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physicsmatt
Personal blog · physicsmatt.com/blogscience
physicsmatt is a personal blog at physicsmatt.com about science. Since 2018, 13 different Hacker News readers have linked it 13 times, most often “Why FTL implies time travel (2016)”. It is still writing: the last post was on 2026-09-16, and you can follow it by feed.
Too few sampled comments (11) to say how often HN recommends it.?
Why HN links it
A physicsmatt blog post explaining why faster-than-light travel or communication would break causality and imply time travel under relativity. HN threads on warp drives and FTL cite it as a clear explanation.
Summarised by Claude from a sample of 11 HN comments that link here. Each comment is marked recommending, critical or neutral (citing without judging); how this works.
Most-linked posts?
- Why FTL implies time travel (2016)12 people · 12 mentions · first linked 2018-07-09
What Hacker News says?
One other mystery not mentioned is the problem of fine tuning. The standard model requires certain parameters (like alpha, the fine structure constant) to have their current values accurate to many orders of magnitude for the universe as we know it to exist. There are two philosophical schools of thought about that -- (a) we're in the universe we're in, so by definition it must exist and there's a selection bias the…
Comment linking /blog/2016/11/17/paper-explainer-precision-corrections-to-fine-tuning-in-susy
on “Mysteries the Standard Model can’t explain”Submitted Why FTL implies time travel (2016) as a story · 12 points
HN discussionBecause of relativity. The speed of light is also the speed of causality. Assuming the theory of relativity isn't totally wrong, then if faster-than-light communication is possible, then so must be time travel. http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on ““I survived the TCP transition” (2013)”Submitted Why faster-than-light implies time travel (2016) as a story · 1 points
HN discussionHere's a good explanation of why FTL causes causality issues. http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... The sonar example works only if everything is traveling in roughly the same frame of reference. You're not accounting for relativity and its effects on space and time in your analogy.
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on “Positive Energy Warp Drive from Hidden Geometric Structures”This is the best explanation I've been able to find: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on “Physical Warp Drives”I'm not sure what that means but like the OP, I think any information transfer at FTL breaks causality. ref: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on “A new model for faster-than-light travel that uses conventional physics”Even if it simultaneity is relative, I still don't quite understand why it is a problem. Every local causality is observed. Take this [1] laborous attempt to explain this issue. They explain that 'you can see the message before it was sent'. NO! You can see the message before light from a distant star system delivers you info about it being sent! Like, a bullet arrives to target's head before he has a chance to hear…
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on “Humanly traversable wormholes (2020)”I don’t think there is a true ELI5 answer, at least, I can’t think of one. The basic concept involves general relativity, and while there are ways to simplify it (i.e. no math) it’s still complicated. This does an ok job: https://physics.stackexchange.com/questions/128075/how-would... Better job, but less accessible: https://physics.stackexchange.com/questions/52249/how-does-f... The classic version involving FTL sh…
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on “DIA Study: Warp Drive, Dark Energy, and Manipulation of Extra Dimensions (2010)”In case someone wants to read a really good explanation about this; http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... Generally speaking, all three of FTL (travel/communication), causality and relativity cannot all be true at the same time. If you can FTL-travel/communicate (they are the same really), then you can come up with a scenario in which an observer can see an event happening, and then see the…
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on “Physical Warp Drives (2021)”A few explanations I've collected: https://www.askamathematician.com/2012/07/q-how-does-instant... http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... http://www.theculture.org/rich/sharpblue/archives/000089.htm... The important bit is that it doesn't matter HOW information is transmitted FTL – it can be via wormholes or magic. For observers in another reference frame it will still result in causality vi…
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on “Scientists announce a physical warp drive is possible”This website explains it much better than I can, but the tl;dr explanation is there's no "privileged frame of reference" in relativity. Even if two frames of reference can be arranged to agree on a causal sequence involving FTL phenomena, a third observer can be constructed that perceives the sequence of events happening out of order; they get the light from effect before they get the light from cause. This creates…
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on “Scientists Are Starting to Take Warp Drives Seriously”If you "instantly" teleported to alpha centauri, then there would be a frame of reference in which you arrive before you left. To another observer, you would travel back in time. It's not something you can disagree on. Instead, convince yourself by studying the spacetime diagram: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... Note that "instantly" is ill-defined in SR. Simultaneity of events is obse…
Comment linking /blog/2016/8/25/why-ftl-implies-time-travel
on “Gödel's Solution to Einstein's Field Equations (1949)”
