Sunday, July 15, 2007
Look into the past with a big piece of glass
The ho-hum telescope is a lot more mysterious than we give it credit for.
How is it that a few pieces of curved glass allow us to look backward in time? Is this a property of light, the telescope or time?
We know time does not require telescopes, because the first known telescope was invented in 1608. But can there be time without light?
These are the kinds of questions that are likely to pop up when the curious non-physicist reads books about general and special relativity. Understanding Albert Einstein's work requires difficult mathematics.
Lacking a readership conversant in Minkowski space, Lorentz transformations, tensor fields, and even plain old integral calculus, writers attempt to explain Einstein's theories by analogy.
For example, space is like a trampoline, Earth causes a dip in the surface, and this curved space bends light. Light is deflected differently depending on the mass of the object it encounters and its distance from the object.
Since light travels at a constant speed, then time must elapse at different speeds depending on distance from the mass. Therefore, an object approaching the mass must shed energy, because it is experiencing slower and slower time.
So in some strange way, what we perceive as gravity is a consequence of mass bending light.
The speed of light also changes depending on what it travels through. This is why a lens is able to focus light and what enabled Hans Lippershey, an eyeglasses maker in the Netherlands, to invent the telescope in 1608.
Using advanced versions of this light-bending instrument we can see galaxies millions and billions of light years away. Or in other words, we can see light that they emitted billions of years ago.
We can look backwards in time. It's just impossible for us to change anything in less than the time it takes for light to travel from the past to the present. If we somehow wanted to change something in a galaxy 100 light years away, which seems unlikely, it would take much longer than a century to get there, because matter cannot travel at the speed of light.
Still, it's amazing that a few pounds of highly polished glass let us view the distant past.