# Time slows down when you approach the speed of light?

## Main Question or Discussion Point

hi

I read a few other post about this but i'm an engineer, not a physicist, not a genius and didn't get much wiser. And if it's possible try not to use to many technical terms when you answer this, please.

As it says in the title "time slows down when you approach the speed of light", it's driving me crazy.

I watched a documentary on Discovery about the 100 greatest physics discoveries and it was a bit superficial.

They said that...
1. Time slows down when you approach the speed of light
2. If you have a satelite moving at 18000 miles/hour around earth you have to account for relativistic effects

Some literature says that it has been exsperimentally confirmed but not how.

I just don't understand, maybe because how i think of time... i see time as a man made thing, if humans cease to exist time has no meaning or also cease to exist. This doesn't mean that all the processes in the universe stops. Like lenght for exsample, what's the meaning of a cm or meter if there are no humans or marketing for that matter, etc.

So why would the processes in your body slow down (and you would age at a lower rate) just because you're moving extremely fast?

Related Special and General Relativity News on Phys.org
mgb_phys
Homework Helper
Mass, length and time all depend on relative motion.
If you move fast your mass, size and measurement of time differ compared to a stationary observer.
It might not be obvious but it is true!

It has been experimentally verified in a number of ways, unstable particles created in upper atmoshpere reach the surface but don't live long enough to do that - unless time for them is slowed down.
The clocks in GPS satellites have to be adjusted to take this effect into account (actualy it's more complicated because they are slowed down by their speed = special relativity, and speeded up because of the lesser gravity = general relativity)

Last edited:
HallsofIvy
Homework Helper
Remember this is "relativity"! You can only "near the speed of light" relative to some other frame of reference. If you speed up relative to another frame of reference, people in that frame, watching you would see your clocks, bodily functions, etc. slow down. You would not notice any change. In fact, since "speed" does not have a direction, you would see their clocks, bodily functions, etc. slow down. And, while it is true that such units of measure as "second" or "cm" or "meter" are 'manmade', time and distance are not. If humans ceased to exist, animals and plants would still be born and die. THAT'S what time is.

Remember this is "relativity"! You can only "near the speed of light" relative to some other frame of reference. If you speed up relative to another frame of reference, people in that frame, watching you would see your clocks, bodily functions, etc. slow down. You would not notice any change. In fact, since "speed" does not have a direction, you would see their clocks, bodily functions, etc. slow down.
Help me out here, HallsofIvy. If I go on a near-light-speed trip and come back to earth, my clocks will have been running slow compared to earth clocks. There should be a clock discrepancy when I get back, with my clock being slow and the earth clock being fast. But you just said speed does not have a direction, which would cause the earth clock to slow down the same amount, and there would be no discrepancy. Something isn't right here.

DaveC426913
Gold Member
But you just said speed does not have a direction...
No, but acceleration does. And the acceleration is experienced by you, not by Earth.

hi

I read a few other post about this but i'm an engineer, not a physicist, not a genius and didn't get much wiser. And if it's possible try not to use to many technical terms when you answer this, please.

As it says in the title "time slows down when you approach the speed of light", it's driving me crazy.

I watched a documentary on Discovery about the 100 greatest physics discoveries and it was a bit superficial.

They said that...
1. Time slows down when you approach the speed of light
2. If you have a satelite moving at 18000 miles/hour around earth you have to account for relativistic effects
They have already answered you, I only add that if you could accelerate (in a reasonable time) to near the speed of light with respect our planet, let's say to 0.999999999999999999999999995 c, then you would reach the present (visible) limit of the univere and back to earth, in one day of your clock, but 28 billions of years on earth (non considering the universe expansion), if it will still exist! Anyway, your biological life would be exactly one year older, not even a little less.

Last edited:
thank you all for your replies... i'm still a bit confused thought, but i'm going to read some books once my exams are over.

i just have one last question... does special relativity hold true for an electron and other quantum mechanical systems?

just a note: all this started when i wanted to explain the electrical properties of carbon nanotubes in depth. I started at Bohr and Einstein and at that time relativity didn't seem to matter, but now i'm back at relativity.

thank you all for your replies... i'm still a bit confused thought, but i'm going to read some books once my exams are over.

i just have one last question... does special relativity hold true for an electron and other quantum mechanical systems?
There are entire branches of physics about it (quantum electrodynamics and quantum field theory).
Anyway, just to say three things:

1. the magnetic field generated by an electric current is a relativistic effect of charges's movement;
2. electrons in an accelerator obeys special relativity laws;
3. the electron's spin (and all the consequences of it e.g. Pauli's principle, the way atoms are made; magnetization of matter) comes from Dirac Equation which is the relativistic generalization of the Schrodinger equation for the electron.

Last edited:
thank you lightarrow.

i was hoping it wasn't, it would have made life easier.

