Does it need light speed or not

[HuaMin]

Hi,
If there is really one way to go back the past, must the way be through light speed or not? If the answer is yes, I really do not believe that the human beings or any animals are able to endure light speed technically!

Best Regards

 

[tiny-tim]

Welcome to PF!

Hi HuaMin! Welcome to PF!

Hi,
If there is really one way to go back the past, must the way be through light speed or not? If the answer is yes, I really do not believe that the human beings or any animals are able to endure light speed technically!

Best Regards

No, that's only in science fiction …

(unless there are "wormholes") there is no way of going back into the past

 

[HuaMin]

Thanks. I know Wormhole is one of the research area in Physics. What speed is needed if one day we will go through the wormhole?

Many thanks

 

[tiny-tim]

wormhole research is on an entirely mathematical basis

you could pass through a wormhole (if any exist, and are large enough not to crush you) at any speed you like, same as you can go to alpha centauri at any speed you like (both slower than light, of course)

 

[HuaMin]

Many thanks. Can I have more details for the wormhole that leads to the past across the time dimension?

 

[Drakkith]

Many thanks. Can I have more details for the wormhole that leads to the past across the time dimension?

You could try here: http://en.wikipedia.org/wiki/Wormhole

Just be warned that time travel is generally frowned upon here at PF, so I would be careful in discussing it. (Meaning I probably wouldn't discuss it here)

 

[HuaMin]

Thanks. I know wormhole does exist only in the multi-dimensional Mathematical models. Do you think it will be technically possible to locate that in reality?

Best regards

 

[Nabeshin]

Thanks. I know wormhole does exist only in the multi-dimensional Mathematical models. Do you think it will be technically possible to locate that in reality?

This actually isn't true. A wormhole requires only the 4 spacetime dimensions we're used to. We usually depict such a structure by embedding it in a higher dimensional space, but this embedding is not required for its existence; it's merely a byproduct of the visualization.

 

[HarryRool]

Seriously, HuaMin, Google is your friend.

There are also quite a few books on wormholes intended for lay audiences:

Cosmic Wormholes by Paul Halpern

Black Holes & Time Warps -- Einsteins Outrageous Legacy by Kip Thorne

Black Holes, Wormholes and Time Machines by Jim Al-Khalili

The Physics of Stargates -- Parallel Universes, Time Travel and the Enigma of Wormhole Physics by Enrico Rodrigo

Unveiling the Edge of Time -- Black Holes, White Holes, Wormholes by John Gribbin

The bottom line is that, despite a lot of theoretical effort, the the existence of wormholes cannot be ruled out. This especially true in the context of quantum theory or when one considers reasonable generalizations of Einstein's theory of gravity.

 

[Ryan_m_b]

It is possible that even with wormholes time travel will still not be possible (or even if it is changing history will not be possible).
http://homepages.mcs.vuw.ac.nz/~visser/general.shtml

One of the truly weird side-effects of the wormhole/warp-drive discussion is that it seems to lead, almost inevitably, to the possibility of time travel. (At his stage the more conservative members of the physics community typically shake their heads in befuddlement and leave the room.) There are roughly four broad classes of response suitable for handling the various types of paradox induced by the possibility of time travel:

The radical re-write conjecture: Grit your teeth and proceed to re-write all of modern physics from the ground up. Painful, very painful. (I'll live with this if necessary-but you'd better give me good experimental evidence before I spend too much time worrying about this possibility.)

The consistency conjecture: Since there seems to be only one universe, insist that it must be consistent no matter what. So if you try to change history you cannot succeed no matter how hard you try, because the past is already fixed. You know that you, the reader, are alive right now, so no-one can ever send a time traveller to five minutes ago to kill you as you pick up your copy of Phlogiston. If someone tries, something must go wrong: the gun must misfire, or the time machine malfumction, or the assasin miss the bus, or any of a potenially infinite list of increasingly contrived excuses. (Not my favourite way of dealing with things; it quickly begins to look like a consistency conspiracy.)

The chronology protection conjecture: A much more conservative point of view. Even though time travel seems to be absurdly easy once wormholes/warp-drives are allowed, there are reasons to expect things to go berserk just at the onset of time travel. We know that gravitational fields distort the quantum mechanical vacuum, and that this vacuum distortion heads off to infinity at the onset of time travel. We suspect that this effect destroys the wormhole/warp-bubble just as one is getting round to building a time machine. (This is my personal favourite.)

