Why do we need particles in our theories?

[Cleonis]

Aren't you still comparing your new meter stick to the lengths of your rubidium cell and mirrors ?

The idea is, I think, that if we would be in radio-contact with extra-terrestrials, would we be able to discuss physics? Would be be able to replicate on Earth their unit of distance? Well, if the laws of physics are the same in their region of the universe then we would. Our unit of length, the meter, is defined in terms of physical properties. One meter of length is defined as the distance traveled by light in free space in 1⁄299,792,458 of a second.
The unit of time is defined as a 'the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.'
The number of cycles can be counted precisely.

Unfortunately, defining a unit of matter quantity in terms of physical properties only is quite difficult. It's difficult to count large numbers of atoms precisely. You would have to narrow down Avogadro's number to a particular value, and then define a unit of mass as Avogadro's number of atoms of a particular element. But the elements come in isotopes, and you would have to know the isotope composition of your sample exactly.

Assuming the laws of physics are the same in different regions of the Universe a specific unit of length can be replicated everywhere, but replicating a specific unit of matter quantity poses challenges.

Cleonis

 

[Cleonis]

My question is this...why do we need particles anyhow...

Hi Lottery,

I have read your messages in this thread, and overall I get the impression that you yearn for exhaustive answers.

As several contributors have pointed out, effectively the concept of particles as fundamental physical constituents has been abandoned. The word 'particle' is still used, indeed the very concept of particle is still used, but the development of quantum physics has placed the concept of 'particle' in the context of particle/wave duality.

Before quantum physics scientists may have expected that their understanding of the world was exhaustive, or nearly so. They may well have expected that at atomic level physics can be understood with the principles that we have intuited from macroscopic physics, the physics of daily life. Quantum physics changed all that; the microscopic world of particle/wave duality is an alien world.

I think that overall scientist have taken a step back, and that they have decided that pushing for exhaustive understanding is like looking for the proverbial pot of gold at the end of rainbow. The question whether particles exist or not is regarded as moot. The concept of particle is used as a mental tool, a tool for visualizing and thinking, just like the mathematical tools that are in use.

Generally I think physicists have retreated to a position where they only demand that the theories account for the observations in a wide scope of applicability. In comparing theories the theory with the wider scope of applicability is regarded as superior.

Cleonis

 

[Andy Resnick]

Aren't you still comparing your new meter stick to the lengths of your rubidium cell and mirrors ? We can define units in terms of each other down to some fundamental levels, but there will always be some arbitrary point of reference. For the sake of argument, even if we could define length, resistance, and temperature in terms of mass, we would still need some lump of mass to define resistance - we just won't need a lump of mass and a random resistor.

Wikipedia talks about this briefly at http://en.wikipedia.org/wiki/Fundamental_unit.

That's incorrect. Construction of a length standard (or a standard clock) has nothing to do with calibration or comparison of lengths (or times)- the standard time or length interval is based on the speed of light, which is independent of clock or ruler. There is no deeper, "more fundamental", operational definition of length and time because the speed of light in vacuum is not based on an arbitrary point of reference.

The mass standard, resistance standard, and temperature standard are qualitatively different because the depend on the existence of an arbitrary object. That object must be *extremely* stable, because any changes that occur to it over time will be interpreted as drifting by the secondary standards (and the instruments calibrated by comparison to the secondaries, etc. etc). This is the motivation for re-defining the kilogram in terms of "more fundamental" concepts:

http://www.nist.gov/public_affairs/gallery/kilogram.htm

Note: it says the kilogram is the only physical standard left, but it's not clear about the other standards I mentioned:

http://www.nist.gov/eeel/quantum/fundamental_electrical/ohm.cfm
http://physics.nist.gov/cuu/Units/candela.html
http://physics.nist.gov/cuu/Units/kelvin.html

 

[kote]

That's incorrect. Construction of a length standard (or a standard clock) has nothing to do with calibration or comparison of lengths (or times)- the standard time or length interval is based on the speed of light, which is independent of clock or ruler. There is no deeper, "more fundamental", operational definition of length and time because the speed of light in vacuum is not based on an arbitrary point of reference.

The mass standard, resistance standard, and temperature standard are qualitatively different because the depend on the existence of an arbitrary object. That object must be *extremely* stable, because any changes that occur to it over time will be interpreted as drifting by the secondary standards (and the instruments calibrated by comparison to the secondaries, etc. etc). This is the motivation for re-defining the kilogram in terms of "more fundamental" concepts:

http://www.nist.gov/public_affairs/gallery/kilogram.htm

Note: it says the kilogram is the only physical standard left, but it's not clear about the other standards I mentioned:

http://www.nist.gov/eeel/quantum/fundamental_electrical/ohm.cfm
http://physics.nist.gov/cuu/Units/candela.html
http://physics.nist.gov/cuu/Units/kelvin.html

Okay, so they "electronic kilogram" uses the "separate systems in the laboratory [that] determine reference levels for voltage and gravity" instead of a reference lump of matter. How is that different than what I just mentioned? We're defining just mass in terms of reference electrical systems. What exactly did I say that was incorrect?

I understand the problem with having a lump of mass as the standard. You're talking about engineering practical methods for coming up with stable ways of maintaining the standards. That doesn't change the fact that when you make a meter stick you will do so by comparing it to something, which is all that's been suggested. There's literally no other way to do it.

 

[Dale]

To the OP, it is not that we "need" particles in our theories, but the modern theories with the most accurate predictions have particles. If someone makes a more accurate theory without particles then that theory will overtake the particle-based theories. That is just how science works.

 

[Thermodave]

I see nothing wrong with empirical formulas. They tell us a great deal and they create a path forward for more rigorous theory. The ideal gas law was developed by completely empirical methods but was later proven with kinetic theory and can be derived with modern thermodynamics given the same assumptions.

 

[Andy Resnick]

Okay, so they "electronic kilogram" uses the "separate systems in the laboratory [that] determine reference levels for voltage and gravity" instead of a reference lump of matter. How is that different than what I just mentioned? We're defining just mass in terms of reference electrical systems. What exactly did I say that was incorrect?

I understand the problem with having a lump of mass as the standard. You're talking about engineering practical methods for coming up with stable ways of maintaining the standards. That doesn't change the fact that when you make a meter stick you will do so by comparing it to something, which is all that's been suggested. There's literally no other way to do it.

You incorrect because the unit of length does not require a length standard. The measurement of a second does not require a time standard. I can construct a primary standard of length or time in my lab; not so mass or resistance.

 

[Bob S]

Charge holds everything together. Only particles have charge. So we need particles.
Bob S