I started with a book on quantum mechanics and as i incountered subjects i had little or no kwonlegde of i read about them. I know i still lag knowlegde about quite alot of physics subjects, so please bare with my stupid questions.

OH... and happy new year everyone

Last edited:
thank you lightarrow.

i was hoping it wasn't, it would have made life easier.

I started with a book on quantum mechanics and as i incountered subjects i had little or no kwonlegde of i read about them. I know i still lag knowlegde about quite alot of physics subjects, so please bare with my stupid questions.
No, don't worry!
OH... and happy new year everyone
The same to you!

For further reading on the subject of time and it's complexities I would suggest "About Time" by Paul Davies. It's an excellent book and Mr. Davies is very good at explaining the complexities of time in an easy to understand writing style.

pervect
Staff Emeritus
Help me out here, HallsofIvy. If I go on a near-light-speed trip and come back to earth, my clocks will have been running slow compared to earth clocks. There should be a clock discrepancy when I get back, with my clock being slow and the earth clock being fast. But you just said speed does not have a direction, which would cause the earth clock to slow down the same amount, and there would be no discrepancy. Something isn't right here.
Have you seen a space-time diagram? It's just a plot of position vs time, basically. Usually the time axis goes up the paper though. Think of a huge piece of paper scrolling downwards, and the rockets being pens that move left and right. Then the rockets (pens) draw a space-time diagram on the sheet of paper as the pen marks their position as a function of time.

These are maps, of course, where time is represented by a spatial dimension. The map represents the territory, but the map is not the territory.

Now, in the twin paradox, the two twins will draw a triangle. One twin, who does not accelerate, draws a "straight line". The other twin, who does, draws two different straight lines - sides of the triangle.

On these diagrams, it is important to know that the time elapsed by a clock following a particular path through space-time between two events will be represeted by a "length" function related to the "length" of the path. The "length" is, however, not measured in quite the usual way, it is computed by the formula for the timelike Lorentz interval. This means that one takes dt^2 - ds^2 as the interval squared, where dt is the time interval, and ds is the space interval. This is very similar to the pythagorean theorem, but different because of a minus sign.

On the plane, there is an identity known as the "triangle identity" that says that the shortest distance between two points is a straight line. So if you have a straight line connecting two points, it will be shorter than a path that follows two sides of a triangle.

In space-time, the twin paradox says that the longest time interval between two points is a straight line. This has also been called the principle of maximal aging, or sometimes extremal aging - google for some recent posts about this. (I'll post the link if needed). This principle of maximal aging is analogus to the way the shortest distance between two points is a straight line.

So some of the details are involved, but the end result is simple. In *flat* spacetime (this works in SR, not in GR), the twin that follows a straight line on the space-time diagram is the one who experiences the most proper time. The twin who follows a different course takes a shorter amount of proper time.

ok, so it wasn't my last question. I may have been taking the "relative to the observer" part a little to lightly.

but first thank you dbecker for telling me about that book.

Everything is relative, right?

If there was a space that is completely empty, no vacuum and no background radiation, just complete nothingness and you place one object in that space.

I first thought that it would not be possible to move at any speed because it would be relative to nothing, but wether or not that space is infinite or finite and no matter what its shape might be that object will always move relative to the center of that space, right?

And even if you remove that object and wether or not the space is constant or expanding, then the space would still be relative to its own center.

Last edited:
ok, so it wasn't my last question. I may have been taking the "relative to the observer" part a little to lightly.

but first thank you dbecker for telling me about that book.

Everything is relative, right?

If there was a space that is completely empty, no vacuum and no background radiation, just complete nothingness and you place one object in that space.

I first thought that it would not be possible to move at any speed because it would be relative to nothing, but wether or not that space is infinite or finite and no matter what its shape might be that object will always move relative to the center of that space, right?

And even if you remove that object and wether or not the space is constant or expanding, then the space would still be relative to its own center.
Ok, you have to remember that "space" must have a physical meaning (in physics). How would you establish the centre of the space? You could think to put a flag in some point and define that as the centre, but how do you know that flag won't move with respect to that "ideal" point? How can you measure its speed to that point? It's impossible. The fact is that ideal point doesn't exist in the physics realm and you are left with the flag (a physical object) with respect to which you can measure distances and speeds.

HallsofIvy
Homework Helper
ok, so it wasn't my last question. I may have been taking the "relative to the observer" part a little to lightly.

but first thank you dbecker for telling me about that book.

Everything is relative, right?

If there was a space that is completely empty, no vacuum and no background radiation, just complete nothingness and you place one object in that space.

I first thought that it would not be possible to move at any speed because it would be relative to nothing, but wether or not that space is infinite or finite and no matter what its shape might be that object will always move relative to the center of that space, right?

