Time travel?

Question

what ideas do people have on time travel? why can/cant it happen etc....

Answer 1

i bet it can happen

Answer 2

There's nothing in modern physics to forbid it, but also nothing to support a particular mode of doing it. So it's conceivable that we could discover it eventually, but it's possible that we aren't capable of comprehending it, just as we aren't capable of perceiving any more than the 4 dimensions of spacetime that we experience even though several other dimensions appear to exist all around us according to modern theory.

But let's say that you find a way to teleport through spacetime to an earlier period. (We'll even assume that this teleportation device allows you to zero in on the right location so that you don't have to worry about figuring out where in the universe the Earth was during your destination.)

And lets say you kill your grandfather. Okay, fine. Nothing in physics forbids that either. You've just created a new timeline in which you will never be born. The original timeline still happens, and you are still born there and you still come back and kill your grandfather. But this new timeline also happens where you are never born and this timeline goes on without you.

In this timeline, you popped into existence from basically nowhere. You didn't exist (yet), and then you suddenly sprang into existence. It doesn't matter where you came from - by coming into existence you have started a new branch of spacetime.  (Even spontaneous popping into existence of complex things like people is theoretically possible, though the odds are vanishingly, inconceivably small. But, a time traveler coming from some future timeline and appearing here could be explained that way.)

There are a mind-bogglingly large number of timelines being generated all the time, at every moment. Schroedinger's cat is both dead and alive until you peek in the box. And then it's dead in one timeline and alive in another.

A particular particle of uranium both decays and does not decay at the same instant. It just depends on which universe you measure from.

This is called the Many-World's Theory. And if particles do exist at superluminal speed (which Einstein predicted they could, and modern theory allows for), then the Many World's theory absolutely must be correct even as strange as it seems. (Note that I'm only saying it must be correct IF there are particles that exist at superluminal speed and that those particles are not forbidden by our theories. Those particles may not, in fact, exist. And if they don't, then Many Worlds may not be a reality.)

Superluminal particles are called tachyons (yes, just like in Star Trek - Gene Roddenberry borrowed the term from real-life physics). Let's say we find a way to transmit and detect tachyons. (This would involve measuring imaginary time - something we currently have no idea how to comprehend. And I mean 'imaginary' in the mathematical sense.)

Anyway, let's go on to say we build a transmitter on Pluto and one on Earth and give each one a receiver too.

Somebody in a base on Pluto decides when to flip a switch to transmit a single tachyon to Earth. Tachyons travel faster than light, and so go backward in time and so the tachyon is received before it is sent. The reception on Earth triggers a transmission of another tachyon back to Pluto. It is received there before it was sent from Earth, and considerably before the person flipped the switch in the first place. Having received this signal, the person decides to be contrary and doesn't flip the switch, and so they never transmit the tachyon that began this series of events.

But the tachyon was transmitted from somewhere, so where did it come from? It came from another timeline where the switch was flipped.

This is a description not of a thought experiment, but of a real-life thing that we could do if we learn how to detect and transmit tachyons. And we wouldn't even need a base on Pluto. With sensitive enough equipment, an experiment like this could be carried out right here on Earth. So, you show me how to find and transmit tachyons, and I show you how to violate causality. In fact, if we ever prove that tachyons must exist - even if we can't find them - then that will prove that the Many Worlds theory must be correct.

And if tachyons exist, then traveling backwards in time is definitely possible because that's the only way they can travel.

Anything going faster than light must perforce go backwards in time. As you accelerate towards the speed of light, time slows down. If you were to reach it - which is impossible because it would require infinite energy and would expand you to infinite mass - then time would stop. (Which, in fact, is the state that photons are in: They don't experience the passage of time at all, even though from our perspective they might travel for millions of years in space. It's all intantaneous to them.) If you kept accelerating past that speed, then time would reverse. Tachyons don't accelerate past the speed of light - they exist there naturally just as photons don't accelerate but naturally occur at that speed.

The problem with detecting a tachyon is that one instant they don't exist because they are in the future and in the next instant they don't exist because they are in the past. The present is an infinitesimal cross section of spacetime and it takes no time to cross it and so there is no way to measure something that does. It co-exists with us not just for a very short time, but for no time at all. (Unless spacetime turns out to be granular in which case there is some unit of time that is the shortest time possible. If that's the case, and if we invent devices capable of sensing it, then we might find that our world is filled with tachyons that we could never see without such a device. But granular spacetime is unlikely. We're probably in an analog world and not a digital one.)

Answer 3

It can happen if u can find a way to find a worm/black hole.

Answer 4

Debates take sides on the probability or impossibility of physical time travel, either forward or backward.  When the real answer to your question really is:

We simply don't know if we can or can't do it.

What we feel is a real probabilty is the fact that space can be bent to allow physical travel to vast distances that would take lifetimes travelling AT LEAST light speed, given we can't travel any faster than that.

Even then, that's still NOT an effort at arriving somewhere forward in time BEFORE you left.

The challenges?  Building a ship that speeds up to 186k m/s.  Have an onboard device that demolecularizes the physical mass of the ship and crew into tachyons---and on target directs those tachyons in a single direction to a destination that MIGHT take the ship FORWARD in time.

But we just atom smashed the ship into tachyons: how can we restore the physical ship's structure?  I don't think letting the ship "cruise" till the tachyons slow down will eventually restore the ship to it's physical state of mass.  Richard Branson might sign up for this attempt---but leave me out of the plan.

Einstein was dabbling into physics theories that might allow work arounds to his established theories of proven fact---shame he never finished work on them.

Even if our government is toying around with the time travel notion---take rest in that we aren't getting anywhere NEAR the ability of such travel.....and likely won't see a glimmer of hope for at least 4000 years or so.

Answer 5

i think it can happen..but we dont have the technology yet.

Answer 6

Of the four dimensions in our known universe, time is the only assymetric one...it can only go forward.  Cosmologists are working on why this is so.  Thus far the best they can come up with is that it's linked to the net positive entropy of our universe.  That is, from the big bang on, our universe has been going from low entropy, at the big bang, to higher entropy.  And time is linked to that increase; time increases as well, which is why we say it has a forward time line.

As time is assymetric, we cannot go back in time.  But we can go forward and do that at different rates.  For example, invoking relativity concepts, a spaceship traveling 1/4 light speed will advance 1.03 seconds on Earth for every one second on the ship.  This means, if the ship is gone a year by its own calender, it will come back to Earth 1.03 years after take off...or .03 years into the ship's future.  

Time travel into the future is possible, but not feasible with today's technology.  The amount of resources to build a ship, train the crew and support staff, and monitor the flight into the future would be overwhelming.  And for what...a crew that would go .03 years or 11 days into their future upon landing on Earth after a year in space by their time frame.

Note: at an acceleration of 9.81 m/sec^2 throughout the flight, such a ship would require almost a year to get up to 1/4 light speed.  Then it would have to slow down at close that same rate (1 g).  So, in the end, the crew would be gone for well over a year, counting speed up and slow down time, to go 11 days into the future.  So the crew would likely be in hybernation as no ship of any doable size could carry almost three years of life sustaining provisions.