Everyone just assumes time travel is frictionless, but that doesn’t make sense. You want to go back ten seconds? Couple AA batteries. Want to go back an hour? Nuclear fission required. Seven days into the past? Microwave electronic resonance craft. A year? Forget about it.
It’s a reference to a patent held by the navy. It’s concentric shells of nickel or something like that, then microwaves bounced inside that cavity. According to the patent it ends up producing power on the order of a hundred atom bombs per second.
If the Multiverse is a thing, then that timeline has happened somewhere and our future selves visited us. Ahahaha. That would be trippy.
In this Multiverse though, it looks like we become the Borg, so they would have already collected all the info they needed and thus have no need to go back in time.
Time travel within the same universe is not possible, it is a fun fiction which is always contradictory in some way. The only time travel possible would be the one that William Gibson uses in The Peripheral. His idea is that every time you go back in time a new parallel universe is created, and it doesn’t impact your current universe because of that.
My theory is that we’re one of the most advanced species in our galaxy, and yet we still can’t reach another solar system. The probability of intelligent life forming from unintelligent life is extremely unlikely, and we had life on Earth for a LONG time before humans evolved. Intelligent life is very difficult to form, you need the perfect conditions and perfect stressors over millions of years. Then on top of that intelligent life which can reach another solar system is even less likely.
There’s life out there thinking the same thing right now:
One of my favourite theory about human evolution is the stoned ape theory. It delivers the conditions of the evolution. Apes were forced outside of the jungle and ventured into open field and had to depend on different nutrition. So they ate some mushrooms and eventually ate ones with psylocibin. Small amounts increase your eyesight - so you can hunt better, bigger amounts are very sexuall arrousing - so more reproduction :)
You don’t even need that hypothetical for it to make sense. Look up how chimpanzees fight against each other for territory. It’s survival of the fittest over millions of years, simple. Cold weather in the north, requiring the development of technology for survival, learning how to use fire to cook clean food, learning how to use alcohol to purify water, etc. We split from chimpanzees millions of years ago, and chimpanzees can only survive in very lush jungle areas because of the food sources they rely on, which is why they’re endangered.
Essentially the type of time travel employed in Harry Potter and the Prisoner of Azkaban. So you wouldn’t be able to change anything, even if you did do things — as if you did something, it had always already been done previously (before you travelled back).
“it’s a fun fiction which is always contradictory in some way” <-- that is not true.
General relativity allows for closed time-like curves and the existence of those means time travel isn’t necessarily impossible, and if that is the case, it has to abide by the Novikov self-consistency principle.
Dumbledore disagrees. The type of timetravel that happens in HP: Prisoner of Azkaban is self-consistent. Same as with Rick’s timetravel in the snake episode of Rick & Morty (specifically excluding the snake time travel, which is an example of non-consistency, leading to endless paradoxes).
Now ofc having a timeturner and/or a timemachine would be the impossible part there, but again, CTC’s are technically allowed by GR.
I’ve begun thinking that time travel can only be possible if all of time were to exist simultaneously. Like a singularity. Then with complete knowledge and ability to influence matter you can rewrite time anyway you would like.
Like spreading the frames of a film out and altering them as you see fit.
I think, lots of people these days have an inflated understanding of time, because of the time travel trope in pop culture, as well as videos and simulations with time sliders being readily available.
Without getting into Einstein’s theories for a moment, time is something we measure, we never modify time in reality.
In particular, we measure the progression of causality. Say, you kick a ball, then the atoms in your foot push the atoms in the ball, the whole ball rapidly accelerates and then takes 0.6 seconds to hit a wall.
You can’t just set the whole process to -1x speed and expect it to happen in reverse. That breaks causality. The ball would need to fly towards your foot without anything giving it a push.
With our time sliders, this may seem like a small difference, but it’s just completely different from anything we’ve ever observed in the universe.
Now, about Einstein’s theories, all of the above is still true. However, Einstein has discovered that travelling through time is possible, but only going forward in time, at a different speed than everything else.
Basically, causality has a speed limit, which happens to be the speed of light. (We believe photons to be massless, so with our usual acceleration formula, any acceleration would make them go at literally infinite speed. They don’t, which is why a speed limit for causality is assumed.)
Gravity and your own speed can influence this speed of causality for you, meaning at a lower speed of causality, things happen less quickly for you and you’d age more slowly.
So, theoretically¹, you can get into an extremely fast spaceship, spend 5 years-as-perceived-by-you in there and then land on an Earth that’s in the year 3000.
That is a one-way trip, though, because going backwards is not a thing.
¹) In practice, you need to go at speeds for this, that are magnitudes higher than anything humanity has ever achieved.
The universe seems to be keyed to disallow time travel. The speed of light limit, in relativity, is sat exactly at the limit where time travel would become possible. Conversely, quantum mechanics does allow for FTP transmission. What it doesn’t allow is information to flow along those links. It’s hit with a 0.5 error rate, which completely blocks FTP communication.
