The horizon problem for faster than light travel

https://eriklentzphd.blogspot.com/2022/01/the-horizon-problem-for-faster-than.html

by hhs • 5 years ago

44 72 5 years ago

eriklentzphd.blogspot.com

awb• 5 years ago

But what is the horizon problem? Is it some single well-defined concept or a catch-all term for many phenomena?

Granted this article is largely above my reading/comprehensive level, but after describing multiple types of horizons and then saying a wrap drive horizon largely resembles a black hole horizon, I can't see where the author actually defines "the horizon problem".

hhs• 5 years ago

“the horizon problem”

It looks like the author was referring to this: “In relativity theory, a horizon is also a boundary on the region which an observer can see. Horizons pose an issue for warp drives as one cannot control the warp bubble if one see or communicate with it.”

And for more context, the author seems to note that faster than light travel is possible under Einstein’s physics: https://www.sciencealert.com/faster-than-light-travel-is-pos...

simonh• 5 years ago

Sure, but only for certain uses of the term ‘possible’. There are solutions to Einstein’s equations that imply faster than light travel or communication, but that doesn’t mean such solutions correspond to physical reality. For example such solutions can include terms in the equations that correspond to negative energy densities, or infinite masses, or trajectories around infinitely extended one dimensional massive objects. Such solutions somewhat stretch the meaning of the word ‘possible’.

slowmovintarget• 5 years ago

The quoted sentence has dropped the word "can't" I suspect.

Reworded: "Horizons pose an issue for warp drives as one cannot control the warp bubble if one can't see or communicate with it."

The problem, then, is that warp drives as posited create KVF horizons that would seem to make them impossible to control, enter, or leave. That would mean warp drive is not permitted in the universe.

Some KVF horizons, like the outer ergosphere of a black hole can be detected, entered, and exited. Conclusion, warp drives are not ruled out on the grounds that they involve horizons because we don't know what kind of horizon they might have.

tl;dr Warp drives aren't ruled out... yet.

pdonis4• 5 years ago

the author seems to note that faster than light travel is possible under Einstein’s physics

He's talking about a highly speculative form of "warp drive" he has proposed that supposedly does not require negative energy, as all previous "warp drive" solutions (such as the Alcubierre drive) did. His speculations have not been accepted as valid by other physicists working in the field. (Also, calling such travel "faster than light" is problematic, since a light beam emitted from inside a "warp bubble" would still outrun the bubble itself. These are really just highly counterintuitive spacetime geometries that are most likely not physically possible anyway because the kinds of materials that would be needed to create them don't exist and can't be manufactured.)

stevebmark2• 5 years ago

Faster than light travel and time travel are both fundamentally impossible in our universe and can't be achieved. I'm sure we'll learn all sorts of cool things looking for ways in. The reality is these will sadly live only in science fiction. There are plenty of things that are fundamentally impossible in our universe, faster than light travel being one of them.

ctoth• 5 years ago

"Heavier-than-air flying machines are impossible." -- Lord William Thomson Kelvin, British mathematician and physicist, president of the British Royal Society, 1895.

Edit: See https://news.ycombinator.com/item?id=27816225 for a discussion of the paper that launched this whole topic.

Edit 2: The actual paper you want is this one by the author of OP, the first one is referring back to this: https://arxiv.org/abs/2006.07125

stevebmark2• 5 years ago

"Inertia is a property of matter" - fundamental unchangeable property of the universe

heavenlyblue• 5 years ago

What about perpetual motion machines though? :)

BurningFrog• 5 years ago

Scientifically speaking, the fact that someone was wrong in the past, doesn't mean someone else is wrong now, even if they use a similar sentence structure.

PS Yes, I am fun at parties, if I ever get invited to one again!

klyrs• 5 years ago

Which ignored the obvious and prevalent heavier-than-air flying birds. We have no such examples to point at with FTL travel.

Datenstrom• 5 years ago

I am not familiar with this field but extraordinary claims require extraordinary evidence and I am not aware of any evidence absolutely ruling out the possibility. From my understanding we are still unsure about very fundamental aspects of physics and the universe so there are certainly unknown unknowns left which could dramatically change our understanding of everything.

munchler• 5 years ago

extraordinary claims require extraordinary evidence

I think you've got the sign on your skepticism reversed. The burden of proof is very much on those who claim that FTL travel is possible.

From Wikipedia:

    The special theory of relativity implies that only particles with zero rest mass (i.e., photons) may travel at the speed of light, and that nothing may travel faster.

    As of the 21st century, according to current scientific theories, matter is required to travel at slower-than-light (also STL or subluminal) speed with respect to the locally distorted spacetime region. Apparent FTL is not excluded by general relativity; however, any apparent FTL physical plausibility is currently speculative.

See https://en.wikipedia.org/wiki/Faster-than-light

roywiggins• 5 years ago

"FTL is impossible" doesn't seem like an extraordinary claim at all, but actually quite an ordinary one. The natural world seems to obey a light-speed limit every time it's been tested.

