Why Going Faster-Than-Light Leads to Time Paradoxes

Is faster-than-light (FTL) travel possible? In most discussions of this, we get hung up on the physics of particular ideas, such as wormholes or warp drives. But today, we take a more zoomed out approach that addresses all FTL propulsion - as well as FTL messaging. Because it turns out that they all allow for time travel. Join us today as we explore why this is so and the profound consequences that ensue.

THANK-YOU to our supporters D. Smith, M. Sloan, C. Bottaccini, D. Daughaday, A. Jones, S. Brownlee, N. Kildal, Z. Star, E. West, T. Zajonc, C. Wolfred, L. Skov, G. Benson, A. De Vaal, M. Elliott, B. Daniluk, M. Forbes, S. Vystoropskyi, S. Lee, Z. Danielson, C. Fitzgerald, C. Souter, M. Gillette, T. Jeffcoat, H. Jensen, J. Rockett, N. Fredrickson, D. Holland, E. Hanway, D. Murphree, S. Hannum, T. Donkin, K. Myers, A. Schoen, K. Dabrowski, J. Black, R. Ramezankhani, J. Armstrong, K. Weber, S. Marks, L. Robinson, F. Van Exter, S. Roulier, B. Smith, P. Masterson, R. Sievers, G. Canterbury, J. Kill, J. Cassese, J. Kruger, S. Way, P. Finch, S. Applegate, L. Watson, T. Wheeler, E. Zahnle, N. Gebben, J. Bergman, E. Dessoi, J. Alexander, C. Macdonald, M. Hedlund, P. Kaup, C. Hays, S. Krasner, W. Evans, J. Curtin, J. Sturm, RAND Corp, T. Kordell, T. Ljungberg & M. Janke.

::References::

::Music::
► Joachim Heinrich - Stjärna
► Falls - Life In Binary
► Chris Zabriskie - Cylinder Two
► Chris Zabriskie - Cylinder Five
► Chris Zabriskie - Cylinder Four
► Falls - Ripley
► Caleb Etheridge - Always Dreaming
► Joachim Heinrich - Y
► Indive - Trace Correction

::Film/TV clips used::
► Interstellar (2014) Paramount Pictures
► Contact (1997) Warnes Bros.
► The Imitation Game (2014) The Weinstein Company
► Star Trek: Generations (1994) Paramount Pictures
► Cosmos: Possible Worlds (2020) National Geographic Fox
► Stephen Hawking's Favorite Places (2016) Curiosity Stream
► Star Wars: The Empire Strikes Back (1980) 20th Century Fox
► Star Trek: Deep Space Nine (1993-1999) Paramount Television
► The Day The Earth Stood Still (2008) 20th Century Fox
► Stargate (1994) MGM/UA Distribution Co.
► Star Trek: Beyond (2016) Paramount Pictures
► Star Trek IV: The Voyage Home (1986) Paramount Pictures
► The Expanse (2015-2022) Legendary Television Distribution
► Star Trek: The Next Generation (1987-1994) Paramount Television
► Avengers: Endgame (2019) Walt Disney Studios Motion Pictures
► Star Trek: Enterprise (2001-2005) Paramount Television
► Back To The Future (1985) Universal Pictures
► Passengers (2016) Sony Pictures Releasing
► The Time Machine (Dreamworks)
► Alien: Covenant (2017) 20th Century Fox
► Star Trek (1966-1969) Paramount Television
► Tenet (2020) Warner Bros. Pictures
► Loki (2021-) Marvel Studios
► Somewhere In Time (1980) Universal Pictures
► The Assassination of Jesse James by the Coward Robert Ford (2007) Warner Bros. Pictures
► Star Trek: Into Darkness (2013) Paramount Pictures
► Logan (2017) 20th Century Fox
► Star Trek: First Contact (1996) Paramount Pictures

::Subtitles::
► German by Frischholz Christian

::Chapters::
00:00 Introduction
06:52 Space Time Diagrams
12:51 Causality Violations
17:01 Paradoxes in Time
24:28 Outro and Credits

#FasterThanLight #Alcubierre #CoolWorlds

[realbrickbread]

I like to imagine that as soon as we discover FTL and break spacetime, the simulation crashes and the higher beings are annoyed they have another bug to fix

[bachelorchownowwithflavor]

"The universe is not required to be in perfect harmony with human ambition." -Carl Sagan

[nativesugarshack]

It's very rare and special when someone can explain such a complicated topic in an easy to understand and follow way. Thank you.

