Is Time Travel Really Possible?
By Donald Kaiser – Feb 1, 2011
Simple concepts that we often take for granted can be quite complicated once we really start to think about them. One example is the concept of time. Most people would agree that our brains are basically hard-wired to deal with time. Temporal considerations are essential to understand motion and other real-life concerns such as cause and effect. For example, when we observe a falling rock that might cause us harm, our brains seem to automatically gauge the distance it moves relative to our own position and calculate the time before it hits us. Sometimes, our response in such a situation is a rapid reflex, instinctively performed without any conscious effort whatsoever. How can we do that? Some people might think we can do that simply because those that do survive, while those that don't get hit by rocks. Other people might believe that we can do that because someone is looking out for us.
If someone is looking out for us by such instantaneous intervention, our own ability to understand time may not be necessary at all. In a similar sense, some philosophers believe that only the present exists and there was no past and there will be no future. It's sort of like the old question, “if you find a tree lying on the floor of the forest, did it fall?” It seems logical, but all we know for certain is that there's a tree lying on the forest floor. (Hopefully, there's nobody under it.)
For the sake of discussion, let's just assume that there was a past, there is a present, and there will be a future. This doesn't seem too far-fetched because when I looked in the dictionary, I found the following definition:
“Time is a non-spatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.”
So even the dictionary uses the terms past, present, and future and it seems to suggest that going back in time may be possible by defining time as merely “apparently irreversible.” I wonder who wrote this definition. Do you think they said that time is “apparently irreversible” because of some real evidence that time may be reversible? Maybe they had just seen the movie “Back to the Future?” Anyone who saw that movie would be aware of the history-altering concerns that time travel presents. I think that in a world of time travel, the hardest profession would be history, not physics.
In the definition above, the term “non-spatial” distinguishes time from space. It's generally believed that the universe has three spatial dimensions (x, y, z) and one time dimension, commonly referred to as the fourth dimension. I've always wondered about this. How come space gets three dimensions but time only gets one? It doesn't seem fair does it? I think time should get at least two dimensions: 1) backward time (into the past) and 2) forward time (into the future). All things being equally, maybe we could even give time a third dimension for the present but I'm not sure where that would be going. By definition, as a “continuum,” time always has to be going doesn't it? But maybe that's OK. If forward or backward time stopped, wherever it stopped would be the present. Therefore, present time would simply be the time until either forward or backward time started-up again. So, space has three dimensions (x, y, z) and time has three dimensions (backward, present, and forward). Hooray! All's fair in the universe!
Based on this crystal-clear logic, I think we need to make a new definition for time:
“Time is a non-spatial continuum in which events occur in reversible succession into the past or into the future, or frozen in the present.”
In physics, there are basically two ways to describe how things work: 1) Classical mechanics which works fine for normal stuff like people, cars, baseballs, airplanes, etc. and 2) Relativity which we need to explain super-galactically huge, sub-atomically small, and especially really fast (speed of light) stuff. Classical mechanics and relativity differ because of time which is constant for normal stuff, but for relativistic stuff, actually goes slower or faster depending on an object's velocity relative to the speed of light and the strength of gravitational fields. Time that varies because of an object's velocity relative to the speed of light (relativistic time) cannot apply to the dimension of present time because present time is only for objects that are frozen in the present and therefore have no velocity. But relativistic time should be relevant to the other two time dimensions (backward into the past and forward into the future). I warned you this might get complicated.
Because relativistic time depends on an object's velocity relative to the speed of light, space and time are actually inseparable for relativistic stuff and this property is commonly referred to as the space-time continuum. So, for relativistic stuff, we need to again revise our definition of time:
“Time is a space-related continuum in which events occur in reversible succession at a rate that depends on an object's velocity relative to the speed of light or the strength of gravitational fields.”
So, our definition of time allows for time itself to proceed forward into the future or backward into the past. But what about us? Can we travel forward into the future or backward into the past?
According to Wikipedia, physics has neither proved nor disproved these possibilities but physicist Stephen Hawking, one of the most creative and brilliant thinkers of our time, published his chronology protection conjecture in a 1992 paper which strongly implies that time travel is impossible according to the laws of physics at all but submicroscopic levels. Presumably, “submicroscopic” refers to the atomic level where relativistic effects are significant and interesting properties like quantum entanglement could allow for communication between spatially-separated, quantumly-entangled particles. But even at this level, no one has reliably suggested how communication between temporally-separated, quantumly-entangled particles might occur, let alone time travel itself. In Hawking's own words:
“It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.”
Whether or not this will stand the test of time remains to be seen.
