“The only reason for time is so that everything doesn’t happen at once.” -Albert Einstein
Now that you know how many galaxies are in our expanding Universe, you might be wondering about their speeds.
After all, since the Universe is expanding, that means that the farther away a galaxy is from us, the faster it’s speeding away from us.
What’s more than that, since the expansion itself is accelerating, galaxies speed away from us ever faster as time progresses.
It should come as no surprise, then, that galaxies that we see moving away from us at high speeds have their light shifted into the red portion of the spectrum.
Well, that’s not such a big deal. We know why this happens: when objects that emit light move towards you, their wavelengths get compressed and the light appears bluer. When they move away from you, their wavelengths get stretched and the light appears redder.
And the faster this light source moves, the more severe the shift in wavelength.
But think about this for a moment. If an object appears to move away from you faster the farther away it is, aren’t we — at some point — going to start seeing objects moving away from us at recessional velocities approaching the speed of light?
Well, you know that a number of things happen when you approach the speed of light: these are perhaps the two most counterintuitive things about special relativity.
If you’re stationary and an object is moving — relative to you — at some significant fraction of the speed of light, you’ll notice two very bizarre things about this fast moving object: its length is contracted in the direction it’s moving, and time runs slow, or dilates, for the moving object!
You may immediately wonder if we can see this happening for distant galaxies!
Well, the length contraction is going to be impossible to measure, because we can only measure lengths in the direction perpendicular to the line-of-sight, but the expansion away from us happens parallel to the line-of-sight. So that’s out.
But what about time dilation? Is that present, or not? Let’s think about what we expect first, based on what theory tells us.
The galaxy in question isn’t actually moving, relative to the objects in its local spacetime vicinity, at relativistic speeds; what’s actually going on is that the space between us and this distant galaxy is expanding. And that expansion of space is what stretches the wavelengths of the photons, making the light appear redder.
But when this light was emitted, the “time” from the peak-to-peak of each wave was much shorter than the time you’ll observe by time those peaks get to you. So even though the galaxy in question isn’t physically moving relativistically, you should still see time dilation. So can you? What would you look for?
For example, we know that spiral galaxies rotate; you might wonder if it’s possible to see their apparent rotations slow down. Unfortunately, the relationship between a galaxy’s brightness and its rotational speed is different in the past than it is today, because spiral galaxies evolve over time.
You might think to look at quasars, instead, since they’re extremely luminous objects and visible at great distances. However, as the main scientist who studies this notes, the environments in which these quasars reside and the sources of variability (e.g., gravitational microlensing) are not constants between very distant and more nearby quasars.
Gamma-ray bursts are another candidate, because you can see them so far away, but what we’d really like is a very well understood class of objects, with uniform properties over time, that we can observe at extremely high redshifts. If we can measure whether that time gets dilated (i.e., lengthened) or not, that should test this once and for all!
Type Ia supernovae! These objects have a very well-known and well-studied timescale over which they brighten, dim, and fade away.
It’s really remarkable; see for yourself.
So if we see a distant, highly redshifted supernova, its light-curve should be stretched out over a longer span of time!
What do we find?
Believe it or not, we’ve got a bunch of them! The first one — a supernova moving away from us at nearly 50% the speed of light — came back in 1996! Then came another, and by time you get to today, we’ve got a whole slew of them, and can see, incontrovertibly, that time really does run slow for these distant galaxies!
The red line is the prediction without time dilation, the blue line is with. So this is really happening!
The amazing thing is, if there’s an observer in those galaxies with an ultra-powerful telescope, capable of viewing us, we’d appear to be running slow, while they move at normal speed to their own eyes!
So when you look at an ultra-deep, distant object, you’re not only seeing it billions of years in the past, you’re also seeing it in slow motion! And as you chew on that for awhile, know that billions of light years away, someone might see you chewing on it for a whole lot longer!