When I was little I used to wonder if a long time ago a distant alien race had noticed our little planet and set up a gigantic mirror pointing towards the earth such that we could use superpowerful telescopes to look into our planet’s past. Mostly I remember thinking that it would be cool if this were true and we could see dinosaurs (that was, I believe, the complete and total extent of my own version of the dinosaur fetish that seems to infect so many children.) Of course I was delighted when I discovered many years later the writings of Jorge Luis Borges, who had quite a fetish for mirrors. A memorial quote of Borges on mirrors is from “Tlön, Uqbar, Orbis Tertius”
Then Bioy-Casares recalled that one of the heresiarchs of Uqbar had stated that mirrors and copulation are abominable, since they both multiply the numbers of man.
While this is not a quote I can exactly sympathize with, the logic of mirrors holds, I think, some strange consequences. For example, suppose you want to freeze a moment of your life for future generations to look back upon. Well, simply launch a mirror away from you at as close to the speed of light as you can possibly manage. Then some future generation will be able to use this mirror to look back at this moment. (Of course the closeness to which you can launch this mirror to the speed of light will effect how much you have “frozen” this instant.) Of course, you could just as easily take a picture of the moment. But the mirror trick affords a certain sense of security: as long as no one can launch a mirror faster than yours, your mirror is safe. And this only gets better as time goes on (they need a faster mirror than yours to catch up to yours.) Of course the size of the mirror needed might be a little extravegant, and certainly gets worse as a function of time. Why Kodak hasn’t marketed this to cult leaders who wish to preserve their teachings, however, I do not know. 🙂
The reason for this post is completely a function of associative memories: yesterday I was bored, and so I calculated for myself that if you move approximately 1-10^(-39) percent of the speed of light as compared to everyone else, then the event in your frame which has coordinates x=1cm and t=0, would correspond to an event which happened about 14 billion years ago, i.e. at the begining of the big bang. In other words there are reference frames where what is next to you happened at the big bang.
So what physicists/(whatever job title describes what I am) do when they get bored? Well it appears to me to be the same things we thought about when we were little. But now they just involve numbers.
> So what physicists/(whatever job title describes what I am) do when they get bored? Well it appears to me to be the same things we thought about when we were little.
I remember very well that as a kid I was wondering at which speed ‘I’ was actually moving. Let me explain: Brains are made of electrons, photons, etc. each one moving at high speed. I knew about relativity and was wondering why ‘I’ do not perceive time as delayed.
I still can reproduce this feeling of not being able to locate my ‘I’ if I close my eyes and remember this way of thinking.
Heh, I had this same idea a couple of months ago! Except that I wasn’t thinking of it along the lines of aliens aiming mirrors at us but in terms of visible light being reflected by objects in outer space. A few weeks later I read an article that said something about astronomers being able to see into the past by looking at the reflection of light from a nearby gas cloud.
I would think that reflected rays would be ‘dopplered down’ to uselessness if your mirror passed a certain velocity.
Microwave wavelengths are in cm, and visuals are in nm, so sqrt(c+v/c-v) = 10^7, so once you get to within about one part in 10^-14th of c, any infomation would get lost in the CMB noise.
Ah, good observation! I suppose that means that if I want to use the mirror to communicate with the far future, I won’t be able to show them directly, but will have to encode the information at extraordinarily high frequencies!
By the way, UW is doing some kind of open house thing this weekend. Do the math, phys or cs folks have any events open to the public?
Hey andy, not sure: will dig around and find out.
In principle what Rafael mentions is possible, but I don’t know of any specific, useful examples. In other news, andy.s is right about doppler; in fact I think you get the doppler effect essentially squared because the mirror is receding from the emitter and also the receiver. But it won’t be confused with CMB, since the mirror will (presumably) block that. It will reduce the resolution of your image, however, because photons can’t resolve things much below their wavelengths. Oh and unless the mirror is super-cold, its own thermal radiation will out-shine your reflection.
It would be interesting if the aliens recorded some images or data of our planet and sent it back via a laser beam, radio, or some other kind of directed communication. Maybe they couldn’t take pictures of dinosaurs, but we would be interested in whatever kind of data they could collect about our planet in our past. If a sufficient number of alien civilizations at different distances all got the same idea we could be receiving tons of data about the early Earth! Seems like a very natural thing for them to send (besides information about themselves)…