And even if you remove that object and wether or not the space is constant or expanding, then the space would still be relative to its own center.
"If there was a space that is completely empty", then neither you nor the object would exist! If the space is "completely empty" except for you and the object, then the object would move relative to you. There is no reason to think that space, whether finite or infinite, has a "center".

thank you Ivy... i think i got a bit too philosophical.

what i really wanted to ask was just "is there anything i the real/physical world that is not relative", cause i can't think of something that isn't.

Anyway none of you have to be more specific than yes or no, but if yes you could provide just one exsample.

And at least for my sake that will be the end of this thread. I have read about more stuff, the last couple of weeks, than my brain can contain in such sort time, i can hardly remeber a page at time right now.

Thanks to all who have helped in this thread.

HallsofIvy
Homework Helper
Well, in a sense, while velocity is "relative" to some frame of reference, acceleration is not! If you feel a force, given by F= ma, then you will feel a force in any frame of reverence. Of course, if you are will to ascribe that force to some outside influence, such as "gravity", then acceleration is "relative" again!

ok, so it wasn't my last question. I may have been taking the "relative to the observer" part a little to lightly.

but first thank you dbecker for telling me about that book.

Everything is relative, right?

If there was a space that is completely empty, no vacuum and no background radiation, just complete nothingness and you place one object in that space.

................................

Here is something to ponder. With just one object in an empty universe it would be impossible to tell if it had linear motion or not. However it would be possible to tell if that lonely object was spinning or not. If you had just 2 objects in our otherwise empty universe it would be impossible to tell which one was stationary and which was moving. However, if one of the objects was spinning, you would be able to tell which one is spinning. For practical purposes assume an object is a large body that holds an observer who has a light source, clocks, rulers, mirrors and a few other bits of lab equipment to make measurements with.

DaveC426913
Gold Member
Here is something to ponder. With just one object in an empty universe it would be impossible to tell if it had linear motion or not. However it would be possible to tell if that lonely object was spinning or not.
That is because anything larger than fundamental subatomic particles are not single particles at all; they are composed of two or more particles, and you're back to comparing theit relative orientation.

But if you did have a single subatomic particle, no you would not be able to tell if it were spinning.

That is because anything larger than fundamental subatomic particles are not single particles at all; they are composed of two or more particles, and you're back to comparing theit relative orientation.

But if you did have a single subatomic particle, no you would not be able to tell if it were spinning.
The point I was trying to make is that there appears to be something fundementally more absolute about rotation compared to linear motion. If the Earth was in an otherwise empty universe , you could launch rockets and carry out all sorts of experiments and measurement of "relative orientation" and still be unable to determine if the Earth had absolute linear motion, yet you could by assuming the simplest possible laws of physics infer that the Earth had absolute rotation relative to the vacuum of space.

The point I was trying to make is that there appears to be something fundementally more absolute about rotation compared to linear motion. If the Earth was in an otherwise empty universe , you could launch rockets and carry out all sorts of experiments and measurement of "relative orientation" and still be unable to determine if the Earth had absolute linear motion, yet you could by assuming the simplest possible laws of physics infer that the Earth had absolute rotation relative to the vacuum of space.
But that is wrong according to Mach's principle (and so, I think, according to GR, but I'm not sure): in the absence of other objects which creates space-time itself, you couldn't say if that only object is spinning or not.

Last edited:
thank you Ivy... i think i got a bit too philosophical.
Your question was not philosophical at all. See post n. 21.

They have already answered you, I only add that if you could accelerate (in a reasonable time) to near the speed of light with respect our planet, let's say to 0.999999999999999999999999995 c, then you would reach the present (visible) limit of the universe and back to earth, in one day of your clock, but 28 billions of years on earth (non considering the universe expansion), if it will still exist! Anyway, your biological life would be exactly one year older, not even a little less.
Sorry, it should be one day of course.

jtbell
Mentor
Everything is relative, right?
No, there are quantities that are the same in all inertial reference frames, and they are very important in relativity!

For example: measure the energy E and momentum p of an object. Different observers (moving relative to each other) will get different values of E and P. Nevertheless, they will all calculate the same result for the quantity

$$m = \frac{\sqrt {E^2 - (pc)^2}}{c^2}$$

which we call the invariant mass. It's also known as the "rest mass" of the object.

Another example: various observers measure the position and time of two different events. Event 1 occurs at position $x_1$ and time $t_1$. Event 2 occurs at position $x_2$ and time $t_2$. In general, each observer will measure different values for the x's and t's. Nevertheless, they will all calculate the same result for the quantity

$$s = \sqrt{c^2 (t_2 - t_1)^2 - (x_2 - x_1)^2}$$

which we call the invariant (spacetime) interval between the two events.

to my own post #16
sorry i forgot to say thanks to lightarrow for post #14 i totally missed that.

thanks again everyone... i won't ask anymore questions for now.