Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Sight and light travel are not the same.

  1. Nov 29, 2016 #1
    You think Hubble's deep field is seeing what happened 14 billion light years ago and not zooming into what is currently happening? With the trillions of suns in each galaxy why is there so much darkness within a galaxy let alone inbetween galaxies? If Hubble was stronger it would see farther. If (visible) light could travel 14 billion light years, you would be blinded by the trillions of suns in the trillions of galaxies all coming at you at once. And if light scatters outward in all directions then how do you see an object, that far away (as a whole). Sight and light travel are not the same. I don't believe a telescope is seeing into the past, I would say it zooms into seeing the limited distance light can travel from an object.
     
    Last edited: Nov 29, 2016
  2. jcsd
  3. Nov 29, 2016 #2

    phyzguy

    User Avatar
    Science Advisor

    You have a lot of misunderstandings here. Let me try to list a few:

    (1) A light year is a measure of distance, not time, so it makes no sense to say "billion light years ago".
    (2)Much of the light we are seeing in the Hubble deep field was definitely emitted billions of years ago. Not 14 billion years ago, because the universe is not that old, but many billions of years ago.
    (3) Visible light can definitely travel billions of light years. You are not blinded by the huge number of suns because only a very small fraction of the light emitted by each star reaches your eye.
    (4) You do not need to see all of the light emitted by an object in order to perceive it as a whole. Think of the light bulb in your room. Only the light emitted in the direction of your eye enters your eye, but enough light reaches your eye that you can perceive the light bulb as having a physical extent.

    Ask yourself, what are the odds that the many people who have studied astronomy over the centuries (millions of people, maybe more) have it all wrong and you have it right? Isn't it possible that you are the one who is confused? Try to set your prejudices aside and learn why astronomers believe the things they do.
     
  4. Nov 29, 2016 #3

    Drakkith

    User Avatar

    Staff: Mentor

    Yes, that's right.
    Because space is big. Really, really big. Space is so big that every single star except for the Sun has an angular diameter of a tiny fraction of a degree across. Betelgeuse, the star with the larger angular diameter other than the Sun, is only ##1.5 x 10^{-5}## degrees across. So the vast majority of the time your line of sight is not falling on a star, but continuing on past it.
    By itself, "stronger" is a meaningless word when it comes to optics. If what was stronger? The magnification of the system? The light gathering capability? The power of the primary mirror? The HST is limited by the signal to noise ratio of its main sensor. This in turn depends on a huge number of factors. A more sensitive detector would enable it to gather light more efficiently and increase the SNR for any given exposure time. Less dark current or bias signal reduces the noise and leads to a better SNR. Increasing the diameter of the mirrors while keeping the focal length the same would put more light onto the sensor and increase the incoming signal, leading to a higher SNR. And there are several more factors.
    We would be if it weren't for the fact that the intensity of the light falls off as ##\frac{1}{r^2}##. For example, the intensity of the Sun's light here on Earth is about 1350 watts per square meter at the top of the atmosphere. At a distance of 4 light years, about how far the nearest star to our solar system is, the intensity has dropped to ##1.56x10^{-11}## of what it was initially, which is about ##2.1x10^{-8}\frac{W}{m^2}##. And that's just at 4 light years. The Alpha Centauri system is actually a little over 4 light years away, so there aren't even any stars within 4 light years, and only a handful of stars within 10 light years. The inverse square law is a harsh mistress.
    Simple. Your eye (or the optics in a telescope) catches a small portion of the outgoing wavefront.
    I don't believe anyone who knows what they are talking about is claiming that they are.
    That is incorrect. Even your eye itself sees into the past. By the time the light from the lightbulb in your room is emitted, bounces off of the walls and other objects in the room, and then enters your eye to be detected, a small amount of time has passed. So you are already seeing slightly into the past, and you will see a little further if you go outside and look at something in the distance. The Moon is about 1 second away at the speed of light, something which mission control and the astronauts of the apollo program had to deal with during communications.
     
  5. Nov 30, 2016 #4

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    This is either a fundamental misunderstanding or a personal theory. Either way, it's not a suitable basis for discussion. Thread closed.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook