"effectively traveling into your own past"
Can you explain how traveling faster than light leads to that? Perhaps you meant "observing your own past?"
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.
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.
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.
https://old.reddit.com/r/askscience/comments/l9sba/why_does_...
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).
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?
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.
Technically that's only one assertion (that is, the second is a strict subset of the first).
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.
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.
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.