# Time Travel and Multiple universes hypothesis

#### Aontaigh

The Multiple universes hypothesis states that there are infinite number of universes, collectively known as the "multiverse". If a person is about to travel back in time, he will create his own parallel universe upon arrival in the past. So if he kills "his" grandfather, a paradox would not occur because the grandfather that he has killed is not his own grandfather from the universe from which he came, but that of the version of himself in the universe he is now in. I believe string theory also relates to this? What would happen if a person went back in time to meet themselves? Wouldn't this also mean our universe was created by a person who travelled back in time which we would surely have evidence of?

Last edited:
Related Beyond the Standard Model News on Phys.org

#### phinds

Gold Member
The Multiple universes hypothesis states that there are infinite number of universes, collectively known as the "multiverse". If a person is about to travel back in time, he will create his own parallel universe upon arrival in the past. So if he kills "his" grandfather, a paradox would not occur because the grandfather that he has killed is not his own grandfather from the universe from which he came, but that of the version of himself in the universe he is now in. I believe string theory also relates to this? What would happen if a person went back in time to meet themselves? Wouldn't this also mean our universe was created by a person who travelled back in time which we would surely have evidence of?
As far as is known travel backwards in time is not possible and never will be so you can posit whatever answers you like to your questions since false premises lead to any conclusion you like.

#### Aontaigh

As far as is known travel backwards in time is not possible and never will be so you can posit whatever answers you like to your questions since false premises lead to any conclusion you like.
Isn't it mathematically possible to travel back in time?

#### phinds

Gold Member
Isn't it mathematically possible to travel back in time?
It might be. There are things that show up in math that do not show up in reality. The map is not the territory.

#### Aontaigh

It might be. There are things that show up in math that do not show up in reality. The map is not the territory.
If I am correct, no law of physics says travelling back in time is impossible? Einstein's theory of special relativity dictates that an object gains mass as it accelerates. The faster it goes, the more massive it becomes. The more massive it becomes, the more thrust or push it requires in order to maintain its speed. According to relativistic predictions, if an object with any mass were to achieve the speed of light, its mass would become infinite. But assuming that the object did achieve infinite mass during light-speed travel, to keep moving, the power behind its thrust or push would need to be infinite as well. No force in the known universe can achieve this, short of space-time itself.

Would it be safe to assume it's technically possible but basically impossible and will never happen if you get me?

#### HallsofIvy

Homework Helper
If I am correct, no law of physics says travelling back in time is impossible? Einstein's theory of special relativity dictates that an object gains mass as it accelerates. The faster it goes, the more massive it becomes. The more massive it becomes, the more thrust or push it requires in order to maintain its speed. According to relativistic predictions, if an object with any mass were to achieve the speed of light, its mass would become infinite. But assuming that the object did achieve infinite mass during light-speed travel, to keep moving, the power behind its thrust or push would need to be infinite as well. No force in the known universe can achieve this, short of space-time itself.

Would it be safe to assume it's technically possible but basically impossible and will never happen if you get me?
There are several unspoken assumptions in that. You seem to be assuming that
1) If you went faster than light, you would go back in time.
2) Going faster than light is the only way to go back in time.

And concluding that "because it is impossible to go faster than light, it is impossible to go back in time".

#### phinds

Gold Member
Would it be safe to assume it's technically possible but basically impossible and will never happen if you get me?
It is not TECHNICALLY impossible for a massive object to go at c, it is PHYSICALLY impossible.

#### phinds

Gold Member
If I am correct, no law of physics says travelling back in time is impossible?

#### ChrisVer

Gold Member
No PHYSICAL mass is going to infinity as the particle reaches v=c. The mass that does is the relativistic mass [reference dependent].
It's not theoretically impossible to reach v>c for that reason tachyons were initially studied... in that case however, the rest mass squared becomes negative. That the tachyon happens to have negative mass squared [or imaginary mass] doesn't mean that the propagation of the particle is higher than c [as naively was seen from the superluminal velocity], but the $m_{0}^{2} <0$ represents an instability to the tachyon condensation. The propagation velocity for the particles then remain subluminal.

http://en.wikipedia.org/wiki/Tachyon
http://en.wikipedia.org/wiki/Tachyon_condensation

#### cosmik debris

General Relativity has solutions with Closed Time-like Curves (CTCs) but require forms of exotic matter. Quantum Mechanics with a technique called Postselection can also have CTCs where some effects seem to propagate backwards. The latter, because of the probabilistic nature of QM, may imply some latitude in the paradoxes created by time-travel. All speculative and seemingly only possible in the quantum realm.