General relativity does allow for a few time travel options. However, these are sat well off in the sticks, where quantum relativity would dominate. Since we don’t have such a theory yet, our predictions are likely wrong. Even within these theories, a time machine would require a “closed timelike curve”. These can, in theory be made using several rapidly rotating black holes. Any ship traversing it, would never be able to leave before the time machine was built.
Basically, time travel is almost certainly blocked by our laws of physics. Any loopholes would be limited to the lifetime of the “machine” and would require stellar level engineering for even a few seconds of travel.
It seems to allow it, in a sense. The errors are also left on the transmission end. By transmitting them normally, the 2 signals can be combined to recreate the data. Something is shared, at some point.
It’s definitely a “we’re not sure what’s actually going on” type situation though. Either both ends are drawing on some (otherwise) hidden data layer, or FTL transmission is allowed, so long as no information flows (information as defined by quantum mechanics). It just turns out that weird entanglement based systems are the only ones (we’ve found so far) able to send infomationless transmissions.
Both solutions would give deeper insights into reality, and its underpinnings. Unfortunately, we’ve not actually teased out which is happening.
My gut feeling is that the speed of light is a side effect of a fixed/stable causality across all rest frames. Hidden information seems to be a lot more cumbersome.
I’m not sure what you mean. If something is “shared”, but this something contains no information, how can we know that it was shared? In what sense does this something even exist?
The perfect correlation of entangled particles is well established, and very cool, but perfect correlation does not require sharing of “something”. The perfect correlation is baked into the system from the start, from local interactions only.
Indeed. I’m not completely sure what point you are trying to make, but my point is not a hidden variable point. The states can be in a perfectly correlated superposition without any hidden variables, and still not “share anything” upon collapse into an eigenstate.
I will concede that it looks a lot like one particle somehow tells the other “hey, I just collapsed into the |1> state, so now you need to as well”, but at a closer look this seems to happen on its own without any such message being shared. In particular, while the collapse of one state causes the collapse of the other, there is no physical way to distinguish between a state that was collapsed due to entanglement, and one that wasn’t. At least not until you send a sub-FTL signal to explain what happened.
So if physically, the state of particle 1 before and after particle 2 was measured is indistinguishable, how can we say that “something” was shared from particle 2 to particle 1?
I mean you can setup a source of entangled particles and two very far detectors that would do measurements roughly at the same time on each particle in such a way that information traveling at the speed of light wouldn’t have time to travel the distance between both detectors.
You can then just gather roughly simultaneous measurements and at a later time join the datasets from both detectors to see what one measured vs the other for each pair.
If I understand correctly the current observations show that collapsing the state of one of the particle influences the other all the way at the other detector. Since there’s no hidden variables that predetermine the result of measurements while the result of the collapse is random, and the fact that particles still respect the correlation over any distance is why there seem to be a FTL communication between the particles.
Something has to be communicated between the particles for the influence to work FTL, but it also seem we cannot leverage this phenomenon to send “actual information” this way :/
edit: Important point with that experiment: once the particles have been observed, if you try the experiment a second time using the same particles, then you’ll get different results, this time in line with hidden variables because the particle’s state already collapsed.
I fully understand the concept of entanglement and the experiments you mention, but I’m still to understand what you mean when you say “something” is being transmitted between the particles.
As you say, this “something” cannot contain information, and it also cannot influence the particle physically, since there is no way to distinguish the physical state of the particle before and after it receives this “something”. So the signal contains nothing, and has no effect on physical reality. That sounds a lot like “nothing” rather than “something”.
I completely get the argument that somehow the two particles must agree on what result to give, but in the theory this is just a consequence of how entanglement and measurements work. No transmission required.
The message transferred between the particles supposedly FTL does contain information though. What I meant was that we cannot encode our own arbitrary information on top of it. The message has a physical effect on reality, without it the state we find the particles in cannot be respected.
Just reconsider this: If we agree that the result of a measurement is totally random (no hidden variable predetermining the result of the measurement) but that once we measure and know the state of one particle then we know with certainty the state of the other particle (entanglement): information about the collapse of the first measured particle was shared to the other so that it’s no longer random.
edit: If your argument is about “sharing information doesn’t imply transmission” then let’s stop here and leave this thread agreeing that “information was shared” :)
I have no opinions on what shape the information sharing takes. Nor am I interested in guessing.
The “message” does not have any local effect on reality - when you measure your particle, you have no way of figuring out if its partner was already measured elsewhere. The effect it does have is on the global state, maintaining the correlation that was encoded from the start.
If you write up the density matrix for the system before and after measurement of one of the particles, you can see that while the density matrix changes, it does not change in a way one can measure.
What I will concede is that before the first measurement the global state is |00>+|11>, afterwards it is |00> or |11>. This projection appears to happen instantaneously, no matter the distance, which is indeed faster than light.
