What Are the Fundamental Assumptions of Modern Physics?

[fractalzen]

It seems to me that classically physics is based upon the idea of measurement, that the length of a meter, the duration of a second, and the mass of an object is the same for one person as it is for another. Wheeler in his Geometrodynamics further was able to express mass and time in terms of length. So if we use Wheeler we can say that the basic assumption of classical physics is that the meter is the same length for me as it is for you. Classical physics assumes that in the "real" world things have fixed length, and don't constantly change lengths as might be possible in a dream.

So along comes relativity and quantum mechanics, and now the length of meter is no longer the same for everybody, but varies according to relative velocity or its position in a gravitational field or its quantum pdf.

Okay so now what? What is the basic assumption of "modern" physics? It seems to me that energy is a more fundamental concept than length, but isn't energy defined in terms of length, mass, and time? So is modern physics still using the "my meter is the same distance as your meter" assumption? If not, what has replaced it?

**/ The idea that everything that I am aware of in the vast universe around us is taking place inside my, by comparison, infinitesimal brain, indicates that perhaps size and distance might be viewed as constantly varying qualities rather than fixed quantities. **/

 

[mfb]

What is the basic assumption of "modern" physics?

Something like this:
There are inertial observers. The speed of light (measured as in classical physics) is constant for all inertial observers. (special relativity)
Quantum mechanics
Gravitational acceleration does not change physics locally, energy density bends spacetime. (general relativity)

but isn't energy defined in terms of length, mass, and time?

In the SI unit system. But this is completely arbitrary. You could use energy as fundamental unit, and express length as $1m=1s\sqrt{\frac{J}{kg}}$. In particle physics, it is common to express mass, momentum and (sometimes) time and length in terms of energy.

 

[DennisN]

I'm not sure what you are asking, there is no clear fundamental definition of energy. Energy comes in many various forms and can loosely be described as the capacity to perform work.

As an example, the energy of a massive particle at rest is E=m_oc², which is basically the mass multiplied by a constant; the units are obviously in terms of kg,m,s but the constant is c² m²/s².

Futher there are many other "assumptions" of modern physics like (intrinsic) electric charge, spin (intrinsic angular momentum) and a bunch of other physical constants. The masses of all particles are also "assumptions" in the sense that they do not automatically "pop out" (yet) from a deeper underlying theory.

"...perhaps size and distance might be viewed as constantly varying qualities rather than fixed quantities."

In what respect would they vary and why would you think so? Are you experiencing that your height is constantly varying?

(I saw mfb had already replied about relativity)

 

[Crazymechanic]

Well in strong gravity height can indeed vary as mass is "squeezing" particles more tightly together and the size they took up here on Earth would not be the same on some other much larger planet or object.

 

[pmacias]

I would first like to clarify that classical physics is not solely based on the idea of measurement. It also includes principles such as Newton's laws of motion, conservation of energy, and the law of gravity. However, measurement is an important aspect of classical physics and the assumptions you have mentioned are valid.

With the development of relativity and quantum mechanics, our understanding of the universe has evolved and some of the assumptions of classical physics have been challenged. For example, the concept of a fixed length has been replaced by the idea of relative length, where the length of an object can vary depending on its relative velocity or position in a gravitational field. This is known as the theory of relativity.

In terms of the basic assumption of modern physics, it is difficult to pinpoint a single concept as modern physics encompasses a wide range of theories and principles. However, as you have mentioned, energy is a fundamental concept in modern physics and it is defined in terms of length, mass, and time. This does not mean that modern physics is limited by the assumption that "my meter is the same distance as your meter". Instead, it takes into account the variations in length, mass, and time that occur in different frames of reference.

Furthermore, modern physics also acknowledges the limitations of our understanding and continues to push the boundaries of our knowledge. The concept of size and distance constantly varying is not a new idea in modern physics. The theory of quantum mechanics, for example, suggests that at a subatomic level, particles can exist in multiple places at the same time. This challenges our traditional understanding of size and distance and opens up new possibilities for further exploration and understanding.

In conclusion, the assumptions of modern physics are constantly evolving and expanding as we continue to explore and understand the universe. While measurement is still an important aspect, it is not the only defining factor in modern physics. Rather, modern physics takes into account the complexities and variations of the universe and strives to uncover the fundamental principles that govern it.