Want to know what really are Particles?

[Moni]

May it seems very basic question, but after a post from general physics I want to know what really is Particles?

what they are made of? Energy? but few of them have mass: like alpha, beta...radiations then...

 

[Nereid]

definitions

"Particle" is a word with many meanings, even in physics.

Many experiments have shown that 'things' like light, electrons, neutrons, ... even atoms ... have both a 'particle' character, and a 'wave' character. Quantum mechanics provides a powerful description of these characteristics (and many, many others); indeed, AFAIK, there are no experimental results which contradict QM.

In this sense, you and I, our monitors, desks; the fruit on our tables, ... even the Earth and the Sun ... are 'particles' different from photons, electrons, neutrons and atoms 'only' in our 'wavelength'.

Someone with a better understand of QM than I will undoubtedly provide a description of what the QM particle entity 'really is'.

"Particles" also refers to 'fundamental particles' or 'elementary particles', which are the basic components of all 'things' in the universe. The 'particles' in this exclusive group have changed over time; not because the universe has changed, but because we understand the nature of things like protons and electrons better (protons are not 'elementary' but electrons are).

In the Standard Model - which accounts very well for an astonishingly large number of experimental results - there are two classes of particles - quarks and leptons. There are also 'force carriers' - the photon, gluons, and 'intermediate vector bosons'. These mediate the electromagnetic force, strong force, and weak force (respectively); another way to say this is that an EM, strong, and weak force interaction involves an exchange of these particles. (There's also the Higgs particle; later).

Quarks and leptons (except, possibly, some flavours of (some) neutrinos) all have mass; so do all the force carriers except the photon.

Hyperphysics has a good overview of all of this; find the 'particles' page and explore the links from there:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

What about the graviton, the 'carrier' of the gravitational force? Very good question!

String Theory (M Theory) may provide an extraordinarily complete explanation of all particles - including the graviton - and their properties, interactions etc ... in terms of the vibrations of 'strings' (actually branes) in 11-dimensions.

'What are they made of??' If String Theory is correct, 'strings'. In the Standard Model, your question can only be answered in terms of their properties (e.g. charge, mass, spin, forces they feel).

 

[Moni]

Originally posted by Nereid

In the Standard Model - which accounts very well for an astonishingly large number of experimental results - there are two classes of particles - quarks and leptons. There are also 'force carriers' - the photon, gluons, and 'intermediate vector bosons'. These mediate the electromagnetic force, strong force, and weak force (respectively); another way to say this is that an EM, strong, and weak force interaction involves an exchange of these particles. (There's also the Higgs particle; later).

Hyperphysics has a good overview of all of this; find the 'particles' page and explore the links from there:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

What about the graviton, the 'carrier' of the gravitational force? Very good question!

String Theory (M Theory) may provide an extraordinarily complete explanation of all particles - including the graviton - and their properties, interactions etc ... in terms of the vibrations of 'strings' (actually branes) in 11-dimensions.

'What are they made of??' If String Theory is correct, 'strings'. In the Standard Model, your question can only be answered in terms of their properties (e.g. charge, mass, spin, forces they feel).

Hmm...thanks Nereid, you've given a brief explanation :)

In many parts I can't get all the concept :(

As far as I know in QM particles are considered as Packets of energy...that means they carry energy!?

Standard model says, about "Force Carriers" How they carry forces?
Do we carry force?

Hmm...I am interested what the "String" theory experts say ?

 

[marcus]

Originally posted by Moni

Hmm...I am interested what the "String" theory experts say ?

Hi Moni. The topic of "particles" is interesting. A PF poster who has, in the past, posted stringy discussion recently had this to say about particles:

"The concept of field is more fundamental than that of particle in that the latter is an approximation valid only in situations where spacetime curvature may be neglected, as is the case in conventional QFT in flat spacetime. The reason is that the properties of mass and spin classifying particles depend for their definition on global poincare invariance which holds only in minkowski space. From this point of view, particles can be thought of loosely as local excitations of fields since classical curved spacetimes are always locally flat. In fact, the relation between the particle concept and spacetime geometry means that particles are actually an observer-dependent concept: Observers undergoing acceleration in minkowski space will detect particles not seen by inertial observers."

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For more discussion by Jeff and Paladin, see
https://www.physicsforums.com/showthread.php?s=&postid=129266#post129266
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I believe this is a generally accepted view among the physically sophisticated. It doesn't have anything to do with string theory per se (a recent post 1980 invention). Just with standard quantum field theory which goes back much farther.
I've seen similar or corroborating statements over the years, from people who ought to know what they are talking about.
Particles do not exist. They are convenient approximations of field behavior which one can use in talking about fields.
They exist mathematically in "flat" spacetime. But the space we live in, and the sun and planets live in, is not flat but curved.
So strictly speaking particles do not exist in the sun or the planets or on the surface of the Earth or any real place.

Particles are an extremely useful mathematical idealization which does not correspond to reality and, of course, when physicists speak to lay-people they always talk as if particles exist because we instinctively like the idea. It is a little like fruit and nuts that our ancestor monkeys picked off the trees and we are tuned to be alert to that kind of image---a cherry or an apricot, a proton or a quark. Therefore because it fits nicely in the mind we believe devoutly that particles exist. Physicists realize they do not, but are usually quiet about it.

Or they talk about it using big words which decently hide the naked fact of non-existence. This way they avoid shocking people.

