote by Bill_K View Post
This always makes a nice story, but in fact the "calculation" was nothing more than a wild and silly guess: one Planck energy per cubic Planck length.
I think you're missing the point of the calculation. Given that assuming quantum field theory holds to arbitrary energy scales gives an infinite value for the vacuum energy, you instead just take the integral to as far as you think you can trust your theory and count on some "new physics" to suppress the contributions to vacuum energy at higher energy scales. This was thought to be the Planck scale. Because of how poorly the calculation fares, the conclusion is that either the quantum field theory you're using fails much earlier than we expected it to, or else something miraculous has to happen at higher energies to almost exactly cancel out out the lower energy contributions to vacuum energy.

It's not a "wild and silly guess", it's learning something from the fact that, one way or another, your working assumptions give an unworkable result. You don't think that's a valuable lesson?
barbacamanitu
#9
Jan17-13, 02:38 PM
P: 29
Ok, I want to throw out another crazy thought I had just in case it's also a widespread theory.

Has anyone ever postulated that if you could zoom in close enough to the start of the Big Bang, and chance the scale enough, that an entire universe would fit inside? Not in the sense of a infinitesimally small point, but an actual universe with galaxies and planets all spaced out somewhat like they are now. Even time would be scaled down to a near halt from our perspective, but from inside this "zoomed in" perspective, time would be flowing differently. Even the speed of light could theoretically be different.

This would solve the problem of needing something before the big bang, since time only gets closer and closer to 0 but never reaches it.
Naty1
#10
Jan17-13, 02:43 PM
P: 5,632
.... but in fact the "calculation" was nothing more than a wild and silly guess:
Bill_k knows a lot more about this than I, but Bianchi and Rovelli takes a different view from Bill here:

http://arxiv.org/PS_cache/arxiv/pdf/...002.3966v2.pdf
Page 5 of 9
IV. THE VACUUM ENERGY IN QUANTUM FIELD THEORY


Their conclusion it seems to me :

... there is still much we do not know in QFT, regarding the mass of the Higgs, the mechanisms that protects Lambda from scaling, and certainly more........it is a mistake to identify the cosmological constant  with the zero point energy of a QFT,
Naty1
#11
Jan17-13, 02:58 PM
P: 5,632
Do virtual particles exist long enough to have even a slight gravitational effect on other things?
I did not think virtual particles have been experimentally confirmed. Some think the Casimir effect does, many do not.

What is a virtual particle.....

http://en.wikipedia.org/wiki/Virtual_particle


.... Virtual particles are also excitations of the underlying fields, but are "temporary" in the sense that they appear in calculations of interactions, but never as asymptotic states or indices to the scattering matrix. As such the accuracy and use of virtual particles in calculations is firmly established, but their "reality" or existence is a question of philosophy rather than science.
In fact virtual particles need not 'exist', that is be detectable, for vacuum energy to have a possible effect on gravity. In fact it is thought the effect of vacuum energy is repulsive, the opposite of what you are asking!
Naty1
#12
Jan17-13, 03:04 PM
P: 5,632
...Has anyone ever postulated that if you could zoom in close enough to the start of the Big Bang...... that an entire universe would fit inside? ,,,,,an actual universe with galaxies and planets all spaced out somewhat like they are now.
That is completely inconsistent from the way this universe is believed to have evolved...from primordial fluctuations emanating from an energy level fantastically high
and very unstable to a far more stable environment we observe today, where forces can evolve, particles can form, atoms can assemble and slowly form stars, planets, galaxies, etc.

So if anyone has such a model, I'd be interested.
barbacamanitu
#13
Jan17-13, 05:01 PM
P: 29
Quote Quote by Naty1 View Post
That is completely inconsistent from the way this universe is believed to have evolved...from primordial fluctuations emanating from an energy level fantastically high
and very unstable to a far more stable environment we observe today, where forces can evolve, particles can form, atoms can assemble and slowly form stars, planets, galaxies, etc.

So if anyone has such a model, I'd be interested.
I'm just speculating here, but is it not conceivable that this fantastically high energy is only high relative to our notion of energy? This is sort of a philosophical question, but since our concept of energy and order is derived from observing the universe, then how can we say that the energy at the start was high?

If the size of your tape measure changes, then your measurements change. That's sort of the point. If we were a Planck length tall, who knows what type of measurements would be taken.

Now that I have committed heresy, I'd like to see if anyone understood me at all.
Naty1
#14
Jan17-13, 05:13 PM
P: 5,632
I'm just speculating here, but is it not conceivable that this fantastically high energy is only high relative to our notion of energy?
It's good to have ideas...questions...it's also good to do a little reading to see what current science says and if THAT doesn't make sense, then ask questions here.

Of course that early energy was high relative to our energy observed now.

Before the early universe cooled sufficiently to allow energy to be converted into various subatomic particles, including protons, neutrons, and electrons, NOTHING massive could have formed.

try checking the first section or two here:
http://en.wikipedia.org/wiki/Big_Bang
barbacamanitu
#15
Jan17-13, 05:48 PM
P: 29
I did not take cooling into account there, you're right.


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