Why time slows down when approaching the Speed of Light.
Thu, July 12, 2007 at 3:48 PM 
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. Always.
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.
Space and Time do not exist seperately, the are parts of the same thing, Spacetime.
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.) That's why poton's from the early universe don't 'fade out' or do something else. They can't, since for them, time is at a standstill.
The universe ages, light does not.
83 Comments | Share Article | posted in Science | tagged Light Speed, Time Dilation, Why Time Slows Down Near Light Speed
Reader Comments (83)
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.
A few questions. First, like hydro asked, does light move through time at all when it is slowed slightly by air, water, etc, or is it that light simply changes form of evergy when absobed into atoms and is released, causing it to slow? And slightly unrelated, how is it that when you approach the speed of light, your mass has a greater value? And third, what does one mean the speed of light? As I understand, light moves in waves. So if you say the speed of light is how long it takes to reach a point going straight that would be wrong. Picture this: two cars on two different roads, both travelling from point A to point B equal distances apart. But the first car goes on a straight road and the second car goes on a curvy road like the shape of a wave. To arrive at their destinations at the same time, the second car would have to be travelling much faster. So I would assume the scientist who measure the speed of light must take in to account the wave motion photons make.
ok so gimme some ratios howmuch moving do i need to do to slow down my time speed visually
This is a profoundly silly idea. Time only exists as a reference to existence. Call that whatever you want. I am not a scholar of any type so I tend to think if it in terms of atomic half-life. Time is not a substance nor is it a field in which we "travel". It is a system of measurement and nothing more. If I had the magical ability to move at the speed of light and chose to do so in a straight line towards Polaris for exactly sixty seconds and then made the return trip (this is also assuming I had the ability to chart the natural motion of all the relevant astronomical bodies) at exactly the same speed only three things would be remarkable. 1. I would have traveled an amazing distance called "two light minutes".
2. Exactly 120 seconds would have passed no matter where you are perceiving this event from in this universe.
3. No one would have seen it (even me) because no light would be able to bounce of of me for anyone to see and no light could reach me for me to see anything other than what was directly in front of me.
I like to think this whole topic is one gigantic (and REALLY REALLY funny) joke by a man who has been reputed to have had a very bizarre sense of humor. I refuse to believe that Einstein was some sort of physics based wican.
The worst part of all of this is that the author of this article didn't even try to show how it would be possible for any of this take place. I would be more likely to believe that the object moving at the speed of light had its rate of atomic decay slowed. That would at least have some basis in reality, but upon returning from my little theoretical jaunt I would find that some amount of time OTHER that 120 seconds had passed? Nope. Total science fiction.
Most things in the universe you can either smell, taste, see, hear, feel or detect. Even those things you can not see, like air, you can feel and measure. Electricity, radio waves, microwaves, gamma rays or any of the radiation found in the electromagnetic spectrum can be detected and measured. You can not smell, taste, see, hear, feel or detect time. Saint Augustine (354 - 430 AD) wrote about time by saying, "If no one asks me, I know, but if any person should require me to tell him, I cannot." Julian Barbour is a British physicist . He is the author of the book, “The End of Time: The Next Revolution in Physics”. Julian Barbour says in his book time does not exist. From all my study of time I have found no evidence time exist. Time is so arbitrary and was invented by man based upon the rotation of the earth against the sun or a star. Clocks are not time. When this country goes on Daylight Saving Time most of the country advances the clock by one hour while other parts of the country stays on the same time. How do you advance time in one part of the country and keep other parts of the country on the same time? I have been in meeting where people attending the meeting and some will say, "That meeting went fast" other will say "That meeting went slow". How can time go fast and slow at the same time? If you are at a sporting event and the clock is stopped, does this stop time? My question is this: If time does not exist how can it slow up?
Harry Spangler
Phoenix, Arizona
I think there's a little inconsistency here with the statement about the proton not aging. A proton traveling at the speed of light doesn't age, but the person traveling to
Alpha Centauri ages by 3 months? I think the proton statement seems wrong. Within it's reference, light from alpha Centauri should age 3 months just like the twin since both are moving at the speed of light. I understand that they are apples and oranges, but take a proton and a person, traveling at the speed of light, why does one age while the other doesn't, within the same plane of reference?
