Physicist: It already has! Or rather, it already will be.
Once time travel has/will have been invented, you’d think that said inventor could just go back in time and show off their invention, or give it to some ancestor (or even become some ancestor) so they could be born into old money.
But despite being both common and world-changing, time travel is intrinsically very low-key. In the 20th century the world population increased from 1.6 to 6 billion people, and even though time travelers account for about 3 of those 4.4 billion new folk, evidence for their presence is almost impossible to find. It turns out that time machines are just like every other machine; they don’t exist if they’re not invented. So whatever else anyone does with a time machine, it didn’t/won’t affect the invention of time machines themselves.
In 1992 Steven Hawking derived the “chronology protection conjecture“, which posits that “closed time-like curves” are impossible. Moving along a time-like path is what you (and every other chunk of matter in the universe) are doing right now; moving slower than light and experiencing time in the usual way. Moving along a closed time-like path is like going for a walk in the woods and following a trail that returns you home yesterday. Hawking showed that closed time-like curves produce “feed back” that destroys everything involved. In other words: Timecop rules.
Ever empirical, on June 28th, 2009 Doc Hawk threw a party for time travelers and (to ensure only time travelers showed up) he kept it secret until June 29th, when he sent out invitations. Save the date!
To his bemused shock, Hawking’s soiree was very well attended. He claims to have met “people” from as far afield as 70189324233 AD, the year in which the invitation, as well as the spaciotemporal coordinates of Earth, were unceremoniously overwritten and forgotten during “The Great System Update”.
Left: Steven Hawking, blocking the photographer from getting into the party behind him. Right: The invitation he sent out the next day. Hope you can/did make it!
At his party, the Hawk discovered three things. First, time travel is not just possible, but easy. Second, closed time-like curves are impossible, but that’s not how time travel works. And third, time travelers don’t leave much evidence behind, because they couldn’t if they tried (and don’t when they do).
Hawking later wrote, “Dear Diary, [I] wasn’t sure about actually buying champagne for the affair, since I knew (or thought I knew) that this was all a [silly stunt]. I’m glad I did! Time travelers are a cagey lot and the evening didn’t really get into full swing until the 7th or 8th crate was opened. A man (perhaps?) who introduced himself as the Designate Demithrall of the North Antarctic Seasteader Federation in 4372, mentioned that the key to time travel is my own work on imaginary time and that it’s ‘obvious really, if you think about it’. This is remarkable! But in the sober light of da [sic.] I can’t help wondering if the Designate Demithrall wasn’t drunk or sarcastic or both. Forty-forth century humour is really hard to read.”
When a time traveler intends to give instructions to someone in the past to help them be the first person to build a time machine, they inevitably and accidentally don’t. The retro-self-cohesion principle of the time-line prevents grandfather paradoxes, so neither time travelers nor machines can change the logic of their own history. In other words: not Back to the Future rules. For example, if you go back in time to kill your own grandfather, then you won’t exist to go back in time and do said killing. You have to come from somewhen. Inescapably, you’ll either get the wrong guy or fail to get the right guy. In other words: Bill and Ted rules. Time travel is possible, and even common, but you can’t change things so much as confirm them. In the archetypal example, Rufus goes back in time to ensure the Wild Stallions succeed in bringing about peace and enlightenment throughout the universe, and he knows they do because he was/will be there to help.
The “grandfather paradox”. Like all paradoxes, this only shows up on paper. It can’t happen in reality.
In the same way that you don’t (presently) worry about your murderous unborn grandchildren, the inventor of time travel is immune to hints. No matter how many time travelers they may incidentally meet, none of them will ever get past general pleasantries; the topic of time travel is logically verboten. The same holds for common knowledge. Presumably, the reason that you can’t go online and find the schematics for a (functional) time machine is that the future inventor of time travel doesn’t live in a cave. The first thing they’re likely to do before getting down to work is a quick internet search to see if they’re reinventing the wheel (or flux capacitor), so all the universe must conspire to make that internet search fail. Like time travelers themselves, the idea has to come from somewhen. Being aware of this tautological time travel truth, and possibly having read Hawking’s published diary, the Designate Demithrall was most likely safeguarding the logical consistency (and existence) of the very conversation he was in by filling it with sarcasm and misdirection.
So if you ever meet anyone who claims to be a time traveler and makes no attempt to support their claim, then they’re probably telling you the truth. Time machines are more common then cellphones, but they’re literally impossible to talk about. And if you yourself are a time traveler, remember that we ran/will run out of prosecco about halfway through Hawking’s thing, so BYOB.
