Physics, Astronomy & Cosmology / Re: If nothing is faster than light, how can we look back in time with a telescope?« on: 24/05/2018 12:39:03 »
Quote from: Stephen
If nothing is faster than light, how can we look back in time with a telescope?An example: Chinese observers saw a "guest star" in the sky in the year 1054.
- It was bright enough to see in the daytime.
- We now know that this was caused by a supernova about 6,500 light years away.
- The expanding fragments are now known as the Crab Nebula.
- The supernova actually took place around 5,500BC (roughly 1054 minus 6500 years).
- So when the astronomers saw it in 1054 AD, they were looking back in time about 6500 years.
But supernovae also emit some other particles that travel at well below the speed of light. This probably includes radioactive iron nuclei, 60Fe.
- If these are traveling at (say) 2/3 the speed of light, they will take about 10,000 years to arrive at Earth (compared to 6,500 years for light)
- We can expect these to start arriving around the year 4,500 AD.
- So assuming that scientists then have radiation detectors in space, they would see this increase in subatomic particles.
- They would be effectively looking backwards in time at the same event - only now they will be looking back almost 10,000 years
- We can see a sprinkling of 60Fe on the sea floor, hinting at nearby supernovae in our galaxy. This isotope has a half-life of about 2 million years, so we know that this did not occur naturally on Earth.
So far as we know, there is nothing that travels faster than light, so no high-technology astronomers on Earth could have seen the Crab Nebula explosion before 1054.
- There is one subtle exception: Neutrinos. It is estimated that a supernova in the core of a giant star may take from a few hours to a day to break through the surface of a star, emitting an intense flash of light, X-Rays and gamma rays. However, neutrinos (which travel imperceptibly less than the speed of light) can make the same trip from core to surface in a few seconds. So neutrinos might arrive at the Earth from the core of the supernova a few hours before the flash of light from the surface of a supernova. With today's neutrino detectors and X-Ray/Gamma ray detectors, I expect we would be able to measure this time difference if another supernova exploded to us as close as 6500 LY.
edit: Correct formatting of the iron isotope, as highlighted by PmbPhy.