What may add the arrow of time is the second law of thermodynamics: a statistical evolution of a system is not symmetric.
I wonder now how a violation of causality could either circumvent of contradict the second law.
To add something substantive, my question to the thread is: anyone here have thoughts on how these questions relate to McTaggart's unreality of time? https://en.wikipedia.org/wiki/A_series_and_B_series
Moving energy around through time may serve to increase entropy overall.
At some point a particle pops up into existence in B, increasing B's pressure.
Some time later, a particle from A disappears, decreasing A's pressure.
This pressure differential would cause flow from B to A through X.
Alternatively, a particle randomly disappears from B, decreasing B's pressure.
Some time later, a particle appears in A, increasing A's pressure.
This would cause flow from A to B through X.
The more particles that cross from A to B via Y, increasing B's pressure, the more likely it is for particles to pass from B to A via Y, keeping these two effects equal.
These two pressure differentials would cancel out for large numbers of particles.
You've essentially just hooked up the system to itself, but at a future point in time. Unless the overall pressure of the system is changing, there is nothing to induce the flow. If you are increasing the pressure, valve Y is essentially just a standard one-way valve.
You can't be a scientific american if you keep using the science words bad.
It's like cargo culting science: Use these words correctly if you want to seem to know what you're talking about!
But anyone who actually knows what they are talking about would use these words correctly, or at least not be mistaken about their meaning.
So this article is teaching people that have no idea what they're talking about, how to sound like they do.
Maybe it's time we start using another language for science. Clearly it's confusing the Scientific Americans.
If we perceived the world through sonar then causality would be violated by anything supersonic. If you were shot then you'd feel the bullet first and then "see" it moving back into the gun.
AFAICS if information can travel at >c, then physical objects traveling at >c doesn't violate causality
If you draw a tachyon on a spacetime diagram going faster than the speed of light, you can create situations where the tachyon travels back in time for certain observers.
> AFAICS if information can travel at >c, then physical objects traveling at >c doesn't violate causality
If information can travel faster than c, then you can create situations where it's also violating causality.
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.
You need to impose additional constraints on the theory (e.g., real mass conditions) to get rid of them.
It's clear that your point of view matters in physics, but it's also clear that I have no idea what a "point of view" means in this context.
If I am stationary and measuring you flying past me in a relativistic train, I would measure you in a coordinate grid say, centered on myself. You may be watching me from your train and doing the same, but from a reference grid stationary with respect to you but moving with respect to me. Even if we both agreed on identical measures ahead of time, your measurement of me from your reference frame and mine of you from my reference will be off from a measurement we do ahead of time in the same reference frame as a calibration. However, that difference comes out in the wash through the magic of the Lorentz Transformation. Using the Lorentz Transformation, I should be able to negotiate transforming a measurement I do of you in my own reference frame to what it would look like from yours and you should be able to transform yours to mine, through which we'd both come to the same numbersassuming we're in spacetime that shares the same general degree of curvature (same gravity). This can even be generalized to arbitrary degrees of spacetime curvature. I.e. if your relativistic train is racing past a black hole with a near tangent intersect, and I'm stationary with reference to the black hole centric reference frame but further out in the system, as you blast through the high curvature part of your trip your clock may seem to de-sync from mine. This too, however, can be mitigated through copious amounts of math I don't get for more than half an hour after looking away from a text book. I just know if I ever have a problem to solve that involves hitting that book again, I have either made some terrible or awesome decisions in my life.
(Again I may have misunderstood, but the frame of the CMB sometimes gets talked about as if it might be a preferred frame).
If light is just information, wouldn’t it just be a matter of seemingly getting something very fast?
Everytime I think I get it, I don’t!
But a decent way to beat Einstein would be find and implement an actual FTL engine: after all, if you can do it then obviously the theory isn't correct (alternately: the universe possible does not care about causality as much as we do in our little bubble of perception).
True, but if the energy requirements for warp drives continue to be measured in Jupiters, reaction-mass is the least of your worries.
