Rethinking the origins of the universe

Well, "linked" is vague. They aren't linked—they are the same thing! Gravity is an effect mass has on the upholstery of the universe. Time runs slower for us here inside the gravity well of Earth than it does for astronauts in zero gravity.

You can't create time distortions without gravity distortions, and you can't create gravity distortions without instantiating mass, so... big massive black holes have to be running slow; if they exist at all and aren't just cosmic bees sitting at the 2D boundary of the universe and projecting nothingness into our inflated space.

IANA physicist, but I have a pretty good intuitive understanding of relativity. Yes, light paths are reversible. The reason you can't get a beam of light out of a black hole by reflecting an incoming beam is that from the point of view of an observer outside the event horizon, and inbound beam never makes it past the horizon inbound because of time dilation. At the horizon, time stops (relative to frames of reference outside the hole).

> They aren't linked—they are the same thing!

If you're speaking of time and space, no, they're different -- they're certainly all part of one spacetime, but they're distinct. This distinct:

t' = t √(1-v^2/c^2)

t = time

v = velocity

c = speed of light

t' = time at velocity v relative to velocity 0

Put into words, this special relativity relationship shows that an increase in velocity (movement in space) causes a reduction in time's rate of passing (movement in time). The general relativity version has more terms, and shows a relationship between mass and both space and time.

So space, time and mass aren't the same thing, but they're part of an interrelated system, one easily described mathematically.

> Time runs slower for us here inside the gravity well of Earth than it does for astronauts in zero gravity.

Yes, true, but astronauts in orbit aren't in zero gravity, they're in free-fall. The gravitational force at typical orbital heights is nearly as strong as it is at the surface.

I'm not a physicist. I imagine that if I were, this would be exciting news (that is, an exciting possibility).

I am a literary critic, though, and I have to say that it's bittersweet news at best. Black holes are so much a part of the space-age imagination. The article mentions Hollywood, but black holes are woven deeply into our everyday metaphors and have been for decades. They have stood among the most wondrous things in the universe -- an awe-inspiring thing.

It reminds me a bit of when the press reported that the Catholic Church had "gotten rid of Limbo" (the actual case was a bit more complex, and involved some fairly weighty theology).

Then again, we all still talk of things "being in Limbo." Perhaps that next project will still be a black hole, even if it turns out that there's no such thing.

If the spacetime is static, then yes, the trajectory of a light beam (or anything else) is reversible. The question you pose is an excellent one and is one of the reasons that many physicists believed that black holes could not exist after Karl Schwarzschild discovered the Schwarzschild metric that describes spacetime around a spherically symmetric black hole.

So what happens if you direct a beam of light radially into a black hole? A distant observer will calculate that as the light beam descends into the black hole it will be (according to a distant observer) progressively blueshifted until it reaches the event horizon, at which point it will have blueshifted to infinite energy. In fact, according to the calculations of a distant observer, the light beam will never cross the event horizon, so the statement that the light beam is reversible remains true. The reason that the light beam (according to the distant observer) doesn't cross the horizon is that if you look at the metric, it states that ds^2 = (1 - R_Schw / r) c^2 dt^2 - 1 / (1 - R_Schw / r) dr^2, where R_Schw is the radius of the event horizon. Notice that at the event horizon r = R_Schw and the dr^2 term diverges. The distant observer therefore concludes that there is a singularity at the event horizon. This analysis lead physicists to initially conclude that black holes were unphysical.

A more careful analysis reveals that this isn't the case, however. We have been using this metric which describes the shape of spacetime according to a distant observer. But what we're really concerned with is not what a distant observer calculates, but how a local observer perceives spacetime. If we now switch to a coordinate system in which we are free falling into the black hole along with the beam of light, we find that nothing remarkable happens when we cross the event horizon. The light is blueshifted relative to its initial energy, but it doesn't have an infinite amount of energy. Thus there is no true singularity at the event horizon. This singularity that the distant observer found must therefore have been an artifact of the particular coordinate system that the distant observer was using. The free falling observer can then hold up a mirror and send off a light beam and have it return back to him even after they have crossed the event horizon.