Student- Theoretical Physics, University of Nottingham

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## Our brain actually could recognise the effects of General Relativity on a smaller-than-cosmic scale.

I observed this a few days ago. We all know that we must stand away from a moving train because you get the illusion that you are being pulled towards it. Right? Well, 'illusion' puzzled me quite a bit. I did some experimentation regarding this, and I observed that if you are looking at the side of a train, your eyesight perpendicular to the direction of motion of the train, then you'd get the illusion that you are being pulled in. If you face away, surprisingly, you don't get the feeling, even if you you can hear the train. Another observation was that if I stood close to the train, and faced in the direction of travel of the train, I'd get the same pulled towards the train sensation. Now, if I faced the other side ie. if I were looking at a train approach me with a high speed, then I'd get pushed away. It didn't take long for me to figure out that it had something to do with the direction of light being perceived by my brain.

As I began to analyse this a bit more using General Relativity, it struck me that this illusion could probably be explained by GR. In general relativity, mass or energy causes the fabric of space-time to bend. One of the subtleties of this theory is that mass causes light and space-time to curve and light causes mass and space to curve. If I am looking in the direction of motion of the train, then the light from the train (which will be redshifted by an infinitesimally small value) will cause the space to contract. This contraction could be detected by the brain, and since the space between me and the train shrunk a bit, I get the feeling that I am drawn towards the train. The other case, when I am facing in the direction opposite the motion of the train, the light is blue shifted by a small amount, causing the space to expand, and hence this is detected by the brain as push-away-from the train.