LOVE IT!!! Happy AF to you as well 😉
The question of whether time travel will or will not ever be invented is much the same question as “Will my brain ever progress to the point that I will understand the futility of asking such a moronic question?”
It reminds me of the days when everyone was asking Marconi “When are you going to invent a radio?”
No it doesn’t.
Great 1st April joke!!!
There is of course a much simpler answer (from the perspective of FTL physics), time travel is impossible because above the speed of light Einstein was wrong. Below the speed of light and on its ‘mechanics’ side Special Relativity is one of the strongest and most accurate theories in physics. However at the speed of light and above Special Relativity is really no more than speculation.
Time travel as described by science fiction writers requires something called a general time dimension – but if such a thing exists then time travel should be easy and should be observable all the time. Steven Hawkings argument against closed time loops is actually a strong argument against such a time dimension. Another strong argument against it is the whole theory of quantum mechanics, because quantum uncertainty and a general time dimension at mutually exclusive.
In a universe without a general time dimension things like the grandfather paradox simply don’t exist. Paradox itself is an indication of an incomplete or wrong model.
Time travel of a sort is still at the very edge of being possible in a universe with no general time dimension but the time traveler destroys everything in the enclosing volume of the light cone of their jump, replacing it with a new version. The old future they leave behind is completely obliterated and the new past they enter is only some form of recreated copy of a previous past.
A true time jump backwards is still at the verge of being possible but (like Steven Hawkings closed time loops) creates a reverse causality funnel that ends with the universe collapsing back in to the Big Bang. This kind of time loop very probably does exist though because it is a viable finite solution to the Anthropic / Tuned Laws of Physics problem. A universe containing life evolving through billions of cycles of universe evolution.
Some believe that the Bible implies God might exist in the form of light (do photons experience time?).
Yes it’s April 1! But what year?
You never cease to come up with bigger and bigger loads of garbage and absolute nonsense. How unscientific and ridiculous.
I attended an auditorium to watch a grandchild in a play yesterday. I filmed it, of course. Initially I made an error in selecting the definition mode and when I watched it, the video wasn’t quite as sharp as it could have been. Darn.
But then I realized a person behind me was taking cell video just as I was adjusting the camera and would have caught me in the edge of their video frame. I contacted them and got to see me do it.
So then in addition, I began to tell everyone that there was a video of me properly adjusting my camera until I now fully believe it myself because of the Often-ness Effect. Sure enough, when I just watched my video again before posting, the setting is now perfect and I don’t remember when it wasn’t.
I attribute it to the quantum effect. Me observing me making my proper adjustment made the result come out. I learned this trick from watching a video about Hawking, Bohr and quantum mechanics. It’s all in the power of being thee observer, which of course, can fool Mother Nature.
I’m surprised Einstein never realized this, but I guess nobody is perfect. Now I’m thinking about going back and taking some cool pictures of Albert and selling them in 2017. Of course I won’t be able to tell him what I’m doing.
Wes
It’s ironic that the main instinctive argument against time travel is causality because that is what makes time travel possible.
We used to think that a moving, read accelerating body moved through three dimensions. Now we know that it moves through four. We theorize that there are more dimensions but we don’t seem to presume that the accelerating body moves through them.
Think of the moving body progressing parallel universes from ‘now’ to all possible alternate futures that have a non-zero probability. An infinite number.
The link between a -t point in the ‘past’ and a t=0 point is effectively causality.
The relationship between a -t point and a t=0 point on the same causality line is 1:1 but the link between t=0 and any positive t value is one to many, or one to infinity.
The probability of my grandson popping into existance is non-zero as long as time travel is possible. If he were to kill me, the probability of his ‘future’ including his birth reduces to zero. But that doesn’t stop him from existing because in his -t, there occurred the causal event of him popping into existence.
Practical time travel depends on causality because travelling in a -t direction anywhere outside the direct causasality chain means ‘landing’ in a past that is impossible to know and has a non-zero probability of being inconsistent with the existence of matter.
It’s no longer 1 April
@Okieprof No, photons do not experience time, at least not time as we perceive it. From the perspective of a photon everything in its entire existence happens all at once in the same timeless instant.
The main issue with time travel is that we have such a limited and restrictive perception of time that we simply don’t know how time actually works, an unfortunate but predictable consequence of being limited to little more than watching time as it passes by us.
