# Effect of time distortion on light

In a story I've recently started writing, one thing that can be done with magic is alter the rate at which someone moves through time. It occurred to me first that the colors they would be seeing would shift because effectively when they are going faster the waves of light are reaching them slower (from their perspective), which would look the same to them as the light having a longer wavelength.

My question is what this would look like from an outside perspective. I'm assuming they are standing still or moving at up to twice normal human speeds (normal human speeds from the accelerated character's perspective), but even so, I'm assuming that the interactions between light and time accelerated matter are different than between light and normal time matter, or at least they act like one of them is doing something differently. I'm tentatively thinking the same sort of hue shift, but in the opposite direction, but I'm not sure if that's actually how that would work.

• @ARogueAnt. I'm not following why the character would be burning up. If they're moving through time twice as fast, the friction should be equivalent to if they are physically moving twice as fast through conventional means. Commented Apr 24, 2021 at 16:08
• I was thinking that even if they are standing still, the way the light would bounce off of or be absorbed by their skin would be affected by the relative rate of time. Commented Apr 24, 2021 at 16:16
• Even if the time acceleration occurred in a bubble surrounding the recipient, it wouldn't make any visible change to the light passing through it. Commented Apr 24, 2021 at 17:25
• This question is not a duplicate, but I think it offers a perspective that's useful. (Disclaimer: it's my question and I also answered it.)
– JBH
Commented Apr 25, 2021 at 4:33

I don't think it would have any noticeable effect from the outside.

When we observe something, light shines on it and it's sent back to us. That interaction is very fast, but even if that time was altered it would not affect the light emitted back.

Darker.

Let us consider from the perspective of exposure time and a camera photo. Suppose I am taking a picture in low light. Short exposure time means not much light hits the film and my image is dark. I use a longer exposure time, collecting more light over time. My image is brighter.

If I am taking an image on a very bright day I want a short exposure time or the brightness will wash everything out.

Let us think about b (light energy) per t (time). Half the time = half the energy but the same b per t. If I experience time twice as fast then I will get 1/2 b per my personal t. My experience of the world will be darker in proportion to how much faster I go, just as the photo I take is darker the faster my shutter speed is.

I like this for a fiction because the as you speed up and experience the outside world as slower, the outside world also is darker. It gets dangerous.

• Moving objects are not darker, they are blurred or invisible, though. In order to capture a fast-moving object on film (or digital camera), one needs very short exposure times with bright lights. Check bullet flight photos for example. Commented Apr 24, 2021 at 20:44
• @Otkin - true that. I was thinking of a landscape or photo of nonmoving things in low light. To get a picture of something moving in the dark you need to add your own light. Commented Apr 24, 2021 at 20:51
• Darker but hotter. Consider that sunlight has a nominal power density of about 500 watts/square meter, half at visible wavelengths and half in IR. It the time machine is travelling at 100 times "normal", the incident light will be at very far IR/ microwave - but with a power density of 50,000 watts per square meter. The traveller will cook very quickly. Commented Apr 25, 2021 at 21:08
• @WhatRoughBeast - cookage would be true even at a lot slower speeds than 100x if the traveller's personal time is slower than the surroundings. 100x sunlight is a lot. I thought the OP wanted the traveller to speedup personal time Quicksilver style. Commented Apr 25, 2021 at 21:14

Light emitted by the person would be blue-shifted, but most light is going to be reflected light, and that wouldn't look any different to an outside observer. That's because that light would be red-shifted as it enters the bubble, and then blue-shifted back to the original frequency as it exits. Note though that reflection/absorption properties are likely to be different however, as materials have different properties at different wavelengths. For example they may appear naked to external observers if the time ratio is 2x and their clothes are transparent in infrared.

Yes, I think there would be a blue shift. Light emitted by the sped up person (or re-emitted after reflection) would have a higher frequency proportional to how much faster time is passing for them. Similarly they would see things as red-shifted, since the light arriving at them seems (to them) to have been emitted by things that have slower clocks/frequencies than normal.