Originally published at: http://boingboing.net/2016/09/15/adam-samberg-asks-neil-degrass.html
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What happened to the “time travel” answer?
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Ok, Samberg not Sandler. [wipes sweaty forehead]
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The theory of relativity states that there can be no effect without a cause.
For that reason, it’s suggested that time travel to the past is impossible because the effect (you showing up in the past) would not have a cause (you left a future that hasn’t happened yet).
Time travel to the future, as a one way trip, could be possible, however. As you approach the speed of light, time slows down. So if you traveled in a vehicle at near light speeds, several years could go by outside your vehicle while you only experience a few seconds passing. This way you could see the future while you were still young, but you wouldn’t be able to tell the people of your past about what you saw.
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Adam Samberg clearly enjoys listening to Richard Herring’s Leicester Square Podcast.
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Of course time travel is possible! I’m going to travel to the future RIGHT NOW.
(whistles tunelessly while looking at my watch for ten seconds)
Whoa! Here I am in the future. Aw man, Trump’s still around? Dang.
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I have issues with it in either direction… simply because of the law of conservation of energy.
At this moment there is a finite amount of matter in the universe: X.
I contain a certain subset of that X matter: Y.
If I travel fwd or back to a time during my own lifetime, where does the duplicate matter (X +Y) to make up a second me come from? Or do I replace myself? If I replace myself, how is that matter transferred to my new location?
In either case for the last point, what about if I travel fwd or back beyond my lifetime? The matter making up me right now was once (and eventually will be) a part of something else. If I suddenly show up there, where does the matter to make up this sudden instance of me come from? It can’t just be conjured (per the law above) so would it suddenly be stolen from the locations it existed in as a part of the universe where I did not exist prior to my time-traveled arrival?
In any case, we cannot put more matter into the universe than already exists (X), so suddenly duplicating the matter that you consist of seems to violate basic laws of physics.
And as for rearward time travel specifically… I have a simple point as to why I think we can safely say it will never be invented. Because if it ever were, every poignant moment in history would be inundated with crowds of sightseers from every moment in the future. Think about it… if you could, you would go back to witness the big mysteries/events of history.
There would be no room on the grassy knoll for all the people looking to solve that one. That moment exists as one day in all of history, but every person with access to rearward time travel for all eternity would be looking to visit that specific moment. They would all be there at once. But they weren’t… so that means they will never be able to… or else many many many already would have and would have been there already on 11.22.63. 
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Who does Samberg think he is, the keeper of the Bridge of Death?
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He answered it and gave the game away. So somebody had to come back to erase it.
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Does it? I know causality is a pretty big deal for physics as a whole, but I didn’t know it was specifically part of relativity.
General relativity allows for “closed timelike curves”, which on the face of it would permit travel into the past. This has caused physicists some consternation; possible solutions include Hawking’s Chronological Protection Conjecture (the laws of physics forbid time travel into the past) and the Novikov Self-Consistency Principle (the probability of inconsistent events occurring is zero).
Hawking said something similar in A Brief History of Time:
If time travel is possible, where are the tourists from the future?
But pretty much every theoretically possible time machine ever described seems to have the limitation that you can’t travel further back in time than the creation of the machine.* If you built one today, future generations would be able to travel back to 15 September 2016, but no earlier. Hence why no time tourists … yet.
* Also the limitation of being massively impractical, like requiring a black hole or similar spun into an infinitely long cylinder.
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Were you perched on an anti-gravity device?
Neil rocks! Second major humanistic crush today…
Adam Samberg? Of the the group The Lonesome Islet? I love that guy! That song I’m On A Ship was awesome.
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Wow, Youtube has stopped telling people that the video is not available due to geoblocking, it’s just ‘Not available’ now.
He answered that question long before it was asked.
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Instead of tunelessly you could whistle this:
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This is only an issue if you assume the universe is a given set of matter upon which time as an outside force acts. With a space-time model in which time is merely a fourth dimension to the universe then the conservation of energy is not violated by moving matter forward or backward in any direction of time any more than rotating a cube so that you can only see one face destroys all other faces. The matter still exists in the fourth dimensional universe, it is simply in a different position that can only be perceived when we change our viewpoint.
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