## 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”.

Thank you Florin for these interesting blogs. Many years ago a guy dying of cancer wrote an earnest book called The Social Limits to Growth in which he tried to think through how humanity was bound not just by physical limits but also by its own institutions. One point he made was that skills that took more than 25 years to learn could not be passed down as part of culture because society wouldn't pay for an educational system that took more than 25 years to create functional graduates.

ReplyDeleteThe depth and breadth of your knowledge reminds me that what may be true for society is not a constraint on individuals with the temperament, focus, and talent of a genius.

The kind of time travel I enjoy is the telepathy across space and time of a great book.

Thank you Nora for your kind words. I am still learning about physics and math, and I will start a new series on the math required to understand nature in modern way. I was thinking that in elementary school we learn about reading and writing which is 5-6 thousands years old, in middle school, we learn about Pythagoras theorem which is over 2000 years old. Then at the end of high school we may learn calculus which is "only" 400 years old. In college we learn the math of 1800s, in graduate school we learn the math of 1920s. I am proficient in the math of 1950s and scratching the surface of the math from the 60s-70s. Young, one of the discoverers of gauge theory which explains the fundamental forces in the universe (and also a Nobel prize recipient) once said : that there are only two kinds of modern mathematical books: the ones on which you cannot read past the first page, and the one which you cannot read past the first sentence. But the funny thing is that whatever mathematicians discover have an intuitive and deep relevance in the physical world. I hope I will be able to bring that to life.

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