Time travel is a very popular theme in many science fiction movies, TV shows and books. The hard truth though is that there is no proof that the past or the future even exist and I have written about that in another post (A Loophole That Will Let You Travel Back One Day In Time).

To quote that older post:

There is no such thing as time travel and all we have is the ever-evolving now. The reason for saying these things is that there are no experiments which can directly observe what is happening even a brief moment into the future. Imagine you have equipment looking at a radioactive nucleus. There is no test in our current technology that can 'see' that nucleus decaying even one nanosecond into the future. When it happens, it happens and it is basically an unpredictable event. We also can't 'detect' what happened even one nanosecond in the past. Photographs and recordings don't count because they only exist in the present.

Basically there are no experiments in physics that let us 'see' the past or the future other than some quantum weirdness such as the quantum eraser experiment and the fuzziness of the Uncertainty Principle.

Okay so now that is explained I want to go on about the subject of this post.

If time travel did exist then it would violate the cherished and well-established conservation laws of physics. I will try to explain why this is so in the next few sections.

Conservation of Energy

The law of conservation of energy tells us that the total energy of a system stays constant. Energy is never created or destroyed, it is only transformed from one form to another (Einstein's mass-energy equivalence is an extension of this law).

Let us now imagine that we have constructed a device in some lab somewhere that is able to see a few nanoseconds into the past or the future (a time 'telescope' or a time 'microscope' if you will).

This postulated device when looking into the past it is not making some recording from a few nanoseconds ago and simple reporting that, it is instead actually measuring the positions and momenta of particles from the very recent past.

Also, this device when looking into the future is not simply measuring the positions and momenta of particles in the here and now and making a prediction of where they will be in a few nanoseconds, it is actually measuring the positions and momenta of particles from the very near future.

Okay then.

If this device was 'seeing' these particles either in the past, or in the future then it must be doing so using an information carrying particle such as a photon or a tachyon (maybe) to execute this task.

This means that the particle carrying the information will have an energy associated with it and this energy will either be brought forward or backward in time to the current moment. This act will leave an energy deficit in the other time period and provide an energy surplus in the present moment.

This is a clear violation of the law of conservation of energy.

To the past it will look like a mysterious cooling event occurred and that energy just up and disappeared. Ditto for the future.

Conservation of Momentum

A photon is a particle with zero rest mass and it travels at 300,000 km/s in a vacuum. Weirdly though, even though photons have zero rest mass they do possess momentum.

Let us imagine then that a photon is used by the postulated 'time telescope' described above to transmit information from either past or the future. This means that momentum will be robbed from the past or the future and re-appear in the current moment.

So in this case we would have a violation of the law of conservation of momentum.

If any exotic particle is used to help us peer into the past or future and it has a momentum associated with it, then it it will violate this conservation law as it moves through time.

Conservation of Mass and Charge

Let us imagine an improved time device, this one can actually grab a collection of particles from a few nanoseconds in the past or the future and bring them into the present moment.

If the collection of particles is electrically neutral then conservation of charge might not be violated but conservation of mass would definitely be violated. The past or the future would be robbed of mass and it would mysteriously disappear to observers in those periods.

If the collection of particles was not electrically neutral then conservation of charge would be violated. The past or the future would be robbed of electric charge and it would mysteriously disappear to observers in those periods.

Even if the collection of particles were electrically neutral, the process of time travel would not be instantaneous so observers in the past or future might see a brief moment where the measured electric charge would waver and shift if the electrons and protons time traveled out of sync with each other. So even this might construed to be brief violation of conservation of charge.

Closing Words

Time travel is an interesting topic for fiction writers but that is about it and it is very entertaining.

However the hard truth is that there is really no observable evidence for it barring some quantum weirdness such as the quantum eraser experiment and the fuzziness of the Uncertainty Principle.

Also the very existence of time travel would be a mortal death blow to several of the well-established laws of conservation in physics.

If we could 'steal' energy from the past we would solve all of our energy problems but give energy problems to the past

Hmmm, maybe this is what killed the dinosaurs.

Thank you for taking the time to read my post.

Post Sources

https://en.wikipedia.org/wiki/Quantum_eraser_experiment
https://en.wikipedia.org/wiki/Uncertainty_principle
https://en.wikipedia.org/wiki/Conservation_of_energy
https://en.wikipedia.org/wiki/Conservation_of_mass
https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence
https://en.wikipedia.org/wiki/Tachyon
https://en.wikipedia.org/wiki/Photon