#### craigi

The Multiple universes hypothesis states that there are infinite number of universes, collectively known as the "multiverse". If a person is about to travel back in time, he will create his own parallel universe upon arrival in the past. So if he kills "his" grandfather, a paradox would not occur because the grandfather that he has killed is not his own grandfather from the universe from which he came, but that of the version of himself in the universe he is now in. I believe string theory also relates to this? What would happen if a person went back in time to meet themselves? Wouldn't this also mean our universe was created by a person who travelled back in time which we would surely have evidence of?
Objections to time travel aside, you misunderstand the concept of the multiverse.

The only description of a multiverse that is similar to what you refer to comes from the Many Worlds Interpretation of Quantum Mechanics. Here, it's important to note that unconnected parallel universes are created as part of quantum mechanics and are not created by a person. One universe splits into multiple universes, but there is no way to travel from one of these universes to another.

It would help a lot, if you could tell us what you've been reading or watching that suggested this to you.

Last edited:

#### Chronos

Gold Member
Why would a multiverse that has no observational consequence in our universe be relevant?

#### craigi

Why would a multiverse that has no observational consequence in our universe be relevant?
It's not clear whether you're asking about relevance to the thread or relevance to physics in general.

In the case of the former, the originial post seems to have arisen from a popular account of the work of David Deutsch: Quantum mechanics near closed timelike lines. http://journals.aps.org/prd/abstract/10.1103/PhysRevD.44.3197

In the case of the latter, there is a common misconception that the Many Worlds Interpretation proposes a multiverse. This isn't the case. The multiverse is a consequence of Everett's observation that wavefunction collapse, which gives rise to the measurement problem, amongst other issues, was an unecessary assumption in the Copenhagen Interpretation. Everett actually called his interpretation, the Relative State Forumulation. Primairily, he noted that the act of measurement was a binding of measurement apparatus and measured object. In modern terms, this can be seen as the equivalence of measurement and entanglement, through the process of decoherence. It was under later popularisation of his work, that his interpretation aquired the MWI name. It seems to me that had the original name been kept, then there would be much less supposition about what the interpretation actually proposes. Obviously, we can add back in another assumption, as many have tried, to make the multiverse go away, but there is no evidence to support such an assumption.

Last edited:

#### arivero

Gold Member
If a person is about to travel back in time, he will create his own parallel universe upon arrival in the past.
So the person has not travelled back in time, it has just been a travel forward to other universe which has its own chronology, delayed some years from the original :tongue:

#### ChrisVer

Gold Member
It doesn't make sense, you can of course do it... you won't have a problem since your time is still moving forward [you don't turn from a middle aged person to a child, a baby, a sperm and to nothingness as you travelled back in time]

#### craigi

It doesn't make sense, you can of course do it... you won't have a problem since your time is still moving forward [you don't turn from a middle aged person to a child, a baby, a sperm and to nothingness as you travelled back in time]
There exist solutions to the Einstein Field Equations which permit timelike lines into the past. This presents problems for causality. As far as I understand, there are 3 possible resolutions:

1) It's just not possible. GR is incomplete and some, as yet unknown, physics stops it from happening.
2) Only self-consistent loops are possible.
3) Travel to the past occurs, but to a parallel universe with no causal link to the travelling object.

#### Lord Crc

There exist solutions to the Einstein Field Equations which permit timelike lines into the past.
Possibly a quite stupid question: what about energy in a universe with a CTC? Are there some restrictions to how these curves can behave so to speak, preventing infinite power to accumulate?

#### cosmik debris

There exist solutions to the Einstein Field Equations which permit timelike lines into the past. This presents problems for causality. As far as I understand, there are 3 possible resolutions:

1) It's just not possible. GR is incomplete and some, as yet unknown, physics stops it from happening.
2) Only self-consistent loops are possible.
3) Travel to the past occurs, but to a parallel universe with no causal link to the travelling object.

The Postselection CTCs in Symmetrical Time QM seem to have a property that amplifies events that would have happened anyway, this prevents the various Grandparent problems. Another way to get around these paradoxes is to assign probabilities to the events e.g. assign 50% probability to grand patricide and 50% to being born, this is at least consistent.

#### Sousf

This is my own view on the topic
Say if you are able to travel back in time(if you could built a machine)You wouldn't really be travel back in time in the same universe you are in but you travel into a different universe that have the exact replica of yourself except different time.
So if you are to kill your grandfather at that time,it would not have any affect on the future of your world but only the other parallel world future you change

This is something that come to me when I was taking a shower lol
This might also be very very very stupid so if I'm very wrong don't judge (just give me advise)
Thank you

#### Sousf

I might have misunderstood the topic lol

#### phinds

Gold Member
I might have misunderstood the topic lol
You might also not have been paying any attention at all to the thread, since that suggestion has already been made.

#### cosmik debris

It may be worth pointing out to those who like to think about this that, in TSQM at least, there are different types of CTCs. In one of them there is no memory of the future state and in another there is. The latter may be used for some form of computation while of course the former can't.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving