478 votes
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Is there a single historic event which could be changed to cause the USA to use SI units?

Stop one ship from either sailing into a storm or (later) being taken over by pirates. In 1793, Thomas Jefferson ordered a set of instruments from France that would let the US calibrate to SI units, ...
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467 votes

How do you prove you're from the future?

I'm going to suggest something rather different, given that he's from not far (2100) in the future. All he will need to bring back is the names of his parents/grandparents. With DNA testing of them ...
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  • 4,490
263 votes
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If time travel is possible in the future, no matter how distant, why haven't they come back to tell us?

In all probability, we, 21st century barbarians, will not be very well loved by our 23rd century descendants. Non-historians of that age will probably group us together with the generations of the ...
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  • 68.3k
220 votes
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What cheap modern items can I use to bribe medieval people?

Rubber No seriously a cheap waterproof rain slicker or pair of rubber boots will bribe the average person very easily, waterproof materials were nonexistent. It is also really easy to demonstrate and ...
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  • 69.2k
202 votes
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How do you prove you're from the future?

He should predict Solar Weather and/or Solar Events. Predicting Earth weather is a complex process, and he's introduced a new variable - himself. And not that he'll have a huge effect, but you never ...
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187 votes

How do you prove you're from the future?

There is one more very convincing thing he can bring from the future: actual copies of items from the present. If someone came up with an aged Mona Lisa, the bones of Barrack Obama and the Tiffany ...
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  • 1,980
186 votes

If time travel is possible in the future, no matter how distant, why haven't they come back to tell us?

In the film Primer, there was a time travel machine that could only jump as far back as when it was first turned on. This puts a practical limit to how far back you can jump to the point where time ...
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  • 3,337
185 votes

Modern warfare theory in a medieval setting

The question shows a misunderstanding of why the Middle Ages were medieval. It's not that the people who lived in those times did not know any better. They did. In particular, they had good Roman ...
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  • 73k
132 votes

How could a time traveler have prevented climate change

Go back to the cold war era and start a "Russians are trying to warm the planet" scare. You will need a lot of money to fund some big advertising campaigns. You also want to seed a few specific ...
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131 votes
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Plausible reason why my time machine can only go back a certain amount of time?

There's an old fashioned option You can't travel back in time to a point before the invention of the first time machine. That means of course that the first person to invent a time machine couldn't ...
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  • 114k
129 votes

If time travel is possible in the future, no matter how distant, why haven't they come back to tell us?

Because the earth orbits the sun at 30km a second, the solar system is moving at 20km a second and the galaxy is moving at 230km a second! Any time travel will result in the people being dumped out ...
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  • 5,443
118 votes

What cheap modern items can I use to bribe medieval people?

There are a few options: Aluminum. Until modern times, it was essentially impossible to produce in any significant quantity, making it far, far, far more valuable than gold. It is also a useful ...
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  • 3,725
117 votes

What steps are necessary to read a Modern SSD in Medieval Europe?

If they are just bringing the drives, and not the attached computer, this is pretty much impossible. The infrastructure simple isn't there in 1300 to refine anything to the necessary purities to even ...
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  • 5,542
116 votes
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How far back in time would English be understandable to a modern speaker?

It's a lot more complicated than it sounds. Not just for the fact that the language changed considerably over time, but because it is highly dependent on in which circles you commune. The higher the ...
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  • 6,736
114 votes

How do you prove you're from the future?

The critics provided the proof themselves Before he travels back in time, he uses his machine to send along a "parcel of proof" to some point in time after his own arrival. He can then predict that ...
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  • 5,667
112 votes

Plausible reason why my time machine can only go back a certain amount of time?

A previous time machine crashed and exploded on August 12th 1941 during a test flight. Maybe sabotage or a design fault. The explosion and shockwave will exist forever at that point in time. The ...
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  • 1,309
107 votes

How would you bring wealth back to the past?

Knowledge. Bringing wealth to the past requires simply knowledge. You have the outcome of horse races, lotteries, inventions, share prices, stock market falls etc etc etc. Your head could easily ...
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106 votes
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My time machine needs to know when it is

If you can deploy a good quality, wide-angle telescope and take photos of the night sky over a period of more than a few days, you can determine the time down to seconds over a wide range of history. ...
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105 votes

How would people in 17th century Salem react to modern day clothing

Zippers! Zippers have not yet been invented. Do keep that in mind. If there are any obvious zippers, this will be a source of wonderment. People were a lot more sharp eyed than you think back then, so ...
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104 votes

If 120 experts in 12 different fields were sent back 10,000 years, could they recreate the 21st century in 100 years?

You can't get anywhere close to that technology level. The best you could do is work to increase their life expectancy. Teach them farming. Teach them how to get water from a river into their ...
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  • 6,244
101 votes

How would a preemptive crime fighting group prove they stopped a criminal?

Arrest that newborn baby? I think you mean give that newborn baby a college scholarship. If the AI so so omniscient it can predict crimes decades in advance, even knowing the rippling effects of the ...
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  • 2,685
100 votes
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Why buy tech from the future?

Yes, 100% Yes Just because the stranger doesn't know how to build it or how it was manufactured doesn't mean that you can't reverse engineer it. With 10 devices, you even have some spares so you can ...
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  • 15.1k
100 votes

Modern warfare theory in a medieval setting

Honestly the 'modern' military theory that would give Lord Gary the biggest advantage would be modern attitudes towards hygiene. It was extremely common in the time period you're talking about for a ...
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98 votes
Accepted

I'm on the Titanic, what should I do to survive?

Befriend and hang out with fellow passenger, Margaret (Molly/Maggie) Brown then follow her to lifeboat #6 which historically will have available seats.
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96 votes
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Would relatively primitive people really confuse technology with magic?

As far as I can determine, the closest we have to hard data on this (which is not that close) would be first contact with people who have been isolated for a long time and have not developed or used ...
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  • 1,547
94 votes
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What would be the impact of a modern programmer and laptop being dropped into World War II, possibly breaking Enigma?

The biggest advantage I see being given here is simply processing power. You have a machine that can run calculations at a speed that would beggar imagination for someone living in that era. In ...
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93 votes

If 120 experts in 12 different fields were sent back 10,000 years, could they recreate the 21st century in 100 years?

No. But I'm answering this from a different perspective: world population. 10,000 years ago, the world population was around 10-20 million. And this population was spread across the globe. Your ...
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  • 6,112
89 votes
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If I went back in time, how could I handle being among the Black Death and survive?

If you have European ancestry, are aged between 19 and 45, physically fit, and are able to maintain a healthy diet, the chances of you being brought down by the plague would be very little. Read this ...
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88 votes
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Are there any ways to allow some form of FTL travel without allowing time travel?

As you said, any FTL drive is implicitly a time machine. Let’s review the reasons why. As a prerequisite you need to understand spacetime and the ideas of space-like and time-like intervals. first ...
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  • 68.5k
87 votes

Why can't modern fighter aircraft shoot down second world war bombers?

They Could, But They Won't: Sorry to say, I don't think that anything short of time travel handwavium will get your planes invisible to modern air defense systems. BUT, if you're willing to overlook ...
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