Sure, it might be wrong, but as far as claims go it has a lot going for it, both experimentally and theoretically.

psyc2• 5 years ago

After developing some intuition for relativity you realize that asking whether FTL is possible is more akin to asking whether you can remain inside a circle after traveling radius + 1 from the center, than it is to asking whether a car can be modded to exceed its top speed. It is nonsensical, not logically coherent within all currently known physics and it goes to the fundamental nature of motion itself.

It can't be ruled out, but neither can Russell's Teapot.

madaxe_again• 5 years ago

Plenty of current physics allows for the possibility - even special relativity allows for it.

Personally, I think that the “speed of light” is a misleading misnomer. It’s the speed of information propagation in flat space - and even that’s a misnomer, as speed is a very human concept, as is time, as is space.

For all we know we’re waddling around in flatland, thinking that what we perceive is what is. More likely than not, most everything we know isn’t even wrong.

To borrow Haldane - “Nature is not only queerer than we suppose, it is queerer than we can suppose.”

teaearlgraycold2• 5 years ago

Not sure if it matters too much from an individualistic perspective. Near light-speed travel will allow a person to go anywhere instantaneously, they'd just have to contend with leaving the time and space they know behind them.

amelius• 5 years ago

Near light-speed travel will allow a person to go anywhere instantaneously

Only if you ignore the enormous acceleration/deceleration forces needed for this.

mulmen• 5 years ago

Only for sufficiently small values of anywhere. Even near-light-speed communication in fiber optics has detectable and significant delay just in crossing a continent. A trip made physically by humans every day.

Near-light-speed travel to the Moon would take a few seconds. Near-light-speed travel to Mars would take longer than my drive to the grocery store. These are not "instantaneously."

yumraj• 5 years ago

Faster than light travel and time travel are both fundamentally impossible in our universe and can't be achieved.

That may very well be, and true in our lifetime, but given how little we know about a lot of things, and that in many cases we don't even know what we don't know, I would hesitate in making an absolute statement.

willis936• 5 years ago

It's been proven rather thoroughly though. Look at Kip Thorne's body of work. His book: Black Holes and Time Warps is accessible.

c0 is the speed of light only incidentally. It is the speed of causality first. You can warp spacetime but you can't change the speed limit.

You might think there would be a hypothetical way to cheat it from some unknown phenomenon, but then any occurrence of that phenomenon would destroy itself in zero time.

senectus12• 5 years ago

Doesn't the concept of Wormholes, folding space or Warp engines bypass FTL blocks?

You're essentially relocating from one location to another by bypassing the distance... Leaving this universe long enough to travel a comparatively smaller distance/time in another then pop back into our universe a long long way but short amount of local time later.

Sure you dont travel FTL but you do arrive in the location Faster than light would take to travel in mundane space.

mannerheim6• 5 years ago

I believe all of those violate causality, at least if you have more than one of them (more than one pair in the case of wormholes).

kingcharles• 5 years ago

Well, technically time travel forward is easy.

And to say that backwards time travel isn't possible at this point doesn't make much sense. We don't even have a clear understanding of what time is, or why it goes forwards.

And the singularity should be around the corner, and that might present a computerized solution to emulate reverse time travel.

woah• 5 years ago

Nice! Great solution to a common inconvenience.

dandotway• 5 years ago

While faster-than-light (FTL) travel is impossible through spacetime as PhD physicists know it (i.e. as "Relativity-confirming experiments reveal it"), there are forms of apparent FTL that have mainstream PhD acceptance, because they don't allow information to travel from point A to point B faster than light and thus do not violate causality.

The expansion of the observable universe itself being apparently FTL is perhaps the most interesting. If we point the Hubble telescope (and soon JWST) towards opposite ends of the observable universe, we observe extreme redshift galaxies receding away from each other in opposite directions greater than twice lightspeed if we measure according to "distance in light years" from earth. But no photons or information from a galaxy at one side of our observable universe can be sent to a galaxy at the opposite end: they are receding away from each other too rapidly, these two galaxies exist Beyond the Cosmological Event Horizon relative to each other and are forever unknowable to each other, and thus causality is not violated.

An advanced alien race in galaxy A might be able to beam a message to earth before galaxy A vanishes Beyond the Cosmological Event Horizon within the next 100 million years earth-time, and likewise galaxy B might beam a message to earth within the next 100 million years earth-time, but earth can't relay the message from A to B or B to A because they have already forever receded from each other and will soon both likewise forever recede away from us.

lumost• 5 years ago

There is no restriction on relative velocities between objects from a given observer as long as all objects have velocities less than the speed of light. You can point laser pointers in opposite directions and the "front" of the laser pointer will recede at "2x" C from the opposite pointer.

In the case of the expanding universe, our standard "flat" space time represented by minkowski space is warped into de-sitter space. As distances increase apparent acceleration increases up to and beyond the speed of light due to the expansion of space time.

The horizon problem for faster than light travel

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