What is time? We think of it as something, if not physical, then at least real. We know there was a past because we remember it, and we believe there is a future because for every moment we've experienced so far another has followed after. It's said that it flies when you're having fun or stands still at emotionally important moments. But, for the most part, it moves ever onward at a rate of one second per second. We, each of us, are mundane and slow-moving travelers through time.
The trouble is, one second isn't always the same as another. Time moves at different rates depending on an individual's or an object's circumstances. It's an idea that science fiction can have a field day with, as seen in the new Amazon superhero series Invincible.
**Spoilers for Episode 2 of Invincible below.**
In the second episode of Invincible, based on the Robert Kirkman comic of the same name, aliens arrive through a portal to attack Earth. A small group of heroes attempts to hold them back but it's quickly apparent they are going to lose. The aliens just keep coming and their tech is too impressive. Then something changes. The alien soldiers shrivel and weaken as if under the influence of some chemical weapon. They retreat through their portals and disappear.
An examination reveals the aliens weren't defeated by a toxin, and they weren't defeated by Earth's atmosphere, germs, water, or any of the usual culprits. They were defeated by time. Specifically, time runs faster for them. They simply fell to old age before the battle was done.
There's a throwaway reference to "tachyons," science fiction's favorite temporal explanation, but it's otherwise accepted at face value that time runs at a different rate wherever they come from, and the effect of that followed them.
Science fiction likes to toss around made-up words or jumbled-together, scientific-sounding jargon, but this isn't one of them. While tachyons are a legitimate scientific concept, they've yet to be observed.
Relativity insists that regular matter cannot exceed the speed of light in a vacuum. As matter approaches the speed of light, acceleration becomes more difficult, requiring increasing energy until it becomes infinite. Particles with no mass can travel at the speed of light, but don't exceed it, except in very specific circumstances. Tachyons, if they exist, behave in the opposite manner. They always exceed the speed of light and would require infinite energy to slow down. Such a particle would reverse causality, appearing prior to its creation, at least from our perspective.
While the speed of light in a vacuum is fixed — a sort of universal speed limit — light does slow down when traveling through different mediums. For instance, when traveling through water, light slows down to about 75 percent of its usual speed. It's why the image of an object in a container of water refracts, like the fractured straw in a glass. A charged particle in a container of water can travel faster than the light in the same container. When that happens, the particle has exceeded light speed. Sort of. It doesn't violate relativity, but it does make for some interesting interactions.
As the charged particle moves through the medium faster than the local speed of light, it emits radiation in a trailing cone. You can visualize this as similar to a sonic boom, only with light. The resulting radiation, known as Cerenkov radiation, glows a soft blue.
This interaction could give us evidence of tachyons if they actually exist. Such a particle, traveling faster than the speed of light, should cause that same Cerenkov radiation even in a vacuum. To date, no such interaction has been observed.
If the existence of tachyons were confirmed, it would change the way we understand the universe, but they wouldn't have any relationship to an individual's subjective experience of time. If tachyons don't impact our subjective experience of time, what does?
SPEED AND GRAVITY
We've explored time dilation by way of speed before and won't rehash it here, as it isn't really relevant for this scenario anyway. The setup of our alien adversaries is one of location, and that can be influenced by gravity.
It helps to think of time not as something separate from space, but something inherently intertwined. Gravity distorts space, and when you distort space you also distort time. The greater the gravitational influence, the longer it takes for the clock to tick.
Unlike the case for tachyons, this notion is supported by more than just math. A clock at the top of a high building runs more quickly than one at ground level. The effect is minuscule but real. It isn't something you'd notice as you're moving through your day-to-day, but it's enough that we have to account for it in certain situations.
GPS satellites, which orbit the Earth hundreds of miles above the surface, experience time faster than we do on the ground (the whole truth is a bit more complex because they're also moving at high speeds). Given that those satellites depend on precise calculations in order to operate correctly, any divergence in time causes problems, especially extrapolated over long durations. In order to counteract the impact of time dilation, those satellites have to correct for the dilation.
Gravitational fields are variable throughout space. The larger the planet, star, or black hole, and the nearer you are to them, the larger the gravitational influence they have on nearby objects. As you approach, time slows down. It's why Matthew McConaughey was so weepy in Interstellar. While he was out there trying to save humanity in the time-library, the clocks back home kept ticking.
In the simplest terms, the stronger the gravitational field, the slower time progresses. The opposite, obviously, is also true. As a result, an alien race from a planet dramatically less dense than Earth and with no other significant gravitational influences would experience time more quickly than we do on Earth. Though not at a rate different enough to account for what we see on screen. Even an incredibly small planet would have enough gravitational variance to be significant.
It is possible then, that those alien enemies, after returning home through their portals, could experience more subjective time than we do but it wouldn't be much, and that effect wouldn't follow them to Earth. Once they arrive, they'd be subject to the same distortion of space-time as we are. Shared gravity and relative motion mean a shared experience of time.
We'd have to kick their alien butts in the usual way. Waiting for them to age out isn't an option.