The Grandfather Paradox The Grandfather Paradox is a concept in theories about time travel. More specifically, the paradox introduces the dilemma that if you were to travel back in time to a point before your grandfather met your grandmother, and you happened to prevent them from meeting (i.e. killing your grandfather), you would, by extension, prevent yourself from being born. But, if it was actually the case that you were never born, how could you have traveled back in time to commit this act in the first place? And if you didn’t go back in time to kill your grandfather, then you would still be alive, meaning that you would be able to go back in time and kill your grandfather. This concept creates a seemingly unsolvable loop- a paradox. In short, the Grandfather Paradox serves to illustrate the complex contradictions that arise when we think about altering events that have already occurred. It suggests that if time travel were possible (if we were to come up with a time machine to the past, today), there would need to be some sort of mechanism that prevents this contradiction from happening. Some hypotheses lie along the lines of alternate timelines/parallel universes, but the backing to these theories are far from absolutely conclusive. Instead, one could argue that it’s equally as easy (or easier) to conclude from this paradox that time travel is simply a physical impossibility.
The Speed of Light When we look at an object in space (a star, for example), what we actually are seeing is the light emitted from such object. When emitted, light travels through space at a finite speed (approximately 300,000 km/s), meaning that it takes some ‘X‘ amount of time for this light to reach us. Generally, X is dictated by the distance between the observer (us) and the source of light. Because of this, the farther an object is, the longer it takes for its light to travel to us. Some examples include the Sun, which is 93 million miles away, and has its light take 8m20s to reach observers on Earth1. The Moon, on the other hand, is much closer at an average distance of about 240,000 miles, resulting in a “light travel time” of around 1.33 seconds. On much smaller scales this truth holds, but is considerably more difficult to notice. Due to this discrepancy between the time the light is emitted and the moment where it reaches the observer, whenever the light arrives at us, we’re actually observing this object as it was in the past. In the case of the Sun, we see it as it was 8m20s ago (8.33 light-minutes). In the case of the Moon, we see it as it was 1.33 seconds ago (1.33 light-seconds). Andromeda (the closest galaxy to our own Milky Way) is roughly 2.5 million light years away (Image 1), meaning the light takes 2.5 million years to travel to us.
(Image 1 displaying the distance, in light years, between the Milky Way and Andromeda galaxies. Image taken from http://www.waitbutwhy.com)
Observing the Past Through the Speed of Light Now imagine a hypothetical alien civilization in Andromeda. If they were to have a powerful enough telescope, they would be able to observe Earth as it was 2.5 million years ago. They would be observing the Earth at/around the time of an Ice Age, allowing them to gather insights on aspects of humanity’s past. This is because the light (and the information it carries) from Earth during that period would just now be reaching Andromeda’s point in space. However, such alien civilization would only be able to observe this past- they have no ability to interact with it. In a way, this hypothetical is a form of a time capsule, where information about the past is encoded in the light that travels across the Universe. And at the same time, every different point of view (Earth, the Sun, Andromeda, a Supercluster, or ANY OTHER PLACE IN THE UNIVERSE) serves as its own time capsule. While holding the Grandfather Paradox to be true, we can consider this idea of light travel-time as a loophole around the contradictions of the paradox. If us, humans, were to somehow, magically teleport to Andromeda today and looked back at Earth, we would see it as it was 2.5 million years ago. This ability to observe the past serves as the closest fathomable alternative to traveling in time, allowing you to observebut not interact with the Universe of the past.
This is my blog for my astronomy class about the solar system. While there aren’t any black holes in our galaxy, I used to be obsessed with them back in elementary school.
Here is a hyperlink to a website that kind of freaked me out when I was younger called This Person Does Not Exist.
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My name is Matthew, and I’m a student at Vanderbilt University. I love singing, and you can find a video of a Barbershop Harmony Performance I was in at the hyperlink below at 19:42.
Hello, my name is Alek! This picture of a piano represents my primary major here at Vanderbilt (as I am a piano major at Blair). Looking forward to a great semester!
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Hi! My name is Mercedes. I am a transfer student new to Vanderbilt. I love to travel and explore. This picture is from when I lived in the PNW. My father is in the military (I personally do not recommend that path, but feel free to investigate your options here). His job has given me the opportunity to explore much of the United States.
I have little to no experience in astronomy, or with a telescope, but I am happy to begin learning!
Hello, my name is Leo Silva and I’m a senior here at Vanderbilt. I major in H&OD and minor in Astronomy (completely unrelated). For many years now I have loved the puzzling nature of learning about space, especially when it comes to pioneering studies of topics that could bring forth answers to some of our most complex problems. As an astronomy geek, I love telescope stargazing and astrophotography– neither of which I actually properly know how to do. Nonetheless, since the Moon is extremely easy to spot, I have attached a picture I took of the moon through my telescope a few days ago using a 25mm lens. Note that clouds are obstructing the view, making the top half of the Moon (in the image) appear differently. On a clear night, the Full Moon becomes much more visible, allowing the observer to hone in on the specific visible features of the surface. This NASA webpage gives a bunch of additional insight on how, when, and what to observe (on) the Moon. Using a smaller lens (10mm, for example) would allow you to zoom-in even closer and much more easily make out the features of the Moon.
Picture taken on an Orion SpaceProbe 130ST Equatorial f/5 Newtonian Reflecting Telescope
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