Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 22/02/2014 20:12:54
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I said to my mrs that time travel is impossible, she said no it is just improbable. Who is right? I told her she was being pedantic.
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My vote is for time travel being impossible.
Time seems to be something that many physicists discuss, but few define. Perhaps one could think of it as a rate of energy propagation or conversion.
Think of your Grandfather clock in the living room. Make the pendulum shorter, and time runs faster. Make the pendulum longer, and time runs slower. Perhaps if one makes the pendulum long enough, the device will loose the ability to count time. However, to make the clock count time backwards is far more complex, as it contains a one-way ratchet mechanism that wouldn't simply slip backwards.
In chemistry, many reactions are more or less reversible. However, the more complex the reaction, the harder it is to reverse. Ever try unburning a log in the fireplace, then uncutting the tree it came from?
Shatter a window with a hammer, and it could potentially be welded back together to make a real piece of glass, but the process would be quite different from simply moving the hammer backwards.
Anyway, even if the forward movements of "time", whatever that is could be halted, and potentially reversed, there is nothing to indicate that the reactions in "reverse time" would produce the opposite effect of your normal "forward time".
For true "time-travel", one would need to make part of a system experience "reverse-time", while another part experienced "forward time". As a lonely astronaut, in a space ship, the goal would be making a system that one's space capsule continued to experience forward time, while the rest of the universe experienced reverse time. That would seem far less plausible than somehow applying a reverse-aging process to an individual (which could also affect the person's memories and experiences).
Of course, stasis or hibernation might halt time for an individual, while the rest of the universe moved forward in time, creating a time gap.
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Time travel is normal - provided you only want to move forward in time, at 1 second per second.
If you want to move forward in time at some different rate, this is theoretically possible but impractical with our current technology: you just need to accelerate to near the speed of light or fly close to a black hole. This would require a chemical rocket bigger than the Earth, but may be possible with an engine powered by antimatter.
The more interesting case is traveling backwards in time - and that has a lot of unknowns.
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As Evan says, there is a vast difference between travelling forward through time at one second per second and going forward through time at any other speed. Also, trying to travel the other way through time is probably impossible.
We all travel through time at one second per second without even having to think about, to vary that even by the tiniest amount requires enormous effort and expense. Last year (2013) I read that the Astronaut who, to date had spent the most time in relatively high speed space flight could claim to be about one-fifth of a second younger than he would have been had he remained on Earth.
Obviously there are scientists who believe it may be possible to travel backwards through time, but I think they are in the minority, for very good reasons.
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Northern Tree Frogs have naturally developed a form of cryostasis, typically for a few months, but potentially they could use it to jump forward in time for a few decades with essentially no aging during the gap period.
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Northern Tree Frogs have naturally developed a form of cryostasis, typically for a few months, but potentially they could use it to jump forward in time for a few decades with essentially no aging during the gap period.
Would you equate that with time travel, or is it just a specific type of longevity?
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Northern Tree Frogs have naturally developed a form of cryostasis, typically for a few months, but potentially they could use it to jump forward in time for a few decades with essentially no aging during the gap period.
Sometimes these threads go in fascinating directions and you learn something new.
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Would you equate that with time travel, or is it just a specific type of longevity?
As I mentioned, one has to come up with a good definition of "time".
A crystal oscillator will create a power oscillation that is used to count time. An atomic clock uses the hyperfine transition of hydrogen, cesium, or other elements to calibrate the crystal oscillators. Both vary their measurements based on their energy states, and in particular, velocity. Are they also temperature sensitive?
In humans, or animals, perhaps one could consider time as metabolic activity and cellular replication cycles which also affect telomeres.
So, in a very real sense, in the northern tree frogs, and Siberian salamanders, reducing the temperatures to just above freezing (with "antifreeze"), will reduce the cellular metabolic rate to zero, or nearly zero. So, the internal clock of the organism essentially stops.
Thus, I would consider it jumping forward in time.
Say one had cryopreserved your favorite historical figure... say bringing Julius Caesar forward from 44BC to 2014. There would be no difference between a cryopreservation method of stopping his internal time clock and any other method of forward time travel, moving him forward two millennia.
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As I mentioned, one has to come up with a good definition of "time".
It is what elapses between sequential events.
A crystal oscillator will create a power oscillation that is used to count time. An atomic clock uses the hyperfine transition of hydrogen, cesium, or other elements to celebrate the crystal oscillators. Both vary their measurements based on their energy states, and in particular, velocity.
