Why Physicists Think Faster Than Light Travel Is Impossible

Veritasium's Video:

Sabine's Video:

0:00 Can Anything Go Faster than Light?
0:48 Why Faster than Light Travel Breaks Everything
2:59 Einstein's Understanding of Reality
5:09 Ad Segment
6:05 What Happens When You Travel At Light Speed
7:49 Why Information Can't Travel Faster Than Light
10:16 Faster Than Light Is Possible

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[SpaceOddity]

Love the use of the Serenity spaceship 👌

[grayaj]

Sabine's argument isn't just throwing spaghetti at the wall, though. She points out that what happens as something accelerates to c is a singularity in GR. Everywhere else in pop physics, people say singularities just mean you don't really understand what's going on -- like the singularity inside a black hole. Why (she asks) is the singularity at c in GR taken as inescapable but others are presumed to be gaps in understanding? While not compelling of an answer allowing FTL, it's at least a fair question.

[simontanguay]

The santa joke with quantum tunneling said with a perfect straight face. Peak comedy right there.

[kdeuler]

To paraphrase Fermi, if posterity figures out how to travel faster than light, then where are they?

[TerryBollinger]

Ah, you are a bold person! The directional lightspeed situation is delightful since special relativity _requires_ “reciprocal equivalence” (my phrase), meaning that given some number _R, _ the Ratio, there is _no_ local test one can perform to disprove that light travels at _cR_ in one direction — any direction — and _c/R_ in the opposite direction.

For example, if you are talking to someone 1 meter in front of you, you _cannot devise_ any physics test to disprove that light takes one billion years to travel from your face to theirs, yet is, for all practical purposes, instantaneous in the opposite direction. It cannot be genuinely instantaneous since it must be a reciprocal of the other speed of light for the math to work out correctly. But you can get so close that it makes no practical difference.

What’s extraordinary is that this is not just some mathematical abstraction but an absolute requirement of how special relativity works. Even though the tests have no meaning to you, there could be an observer for whom they represent very real delays, in which time for you is asynchronous to an extraordinary degree.

I’m working on an Apabistia Note called _Einstein’s Unfinished Homework_ that starts with the same lighting and train example. The part Einstein never finished was how to _calculate_ the delay in the flashes of lightning. The missing math is to multiply the signed distance between flashes by an “age gradient” alpha, which is:

α = -βγ/c

For a train 0.3 km long traveling at 0.6 c with flashes that appear synchronous _inside_ the train, anyone on the embankment sees the back flash occurring 0.6 microseconds before the front flash — that is, the back always appears _older_ than the front.

Einstein never finished his thought problem by writing down the equation because the concept of an _internally asynchronous_ object went a bit too far. If you follow that idea to its logical conclusion, the very concept of an object breaks down into a sequence of probabilistically linked but otherwise isolated spacetime events. That’s going a bit too far from the classical viewpoint for most people, even now, let alone a century ago.

Object-ness is one of our most fundamental assumptions about the nature of the universe. It is also, alas, flatly incorrect. There are _only_ spacetime events, and the only self-consistent, non-paradoxical separation metrics possible are the ones that separate such spacetime events, not objects. We humans, in particular, love to string events together in ways that create what we think of as objects, but that’s not the deeper fabric.



Regarding going faster than light using quantum mechanics, I remember that tunneling issue from Sabine’s video. I believe I both commented on it and emailed her directly.

The correct resolution is oddly simple: While the tunneling is, indeed, “instantaneous” (wow, that word _so_ oversimplifies it), the spread of the Schrödinger wave that enables the tunning is bound by the speed of light. Without that wavefront first passing by, there is no tunneling and, thus, no dissemination of information faster than lightspeed. Any mathematical analysis more complicated than that is mostly noise.

In other words, you can’t send information faster than the speed of light, not because tunneling is too slow, but because the Schrödinger wave expansion is too slow.

Finite Schrödinger wave expansion brings up another rather critical issue, which is that quantum mechanics is _no more fundamental than classical mechanics, _ or for that matter, special relativity. All three are pieces of a single fabric, all three are emergent, and all three have limitations.

The critical limitation in the case of quantum mechanics is the non-existence of Hilbert space… which, unfortunately, is also the foundation of most modern quantum mechanics.