 

[Nereid]

What's the current status of 'wave-particle' duality then? Just another good approximation in the (a) limit?

In the 'fields more fundamental' view, as summarised by lethe, how does one refer to quarks and leptons (and photons and gravitons)? Does the term 'particle' still survive, perhaps just as a shorthand?

Another common term used by folk trying to understand 'what (a) particle really is!' is 'energy'. What say you to questions by searchers without physics degrees to them?

Another example: https://www.physicsforums.com/showthread.php?s=&threadid=11142

That's a great thread you linked to! Thanks for posting it.

 

[marcus]

Originally posted by Nereid
What's the current status of 'wave-particle' duality then?

so many layers to the onion!

it is one question to ask "how are physicists talking these days? what words do they use and how do they use key ones like particle?

it is another question to ask what nature is made of

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I will tell you what I think about nature, and say nothing of the semantic fashions of some group of physicists (relativists use words differently from particle theorists so one would even have to divide up the sample)

I think the sun is made of protons and electrons, a great stew of them, like caviar is made of fisheggs, and some other species of things too like helium nuclei, but mainly a great lot of protons and electrons.

And yet if I want to think more fundamentally I must say to myself NO those protons and electrons do not exist---they are just convenient mathematical approximations that I am using to help me think about the sun.

So if I want to think more fundamentally about the sun I must imagine fields. And the sun certainly does not live in flat Minkowski space!
So to mentally construct my picture I must begin with the gravitational field------a superposition of quantum states of the gravitational field.

On a representative quantum state of the gravitational field, I can picture lots of other fields defined. A typical quantum state (or network) provides enough geometry so that I can visualize other fields defined on it.

The way I define a field on a quantum state of gravity may actually be to place particle labels at individual nodes of the network, anyway I assume I have some proceedure maybe the one described in detail by Rovelli in Chapter 7 "Dynamics and Matter".

The general idea (Rovelli page 7)
"In Newtonian and special relativistic physics, if we take away the dynamical entities---particles and fields---what remains is spacetime. In general relativistic physics, if we take away the dynamical entities, nothing remains...

...physical entities---particles and fields---are not all immersed in space and moving in time. They do not live on spacetime; they live, so to say, on one another.

It is as if we had observed in the ocean many animals living on an island: animals on the island. Then we discover that the island itself is in fact a great whale. Not anymore animals on the island, just animals on animals."

So we have come full circle and I am using particles as labels to apply to nodes of a quantum state network. I picture one representative quantum state of the sun as a vast network with jillions of nodes and links----honeycombing the sun that I see---and this network describes first of all the curved geometry, and then, by the labels or colorings on it, it describes matter fields overlaying the geometry.

And I imagine a superposition of many of these quantum states all more or less the same but differing in random details, such that if I back off and don't look too carefully it will appear to be the familiar caviar stew of protons and electrons. The network picture blurs into the "normal" one.

And if I back off still more, I see the sun.

Well Nereid, you asked. That's particles as I see them---not as fundamental as fields but too useful to give up

 

[Nereid]

So that's what's meant by 'spin-foam'?

Let's see if jeff, SelfAdjoint, Tom, ... have a different view. No wait! SelfAdjoint already posted an answer to Moni's question (on the Strings, Branes & LQG board): " Very briefly, the strings vibrate, and it's the vibrations that generate particles. Not the strings themselves but the way they vibrate. The vibrations are quantized - grouped in levels like the energy levels of electrons in atoms, and for example the graviton comes from combining two sets of vibrations at the second level. (In bosonic string theory with closed strings)."

In other words, what particles 'are' depends on the theory you use. As QFT, GR, and the Standard Model are all well tested, and pass the 'account for observations and experimental results' test with flying colours, it'd be accurate to say a particle 'is' whatever QFT, GR, and the Standard Model describe them as. Ditto 'energy'.

Moni -> if you thought QM was weird, you should try getting your head around some of the hot concepts in physics today, esp M-Theory - zero-branes, 7 curled-up space dimensions, the holographic principle ... [b(]

 

[marcus]

Originally posted by Nereid

In the 'fields more fundamental' view, as summarised by lethe, how does one refer to quarks and leptons (and photons and gravitons)? Does the term 'particle' still survive, perhaps just as a shorthand?

I think it is not quite enough to say that what particle is depends on what theory you are using.

There seems to be a fair amount of consensus that "fields are more fundamantal"

the point being that the convenient shorthand (as you suggest)
concept of a particle only exists in flat space.
As far as we know flat space (the Minkowski space on which quantum field theory and much else is defined) does not exist. Everywhere we look space is at least slightly curved, and (cosmologists tell us) is expanding---whereas Minkowski space is not curved and does not expand.
Although it can serve as a fine mathematical approximation, there is something false about it.

The same would be true for any physical theory limited to flat space and dealing only with particles. Particles, although extremely helpful conceptually, are idealized approximations of stuff happening to fields. The fields have a firmer more fundamental existence. Or so I have gathered from listening to a wide variety of knowledgeable people over the years---and some as recently as Lethe! (I guess he is a young grad student somewhere in Midwest, like Madison. Not the most venerable and august authority but not a bad one either!)

So I think the "fields more fundamental" thing is recognized across different theories and disciplines. It may indeed be wrong, as any of our ideas can be! But it deserves to be highlighted and not plastered over.