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Thank you so very much for your encouragements, dear Marie!

Can someone please explain to me how a particle has mass?

The terms mass, matter and energy can be confusing because they are often used to define each other.

Mass is the measure of the matter of an object and matter occupies space and time. Energy is the amount of work required to change the state of a system.

Energy is also used as a substitute term for relativistic mass which is the observed or apparent mass as an object approaches the speed of light and thereby increases mass.

To an observer, the relativistic mass increases as the velocity of the object increases whereas the invariant mass is the rest mass of the object itself. And inertial mass is the resistance of the object to changing its state of motion when force is applied to it.

For massless particles (e.g. photons or light) the energy of the particle is its momentum times the speed of light. Massless particles do not have a rest frame, they are always moving at the speed of light regardless of the frame of reference.

The photon travels a "null path" - for it no time elapses.

Matter constitutes the observed universe (space/time) and matter density (mass relative to volume) in the universe is called the critical density.

Antimatter is matter composed of the antiparticles of the particles that compose matter. The antiquark is the antiparticle of the quark, the positron is the antiparticle of the electron, etc.

When matter and antimatter collide, they are mutually annihilated, and energy is released in a burst of radiation. Matter is also created by energy in pairs as in the case where two or more photons interact to create a new fermion/antifermion pair.

Even so, that 5 percent of the critical density called “ordinary matter” has not yet been created or observed (Higgs field/boson) despite many attempts. The new equipment at CERN may yet observe the Higgs but so far, no cigar.

However, physicists love a good mystery and already have other theories for what we call mass:

The Mysteries of Mass

It is possible that the particles we see are all actually massless, their apparent masses corresponding to extra-dimensional momentum components we can't as yet detect.

Five Dimensional Relativity and Two Times

It is possible that null paths in 5D appear as the timelike paths of massive particles in 4D, where there is an oscillation in the fifth dimension around the hypersurface we call spacetime. A particle in 5D may be regarded as multiply imaged in 4D, and the 4D weak equivalence principle may be regarded as a symmetry of the 5D metric.

Or to put it another way, the particles we observe in four dimensions may actually be a single particle in a fifth time-like dimension multiply imaged 10⁸⁰ times.

It was Einstein's dream to transmute the base wood of matter to the pure marble of geometry.

I agree with Einstein, the geometry (space/time) is the key. In that regard, you can think of high gravity regions (ordinary matter and dark matter) as space/time indentations and conversely, negative gravity regions (dark energy - space between galaxies) as space/time "outdents."

Likewise, in that respect, I think of particles as placemarkers in the dynamic fabric of space/time.

Thank you for breaking it down for me! There's a lot in the terminology that gets confusing. Just to be clear on one point. You stated: When matter and antimatter collide, they are mutually annihilated, and energy is released in a burst of radiation. Matter is also created by energy in pairs as in the case where two or more photons interact to create a new fermion/antifermion pair.

So when an particle and an anti-particle collide, they cease to have mass, are transformed into massless particles and all that you have left is energy?

I think that part of my mental hang-up is that my brain keeps equating mass with weight. I have to accept that weight is nothing more than the effect of gravity on mass. No gravity - no weight. Mass is nothing more than the area take up by an object in space. Time and gravity affect the object in different ways, relative to the objects position and motion.

Is that right?