Hilbert space simplifies quantum states by assuming they are preexisting and infinitely precise. That’s nonsense since nothing in the physical world works that way. Schrödinger waves are laboratory phenomena, and all observations of them, including in commercially available quantum encryption devices, require them to start at a precise location in classical space and then spread at no faster than the speed of light. That critical limitation time-spread limitation _cannot_ be adequately represented using only a Hilbert space.

Thus, laboratory-consistent Schrödinger waves are never infinite in scope and always require a finite time to traverse space and become “more” perfect. That is the core of why quantum mechanics cannot be the deeper infrastructure of the universe.

More later, maybe, I must leave for the airport early tomorrow.


(Two PDFs of this 2023-06-01 comment are available at sarxiv dot org slash apa)

[DavidHunter]

Would be funny if at some point in the future we discover that Einstein wasn’t quite right. Just like Newtons laws seemed perfectly adequate, we realised eventually that they weren’t. A good approximation and useful for many calculations, but ultimately not the actual truth of reality. Wonder if einsteins laws will ever see the same moment. I wouldn’t be surprised if they did.

[pallharaldsson]

4:44 "paradox .. but what's not counter-intuitive is making sense of Investments". On the sponsor then mentioned Masterworks and not needing millions of dollars to buy art, that seems more counter-intuitive to me. Art prices are crazy high because rich people want their art in the home (and claim ownership). But to own a "part" of it, i.e. just a record in a blockchain stating that, and never having it seems to me counter-intuitive, why people do buy and can't even have the real art shown. I realize you need Sponsors, but it seems like a scam (by them) to inflate prices even further.

[JHandle]

9:21 All of these Minkowski diagrams being shown are vastly wrong even from just a cursory inspection. Crucially, each observer disagrees on whether each message is instantaneous or traveling backwards in time. If both messages are instantaneous from the perspective of Earth, and not for the ship, then there's no time paradox. And if both messages are instantaneous from the perspective of the ship, again, no paradox. It's only when you bizarrely decide to have one message be instantaneous for Earth and the other for the ship that suddenly we have a time paradox.

The reason for this error is the desire to maintain symmetry between the experiences of the Earth and the ship. But FTL communication by its very nature breaks symmetry. It doesn't break causality, just symmetry. Now, many physicists seem to think this is equally impossible, but that is only if there is no aether. Reintroduce the aether into relativity, and it decides which observer has the _correct_ frame of reference. If the aether is stationary with respect to Earth, then Earth will see the messages as instantaneous, and the ship will think it is communicating with the future Earth (but be unable to communicate with what it considers to be the present Earth). If the aether moves with the ship, then vice versa. And if the aether moves independently of the Earth and ship, then the Earth and ship can determine the velocity of the aether by observing the difference between what they would consider instantaneous communication and the actual communication they get.

"But, " you say, "the aether was disproved by the MM experiment!" Actually, the MM experiment never showed the aether to not exist; that was simply the incorrect interpretation that many took from the results, and now it is taken for granted that there is no aether. In reality, the experiment simply showed the aether to be immeasurable. And if it's immeasurable, it may as well be not come into play in any future discussions or mathematical equations. So people forgot about it. But there's one way that the aether COULD be measurable, and it's if we had FTL communication. So any discussion about potential FTL communication is also a discussion about a potential aether (in my opinion, the aether is already necessary just to justify absolute acceleration, but many fools seem to think that absolute acceleration can also be taken for granted without any care in the world for the fact that fundamental particles don't have accelerometers and have no absolute notion of acceleration, only relative acceleration, but we can just brush that under the rug, right?). A universe with positive or negative spatial curvature (like a sphere or a saddle) would also permit measurement of the aether's velocity even without FTL communication (if you want an explanation why, I can do it with a variation on the twin paradox thought experiment, but I don't feel like writing it out right away), though every observation we've ever made on that front has indicated the universe is flat, so that appears to be a fun fact, but a dead end.

Once we revive the aether, the speed of light is no longer a constant for all observers (I mean it kind of is, but if you have a ship moving at 90% c relative to the aether, then light really will only move a bit faster than that ship in the same direction). Instead, the speed of light can be measured by using the instant FTL communication method as a measuring stick. Now of course, because everything we see, touch, and sense is electromagnetic (i.e. light-based) in nature, when we measure the speed of light using normal clocks and measuring sticks in any reference frame, we will always measure a constant speed of light as has been shown by MM. But if we use an FTL clock or measuring stick (doesn't really make sense for instant FTL, so let's make it near-instant FTL instead) as the standard, then we _will_ measure the speed of light changing under different contexts.

