# I Verifying claims about backwards time travel

1. Nov 19, 2017

### girts

So I understand the part about traveling to the future when a person reaches near the speed of light (as a physical object could not reach the very speedlimit itself)
but how does the backwards travel works? Please explain whether the author is completely making this up or does this have some (any) merit for that matter?

https://www.forbes.com/sites/starts...d-really-physically-be-possible/#674067f012db

thanks.

2. Nov 19, 2017

### Orodruin

Staff Emeritus
You are always traveling into the future. At a rate of 1 second per second.

Regarding wormholes to the past, nothing has been observed that supports their existence.

3. Nov 19, 2017

### Staff: Mentor

The author is not completely making it up. However, as the author mentioned, all of this speculation hinges on the idea of negative mass, which has never been observed.

4. Nov 19, 2017

### m4r35n357

AFAIK negative inertial mass implies motion opposite to an applied impulse, and negative gravitational mass implies gravitational repulsion.
Is there even a definition of negative mass that takes the equivalence principle into account (either to accommodate or reject it)?

5. Nov 19, 2017

### Staff: Mentor

Neither of those things are what "negative mass" means in this connection. When we are talking about what is required to hold a wormhole open, "negative mass" means "a stress-energy tensor that violates the energy conditions"--specifically, IIRC, the null energy condition. One way of stating this in more physical terms is that the stress-energy in question must have a tension in its rest frame that is larger than its energy density--heurisitically, the tension is what holds the wormhole open, and the tension being larger than the energy density is what keeps the wormhole from collapsing due to its own gravity.

6. Nov 19, 2017

### m4r35n357

OK, thanks for that!

7. Nov 19, 2017

### Jacinta

If you'd need the speed of light to travel forwards in time, would that mean you'd need the opposite of the speed of light to travel back in time?

8. Nov 19, 2017

### Staff: Mentor

You travel forwards in time even at rest.

9. Nov 19, 2017

### Jacinta

But you'd travel even farther into the future at the speed of light.

10. Nov 19, 2017

### Staff: Mentor

But you don’t “need” it to travel forward in time. And in fact it is impossible for an object with mass to achieve.

11. Nov 19, 2017

### Jacinta

You might if you'd want to travel faster into the future then 1 second per second.

What about de-massifying the object? Never say never.

12. Nov 19, 2017

### Staff: Mentor

We don’t need to say never, just that it is not a suitable topic for PF until it is published in a peer reviewed paper.

The stuff that @girts is talking about from the article he read is discussed in the literature, and he is carefully asking to learn the current state of science. Please do not confuse his legitimate query with personal speculation.

13. Nov 19, 2017

### Staff: Mentor

Several points, in no particular order:
1) Travel at the speed of light is impossible (except for propagating light and gravitational waves) so we have to say "at a speed very close to the speed of light" instead of "at the speed of light". This isn't just a quibble. All of relativity is based on assumptions that are equivalent to saying that travel at the speed of light is impossible, so any attempt to apply the relativistic time dilation formulas to something moving at the speed of light is inherently self-contradictory - it's like starting a mathematical proof with "consider the two integers $a$ and $b$ such at $\frac{a}{b}=\sqrt{2}$.
2) It makes no sense to speak of travel at any speed without saying what it is relative to. Right I am at rest relative to the chair I'm sitting in, moving at several kilometers per second relative to Mars, and moving at very close to the speed of light relative to a particle zooming through the Large Hadron Collider at CERN. So which of these speeds would I use to decide how quickly I'm travelling into the future?
3) The answer to #2 is "none of them". I am moving into the future at one second per second no matter what my speed is relative to anything or anybody else anywhere in the universe.

14. Nov 20, 2017

### 1977ub

Is it possible to have a wormhole scenario where causality doesn't end up violated - or at least could easily be violated?

15. Nov 20, 2017

### jbriggs444

We do not know if closed timelike loops are achievable. There are speculations about how closed timelike loops can avoid leading to consistency problems. The references I can dig up do not meet PF standards, though they appear reputable.

16. Nov 23, 2017

### nitsuj

How, beyond math with coordinates, could someone determine they are traveling backwards in time? I can tell slower, or faster...but not "backwards" or "forwards", geometrically speaking; as I don't think anyone is imagining happenings in "reverse" such as unbreaking eggs.

17. Nov 23, 2017

### Mister T

By looking at a clock?

I am. Unscrambing and uncooking, too.

18. Nov 24, 2017

### nitsuj

I believe the clock doesn't measure direction of time, yes it's incremental. However so is a ruler regardless of direction. And proper time is proper time, I'm guessing you mean looking at some distant clock.

I know for certain that my mechanical watch CANNOT physically tick backwards. By design the hands move clockwise.

Last edited: Nov 24, 2017
19. Nov 24, 2017

### Staff: Mentor

By the fact that your worldline passes through the same event multiple times. In other words, your worldline is a closed timelike curve. That's what the article linked to in the OP is talking about.

The OP article certainly isn't.

That should be a separate discussion in a separate thread. Please keep this thread focused on the OP question.

20. Nov 24, 2017

### girts

ok as I already thought this is possible in a sense that our current best working theories that govern this are not against it but that doesn't necessarily mean that it is physically possible, would that be a fair argument? Given the fact we are still trying to produce a net positive fusion reactor and only recently began exploring our own solar system with robotic spacecraft I would say even if this negative mass would one day be observed, how likely is the chance of creating something like this? Pretty small to say unrealistic?

I can understand the concept of traveling into the future as that even doesn't sound so unrealistic once you think about it, quite frankly i think of it as not so much traveling into the future as simply slowing down the rate at which the traveler ages as he/she approaches near lightspeed while the ones who aren't traveling at that speed relative to the traveler age much faster, so by the way does that mean that everything is going much slower for the traveler in the spacecraft or train or whatever is traveling near lightspeed, does it also mean that say the rate at which caesium 133 electrons transition in a atomic clock also decreases and hence its output shows the slowing down of time?

Ok if this is correct then I think I got future time travel, but then , pardon my ignorance in terms of that I haven't read all the scientific literature regarding this topic beforehand, but how can one travel backwards in time? because what has happened has happened that would imply one can reverse that but doesnt that break some basic rules? the article mentions this but honestly not in a very bright or explanatory way so I am asking here.
Say for example I make an antenna and transmit a wave and I've done that say 40 years ago and it was a Beatles record, now since that antenna has long been switched off , theoretically there should be somewhere among the background "noise" of space still my Beatles "airwaves" expanding and decreasing in amplitude as the waves get further away from the source, now how can I move to a point where I am back again at the place where the waves originated near my antenna? If the antenna is not there anymore nor is the music source nor the amplifier and wires?

I could maybe figure this out myself but I believe this one will be more fun asking you guys first.