Sci-fi and time dilation

Someone will tell me I have this wrong I am sure. But I'll try explain the weird dream I had the other day.

Firstly, time dilation is a thing, the fact that some things (gravity, for example) can mean that in one place time may go at a different rate to somewhere else.

One of the thoughts that then occurred to me is that time dilation must change over a distance, creating a gradient - a rate of change of time over distance. One that would be a great one for xkcd What If? is what sort of time dilation gradient could a person tolerate?

My thought was how the human body could possibly cope with passing through some level of time dilation - what gradient would make sense before blood pressure either way is fatal, etc.

Now, lots of sci-fi has time dilation, for various reasons. Sometimes (like a Star Trek TNG episode) there were these bubbles of different rates of time. Sometimes (like a Star Trek Voyager) there were people walking in one time frame around a room of others apparently "frozen" (but just really slow). In StarGate Atlantis they had a portal that transitioned through a time dilation (but supposedly protected you from the transition effects). The idea of "stopping time" for everyone in an area apart from the protagonist, or just for one person, is very common in sci-fi. The SG-1 I am watching now has time dilation in the SGC over a distance of a few floors of the facility.

The huge issue that occurred to me first was energy - sonic boom type stuff but with heat and light. If an environment has normal time, and is, for example, in daylight, but some person is (near) frozen,  the amount of light and heat hitting them, from their point of view, would have enough energy to vaporise them very quickly. Time dilation enough to be noticeable would quickly cause severe suntans from the normal light fittings.

But what really struck me, in this dream, was frequency. Surely if someone is in a time frame that is, say, 40% slower than those around them, they would see things in a very odd way. All red would have turned to blue one way, or blue turned red the other way. If someone is half or twice as fast as their surroundings then they will not be able to see or be seen as light will be shifted out of the visible both ways. Indeed, at any notably larger differences you are either microwaving or x-raying someone just by carrying a torch.

Actually the x-ray one is amusing, as normal light could enter the dilation area, pass through as x-rays, a person, and come out converted back to normal light the other side, so you would see people as skeletons illuminated from behind if they were in a time dilation bubble of some sort. The microwave version is probably less of an amusing sight.

Did I get that right?


  1. IIRC the flashlight as weapon with time dilation was a plot point in one of Niven's Known Space stories. I think it was The Long ARM of Gil Hamilton, but could be wrong.

  2. Oh there's a lot to unpack here. Firstly, there are *two* sources of time dilation, acceleration and the local gravitational gradient. The second is a property of location, but the first is a property of rate of change of velocity. (They both point in the same direction, because of the principle of equivalence, so they cannot "balance out", they can only make each other stronger).

    Simply being in a sufficiently dilated environment can eventually kill you from blueshifted light, but it's hard to see how you could get into such an environment without getting into it killing you in the first place: in both cases the harm is likely to come from other sources, like being hit by objects moving at near-lightspeed with respect to you, being irradiated by perfectly ordinary stationary interstellar hydrogen ions, or being torn apart by the nearby black hole or neutron star's tides.

    But for a real *gradient* you need to be able to *sense* the difference: we're not talking the external world being dilated, we're talking about parts of your own body running at different speeds to other parts. This would obviously be lethal if extreme enough, but it is thankfully also not a problem in practice. Time dilation due to acceleration can obviously not do this unless different parts of you are accelerating at different rates, in which case you have much bigger problems. By the principle of equivalence, the same applies to time dilation due to the local gravitational gradient: if you are in a place where the local gravitational gradient is changing fast enough across your body for this sort of thing to be noticeable, you will be ripped to bits by tides more or less instantly. Even instantly-lethal 50,000G tides across your body -- the sort of thing achievable via acceleration, briefly, by some gun-launched unmanned drones, and otherwise only by loitering close to neutron stars and small black holes -- would cause a time dilation difference of fractions of a second per year. This would not be the thing that killed you :)

    Its also true that gravitational gradients can be caused by many things: only one of them is the presence of mass. Gravity is produced not directly by mass but by the flow of energy and momentum through spacetime. There are sixteen components in the stress-energy tensor and only one of them is mass... mind you, the others are usually negligible: the contribution of, say, the pressure inside the Earth to its own gravity is very small. (But this does become very significant in large stars and is one of the things that forces large-enough neutron stars to collapse to black holes. Again, if you're in a position to care about this affecting you directly, you have bigger problems.)

  3. You may enjoy "Mr Tompkins explores the atom" - a book predicated on the effects of changing various physical constants in Mr Tompkin's world. (See Wikipedia for more info).


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