The boring physics conjecture: Forget all this nonsense. Take a good hard look at the experimental evidence, or rather lack thereof, and move on to greener pastures.

To wrap up then, what we are doing is laying the groundwork for Planck scale physics, and groping our way toward the as yet ill-perceived theory of quantum gravity. We are surveying the lay of the land, and even if we do not yet have definitive answers, we are trying to at least formulate the right questions. In particular, whatever the true theory of quantum gravity is, at energies below the Planck scale it must reduce to semiclassical quantum gravity. So any question you can ask about semiclassical quantum gravity must be askable and answerable in the full-fledged theory of quantum gravity. Whatever the theory of quantum gravity ultimately proves to be, it must (among many other things) be able to give solid answers concerning the existence of, and properties of, wormholes, warp-drives, time machines, and other exotica. The author:

Matt Visser grew up in Lower Hutt and studied undergraduate physics at Victoria University. He obtained his Ph.D. at the University of California at Berkeley. He is presently Research Assistant Professor of Physics at Washington University in Saint Louis.

 

[georgir]

...I really do not believe that the human beings or any animals are able to endure light speed technically!...

This seems the most misguided part of your post to me. Speed does not have any effect on what a traveller feels, only acceleration does. And even acceleration would not, if it were uniform, as in from a gravity field, as opposed to spread around along the body through stress and elastic forces.

 

[Ryan_m_b]

And even acceleration would not, if it were uniform

Perhaps I'm misunderstanding something but this doesn't seem correct. The human body can only widthstand certain accelerations for certain amounts of time without health complications.

 

[jbriggs444]

That is because the acceleration is non-uniform. Take, for instance a high G decelleration in a rocket sled. It is the lack of uniform acceleration that kills you -- for instance your blood, your retinas or your aorta failing to decellerate along with the rest of your body.

If, hypothetically, you could decellerate your entire body uniformly (e.g. in a high strength, uniform gravitational field) then you would feel nothing out of the ordinary.

 

[Ryan_m_b]

That is because the acceleration is non-uniform. Take, for instance a high G decelleration in a rocket sled. It is the lack of uniform acceleration that kills you -- for instance your blood, your retinas or your aorta failing to decellerate along with the rest of your body.

If, hypothetically, you could decellerate your entire body uniformly (e.g. in a high strength, uniform gravitational field) then you would feel nothing out of the ordinary.

*smacks forehead. That's pretty obvious now

 

[HuaMin]

That is because the acceleration is non-uniform. Take, for instance a high G decelleration in a rocket sled. It is the lack of uniform acceleration that kills you -- for instance your blood, your retinas or your aorta failing to decellerate along with the rest of your body.

If, hypothetically, you could decellerate your entire body uniformly (e.g. in a high strength, uniform gravitational field) then you would feel nothing out of the ordinary.

Many thanks to you all.
Jbriggs,
Is there any evidence/lab completed to prove such things?

 

[jetwaterluffy]

That is because the acceleration is non-uniform. Take, for instance a high G decelleration in a rocket sled. It is the lack of uniform acceleration that kills you -- for instance your blood, your retinas or your aorta failing to decellerate along with the rest of your body.

If, hypothetically, you could decellerate your entire body uniformly (e.g. in a high strength, uniform gravitational field) then you would feel nothing out of the ordinary.

Not true. A high strength, uniform gravitational field will likely kill you over a long time period. High gs in a rocket sled don't count because they are over a short peroid of time. E.g a slap is 100g or something, and that doesn't kill you because its over really quickly.

 

[jbriggs444]

Not true. A high strength, uniform gravitational field will likely kill you over a long time period. High gs in a rocket sled don't count because they are over a short peroid of time.

Do you have evidence for your claim here that the equivalance principle is false?

We're talking about free fall in a high strength gravitational field where tidal forces are assumed away.

 

[Drakkith]

Not true. A high strength, uniform gravitational field will likely kill you over a long time period. High gs in a rocket sled don't count because they are over a short peroid of time. E.g a slap is 100g or something, and that doesn't kill you because its over really quickly.

If the field is uniform, and you are in free fall, then you experience no G forces and it will have no detrimental effects. If you are in a high strength field and NOT in free fall, such as standing on the surface of something, then you will promptly be squished into a pile of mush, just like standing on the surface of a neutron star would do.

 

[HuaMin]

Thanks a lot to you all. Is it possible to technically make the wormhole a "free fall" state or not?

