486 votes
Accepted

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, ...
SRM's user avatar
  • 25.4k
268 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 ...
Henry Taylor's user avatar
  • 69.2k
224 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 ...
John's user avatar
  • 81k
193 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 ...
Cody's user avatar
  • 3,407
187 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 ...
AlexP's user avatar
  • 89.1k
134 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 ...
John's user avatar
  • 81k
132 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 ...
Chris J's user avatar
  • 5,523
132 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 ...
Separatrix's user avatar
  • 118k
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 ...
TheBlackCat's user avatar
  • 3,775
118 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 ...
abestrange's user avatar
  • 5,598
116 votes
Accepted

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 ...
Fayth85's user avatar
  • 6,776
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 ...
user3161729's user avatar
  • 1,309
109 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 ...
Thorne's user avatar
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109 votes
Accepted

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. ...
Mark Olson's user avatar
  • 14.3k
106 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 ...
Trevor's user avatar
  • 6,474
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 ...
Erin Thursby's user avatar
  • 32.1k
104 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 ...
Morris The Cat's user avatar
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 ...
mjt's user avatar
  • 2,695
100 votes
Accepted

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 ...
Shadowzee's user avatar
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98 votes
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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.
Henry Taylor's user avatar
  • 69.2k
97 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 ...
overlord's user avatar
  • 6,262
90 votes
Accepted

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 ...
Youstay Igo's user avatar
  • 20.6k
90 votes
Accepted

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 ...
JDługosz's user avatar
  • 69.4k
89 votes

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

Here's another option that is often forgotten: Time travel might need an exit system available at the destination. Since we don't have an exit system in our time, time travellers cannot visit us. For ...
celtschk's user avatar
  • 31.4k
88 votes

How can I hide my island in the 1600s?

Publish the highest-quality maps for your area (Google maps.) As more people see that your maps are the best, they will come to rely on them. Obviously, leave your island out. Mark safe waterways far ...
knallfrosch's user avatar
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 ...
DWKraus's user avatar
  • 63.6k
83 votes
Accepted

How to find out a year I'm in?

If you travel in the recent past (say, not before 1930): a shortwave radio. Listen for news bulletins. Works everywhere. Time to determine what year you are in: one or two hours, depending on what ...
AlexP's user avatar
  • 89.1k
82 votes

Suitable jobs in Ancient Rome for time travelers

Spice Trader First they pick up lots of spices at their local Mall-Wart. Then they carry them back in time to when they were fantastically valuable. Profit! Traders come from faraway countries and ...
Stig Hemmer's user avatar
81 votes
Accepted

How could we time travel backwards without killing everyone with germs from the future?

I don't think the premise of the question holds water. It seems to assume that pathogens have a single scalar "deadliness" score (like hit points in a game) which is higher for modern germs, such that ...
hmakholm left over Monica's user avatar
79 votes

How can an average person prepare themselves to verify their future time-traveler self's identity?

Predicting Future Passphrases Dialogue: You: How do I know that you're me? Future Self: I've written down a passphrase on this paper. You: I don't have a passphrase. Future Self (hands the paper over):...
Mathaddict's user avatar
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