Why time slows down when approaching the Speed of Light.
Ok, so you’ve heard that nothing can travel faster than the speed of light. (That’s not quite true. The expansion of the universe allows for faster than light travel but that’s another post.) You’re also aware that time slows down the closer you get to the speed of light. You know, the ‘One twin goes off to Alpha Centauri at the speed of light and comes back after 80,000 years but he’s only aged 3 months’ story.
Ever wonder why? Here’s the crib notes.
Everything in the universe always travels exactly at Light Speed.
Time dilation: Special relativity declares a law for all motion: The combined speed of any object’s motion through space and it’s motion though time is always precisely equal to the speed of light.
That’s right, everything. You, me, the computer screen you’re looking at, your grandma’s French toast, Santa Clause… everything.
Everything is traveling through Spacetime: space (the three dimensions we experience and the nine others that m-theory predicts) and time.
Adding the total movement through both space and time always equals light speed. Always. Always. Always.
Since you must travel constantly at exactly the speed of light, when you increase your speed through space, you decrease your speed through time.
Your head (and the rest of you) is traveling through spacetime at the speed of light. But, when you’re at rest (not accelerating) all of your head’s movement is through time, none of it is traveling (accelerating) through space. Every time your head moves (accelerates) through space; in a car, in a plane, in a spaceship… even nodding up and down, some of it’s movement in time is lost since it is now moving through space. Cool huh.
What about light?
Since light waves use all of their motion to travel through space at Light Speed, they have absolutely no motion through Time. Every photon that has ever been produced exists in an ageless state. (To us, the light seems to move through time but to the photon, time is standing still. This is one of the seemingly odd realizations fo Ensteins Theory of Relativity.)
The universe ages, light does not.
Jeff Barson
in Science
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Reader Comments (9)
huh? but can't we slow light down? Wouldn't it travel through time when it isn't going through a vacuum?
I don't think that that was any less confusing than any other explanation that I have heard. In fact I'm am now more confused about the whole subject having read this. Maybe I'm just retarded or something, but I just can't seem to grasp why the faster you go "time" goes slower or that you cannot go faster than the speed of light. In any case, I think this has made me even more retarded; If "light waves use all of their motion to travel through space at Light Speed" and therefore "they have absolutely no motion through Time" then why is it that the light that was shone from faraway galaxies millions of years ago is just now reaching us. If there was absolutely no motion through time then it would seem to me that the light would have reached every destination possible instantly and with no visible effect to the human eye.
SubOne has asked a good question. Here's the answer:
Einsteins Theory of Relativity. Your perception depends on your motion 'relative' to what you're observing. To us, time moves through both time and space, but to the photon, there is only motion through space. Photon's see no motion through time.
Odd, but there it is.
I will show my son your post, he loves science discussions.
The article's author equates "being at rest" with "not accelerating". This isn't correct, as Brian Green demonstrates in the referenced book using the example of a non-accelerating railroad car passing a stationary point on the ground. The article did make me wonder: Since the railroad car accelerated in the past in order to get to its current velocity, was it that acceleration that produced the difference in clock rate from that of the ground-based observer. Did I miss this point from the book, or do I still have something wrong?
"Being at rest" and "not accelerating" are the same thing from that point of view and since all points are relative and equally valid, it seems fair to use the term thus.
Some things are best explained through equations.
The explanation is trivial to anyone with some physics background and not so for laymen but it is still correct when expressed in words.
A point of clarification, don’t be mistaken to think that if you raveled at the speed of light you would have stayed forever young.
In your reference system, the system that travels at the speed of light, you would grow old the same way as if you were at rest because your biological clock will pace the same. In that sense a photon is not comparable to a living creature. So what’s the story? Well in other reference systems that are much slower than yours, things will occur at much faster rate. So it’s better to think about it as if in slower systems times moves “faster”. It is only when you will stop or slow down to meet other people at other systems you will notice the RELATIVE time difference. This is why it is called the Theory of Relativity and the time difference (“dilation”) is noticeable only when you compare different reference systems. If Earth had moved at the speed of light, from our perspective in our reference system everything would stay the same.
So, even if you travel at the speed of light, you do grow old.
Thoughtful comments, but they don't address a point that perhaps the author didn't emphasize enough: 'space' and 'time' do not exist seperately. They are our only possible views of the dimension of SPACETIME. Travel through this dimension ALWAYS (Always. Always.) occurs at a constant rate. Give this rate a numeric value of 1000. This rate of travel is occurring to you and everything you can physically perceive at this total rate, always. If you travel through 'space' by driving your car down the road at a velocity we'll give a value of 1, then your velocity through 'time' drops to 999. Get in a jet and increase your velocity through 'space' by a factor of 10, then your velocity through time becomes 990. The total must and will always add up to 100. This seems to show that 'space' and 'time' are merely inextricably linked, but further extrapolation shows that in fact they are one and the same. We are are unable to alter our 'time velocity' other than by altering our 'space velocity', and in fact can't even avoid doing so. And this causality cannot be reversed (imagine that!). That is, we can't alter our 'space velocity' by altering our 'time velocity'. The cause and effect, for us in this life anyway, is a one-way street.