Any contradiction would be resolved on quantum level even if it can be interpreted at macroscopic scale as breaking of causality.
The important thing for physics would be to better clarify what time is. Why is the speed of light as much as it is, why is there time dilation related to mass and why is the time slowing down for objects travelling faster.
My guess is that the quantum world needs more of its own time to agree when there are more objects (more mass) close to one another, or that a fast travelling object needs to "agree" with its surrounding more frequently, and that overhead is manifesting as slowing of its time.
According to §3.3, they aren't sure. If I parsed it correctly, they haven't mathematically resolved whether closed timelike curves are necessary, or precluded, or neither.
In a warp field, you're just somehow shrinking X,Y,Z while not doing the same to time. I see no reason to think you'd travel backwards in time.
Perhaps the CMB is the preferred frame of reference for all motion and this problem has a trivial solution (I am not a physicist), but everyone I’ve seen explain relativity starts with “there is no preferred frame”, and the rest follows.
Can you explain how traveling faster than light leads to that? Perhaps you meant "observing your own past?"
First of all, it gives you the ability to solve NP-complete problems trivially.
Step 1: you receive a message from yourself from the future. It explains the problem, gives you a potential solution. Let's say you ask yourself to solve boolean satisfiability, gives you the problem statement, and then a potential solution. You check the solution: If the possible solution is correct, you send yourself an identical message. If the solution is not correct, you send the next possible permutation of the answer. So if you sent yourself the possible solution TTFTFTTTFTFFTTTFTF, you send yourself TTFTFTTTFTFFTTTFTT. If there are no further possible permutations, then you send yourself a statement saying that the problem is not solvable. You've created a time paradox; a future in which you send yourself an incorrectly solution is logically inconsistent. The only consistent universes are the ones where you send yourself the correct solution or not-solvable, and all other universes (starting with the one where you sent yourself the message FFFFFFFFFFFFFFFFFF) collapse into the valid one.
No, it doesn't. Even if we will be able to communicate instantly (0 delay = infinite faster than speed of light), we will not be able to send a message to the past (negative delay).
Read the numerical example on https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone
2) It's an unsolved problem, which waits for a correct solution.
3) Next time, please read what you are posting, please.
2) It's an unsolved problem because it's built on a foundation which is also an unsolved problem -- namely faster than light travel/communication. If -- if -- we solve the problem that it's built on, all indications are that sending a message to your past will become possible.
That being said, I expect that it's impossible because I expect that FTL travel/communication is impossible.
2) It's an unsolved problem because formula is defined for [0..c] only and for EM communication only. For a•v = c² it predicts dT=0, so time will stop for unshielded Alice, like it stops for photon, so she will not be able to communicate at all. For a•v > c², the formula is not defined at all (it is the unsolved problem). In reality, we expect to see Cherenkov radiation. In theory, according to this formula, EM processes should go in reverse, which is not possible at all, because it requires all photon emitted by Alice to reverse their direction and hit Alice at her new location.
We're sitting here talking about how 1+2=3 and you're telling us that we're wrong, and that the sky is actually blue.
I have no basis to object to your argument, other than to tell you the thing you're arguing against clearly doesn't have anything to do with what the rest of us are talking about.
I'll add: 3) yes I read it. It's an example Einstein created, so possibly worth responding to the points made there rather than attacking me for "not reading it".
So you put in the numbers in the same way as that numerical example on Wikipedia, and it shows the same thing.
Edit: the only exception I can think of is if causality can be reversed.
https://old.reddit.com/r/askscience/comments/l9sba/why_does_...
Being able to observe your own past is a given after FTL travel, and the most important thing: it's a given that relies on almost no assumptions about the deeper mechanics of the universe. If you instantly teleport to Alpha Centauri, you will be able to see your old Earth photons for years.
> Instead, your past self could observe your future self.
Now that statement (like the Tachyonic antitelephone) relies on some assumptions that we can't test yet. My hunch would be that either an as-of-yet unknown non-local temporal dimension (or synchronization mechanism) exists at the foundations of the universe, or that FTL travel is indeed impossible. One excludes the other would be my intuition.
