Top Ten Things That Would Be Hard To Imagine Without Mass

Mass is a property which prevents you from travelling at the speed of light all of the time, and allows you to travel at the speed of choice that is lower than light speed. So, without mass, you would be completely unable to stop, and could only reduce your momentum upon collision with an object by increasing the wavelength of the wave associated with your motion. But if nothing else had mass, you might as well be constantly stationary, as everything else would be moving at the same speed as you, so you would be unable to move relative to anything, and would see EM waves standing still next to you. The only way in which something would defy this principle would be if two waves associated with your motion were out of phase. Or simply moving in the opposite direction. Imagine it.

Without mass, you simply do not experience time. Nothing would. In a world without time, everything that you would experience would fly past you in literally zero time. That's even hard for me to picture.

If gluons, which drive the strong nuclear force, and W and Z bosons, which drive the weak nuclear force, were to be massless, these forces would, like gravity and electromagnetism, have infinite range. Nuclear fusion, but also radioactive decay, would be a lot easier, and one could theoretically achieve fusion with everyday energy transfers. But beta decay would not occur if W and Z bosons were stable and had infinite range, but if this were so, many other interactions involving the transfer of said bosons could occur at any distances, creating seemingly random changes of leptons into other leptons, possibly from lightyears away.

Gravity is not directly caused by mass, but by energy, which dilates the spacetime continuum. But the absence of mass would take a sufficient amount of this away. Gravity would still exist, but on a much smaller scale. Ignoring the absence of time, time dilation would be miniscule, it would be hard to stay on Earth, hard for Earth to form in the first place, and harder for stars to ignite and produce the heavy chemical elements that we rely on to live. The universe would, overall, progress a lot more slowly. But on the plus side, as you float aimlessly through space, you are less likely to incinerate as you approach a star or to be torn to shreds by a black hole, given that they are much less likely to come to be.

With all objects moving at light speed, exchange particles are hard to picture, as any particles to be emitted from another or absorbed by another have to move relative to that particle, and can only take place at specific angles, but realistically, quantum interactions would only make sense in terms of phase difference in waves. Due to redshift and blueshift, the particles within the interaction would receive energy in a mix of quantities unlike anything that involves mass. The wave-particle duality would have some loophole.

With nothing, not even light, moving relative to you, the conditions required for pair production would be unlikely to arise. So we would be without the fundamental constituents of the atom, and most particles would disobey the exclusion principle, and so, structures from the huge to the tiny would be hard to create. Scary.

Suppose that structures that we know of could exist without mass. The net quantum spin would still be zero, and so, the entire structure would exist within a superfluid state, so any structures that could exist would do so without viscousity. Any transfer of a vector to it would result in all particles following said vector, and always at one speed. Mind-blowing.

Given that light, in a massless universe, cannot progress in the same way that we know of, depending on the dimensional components of its direction, and the structures of an optical medium cannot materialise as we know it, the electromagnetic disturbance which a photon will impose would fail to cause any internal pressure on the given structure. The idea of a refractive index would therefore collapse. Light would either be transmitted or absorbed.

Assuming that the effect of dark matter would still exist, the gravitational dilation would still be miniscule when compared with that of our current universe. The imbalance between it and dark energy would be much greater, so the universe would fly apart as a result.

That's true. The Higgs would not be of much significance without mass. Except as a link in the standard model.