But calling the wave function collapse a signal or a message or a transfer of information is misleading, I would say. In your example, we know that the initial state is |00>+|11>, and that the result of the first measurement is then, say, 1. Then no further information is required to know that the other measurement will result in 1. No messages required, no hidden variables, simply the process of elimination.
I would like to say that this is indeed a confusing subject, but that the math is clear, and that I am arguing what is my impression of the mainstream view in the field.
As I understand, the FTL “transmission” in quantum entanglement is equivalent to just ripping a photograph in half, sticking the halves into envelopes and sending one of them to Australia.
By measuring the envelope you kept, i.e. opening it and seeing which half is in it, you gain instant knowledge, what the other half in Australia is.
This is mostly useless for communication, though, because the person in Australia does not get this information instantly.
In the case of quantum entanglement, the photograph halves are a particle, which has decayed into two particles, each of which have kept a shared property, like a spin of -1 and +1 respectively.
Time isn’t really a thing, right? It’s just the chain of cause and effect, tied inexorably with space. There is only ever the present, the ever-shifting now. The past is a remembered state, the future is merely possible states.
Cause and effect happens more slowly or quickly due to relativity, but it doesn’t go backwards.
Reading that formed an interesting question in my (also non-physicist) mind:
If we can, at most, take advantage of relativity to slow down our own time frame, then time could just be our way to describe the pace of how space changes around us following simple causality.
But if, on the other hand, it is possible to move backwards through time, wouldn’t the universe have to necessarily exist not only as a giant block of eternally changing 3D space, but as a giant block of 4D spacetime one could move around in? And would that mean predetermined past and future, or would that 4D block of spacetime change, too, advancing through meta-time, continually changing future and past of the universe?
ScienceClic has a cool video, stipulating that we live in 4D spacetime and are bound to always move forward at light speed. If we stand still in 3D space, we move forwards in time at light speed. If we accelerate in 3D space, we change out motion vector from only pointing forwards in time to pointing slightly sideways (up to completely sideways, i.e. time stops, if we were able to move at light speed). But there may be now way to do a 180 involving the time axis like we could do involving the other 3 axes.
Location isn’t really a thing right? It’s just a chain of “over here” and “over there” tied inexorably with space. There is only ever “here”, the place you occupy while things move around you. Behind is places you have been, forward is places you might be.
If time travel is possible it’ll probably be limited to the lifeline of the time machine itself. You cannot travel back in time to a point prior to the invention of the first time machine.
GR predicts “closed time-like curves,” but they require weird conditions (an infinitely dense and infinitely long rotating string) or negative energy, and you can only ever travel to the start of the loop at the earliest.
Basically, you can only travel forwards in time because going backwards violates causality
However
Because of the possible 4th dimensional geometry of the universe and what that means for how it behaves over deep time intervals, it could theoretically be possible to go so far forward you end up back in time.
This possibility relies on the accuracy of the theory of the bang and crunch cycle, which basically states that the universe is a bubble of infiniteness and that if you figure out how to ride out a big crunch and following big bang, you can just keep fast forwarding infinitely until random happenstance takes you far enough forward in time to a universe that is identical to ours except you’re arriving to a point that is identical to one that was “back in time” from where you started.
It’s probably one of the more depressing takes on time travel since it’s impossible to ever go “home” but then again in that sense you’ve never returned to the home you left from in the morning, because the one you return to exists forward in time from where you left.
There’s a hard sci-fi novel (1970s or 1980s) called “Tau Zero” that features this idea.
Book summary (spoils all of it)A colony ship with a Bussard Ramjet on each end (debunked theoretical spaceship drive that uses interstellar hydrogen for propulsion, i.e. the faster you go, the more medium you hit and the harder you accelerate) suffers an accident that destroys the deceleration engine. They keep accelerating because they need the engine’s magnetic field to protect them from interstellar dust. First they try reach the void between galaxies to safely repair the ship, when the interstellar medium is still too “dense,” they go for the void between galaxy clusters, then superclusters, then they just stay on the throttle until the big crunch, at which point, in the nothingness after the universe collapsed in on itself, they can finally fix their ship and begin decelerating into the bounce-back big bang of the forming next universe and colonize a planet.
Or they’re the things 99% of people are calling ‘aliens.’
Why an interstellar species would travel light years to come to this pale blue dot in ships that don’t really interfere and look like our own just a few hundred to thousand years more advanced is kind of hand waved away.
But if those sightings are in fact accurate, it sure seems like our narcissistic species would be pretty interested in our past selves once the tech existed.
Listen, the only logical conclusion to stop time tavel fuckery is a cascading series of event that cause all time travelers to be killed before tbey discover time teavel, or otherwise foil the discovery. The result is a nice clean timeline. I know this to be true because that technology would NOT be used responsibly
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