What's the difference between "there" and "then"?

by Zak Martin

Time Travel

One of the most intriguing implications of Einstein's special theory of relativity is that there is no difference between "there" and "then".
Relativity implies that there is no general "now". Every "now" is a local event which has no connection with any other "now". Every object has its own "now".
Before Einstein, time was regarded as universal and constant, independent of the velocity or direction of an observer, and this idea has persisted despite having long been proved to be erroneous.
When his research was leading him to the conclusion that there was no universal "now", Einstein went around for weeks asking: "What is simultaneity?"
He realized that his theory, if correct, meant that no two events happen "at the same time", unless they happened at the same location. Because there is no "same time". This would only be true if time were a field, but it isn't.
Relativity means that every event is like an individual planet, with its own unique gravity.
We say that two events occurred "simultaneously", or "at the same time", but this is merely an observer illusion. No two events are connected (leaving aside quantum entanglement, which is a different kettle of phishiness).
People often say things like: "At that very moment, three thousand miles away..."
But in reality they are talking about two separate and unconnected moments. They might as well be three thousand years apart as three thousand miles. We connect them in our minds and give them a meaningful relationship they do not have.
Since space and time are interchangeable and the same laws of physics apply equally to both, in absolute physical terms there is no difference whatsoever between an event that has happened in the past and an event that is happening at a distance (or, indeed, an event that is happening in the future.)
Let's imagine that you have two telephone lines, one going to the past, and the other going to a distant location on planet Earth. One is connected to your great uncle George, who lived over a century ago, and the other is connected to your aunt Sally in Australia.
Now obviously electronic communication only works in one time-direction (at the moment), so whereas your great uncle George could talk to you (if, for example, his conversation was bounced off an object in space and took 100 years to reach you), you wouldn't be able to talk back to him.
But let's suppose that this little technical obstacle had been overcome, and telephonic communication was possible in both directions - ie, backwards as well as forwards in time. (This is not debarred in physics: we just don't know how to do it. Yet.)
And for the moment let's leave aside the so-called time travel paradoxes ("if you travelled back in time and murdered your own grandfather..." etc.).
You ring up your great uncle George and you say, "Hello uncle George... this is your great nephew/niece calling you from the future. How are you?"
And your great uncle George replies "I'm fine. I've just got home from a ride in our new horseless carriage..."
Of course you are aware, as you talk to your great uncle George, that he is talking to you from his "now" - a time when he is still alive, obviously - whereas you are talking to him from your "now" (a "now" in which he is long dead.) Equally, in his "now", you don't exist yet, so in a sense you are both talking to dead people).
After you finish talking to your great uncle George, you call your aunt Sally in Australia. "Hello aunt Sally," you say. "How are you?" And she replies, "I'm fine. It's a hot day, and I'm just about to head to the beach..."
Your great uncle was talking to you from his "now". You aunt was talking to you from her "now".
What is the difference?
The difference, most people would say, is that great uncle George's "now" is in the past and no longer exists.
But this is wrong, because it only looks at the exchange from one temporal perspective, and gives that perspective an importance it does not have in purely physical terms.
Great uncle George's "now" exists for him, and is just as real as your aunt Sally's "now" is for her.
In reality - in physics - there is no difference between the two "nows".
Great uncle George's "now" is no more "in the past" than aunt Sally's "now". Both "nows" have their own time and location in space, and there is no physical agency connecting them.
Let's imagine that, as you are talking on the phone to your great uncle, time suddenly reverses, and the future becomes the past (and vice versa). Now your great uncle is in the future and you are in the past. Has anything changed?
No! Nothing has changed at all! He's still talking to you from his "now", and you are still talking to him from your "now". It is exactly the same as if you were talking to each other from a distance. Because in physics a distance in space is exactly the same as a distance in time. The two are different aspects of the same spacetime manifold.
In other words there is no difference in physics between "then" and "there".

© Zak Martin


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