Impossibility of Backwards Time Travel

[timetravel888]

Sorry, but I only have very limited knowledge in physics. I came to ponder the concept of time travel.
Backwards time travel has been a topic of discussion for as long as man has pondered his own existence. However, what this simplistic discussion attempts to do is to explain how backwards time travel is an impossibility due to the violation of the law of conservation of mass. For this theory to work, we must assume that the law of the conservation of mass is a given. We must create a closed system with a fixed amount of mass, mass that does not change over time.
As an example, let us assume that our closed system, Earth, has a fixed amount of mass bounded by the vacuum of space. We will call this mass X, which includes the mass of all human beings on the planet. The total mass of Earth X remains unchanged with time. Mass is not created nor destroyed.
Let us assume that someone discovered a way for backwards time travel to exist by creating a time machine. A human with a given mass, Y, uses the time machine to go back in time. For example, this human goes back ten years in time. Now, the mass of the system Earth ten years ago is now X + Y. The mass Y is being added because there already exists the mass of the time traveler in the total mass X. In essence, this is creating mass that did not exist prior. This is violating the law of the conservation of mass. Let us take this example to an extreme. Let us assume that the time machine has a high capacity and can transport all of its occupants back in time. Let us also assume that this time machine can operate at extremely high speeds (i.e. it can go back in time one time per second). Now, the human can go back one second in time, find oneself (or any other individual), and then transport two of the same individual two seconds back and so forth. So, in a matter of one year, one can be with 31,536,000 “clones” of oneself. Now take this example to the extreme with it being able to operate at the millisecond, nanosecond or femtosecond level. One could be able to create a near infinite number of “clones” of oneself, thus creating a near infinite amount of mass. This would be a clear violation of the law of conservation of mass. Mass would be created in our closed system Earth, which is an impossibility. Backwards time travel is in essence functioning as a ‘mass generator’. Backwards time travel is in clear violation of the law of conservation of mass.
Conversely, time travel into the future does not violate the law of conservation of mass. There is no net gain or loss in the total mass. For example, using a time machine or a rocket ship to go forwards in time, a human boards the device today and then travels ten years into the future. At all points along the time continuum, the total mass of the Earth remains a constant as the mass of the human being Y is moving along the time continuum with the Earth. The total mass of the system would still remain constant. Ten years from now, the total mass of the system would still be X. No additional mass is created nor destroyed and thus no violation of the law of the conservation of mass has taken place.
Conclusion: Backwards time travel is impossible as it violates the law of conservation of mass. Conversely, forward time travel is possible as it does not violate the law of conservation of mass.

 

[stevenb]

Definitely good thinking on your part.

You won't get too many PF members disagreeing with you conclusions. Keep in mind that conservation of mass is not really a law of nature. It's really conservation of mass/energy that is an accepted law.

What if someone argues that they have developed a time machine that sends a person back in time, but transfers an appropriate amount of energy from the past to the present in order to maintain a balance of mass/energy? How would you respond to such a person?

Also, keep in mind that conservation of mass/energy or conservation of charge are more than just laws which say that a quantity is conserved in the universe. Instead they are laws that express local conservation in a way that says that any change in a quantity in a region must be accounted for by flow of that quantity through the boundary of that region. This is an expression of your point in a way. Change in a conserved quantity in time is due to change flowing through a boundary and not direct transport to a remote location or a remote time.

 

[JesseM]

First of all, conservation of mass/energy doesn't actually hold in all situations in general relativity (and general relativity is the theory that contains theoretical solutions involving backwards time travel). See this page:

http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

There are some situations where energy conservation is more simple in general relativity though. The above article says:

In certain special cases, energy conservation works out with fewer caveats. The two main examples are static spacetimes and asymptotically flat spacetimes.

One of the time travel solutions in general relativity involves a traversable wormhole, and wormhole spacetimes are usually treated as "asymptotically flat" (meaning they approach perfect flatness as the distance from the wormhole approaches infinity), so the above quote seems to suggest energy conservation would hold in this situation. So how can this be consistent with the idea that wormholes would allow time travel? Pervect's comments here suggest the answer: for a wormhole, whenever an object enters one mouth then the mouth's own mass will increase by the same amount, and likewise whenever an object comes out of a mouth the mouth's own mass decreases by the same amount (with the mass of the mouth possibly becoming negative if enough objects exit it). So, this could explain why there's no violation of conservation of energy even when an object entering one mouth will exit the other in the past.

 

[bcrowell]

General relativity does not have a general law of conservation of mass-energy; all it has is conservation laws that work in certain special cases, such as asymptotically flat spacetimes. In any case, it is not necessarily true that CTCs lead to nonconservation of mass-energy, even in asymptotically flat cases: https://www.physicsforums.com/showthread.php?p=819700#post819700

 

[DLuckyE]

Wouldn't a wormhole be able to violate conservation of energy anyhow? If you'd put one facing up from the ground and the other end facing down towards the first entrance then if you'd throw something in it would be accelerated by gravity forever.

Even worse if you bring time travel into it, then it could be making copies of the item forever.

 

[bcrowell]

Wouldn't a wormhole be able to violate conservation of energy anyhow? If you'd put one facing up from the ground and the other end facing down towards the first entrance then if you'd throw something in it would be accelerated by gravity forever.

Even worse if you bring time travel into it, then it could be making copies of the item forever.

See the thread I linked to in #4.

 

[timetravel888]

I think it is very cool that this topic generated some discussion. I only completed one year of college level physics (and did not do very well in it). I was just watching a program on the Discovery Channel about time travel and came up with this hypothesis/concept. You guys can continue debating this topic because things like wormholes are way over my head.