so where where do electromagnetic waves come from well obviously if we make a light source then we can produce them ourselves but most of the electromagnetic waves on earth originate from the sun and in fact just about all the energy that reaches us from the sun comes in the form of electromagnetic waves because these can travel through space even if there's no medium through which to propagate so as it turns out even though light is traveling at 300 000 kilometers per second that is three times ten to the eight meters per second it takes eight whole minutes for light to reach the earth from the sun just because of the immense distances involved so in fact we're not seeing the sun exactly as it is at the present moment we're seeing the sun as it was eight minutes ago when it first emitted the light that we're now seeing it means that uh when you look at anything really you're not seeing it as it is in the present moment but as it was just an instant in the past and the further away we go the further back into the past we're looking and this brings me to the next slide other stars and galaxies because of course electromagnetic waves are able to propagate through empty space if there's a star or a galaxy way out in the distance where in outer space then light waves from those stars and galaxies will still be able to reach us although they have to travel very very far and they'll end up getting very very spread out by the time they reach us because of that the stars aren't quite as bright as if they were as close as the sun but if we look at any other method of transmission so you know sound waves or mechanical waves we can't actually receive any energy through those mechanisms from these stars or these galaxies in fact all of the information that we have about these celestial objects comes from electromagnetic waves that they emit it's the only source of energy that we can pick up from them simply because they're so far away and so far separated by emptiness and because they're so far away it takes a very very long time for the light for even the closest star to us aside from the sun to reach us it takes light from the sun eight minutes to reach the earth but the light from the next closest star proxima centauri takes over four years and that's bear in mind the closest star other than our sun so what if we go further back what if we go to stars that are not closest to our sun well it turns out that if we look at the very most distant galaxies we don't see anything because it would take longer than the age of the universe for light from those galaxies to reach us so what this means is that we don't see any light from the most distant galaxy we know that probably there but we'll never be able to see them simply because the light from those galaxies has not had time to reach us in addition to that there's also the expanding of the universe and the sheer distances involved which makes them extremely faint and extremely hard to see anyway when we look at the night sky what do we see do we see the stars as they are now as they were in the past as they will be in the future or a combination of all three well it turns out that as we know light doesn't travel instantly it takes time for light to travel now given how far away stars are it'll take a very very long time years in fact before any of the light from those stars will reach us so in fact when we look up into the sky we're looking not at the star as it is now but at the light of the star that it gave off you know four or five years ago if we look at for example uh proxima centauri the closest star to earth it will emit light at some point but we won't see that light until four and a half years later well 4.2 years later the point is there's a four year lag between the star doing something and are seeing it so in fact when we see it we're looking at what the star wars in the past and our answer is b so using this technique to look up at the sky we can we can never in fact see the stars as they are their present moment we have to infer that from what we know about stars [Music] you