Is Time travel Possible? (part 2 of 2)

Here is the conclusion of my interview for a popular science magazine: Science Illustrated in Denmark.

1:

Could you clarify why you find it highly unlikely that time travel is allowed by the laws of nature? Why do you believe, that the merger of general relativity and quantum physics leads to a theory (of everything) that will not allow time travel / closed timelike curves?

2:

When you build your own wormhole, you can only go back in time to the very moment a time loop was created. But couldn’t you – in theory - use an existing wormhole (if you have the technology to open a microscopic wormhole wide open) to go back further?

3:

Delayed choice quantum eraser experiments and the non-locality of quantum mechanics seems to indicate that the quantum world is “above” space and time. So, could we live in a world of self-consistently evolving quantum spacetime fields, which would work as a banana-peel-solution to the paradoxes?

1. Could you clarify why you find it highly unlikely that time travel is allowed by the laws of nature? Why do you believe, that the merger of general relativity and quantum physics leads to a theory (of everything) that will not allow time travel / closed timelike curves?

The answer is a bit complex and I’ll start with a detour. Quantum mechanics teaches us that position and velocity cannot be measured simultaneously with arbitrary precision. This goes under the name: Heisenberg uncertainty principle. Special theory of relativity shows that there is a maximum speed limit in the universe and nothing can go faster than the speed of light. Combining quantum mechanics with special relativity results in something completely new: creation and annihilation of particles (hence anti-particles). Why is this so? Suppose you try to pinpoint the location of a particle with arbitrary precision by putting it into a box and squeezing the box on all sides. By Heisenberg uncertainly principle, when the box is squished to nothing, because the position is known exactly, the velocity uncertainty goes to infinity and will be possible to have velocities higher than the speed of light. Since nothing goes faster than the speed of light, the higher velocity is only apparent because new particles are generated and we detect another particle instead of the original one. The theory for combining quantum mechanics with special theory of relativity is called quantum field theory and is the most successful theory of nature we have so far (the current measurement and prediction accuracy is better than a part in a billion). In quantum field theory, the lowest possible energy state is called the vacuum. Vacuum is not the absence of things, and it is a very violent place where virtual particles and antiparticles get created and eventually destroyed. Quantum field theory obeys a fundamental principle of physics called unitarity which means that information cannot be created or destroyed. A time loop violates unitarity because it can create new information out of nothing. When general relativity meets quantum mechanics, by time machine solutions, or by the simpler example of a black hole, unitarity is violated and information is no longer conserved. One may recall the debate between Leonard Susskind and Stephen Hawking on the black hole information paradox. The same thing is at play here and by its construction the best candidate for unification between general relativity and quantum mechanics, string theory, preserves information conservation and rejects time loops. If information conservation is violated, quantum field theory predicts that the universe is heating up and we simply don’t see this happening. Hawking also proposed a quantum field theory mechanism to prevent time loops. The moment general relativity is about to create a causal time loop, the virtual particles in the vacuum start traveling around the loop draining the energy out of it. The end result is that the time loop collapses. Since we lack the precise unified theory, Hawking’s computation is only speculative at this time.

2. When you build your own wormhole, you can only go back in time to the very moment a time loop was created. But couldn’t you – in theory - use an existing wormhole (if you have the technology to open a microscopic wormhole wide open) to go back further?

Yes in theory, no in practice. A wormhole has a delicate part, its “neck” which by general relativity will collapse very quickly unless it is kept open by negative energy. Think of negative energy as a credit card: you spend what you don’t have, and you have to pay it back eventually with interest (this is also because of Heisenberg uncertainty principle, but this time not for position and velocity but for energy and time). To keep a wormhole open for a decent amount of time, you need to keep feeding it negative energy and every time the interest compounds. Some extremely advanced alien civilization has to maintain the wormhole open for us to be able to see the extinction of the dinosaurs (like rolling the balance from one credit card to another credit card with a higher credit limit). And for that long amount of time, the energy required could easily exceed the entire energy of our galaxy.

3: Delayed choice quantum eraser experiments and the non-locality of quantum mechanics seems to indicate that the quantum world is “above” space and time. So, could we live in a world of self-consistently evolving quantum spacetime fields, which would work as a banana-peel-solution to the paradoxes?

This is correct. And this was shown in a precise mathematical way by David Deutsch using quantum mechanics. There may not be there any “banana peel” but it will feel like it.  However, this only solves the grandfather paradox. The lack of information conservation problem still remains and this is against quantum mechanics.

You can’t have the cake and eat it too:  you can’t have a quantum mechanics solution of the grandfather paradox while rejecting quantum mechanics because of lack of information conservation.

I think is safe to say that we all love “Back to the future” movies. I’d love to have a flux capacitor installed in my car and as a physicist I am saddened by the realization that time travel is almost surely impossible. However, physicists pursue time travel questions because they test the limit of our current understanding and the quest can provide hints of how to uncover the ultimate “theory of everything”.