Next: What if it DOESN'T hit an atom? Imagine photons created right at the edge of the universe, directed outwards. At this point, I am stuck.
The word NEVER implies that this is impossible, I would enjoy an explanation on how you can just assume anything will NEVER happen. You don't seem to be such a scientist now do you?
1st law in scientific text is accept that you can always be wrong about everything, therefore extreme words never and always should be taken with a grain of salt.
Also, I cannot deny time as a measurement, but I would like to see the studies of perception of time on an individual basis. Have you ever been in college and a party starts an hour early? Then everyone there has a miscued reference of time? Why is that? Was it us being in motion for an hour longer? Was our reaction to others change how time actually went by? What about hallucinogens? You can cook, play a game and have 20 different conversations in what feels like an hour, but look at your watch. 5 min have gone by.
Would this mean that the speed in which our bodies/minds are moving/reacting with others could possibly alter time its self?
I have taken a undergraduate GR course which was great, but pretty simple. This was a good way to describe whats happening. Its a pretty obvious statement, but when learning it in the classroom you dont get this thought provoking description out of it. What you get is the same ol' energy is conserved, momentum is conserved, principle of least action, etc.
Its really interesting how fundamental light is to...l everything.
Light is the only thing that really directly interacts with particles. Its almost as if its a communication tool of the particles.
Its also so interesting how we developed eyes to detect the photons....
What about a particle that has exactly zero velocity; it would have experienced the entire existence by now.
A photon has no mass, and conversely this particle would have infinite mass...
I am guessing this would be the entire universe as a whole... or the point where the big bang is exanding from(since presumably its not moving).
When atoms are split, some part of the matter involved truns into energy. The glue that holds the nucleus together is the usual suspect. Perhaps it's because the strength of the glue transfers energy so quickly that a collision of it's atom with another produces surprising off the scale rogue waves of energy that produce a momentary rogue speed of a piece of this glue. Speeding it up to light speed, turning it INTO light and shooting out.
Of course photons are routinely absorbed by atoms. This could certainly be because it slowed down enough to TURN INTO matter.
It should be added that this would not only be due to resultant speed but how fast it is accelerated to that speed. And that the 'glue' is the only thing strong enough to relay this kind of sudden energy fast enough to produce a rogue anything of this scale. This would explain why a large piece of matter is never accelerated to this speed. That is, it would break apart before the energy could be relayed to all parts of it.
But matter turning into radiation is not new. Just that it may be precisely because matter CAN travel the speed of light. It just isn't matter anymore. Whether this is semantics or physics hardly matters. Pardon the expression.
I like seeing how far plain English can go in talking about things. I am not a physicist so I expect the above to be very wrong. In a few paragraphs I know I haven't covered much so there must be several directions that this idea would either benefit from or die a horrible death. I don't care which. I'm asking for it. I am McLovin.
No body can travel as fast as the speed of light. Why? E=mc^2, so energy is mass. Matter constitutes particles, which have mass. The speed of a particle can be described by KE=1/2mv^2. So what you get is: an increase in the velocity of the particle(by adding energy) increases the KE of the particle. But now that energy adds mass to the particle based on E=mc^2. So the apparent mass(it's rest mass plus the energy mass) of the particle is now more. As you keep adding energy, the particle becomes heavier and heavier while increasing velocity. You will reach a point where all energy put into the system simply makes the particle 'weigh' more instead of speeding it up. Richard Feynmans 'Six not so easy pieces' is a short and fantastic book for explaining special relativity.
I am by no means a physicist so I don't konw all the good examples for relativity. you should really read that book, it goes into tons. it's really quick and he explains everything talked about here clearly and concisely. It'll blow your mind =)
so time is whatever you want to define it as, but is usually defined as a relative change in a specific material through some process. (a particle decays into another particle, the earth makes a complete revolution around the sun etc..)
And there is a problem to that theory: things are only in movement relatively to one another. Therefore, the twin who leaves the planet but the other twin as well are moving relatively to each other. So, time should slow down for both of them, in fact whenever something changes direction or speed (i.e., accelerates) everything in the universe should slow down. Which come down to nothing slowing down at all. Do I make sense?