With an aether, gravity stops being spacetime curvature (there is no such thing as spacetime either) and instead becomes a local reduction in the speed of light. In other words, gravity becomes fancy refraction. It still affects particles because particles are electromagnetic in nature, even neutral particles.

[timbehrens]

Einstein reasoning for GR wasn't based on the Michelson Morley experiment. In fact, he knew nothing about it before developing the GR. Not knowing what shows how shallow is your understanding of GR is.

[patrickmchargue]

The light pulse traveling between two mirrors, and what two observers see as the light reflects back and forth is probably the best visualization of why time slows for a moving observer that I have ever seen. Thanks.

[JonathanJollimore-wv]

I think we're trying to measure a speck of dust with the bathroom scale.

[cohomologygroup]

My favorite "faster"-than-light travel is how the cosmological horizon exists because *space* is moving faster than light, and that's allowed.

[khatharrmalkavian]

This is a pet peeve of mine. "It breaks causality therefore it's impossible, " is non-sequitur. It is is not a valid proof that travel/communication faster than light is impossible. It doesn't even imply that it's impossible. It only suggests that it's complicated. The universe is not designed to make sense to us, we are made to try and make sense of the universe of our ancestors.

[theoptimisticskeptic]

Too funny, I just watched Sabine's video and just happened to come across your video right afterwards. Both great videos! Thank you!

[ke_yhwINS]

what thinkable is possible... light cant be the ultimate... what ever you think to.... us
the ultimate reason for everthing... is...?
go back to the first condition... GOOD LUCK

[jnhrtmn]

The light source and the detector were in the same lab, and the medium is OBVIOUSLY massless (a foreign concept), so there are no points with which to define a velocity. Each and every velocity is infinitely relative which is exactly like saying that there is NO DEFINITION for any velocity. Jumping from what I describe to light being constant per the observer is such a ridiculously HUGE leap of faith that I cannot believe people actually believe it, but then again, physics is 100% math with rote memory dependent fools everywhere. Accurate or not, AND AFTER a Doppler effect observed BEFORE the transform equations, Relativity must CHANGE your dimensions in order to CREATE itself which is also saying that there are at least two theories, the one that has you change your data, and the other where you do NOT change your data. Relativity is not fixing something broken, and the NON-transformed reality works just fine without your new numbers. Show me dimensions 1 and 2 outside of math, then send a pendulum into space and explain how time is relative to its velocity. It falls apart fairly easily if you try.

[Izwat]

A: NASA's Juno spacecraft is the fastest man made object ever recorded, at roughly 365, 000 km/h (165, 000 mph) as it approached Jupiter. The fastest launch velocity belongs to New Horizons, which went 58, 000 km/h (36, 000 mph).14 Mar 2017 and Light from a stationary source travels at 300, 000 km/sec (186, 000 miles/sec). so we far away dude stop

[GoingToBeWild]

The measurement of the passage of time for observers like us is dependent on relativity, however it appears the universe itself does not require relativistic comparisons to have a local rate of time change, so the concept of relativity is a result of our inability to conceive of rates of change with respect to nothing, I think this is evidence that time is an emergent property

[georgeshapovalov]

Sigh, the magic, ftl tunneling, "grand unification" etc, yup, gives rise to these funny ideas. But this is because quantum physics/info is severely misunderstood. And we have the present formalist to blame for it - that "wavefunction of the universe" (as a generalization of collapsed and non-collapsed regions, etc). It gives rise that that idea of all-permeating, omniscient entity that knows all is everywhere at once and we just have to find a better way to deal with it. Nope, QI/QP is all about *absence* of information. Its literally the "lazy calc" - the state of the particle is not decided *until it is needed*. That is, until interaction happens, if it happens to be a "classical one" then we have a "collapse", which is simply a computation of the value finally taking place. Anyone who has done any non-trivial computation (the backend coding) or even any nontrivial backend programming that would require optimization of the process should understand it implicitly..

[donkeyhobo]

I don't doubt how physics works but my question is, don't you just have to know how to take control over it? Like we could have known how flying works and how animals fly but what if we never tried to fly ourselves because we couldn't do it naturally? I think we have to learn how to unnaturally take control of physics in order to change it