Can you travel in time?

[hawkingfan1]

Well if you are talking about travelling faster than the speed of light then no. It is rather simple really, according to the equation E=MC2 (sorry, no symbol) the energy is proportional to mass and if you want to travel faster than the speed of light you would need an infinite amount of energy and therefore and infinite mass which is not possible as the universe is not infinite however it may be possible to travel throught time in wormholes, we have cannot prove it but we cannot disprove it. Relativity does not just mean you can't travel in time, it just means you can't travel faster than the speed of light. You can also slow down time by going faster. If you are on a train going at 60mph and an observer outside is merely walking time will be passing slower for the people in the train!

[Mohan.C]

Theoretically, there are particles called the tachyons which have negative mass and so can go faster than the speed of light.
To slow down time we have to go faster than the speed of light. The slowing down of time of a person in a train and a person walking is relative.
However to travel through time, the speed of light I think should be considered. Wormholes are like folded parts of space time fabric, Light travels the straightest path through the space time, that is why light takes the shortest time, but wormholes are like inversions in the space time. Its very obscure and not practically proved, so, hard to explain for me. But, others in the forum who have more knowledge in math can give you a better answer.

[hawkingfan1]

I often talk to my friends and they don't quite understand but I think it is really nice to have someone who understands more about what you're talking about! Thanks very much!

[Mohan.C]

Your questions are very valid hawkingfan1, and I'm very happy I can be helpful... Your friends on the meantime will understand someday or the other...

[PoPpAScience]

Time travel into the past defies Logic! Yes it can be done in mathematical equations for fictional entities. But to recreate the past one would have to force the Universe to put all substance back to where it was on that certain moment one is trying to visit.

The Human perspective, and Human measurement of the passage of Time, is strictly a generalization. "Motion" of "Substance" gives us a view of "The flow of Time". No human can return all "Motion" of "Substance" back to where it was last second, let alone 2 seconds ago.

[PoPpAScience]

I guess I should also add, that we are always traveling in "The flow of Time".

Traveling from the past to the future, is the only perspective we have.

[neutralino]

Theoretically, there are particles called the tachyons which have negative mass and so can go faster than the speed of light.

Wouldn't a tachyon have to have an imaginary (rest) mass?

[hawkingfan1]

Although they are in top set with me they think of me as a nerd which I find rather hippocritical because they themselves could unerstand it if they tried. To Mohan.

[Mohan.C]

Wouldn't a tachyon have to have an imaginary (rest) mass?

Hi neut, whatever I know comes from this blog...

HappytheStripper's Journal

Posted 04-15-2006 at 02:41 AM by HappytheStripper
Basic properties (from a special relativity perspective)

As mentioned above, a tachyon is a particle with space-like four-momentum. If its energy and momentum are real, its rest mass is imaginary. It is difficult, for instance, to interpret exactly what a complex-valued mass may physically mean. One curious effect is that, unlike ordinary particles, the speed of a tachyon increases as its energy decreases. This is a consequence of special relativity because the tachyon, in theory, has a negative squared mass. According to Einstein, the total energy of a particle contains a contribution from the rest mass (the "rest mass-energy") and a contribution from the body's motion, the kinetic energy. If m denotes the rest mass, then the total energy is given by the relation



We take this relation to be valid for either tachyons or regular particles ("tardyons"). For ordinary matter, this equation shows that E increases with increasing velocity, becoming infinite as v approaches c, the speed of light. If m is imaginary, on the other hand, the denominator of the fraction must also be imaginary to keep the energy a real number (since a pure imaginary divided by another pure imaginary is real). The denominator will be imaginary if the quantity inside the square root is negative (recall the problem imaginary numbers were invented to solve), which only happens if v is larger than c. Therefore, just as tardyons are forbidden to break the light-speed barrier, so too are tachyons forbidden from slowing down to below light speed.
The existence of such particles would pose intriguing problems in modern physics. For example, taking the formalisms of electromagnetic radiation and supposing a tachyon had an electric charge—as there is no reason to suppose a priori that tachyons must be either neutral or charged— then an accelerating tachyon must radiate electromagnetic waves, just like ordinary charged particles do. However, as we have seen, reducing a tachyon's energy increases its speed, and so in this regime a small acceleration would produce a larger one, leading to a run-away effect similar to an ultraviolet catastrophe.
Some modern presentations of tachyon theory have demonstrated the possibility of a tachyon with a real mass. In 1973, Philip Crough and Roger Clay reported a superluminal particle apparently produced in a cosmic ray shower (an observation which has not been confirmed or repeated) [1]. This possibility has prompted some to propose that each particle in space has its own relative timeline, allowing particles to travel back in time without violating causality. Under this model, such a particle would be a "tachyon" by virtue of its apparent superluminal velocity, even though its rest mass is a real number.

Causality

The property of causality, a fundamental principle of theoreticalparticle physics, poses a problem for the physical existence of tachyons. If a tachyon were to exist and were allowed to interact with ordinary (time-like) matter, causality could be violated: roughly, there would no longer be a way to tell the difference between the future and the past along the worldline of a given piece of ordinary matter. A particle could send energy or information into its own past, forming a so-called causal loop. This would lead to logical paradoxes such as the grandfather paradox, unless the theory was set up in such a way as to prevent them. At present such a fix is not known: for example, the Novikov self-consistency principle has not been obtained within a quantum field theory, but has to be imposed by hand. At the very least the principle of special relativity would have to be discarded.
Other avenues of speculation involve parallel universes. One can imagine a scenario in which sending energy or information back in time causes history to diverge into two distinct tracks, one in which events reflect the altered information and one in which they do not.
In the theory of general relativity, it is possible to construct spacetimes in which particles travel faster than the speed of light, relative to a distant observer. One example is the Alcubierre metric. However, these are not tachyons in the above sense, as they do not exceed the speed of light locally.

[Mohan.C]

continued.....

Field and string theories

In quantum field theory, a tachyon is a quantum of a field—usually a scalar field—whose squared mass is negative. The existence of such a particle implies the instability of the spacetime vacuum because the energy of the vacuum has a maximum rather than a minimum (at least with respect to the tachyonic direction). A very small impulse will lead the field to roll down with exponentially increasing amplitudes: it will induce tachyon condensation. The Higgs mechanism is an elementary example, but it is important to realize that once the tachyonic field reaches the minimum of the potential, its quanta are not tachyons anymore but rather Higgs bosons that have a positive mass.
It is important to realize even for tachyonic quantum fields, the field operators at spacelike separated points still commute (or anticommute).
Tachyons arise in many versions of string theory. In general, string theory states that what we see as "particles"—electrons, photons, gravitons and so forth—are actually different vibrational states of the same underlying string. The mass of the particle can be deduced from the vibrations which the string exhibits; roughly speaking, the mass depends upon the "note" which the string sounds. Tachyons frequently appear in the spectrum of permissible string states, in the sense that some states have negative mass-squareds, and therefore imaginary masses.