In short; we don’t have enough dimensions to understand this stuff without experimental data, which we can’t get because insufficient dimensions.
One day, someone may figure out how to loophole around this, until that day (or something along those lines) we are sadly restricted to hypothetical arguments on the internet and can go no further.
@ A Time Traveler…
What do you mean, “It’s no longer 1 April”?
We can always go back to April 1, 2019 (or 2017). It’s a simple matter. All one need do is go back in time… like I’m doing now. Or did.
Now there are those that don’t believe we can violate the Arrow of Time; in other words go backwards in time instead of just slow it down by going fast. But it is a simple matter really. All one need do is be able to travel twice the speed of light.
For instance, first travel at the speed of light out into the friendly vacuum of space until one catches up with the light emitted from Ford Theater just before Lincoln was shot (or was/will be). Enjoy the view for a bit. Then quickly go back to earth at the speed of light again, to the theater before anything can happen, and stop Booth by dousing him and his gunpowder with a bucket of cold water.
Saving Honest Abe would be worth it. His next political rival would have at least needed to be somewhat honest to defeat him. After the popular trend was started, I think it likely all politicians would have necessarily been honest thereafter. Or we could keep going back, and do it again, until it finally worked anyway.
Of course we might lose the phrase, “Break a leg”, concerning theater. Others don’t believe Booth started this.
Wes
@Wes
How exactly does one catch up to light by traveling at the speed of light?
…
“How exactly does one catch up to light by traveling at the speed of light?”
@Neruz
First… realize everything I write concerning the first day of April is intended in the spirit of humor.
Then slightly more seriously:
Ok, you got me. A very sharp pointed question. Touché.
I think the Lorentz/Fitzgerald Contraction Ratio rules. Roughly, it seems we could possibly catch light in theory because the distance, to the “light-wavicles” we chase, becomes infinitely short if we leave at exactly the same time.
But since we are leaving well after the assassination, quite some years later, and the light from the Ford Theater is still moving under Einstein’s coordinate system even as we chase it, we must impossibly go even faster to beat the Ford wavicles to some appropriate distant rendezvous to view the living Lincoln.
Furthermore, this appropriate rendezvous must allow a bit of early recreational theater viewing, yet allow enough time to travel back to the theater at light-speed to stop (or at nearly light-speed to nearly stop) the theater managers clock in order to arrive in time to stop the dastardly assassination attempt.
Unfortunately, it is not possible for entire atoms (us) to even reach the speed of light because all generally known propellant forces reduce to electromagnetic in nature and, according to Maxwell, such electromagnetic thrust can only travel at the speed of light, max. Therefore I think any vessel mass at all precludes ever reaching any-speed if the thrust force isn’t pushing at least a hair faster than the desired said any-speed, and any-speed includes light-speed.
Obviously, and slightly less seriously, we would need to use something faster than light for thrust, something like Quantum Entanglement Drive (Bohr) or Spooky Repulsive Action at a Distance(Einstein).
Near April 1, there is the B.S. engineering factor, through and through.
Wes
As a close friend to the Designate Demithrall of the North Antarctic Seasteader Federation, I came back in time to say that I strongly resent the insinuation that he may or may not be drunk during the said party. He hasn’t been sober since the late 60s, for sure.
In the Temporal State, we are not just limited by time, we are also limited by sequence, i.e., the sequence of events. Even if you could physically step into the Sequential State, where there is not limitation of time, you will still be limited by sequence. Now in the Concurrent State, there is no limitation of time nor sequence. Jesus demonstrated that by saying, “Before Abraham was born, I am!” (John 8:56-58). That means he not only existed before Abraham but before his mother. Not limited by time nor sequence. If we had the ability to function in the Concurrent State, we could travel in time and not mess things up, unless we wanted to.
Surely there’s a simple answer to all this time travel stuff. The future does not yet exist and the past has been and gone. We can and do travel into the future whenever we move. We just don’t notice the difference. But if we travelled fast enough, we would notice, and we would see the future evolving.
Travelling into the past is a different problem. Without getting into the realms of parallel universes, it’s not at all obvious that ‘the past’ still exists. To genuinely travel backwards in time within our own universe, we would have to rearrange it into an earlier state, something that the second law of thermodynamics will not allow.
Note, however, that the second law does not preclude rearranging a subset of the universe into an earlier state. Given enough information, a technologically advanced entity (call it God if you like) could, in theory, rearrange a whole planet. Let’s hope it isn’t ours.