Re: How old would those naysayers be?
Is that the equivalent to saying it's impossible to travel at light speed?
No. The equivalent of saying "it's impossible to travel at light speed" is that (the mathematical description that best fits) this universe only allows, indeed enforces, light speed for particles with 0 rest mass. These happen to be photons, the carriers of the electromagnetic force. Matter structure (i.e. spaceships and fleshers therein) cannot be mapped to bunches of photons in any evident way. So no spacecraft at lightspeed.
Addendum 0: Interestingly for a photon, the universe has size zero. Photon trajectories seem to be a very special object that may well indicate a better mathematical description of the whole shebang. Last I heard, Penrose was working on that.
Addendum 1: According to Penrose, an old universe may well see the rest mass of all particles go to zero, in which case everything will be at light speed, which means there won't be any "time" left.
Addendum 2: Why is light speed that special veolcity "1"? The explanation may be at a deeper layer that we don't have access to as yet. Let's grab something from the SciFi bag and listen to Greg Egan (Schild's Ladder) for a few paragraphs:
In the beginning was a graph, more like diamond than graphite. Every node in this graph was tetravalent: connected by four edges to four other nodes. By a count of edges, the shortest path from any node back to itself was a loop six edges long. Every node belonged to twenty-four such loops, as well as forty-eight loops eight edges long, and four hundred and eighty that were ten edges long. The edges had no length or shape, the nodes no position; the graph consisted only of the fact that some nodes were connected to others. This pattern of connections, repeated endlessly, was all there was.
In the beginning? Waking more fully, Cass corrected herself: that was the version she remembered from childhood, but these days she preferred to be more cautious. The Sarumpaet rules let you trace the history of the universe back to the vicinity of the Diamond Graph, and everything you could ask for in a Big Bang was there: low entropy, particle creation, rapidly expanding space. Whether it made sense to follow these signposts all the way back, though, was another question.
Cass let the graph's honeycomb pattern linger in the darkness of her skull. Having relinquished her
child's-eye view of the world, she was unable to decide which epoch of her life she actually inhabited. It was one of the minor perils of longevity: waking could be like to trying to find your way home on a street with ten thousand houses, all of which had once been your own. That the clues on the other side of her eyelids might be more enlightening was beside the point; she had to follow the internal logic of her memories back into the present before she could jolt herself awake.
The Sarumpaet rules assigned a quantum amplitude to the possibility of any one graph being followed by another. Among other things, the rules predicted that if a graph contained a loop consisting of three trivalent nodes alternating with three pentavalent ones, its most likely successors would share the same pattern, but it would be shifted to an adjoining set of nodes. A loop like this was known as a photon. The rules predicted that the photon would move. (Which way? All directions were equally likely. To aim the photon took more work, superimposing a swarm of different versions that would interfere and cancel each other out when they traveled in all but one favored direction.) Other patterns could propagate in a similar fashion, and their symmetries and interactions matched up perfectly with the known fundamental particles. Every graph was still just a graph, a collection of nodes and their mutual connections, but the flaws in the diamond took on a life of their own.
The current state of the universe was a long way from the Diamond Graph. Even a patch of near-vacuum in the middle of interstellar space owed its near-Euclidean geometry to the fact that it was an elaborate superposition of a multitude of graphs, each one riddled with virtual particles. And while an ideal vacuum, in all its complexity, was a known quantity, most real space departed from that ideal in an uncontrollable manner: shot through with cosmic radiation, molecular contaminants, neutrinos, and the endless faint ripple of gravitational waves.