B Time Travel: Is it Physically Possible? | Scientific American

[jedishrfu]

TL;DR Summary

discusses the nature of time travel and how physics approaches it

https://www.scientificamerican.com/article/is-time-travel-possible/

In the movies, time travelers typically step inside a machine and—poof—disappear. They then reappear instantaneously among cowboys, knights or dinosaurs. What these films show is basically time teleportation.

Scientists don’t think this conception is likely in the real world, but they also don’t relegate time travel to the crackpot realm. In fact, the laws of physics might allow chronological hopping, but the devil is in the details.

 

[jedishrfu]

Reading the article left me with a question concerning the twin paradox. Namely one twin goes out at near lightdspeed and returns. He has to accelerate and decelerate twice.

What is the maximum acceleration a human can handle?

From some sources, we can handle 9.8 m/s and for short durations 3x or 4x upto 9x to 10x (really short) that rate. That would mean in a classical sense, it would take the spacefaring twin about a year to get upto near light speeds and a year to come back down and the same on the return trip. Hence 4 years time to travel the journey minimum.

However, the space faring twin wouldn't notice the time to accelerate and decelerate as being a year each right?

If they were traveling at 99.9% lightspeed then only a day would pass for the spacefaring twin instead of a year right?

Just trying to get a handle on the practicalities of near lightspeed travel as connected with time travel.

 

[PeterDonis]

That would mean in a classical sense, it would take the spacefaring twin about a year to get upto near light speeds

Actually it's longer than that in relativity. See below.

the space faring twin wouldn't notice the time to accelerate and decelerate as being a year each right?

The best quick reference I know of for calculating these kinds of problems is this article on the relativistic rocket equation:

https://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html

In the notation of that page, you want to calculate T (the time according to the ship's clock) where a is 1 g and v is 0.99 c. From the table giving typical values for a = 1 g you can see that at 2 years by the ship's clock v is 0.97 c, so it will take somewhat longer than 2 years to get to v = 0.99 c. For the exact number, you can simply invert the equation given for $v$ to obtain

$T = (c / a) \tanh^{-1} (v / c)$.

For v = 0.99 c, this gives $T = 8.096 \times 10^7$ seconds, or $2.566$ years.

For the time $t$ according to Earth clocks, you can use the first equation given since we now know $T$:

$t = (c / a) \sinh (a T / c)$

which for the above $T$ gives $t = 2.147 \times 10^8$ seconds, or $6.083$ years.

 

[Ibix]

If they were traveling at 99.9% lightspeed then only a day would pass for the spacefaring twin instead of a year right?

To add to Peter's calculation, if you accelerate and then do a steady 99.9%c you have a time dilation factor of $1/\sqrt{1-0.999^2}=22.4$, so one year corresponds to a bit over 16 days.

 

[jedishrfu]

The problem is that these solutions are based purely on how to get the math to allow it. We have no idea how to make a wormhole nor how to get its two openings in different spacetime frames nor how to actually travel to the past.

 

[PeterDonis]

We have no idea how to make a wormhole

True.

nor how to get its two openings in different spacetime frames nor how to actually travel to the past.

I don't think this is true; I think at least one of the papers by Kip Thorne and his collaborators on wormhole research in the 1980s and 1990s contained scenarios along these lines. One such is described in Thorne's popular book Black Holes and Time Warps. All of the scenarios assume that we have already made a wormhole and that we can move its openings independently; that's the part we don't actually know how to do.

 

[Herman Trivilino]

If they were traveling at 99.9% lightspeed then only a day would pass for the spacefaring twin instead of a year right?

Just trying to get a handle on the practicalities of near lightspeed travel as connected with time travel.

In the usual statement of the twin paradox the travelling twin ages less than everyone here on Earth. The difference can be large when the speed is good fraction of the speed of light.

But this is not what people usually mean when they talk about time travel. Time travel usually means travelling into the past.

 

[Demystifier]

I would say that "traveling to the past" is an oxymoron, something like "climbing down". I have elaborated this in https://arxiv.org/abs/gr-qc/0403121

 

[Vanadium 50]

that's the part we don't actually know how to do.

That's a little like, "we can have ham and eggs. Well, if we had some ham...and if we had some eggs."

 

[pervect]

The problem is that these solutions are based purely on how to get the math to allow it. We have no idea how to make a wormhole nor how to get its two openings in different spacetime frames nor how to actually travel to the past.

We know a little bit about classical limitations on making a wormhole. Specifically, we know that classically, you'd need a time machine (closed timelike curves) in addition to exotic matter that violates the weak energy condition to make a wormhole with purely classical General relativity, amongst other such issues.

This is a bit oversimplified. See for instance the complete text in the Morris, Thorne, Yurstserver classic paper, https://authors.library.caltech.edu/9262/1/MORprl88.pdf, "Wormholes, Time machines, and the Weak Energy condition", and the footnoted references for the necessary fine print.

Thus, the Morris et al paper suggests not creating a wormhole, but stabilizing one formed through mechanisms involving quantum gravity during the formation process and not classic GR.

 

[MikeS]

I am late to the discussion, but I do have a question related to special relativity.

Assume I am on a starship that accelerated and went fast enough that from my perspective it took only 2 years to travel from Earth to Alpha Centauri. I presume I consumed only 2 years of oxygen instead of 5 years worth of oxygen.

My question is how much fuel did my starship consume-- 2 years or 5 years worth?

 

[Ibix]

I am late to the discussion, but I do have a question related to special relativity.

Assume I am on a starship that accelerated and went fast enough that from my perspective it took only 2 years to travel from Earth to Alpha Centauri. I presume I consumed only 2 years of oxygen instead of 5 years worth of oxygen.

My question is how much fuel did my starship consume-- 2 years or 5 years worth?

No more than it would use in two years in a tethered test, because it runs for no more than two years by its own clocks.

You might not run the engine the whole way, though. You might accelerate, coast at constant speed with your engine off, and decelerate at the end. Then you'd use less than two years' worth - it depends on your mission planning.

 

[phinds]

I am late to the discussion, but I do have a question related to special relativity.

Assume I am on a starship that accelerated and went fast enough that from my perspective it took only 2 years to travel from Earth to Alpha Centauri. I presume I consumed only 2 years of oxygen instead of 5 years worth of oxygen.

My question is how much fuel did my starship consume-- 2 years or 5 years worth?

Sounds like you think that maybe the ship is traveling at a different speed than you are. Good luck with that

 

[jbriggs444]

If the starship uses some sort of rocket propulsion then the [relativistic] Tsiolkovsky equation applies. For constant proper acceleration, the requisite fuel supply is exponential in proper time. Things get very impractical.