Time Dilation

[iceworks]

May/June Sky and Telescope had an article on a cosmic superparticle that travels nearly the speed of light. If it originated in NGC 5128 twelve million light years away, it only experienced twenty minutes in flight due to relativistic time dilation. Can someone explain the physics, equations that are used to come to this conclusion. I presume that time is slowed at the speed of light, and that twelve million light years distance only seems like twenty minutes travel? Also, if scientists could produce high energy particles near this speed, could they "ping" them at distant objects( a bit like radar ) to accurately measure distance?:shrug:

[sjastro]

The concept of time dilation despite being a blow to common sense is very easily explained using simple geometry.

http://library.thinkquest.org/C008537/relativity/math/math.html

From the scientists frame of reference, the high energy particle would take a lot longer to reach it's destination due to time dilation thereby making the ping method unworkable for distant objects.

Regards

Steven

[Ian Robinson]

http://www.walter-fendt.de/ph11e/timedilation.htm

is not bad .... sort of a Time Dilation for Dummys.

No scientists can not ping high relativistic velocity at distant stars or galaxies. It doesn't work that way and the it still takes a long time for the particles to get there , even if they by some happen chance hit something to bounce back from.

[iceworks]

Thanks for the link. I was lousy at physics at high school, and many years later I still am. Does make sense though to my limited grey matter.

[Jeff]

Hi Brett,

Have not re-read the article, but mathematically I think it's best understood as a rearrangement of the Lorentz Transformation equation for time dilation. In this case:

T(superparticle) = T(Earth) x SQRT[ 1 -(v/c)**2 ]

where
v = superparticle speed (relative to us)
c = speed of light

So if the particle were a photon travelling at the speed of light, the the elapsed time experienced by the photon would be zero.

What's also interesting is that the equation has been verified as accurate by a wide variety of experiments.

Amazing thing to get our head around isn't it!

[avandonk]

Do you ask the miriad of particles that make your body how old they feel? The constant quantum interactions of every particle in your body governed by a few 'simple' rules somehow gives you self awareness. Let alone all the messy chemistry of keeping the whole mechanism humming to support this self awareness. It is a meaningless concept to say that a photon crosses the Universe in an instant as it does not care as it can only follow the intrinsic laws of Physics as it interacts with anything in its path. 'Laws' by the way we are only starting to really understand. We may never!

It is interesting to note when you see an object in a telescope the photons 'die' as their energy is transformed for a fleeting instant to a sensation in a mere mortal man as a visual record when they have been travelling according to our time frame for millenia or billenia! The same thing happens in your humble camera to form an image.


Bert

[hector]

But they don't die in vain.

[Mr. Subatomic]

I seem to remember that those photons have the equivelant kinetic energy of a baseball pitched at 90 km/h. If one were to 'hit' us, would we feel it because, unlike light, it has mass?

[JimmyH155]

Are you saying that a photon travelling for 16,000 years from Orion goes into our eyes and dissipates as energy??:shrug: Cant believe the baseball theory. I know I sometimes get sore eyes, but not THAT sore:D:D
So if photons dont hit anything, they just go on and on and on.....
Photons are massless aren't they? And they can turn into energy then, because of E=MCsquared??:shrug:
Surely when the photon hits the objective lens (or an air particle for that matter) it will disappear then....so how do we see it with our eyes????

[bojan]

Photon has energy, and delivers that energy to the retina of your eye, and that is why you see the light. Only thing is, that energy is not that much as baseball.
It does not hit the lens, though.. it passes through it (and it is not so simple because the photon is also a wave, that is why it deflects a little and that is why the lens focuses light).
As far as mass of the photon is concerned, they have zero rest mass.. but because of that they can travel at light speed, and the energy they carry could be of any value, including the energy of the baseball at 150km/h. Such photons, though, would be VERY energetic, and they would have wavelengths close to something like gamma rays or even shorter.. I should sit down and take calculator in my hand to give you the exact value.

[JimmyH155]

So what you are saying, Bojan is that the photon cruises through the air and the objective lens, then when it passes through your retina, it gives off a bit of energy - enough for us to "see" it, then what?? It keeps going????:shrug:

[bojan]

No.
What I said was, photon is stopped by your retina, and it delivers all its energy to it. Some of this energy will be converted to heat (which may be re-radiated as thermal radiation a bit later), some of it will go to chemical reaction in the photosensors of your eye.. so you can see the light.

EDIT:
Of course, some photons (those which did not reach your retina to reveal their existence) are stopped by the lens, or by the molecules of air (especially if they are of such wavelength that they can be absorbed by water vapor or oxygen molecules present in the air). Some photons will be reflected from the lens.. But majority of them will reach your retina, then they will be stopped there and some of them will be detected - if my memory is right, 10% of photons from visible spectrum are detected by retina, producing the signal in visual nerve. The rest of them are simply converted into heat.

[JimmyH155]

Thanks for that, Bojan - just about understood. So on a night of bad seeing it is simply that more photons get stopped by the atmosphere so there are less to get down to your eyes:D

[bojan]

Exactly right.
Also, more of them are deflected by atmospheric turbulence, hence blurred images...

[Mr. Subatomic]

Oops! I meant PROTONS not photons with the equivalent kinetic energy of a baseball.