Gravitational lensing causes light from a single object in distant space to split so that it passes on either side of the lensing object. Any sufficiently massive object, for example, a distant galaxy, can cause this phenomenon
Now I am about to do some nit-picking. You have the right idea about this, but the way you said it might be a little misleading. The light from the distant single object isn't really split. Rather 2 light beams which are diverging at a very small angle will be bent so that they appear to come from different sources. Analysis however shows that they came from the same source.
If one of the routes around the lensing object is more direct than the other, then the light that takes that route will arrive sooner than the light that has taken the longer route. If the source of the light is a pulsar, then pulses of light will arrive at the detector. It has been established experimentally that a pulse of light that has taken the shorter route to the detector will arrive sooner than the same pulse that has taken the longer route. As would be expected, the speed of light has remained constant for both routes; however, the pulse that took the shorter route, and arrived ahead of the other appears to have travelled faster than light. There was no time difference when the pulse was generated, as it was then a single pulse; but there is a time difference at the detector. Not only is the perception of simultaneity different within the frames of reference of the source and the detector, but, it is argued, the first arriving pulse has apparently travelled faster than light, and has, therefore, travelled into the past. Of course, all this is relative, so the argument can be reversed. We can, with equal validity, maintain that the second pulse has travelled more slowly than light, and has therefore, within the terms of this argument, travelled into the future. However, there seems to be something of a paradox here, because, relativity says that the faster one travels the slower time passes, thus the further one travels into the future, yet in the case of the two light beams, it is the slower one that moves into the future relative to the faster one. So, can both of the above assertions be true?
We have to consider the fact that the arrival of the pulses at different times is due to the fact that one has taken a longer route than the other, so neither has actually travelled faster than light. Nor does the fact that the two pulses leave the source as a single pulse actually alter the situation. After all, if we split a single beam of light from a source into two beams, send one directly to a detector and the other, via a mirror placed a light year away, to the same detector, the first beam will arrive two years before the other, but we would not argue that it had travelled faster than light, or that it had time-travelled – or are there those who would?
I don't think that anybody who is a reputable scientist would make any claim for time travel in this case. Well, maybe you could say that the beams traveled into the future. But then everything is traveling into the future. But since Relativity destroyed the idea of simultaneity we observe them at different times and this is perfectly normal.
Bill Gill
