Time Travel And Conservation Of Energy/Mass/Matter:
The possibility of time travel, particularly to the past, has had numerous objections raised to it over time. Perhaps one of the most seemingly difficult to grasp is the objection that time travel, particularly to the past, violates the First Law of Thermodynamics, also known as the Law of Conservation of Energy and Mass/Matter (as energy and mass are equivalent, as shown by Albert Einstein's famous equation e=mc^2). This law states that energy can never be created nor destroyed, but can only be changed from one form to another. The reason some have equated this to ruling out time travel is the following: You are probably aware that you existed in the past, for example, one week ago. Even prior to your conception, although you were not alive, the particles that would later make up your body still existed, but were just scattered around in various places until they later coalesced to form you. So every person comes from matter that already existed, and has since the beginning of the Universe. Let's say you time traveled to the Late Jurassic period. Even though it is at least 144 million years before your conception, the energy that would later constitute your body exists, as tiny particles scattered throughout the world (and possibly throughout the universe -- who knows if some of the particles that would later make up your body came to Earth from outer space?). This, according to some, constitutes a violation of the First Law of Thermodynamics, since you now coexist in the same time period alongside the particles in the past that would later form you, with the result that more energy is being added to the Late Jurassic, while energy is being simultaneously removed from the present Quaternary period, constituting a violation of conservation of energy.
This is the crux of the argument against time travel from violation of conservation of energy/mass. However, I disagree with this argument, and this article will refute this argument by probing more deeply into the logical underpinnings at work beneath it.
The Law of Conservation of Energy simply states that, in a closed system, energy cannot be created or destroyed. A closed system is defined as a system in which no input from outside of the system is received by said system. The issue at relevance here is that different time periods are emphatically, demonstrably not closed systems, due to the simple fact that entities are always, constantly moving forward in time, and, therefore, entering new time periods. Someone inevitably entered Wednesday from the preceding Tuesday; they did not just magically, spontaneously pop into existence on Wednesday. Additionally, general relativity shows that space and time are inextricably woven together, as complementary components of a single, unified system known as spacetime. Therefore, since individual time periods are not closed systems, we do not have to apply the conservation law to particular periods of time, on their own. Considering the entire spacetime continuum, altogether, to constitute a closed system, someone popping into a past time prior to their conception, and existing alongside the particles that would later make up the ovum and spermatozoon that would eventually conceive them, would not be injecting more mass or energy into a closed system, as, without time travel into the past, both the putative time traveller and the particles in the past prior to the individual's conception that would later come to constitute their body already are coexisting in the spacetime continuum -- merely at different times. Travel to the past would merely bring their locations in spacetime into greater proximity with one another, as they are now at the same time, instead of at differing times.
As a thought experiment, let us now envision a wormhole connecting the year 1733 to the year 1725, for example. A person conceived in 1721 who is twelve years old in 1733 and four years old in 1725 would exist in both time periods. Now let's say the twelve-year-old goes through the wormhole, and arrives back in time in 1725 from 1733. When this happens, the twelve-year-old disappears from 1733, and reappears in 1725. While, if we were to consider each of the times, 1733 and 1725, as closed systems, this would, indeed, be in violation of the First Law, since we know that they are not closed systems, we know that this is not a violation. If we are to consider the entire spacetime continuum, as a whole, to be a closed system, then, there is no violation of the First Law of Thermodynamics inherent in this situation, as the disappearance of the time traveller from 1733 is balanced out by his/her subsequent reappearance in 1725. It's just like how removing a peanut from a bag of peanuts does not violate the law of conservation of energy/mass, as the peanut bag is not a closed system, but, rather, part of a closed system. Energy/matter can, indeed, be displaced within a closed system. And being displaced is completely different from being destroyed or created.
Energy can be displaced from one region of a closed system and arrive at another region in its stead. There is, theoretically, no reason that a person could not coexist at the same time as the particles which would later go on to constitute their physique, instead of existing at a different time from them. Only the location of the person along the time dimension would have changed, without creating any new energy, so this would not violate the Law of Conservation of Energy, and, by extension, of Mass and of Matter.
Overall, this argument against time travel, particularly time travel to the past, seems compelling at first glance, but, upon closer examination, its faults become readily apparent. It shall be noted that one may feel tempted to accept arguments against the possibility of time travel due to the fact that time travel contradicts common sense. However, there are numerous statements made by science, some of which are facts, which contradict common sense. Common sense is not always necessarily an infallible arbiter of truth. One must always tread with caution, and think critically about any arguments one finds, and parse them logically, even if they seem to appeal to intuitive notions of common sense. This is how progress is made, and new discoveries that potentially overturn paradigms occur.