In the tachyonic antitelephone example, Bob and Alice send messages that travel at 2.4c, but neither observers are at that speed. Wouldn't the limits of their observation and time dilation cause the messages to appear in order?
If you were to theoretically travel into a black hole, and then somehow escape out of it very quickly, you would see yourself still in the black hole, but you wouldn't see yourself coming out of it, because that light was further from the black hole and thus moving at a different speed?
I feel like this is the part that would probabally be hard (ie not possible) when going faster than light, assuming such a thing is possible.
Also whatever magical space warp it is that lets you go that fast could presumably also obscure your ability to see yourself in the distance.
Slowing down to reach the speed of light might be as hard as speeding up to reach it; I remember it played a role in the theory of tachyons, hypothetical FTL elementary particles which could not be slowed down.
Another question for the thread: Anyone here feel like they can feel the Coriolis effect if they focus enough? I haven't travelled across the equator enough to know if the north/south jetlag is different from east/west.
The coriolis force differential, besides being rather small to begin with, has the additional problem that the coriolis force is proportional to the speed at which you're moving over the earth. If you were to accelerate to a speed where the coriolis force differential might be appreciable (e.g. by sitting in a bullet train or in a plane), it will be completely drowned out by any other momentary forces imparted upon you by the vehicle (trains or planes can move mostly straight, but not in a mathematical-grade straight line).
Technically that's only one assertion (that is, the second is a strict subset of the first).
Here's the grid coordinate system for the streets in my town. One intersection is (0, 0). But in the grid system used by the next town over, that same intersection is (100, 200). And that's fine - we're just shifting the origin, which changes all the numbers.
The same thing happens with velocity. If I'm on the ground, "stationary" means that you're not moving with respect to the ground. If you're in an airplane, "stationary" means that you're sitting in your seat, and you see me on the ground moving toward the back of the plane at 550 MPH. And that's still fine - we just shifted the origin of velocity.
Light is unique in that, when you shift the origin of velocity, it's speed is still the same number. No other velocity transforms that way.
(Well... light isn't quite unique. The speed of light is unique in that way. But gravitational waves also travel at the speed of light.)
Right - this is a subtle but important distinction! Photons don't have primacy, space-time does.
You can calculate the speed of light from other physical, measurable constants without using light (the permittivity and permeability of free space). Those you can measure with inductors and capacitors. So because those measurable physical behaviours are invariant in all reference frames under SR, it follows that the speed of light is also invariant.
You don't have to measure c, or calculate it. The causal chain goes the other way - we assume c is invariant and define capacitance and inductance in terms of it (c, that is).
Edit: I'm totally this guy right now https://xkcd.com/386/
That is what I just said: the meter is defined in terms of c, not vice versa.
c isn't defined as the speed of light - light travels at c, but so does everything else, when you factor in movement through time.
It's not unusual for (anticipation of) future events to affect the present. For example if I think it will rain, I grab an umbrella. While typically my belief that it will rain is based on something in the past (previously weather patterns for example), does it really change anything if a time traveler claiming to be me from the day after tomorrow told me it's going to rain tomorrow?
We only think of causality as going from past to future because that is the way we live. We also find the idea that particles can have an indeterminate location or pop randomly into or out of existence is weird, because our minds which have evolved to experience and understand the macroscopic world have no method of really processing that, but our intuition is not reality. If we can accept that our concept of position of objects in space is merely an approximation of a more complicated reality, the same for position of events in time should not be much harder.
People get bent up about paradoxes, but we have things like the Novikov self consistency principle [0] which allow us to avoid paradoxes even without one way causality.
[0] https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
[0]: https://arxiv.org/abs/2105.03079
[1]: https://eriklentzphd.blogspot.com/2021/05/response-to-new-pa...
If not then the spacecraft is traveling faster than 'c' with regard to everything not in its reference frame (right?). If this kind of thing is possible then the universe is considerably weirder than we thought even after a hundred years or so of QM.