Not true. There being no absolute velocity, neither can be aware of its speed. Are they also temperature sensitive?
Crystal oscillators, yes, so we used to keep the important ones in ovens. Atomic clocks, no, so we use them to calibrate crystal oscillators.
There would be no difference between a cryopreservation method of stopping his internal time clock and any other method of forward time travel, moving him forward two millennia.
Not from his point of view, certainly, but as far as everyone else is concerned his body was still visible the day after he died so it hadn't been displaced along the time axis any further than normal.
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J. Richard Gott, “Time Travel in Einstein’s Universe”, explains how to construct a “gravity well” around yourself such that at the end of 50 years you are 200 years in the future. It is tempting to think that this differs from the cryopreservation method because you experience only 50 while everyone else will experience 200 years; but others will see your gravity well (from a safe distance) throughout their 200 years. Does the fact that you are conscious, whereas the frog is not, really make any difference?
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A thought about the accuracy of atomic clocks, as they have been introduced to the thread. :)
A state-of-the-art atomic clock might be said to be accurate to within 1s in 3X10^7 yrs. But what does this mean?
What is a second?
One second is the time that elapses during 9.192631770 x 10^ 9 cycles of the radiation produced by the transition between two levels of the caesium 133 atom. This is incredibly precise, but is it more than the definition of an arbitrarily designated period that has no meaning beyond that assigned to it by humans?
When we say that a caesium clock is accurate to within 1s in 3X10^7 years, are we saying anything more than that if we record accurately the cycles of the radiation produced by the transition between two levels of the caesium 133 atom; at the end of 3X10^7 years our counting should be accurate to +/- one cycle of the number we would calculate in theory? Since a caesium atom is producing, and measuring the cycles, would we not expect a close correlation using an accurate machine?
Does this actually tell us anything about time?
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Going back to the time travel theory, I have watched a program containing some of the worlds best minds in theoretical physics that suggest that at least the theory of time travel is sound. If you could put enough energy into on single point in space and one single point in time then it will create a tear in space-time that will essentially always be there, and if in 10 seconds time you could traverse through this point of energy you would emerge 10 seconds in the past and it would work even in 10 years.
Now I am not very good at physics and I don't pretend to understand, nor be explaining this theory very well, however I am very inclined to believe it as I also believe the theory of space-time (the "fabric" of the universe). Nothing else has made more sense to me to explain how gravity is so weak yet can affect other masses at such large distances. Thus if the known universe consists of space-time, it isn't so hard to believe that it is possible to bend or tear
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This is incredibly precise, but is it more than the definition of an arbitrarily designated period that has no meaning beyond that assigned to it by humans?
Also, I was just thinking about this and what is time but a system we put in place to explain our universe? Who is to say that humans evolved elsewhere in the universe and their 1 second is completely different to ours? We think of time as 1 second because that is how we experience and explain things. Other animals that live their lives faster probably feel time completely different to us, one second to us may last them 10 seconds from their perspective. If one person was at the event horizon of a black hole and one observing them from far away would both experience time completely differently. If you were to put an atomic clock near a black hole IT would count time differently, so therefore time isn't a constant and is only made real by our perspective. Thus every individual experiences time at their own rate, and the only thing that makes us feel it as a whole is because we keep time with machines and with the passage of the sun and rely on them to tell us all the same thing.
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J. Richard Gott, “Time Travel in Einstein’s Universe”, explains how to construct a “gravity well” around yourself such that at the end of 50 years you are 200 years in the future. It is tempting to think that this differs from the cryopreservation method because you experience only 50 while everyone else will experience 200 years; but others will see your gravity well (from a safe distance) throughout their 200 years. Does the fact that you are conscious, whereas the frog is not, really make any difference?
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You aging is a local phenomena, related to 'c'. Split 'c' in even chunks and you get to a local clock keeping time with, and for, you. That those are constant means only that they are local constants, which probably also was why Einstein called time a 'illusion'. Because when he did he was thinking of the archetypal universe we define. The one that exist 'the same' for us all, and the one we think we agree on. But if you break it down you will find that what we really can agree on are all local definitions, same for us all, from there we can get to physical laws and constants.
So locally defined, which is the way each one of us ever is going to be able to measure anything, time exist, same for us all. But the 'common universe' is a illusion, as a sort of 'global' myth. The universe is really tricky, and has a good joke upon us giving us this illusion.