 

[rbj]

Thanks a lot to you all. Is it possible to technically make the wormhole a "free fall" state or not?

Hua, can you get out of this Star Trek science fiction mode and ask questions about physics?


oh, and to the other guys, jerk (the time-derivative of acceleration) isn't the only thing that will kill you, but sufficient g's of acceleration will too. even if you slowly accelerated to, say, 20 g's (so the jerk is small), and slowly decellerate back to your original velocity (so that living people can examine you), you'll be deader than a doornail.

 

[georgir]

rjb, jetwaterluffy

You are not quite grasping the concept here.
If the force is applied simultaneously and equally on all atoms of your body, you feel nothing, because then there is no relative misaligment or stress between the atoms of your body.
Duration and magnitude of the force are irrelevant. You can go straight from 0g to 200g and live your entire life happily and healthily and not even know that you are accelerating if you never look out the window.

This uniform acceleration can not be done with any kind of engine that we have today, however. All artificial acceleration gets applied to and spread through our bodies with stress and tension and elastic forces, and that is what we can actually feel and what can kill us.

Technically, even gravity fields are not uniform. The forces they produce depend on distance, and are not parallel to each other but all directed to a single point. That can still cause stress in the body and even kill you if the differences are large enough, which they likely are near black holes and such. But in most ordinary situations, gravity fields can be considered uniform and you will never feel them.

 

[HuaMin]

Georgir,
Good day,
Do you think it is possible to make one "uniform" wormhole technically?

 

[Ryan_m_b]

Do you think it is possible to make one "uniform" wormhole technically?

What do you mean by uniform?

 

[HuaMin]

I mean uniform acceleration, uniform gravity fields and etc.

 

[Ryan_m_b]

I mean uniform acceleration, uniform gravity fields and etc.

That's still not making any sense to me. What are the characteristics of a uniform wormhole? Wormholes if they are possible would obey the same laws of gravity as anything else.

 

[georgir]

Georgir,
Good day,
Do you think it is possible to make one "uniform" wormhole technically?

It almost sounds like you are asking me for permission to include that in your sci-fi :p
Well, sure, go ahead. Everything is possible in sci-fi.

 

[HuaMin]

Thanks to all.

Ryan,
According to the laws of Physics, I really do not believe that there is a way to go back to the past and this is what I thought before I raised such thread!

Best regards

 

[tiny-tim]

According to the laws of Physics, I really do not believe that there is a way to go back to the past …

you can't go back to the past

you can't go back at all, you can only ever go forwards ​

of course, you could go forwards to the past if you could find a wormhole

 

[Chronos]

As Fermi would say - 'Where are they [time travelers]?'.

 

[DennisN]

'Where are they [time travelers]?'.

Still in transit? Stuck in traffic?

 

[HuaMin]

Still in transit? Stuck in traffic?

I think their human bodies have totally collapsed during the transit inside the wormhole!

 

[Nabeshin]

That's still not making any sense to me. What are the characteristics of a uniform wormhole? Wormholes if they are possible would obey the same laws of gravity as anything else.

I think what he is getting at is a 'traversable' worm hole, in which the tidal forces experienced by a traveler can be made arbitrarily small. In this case, such a solution does in fact exist. But as with all worm holes, they require exotic matter to exist.

 

[DaveC426913]

oh, and to the other guys, jerk (the time-derivative of acceleration) isn't the only thing that will kill you, but sufficient g's of acceleration will too. even if you slowly accelerated to, say, 20 g's (so the jerk is small), and slowly decellerate back to your original velocity (so that living people can examine you), you'll be deader than a doornail.

As georgir points out, you are being simplistic in your idea of acceleration. You are likely assuming that the rocketship is providing the acceleration, and you are riding along in it. In such a case, you are right, the acceleration of the rocket could kill you if high enough, because it is crushing your body.

But that's not the only kind of acceleration. If, for example, you are falling towards a massive body, every atom in your body experiences the same acceleration - you are in free fall - thus, no net force upon you, even under very high g's.

 

[mjacobsca]

Near light speed velocities may not kill you. But an impact with a grain of space dust might. You would need an immense shield to stop debris from obliterating your vessel at those speeds. That seems as big, if not bigger, than the technical hurdles of accelerating to fantastic velocities.

 

[HuaMin]

Thanks all.
Babeshin,
What are the exotic matter you mentioned? Are they dark matter?