By Donald Kaiser – Feb 1, 2011
Simple concepts that we often take for granted can be quite complicated once we really start to think about them. One example is the concept of time. Most people would agree that our brains are basically hard-wired to deal with time. Temporal considerations are essential to understand motion and other real-life concerns such as cause and effect. For example, when we observe a falling rock that might cause us harm, our brains seem to automatically gauge the distance it moves relative to our own position and calculate the time before it hits us. Sometimes, our response in such a situation is a rapid reflex, instinctively performed without any conscious effort whatsoever. How can we do that? Some people might think we can do that simply because those that do survive, while those that don't get hit by rocks. Other people might believe that we can do that because someone is looking out for us.
If someone is looking out for us by such instantaneous intervention, our own ability to understand time may not be necessary at all. In a similar sense, some philosophers believe that only the present exists and there was no past and there will be no future. It's sort of like the old question, “if you find a tree lying on the floor of the forest, did it fall?” It seems logical, but all we know for certain is that there's a tree lying on the forest floor. (Hopefully, there's nobody under it.)
For the sake of discussion, let's just assume that there was a past, there is a present, and there will be a future. This doesn't seem too far-fetched because when I looked in the dictionary, I found the following definition:
“Time is a non-spatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.”
So even the dictionary uses the terms past, present, and future and it seems to suggest that going back in time may be possible by defining time as merely “apparently irreversible.” I wonder who wrote this definition. Do you think they said that time is “apparently irreversible” because of some real evidence that time may be reversible? Maybe they had just seen the movie “Back to the Future?” Anyone who saw that movie would be aware of the history-altering concerns that time travel presents. I think that in a world of time travel, the hardest profession would be history, not physics.
In the definition above, the term “non-spatial” distinguishes time from space. It's generally believed that the universe has three spatial dimensions (x, y, z) and one time dimension, commonly referred to as the fourth dimension. I've always wondered about this. How come space gets three dimensions but time only gets one? It doesn't seem fair does it? I think time should get at least two dimensions: 1) backward time (into the past) and 2) forward time (into the future). All things being equally, maybe we could even give time a third dimension for the present but I'm not sure where that would be going. By definition, as a “continuum,” time always has to be going doesn't it? But maybe that's OK. If forward or backward time stopped, wherever it stopped would be the present. Therefore, present time would simply be the time until either forward or backward time started-up again. So, space has three dimensions (x, y, z) and time has three dimensions (backward, present, and forward). Hooray! All's fair in the universe!
Based on this crystal-clear logic, I think we need to make a new definition for time:
“Time is a non-spatial continuum in which events occur in reversible succession into the past or into the future, or frozen in the present.”
In physics, there are basically two ways to describe how things work: 1) Classical mechanics which works fine for normal stuff like people, cars, baseballs, airplanes, etc. and 2) Relativity which we need to explain super-galactically huge, sub-atomically small, and especially really fast (speed of light) stuff. Classical mechanics and relativity differ because of time which is constant for normal stuff, but for relativistic stuff, actually goes slower or faster depending on an object's velocity relative to the speed of light and the strength of gravitational fields. Time that varies because of an object's velocity relative to the speed of light (relativistic time) cannot apply to the dimension of present time because present time is only for objects that are frozen in the present and therefore have no velocity. But relativistic time should be relevant to the other two time dimensions (backward into the past and forward into the future). I warned you this might get complicated.
Because relativistic time depends on an object's velocity relative to the speed of light, space and time are actually inseparable for relativistic stuff and this property is commonly referred to as the space-time continuum. So, for relativistic stuff, we need to again revise our definition of time:
“Time is a space-related continuum in which events occur in reversible succession at a rate that depends on an object's velocity relative to the speed of light or the strength of gravitational fields.”
So, our definition of time allows for time itself to proceed forward into the future or backward into the past. But what about us? Can we travel forward into the future or backward into the past?
According to Wikipedia, physics has neither proved nor disproved these possibilities but physicist Stephen Hawking, one of the most creative and brilliant thinkers of our time, published his chronology protection conjecture in a 1992 paper which strongly implies that time travel is impossible according to the laws of physics at all but submicroscopic levels. Presumably, “submicroscopic” refers to the atomic level where relativistic effects are significant and interesting properties like quantum entanglement could allow for communication between spatially-separated, quantumly-entangled particles. But even at this level, no one has reliably suggested how communication between temporally-separated, quantumly-entangled particles might occur, let alone time travel itself. In Hawking's own words:
“It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.”
Whether or not this will stand the test of time remains to be seen.