[MHGFTW]

I watched the entire video, and I get the sense you broke the rules of your own diagram. You drew two diagonal lines at the start, indicating the speed of light. You set the premise that any line between the time axis and this diagonal line is something moving slower than light (STL). Any line between the diagonal line and the space axis are faster than light (FTL). You then start speculating about FTL transmitters able to send a message instantaneously, meaning horizontally to the right. I'm with you so far. This is where you break every rule that has been set. You draw a line that goes downward. We've not established any reason why a line would do that. Your made up transmitter can only go up to a limit of a horizontal line (instantaneous messaging). These paradoxes start popping up because you've suddenly decided this message is being sent to the past. The FTL transmitter is now a time machine for no reason.
Like I said there might be some physics I don't understand. However in the diagram you've been using, there has been zero explanation of why any line would ever go down. The established rules were that lines could only be in the STL or FTL directions. Once you start going downward you break your own rule set.

[krishp]

Does anyone realize how crazy it is that we can even theorize something like this

[JACK_TheAllSeeingEye]

There once was a young lad named Mike.
Who could run faster than the speed of light.
He took off one day,
In a relative way,
And returned on the previous night!

[yapdog]

The universe doesn't care if it makes sense to us.

[AlmightyDude]

The greatest explanation so far. I've been trying for months to understand this by reading forums and watching youtube videos. None were able to explain this. I still need to understand it more intuitively instead of through looking at lines, but this definitely opened my eyes.
This is by far the best and most simply explained video so far on this. Thank you

[mwzorzest]

Well, I don't know about time travel, but a fascinating thing to think about is that if you could travel FTL and have a way to look at earth from any distance, you could go far enough away from earth to see the past.

[shinysheepy]

"It almost seems like a cruel cosmic joke that the universe be arranged such a way that we are able to look out and glimpse its wonders and yet are trapped here by the speed of light." So well said

[CalamitousJonathan]

I enjoy that you are actually including the possibility of breaking the universe in this video. That is why we stopped increasing speed, in my warp engine space ship.

[joshuagharis]

Revisting this. Professor, I just saw an article featuring your terrascope idea, featuring you, of course 😊. I was so excited for you

[tayzonday]

The speed of light should really be taught as the speed of causality. The speed of light is an incidental stenographer of the speed of causality.

[yufansa]

"The cosmic speed limit, the speed of light is so diminutive compared to a galaxy. It almost seems like a cruel cosmic joke that the universe be arranged in such a way that we are able to look out and glimpse its wonders and yet are trapped here by the speed of light. Fated to only ever peer through the bars of our cosmic prison and dream." -- This is so beautifully spoken...

[matzefly]

This video is amazing. Thank you for this simple explanation of such complex issues.
I have discussed your diagrams with a couple of colleagues last night and we had an amazing time 😊

[davidschaftenaar]

I'm probably just not understanding this properly, but: Shouldn't the reply from the STL ship _always_ travel *up* every possible Worldline, no matter how quickly it gets back to Earth? Drawing the reply the way you're drawing it, means that _all_ replies back to Earth (even STL ones) will break causality, because you're sending them backwards through time rather than forwards. While Earth, the ship and Vega are all moving through time at different rates, time is still passing for all three; In your example, you're drawing the reply line back to Earth as though Earth were stationary in time from the moment it sent it's message.

[zadrik]

I like how Terry Pratchett said it better than the way Hawking did: Everything that happens, stays happened.

[caitlyn]

Omg this diagram finally explained why and how space and time are connected. Thank you so much. I always thought I understood this concept but I was just repeating info not fully grasping it.

[KrazyTrumpeter]

My mind is blown. When you switch the communications to "instantaneous" and re-explained the diagram, everything started making sense. I have had so much trouble trying to conceptually understand these potential issues with FTL travel/communication and this has helped so much.

Still hope we figure out a way around it, though! Like you said, it's criminal to be able to see all this cool stuff out there and not actually be able to go see it ourselves.

[rajm]

This is perhaps one of the best explanations I’ve seen so far! Visual representation of this concept at its best!