# If time slows down as velocity approaches C

## Main Question or Discussion Point

I sense this question has been asked to death in these forums so sorry if I'm giving you a sense of Dejavu.

If the relative time of a moving object begins to slow as it APPROACHES the speed of light compared to another reference point, then what happens to the relative time when a moving object IS travelling at the speed of light, ie. a photon?

## Answers and Replies

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DrGreg
Gold Member
I sense this question has been asked to death in these forums so sorry if I'm giving you a sense of Dejavu.

If the relative time of a moving object begins to slow as it APPROACHES the speed of light compared to another reference point, then what happens to the relative time when a moving object IS travelling at the speed of light, ie. a photon?
See What does the world look like in a frame of reference moving at the speed of light? (FAQ written by PF Mentor bcrowell.)

So does time for a massless object travelling at v not exist when v = c? If so, how can it have a velocity?

bcrowell
Staff Emeritus
Gold Member
So does time for a massless object travelling at v not exist when v = c? If so, how can it have a velocity?
When we say time exists, we mean that we can measure it with a clock. I can use a clock to measure how much time it takes for light to get from point A to point B. When we say that time doesn't exist for a ray of light, what we mean is that the ray of light isn't a clock, so it can't measure elapsed time in its own frame of reference.

When we say time exists, we mean that we can measure it with a clock. I can use a clock to measure how much time it takes for light to get from point A to point B. When we say that time doesn't exist for a ray of light, what we mean is that the ray of light isn't a clock, so it can't measure elapsed time in its own frame of reference.
Thank you. I'm going to write that down. Is this the equivelant to what you are saying? We can measure time in a frame of reference only when there is matter (ie a clock) that is at rest in accordance to that frame of reference?

If you travel at the speed of light you still see time moving by at the same rate for you, but an observer watching you at zero velocity will notice time passing by at an undefined (infinitely large) rate as observed by you. so the fact that if you have a clock in ur reference frame which is moving at speed c you can measure time does not make time a meaningful quantity for you.
imagine taking a movie and then playing it at infinite speed, thats how an object at the speed of light will observe the universe.

If you travel at the speed of light you still see time moving by at the same rate for you, but an observer watching you at zero velocity will notice time passing by at an undefined (infinitely large) rate as observed by you. so the fact that if you have a clock in ur reference frame which is moving at speed c you can measure time does not make time a meaningful quantity for you.
imagine taking a movie and then playing it at infinite speed, thats how an object at the speed of light will observe the universe.

No. Time slows down as you move faster.

i stand corrected. the time will appear to move slower for you compared to the stationary world, but you will see time for the sationary observers pass by at an infinately high rate.
let me make it simpler for you
imagine two twins, one of them goes on a spaceshuttle travelling at the speed of light to mars and back on a journey that takes 20 years. once the travelling twin is back he is the same age, but the older twin has watched the other twin travel for 20 years. so the travelling twin has observed the surrounding (stationary) world pass by at an infinitely high rate.

i know im getting a bit too excited about this but einstein once mentioned, that if the hand of god operates at the speed of light, you will not notice it or what it does to your life

WannabeNewton
i stand corrected. the time will appear to move slower for you compared to the stationary world, but you will see time for the sationary observers pass by at an infinately high rate.
Time never moves slower in your reference frame. You measure time as ticking slower in another observer's frame who is not at rest relative to you. So if there is a stationary observer relative to you that means you will measure time as ticking normally for that observer i.e. at the same rate as your own clock. The only time the time dilation would be infinite, if time dilation could even be defined in such a situation, is when the gamma factor blows up which is when the relative velocity of the other observer is v = c which we know is not possible.

i stand corrected. the time will appear to move slower for you compared to the stationary world, but you will see time for the sationary observers pass by at an infinately high rate.
let me make it simpler for you
imagine two twins, one of them goes on a spaceshuttle travelling at the speed of light to mars and back on a journey that takes 20 years. once the travelling twin is back he is the same age, but the older twin has watched the other twin travel for 20 years. so the travelling twin has observed the surrounding (stationary) world pass by at an infinitely high rate.
Thank you but I understand the basic principle you are attempting to explain. I want to know what happens when something IS going the speed of light, not approaching. All massless particles travel at the speed of light. My question is in a frame of reference where a massless particle is at rest, does the definition of time break down? Does time in this frame of reference exist when v=c? I feel it does not.

pervect
Staff Emeritus
ecneics said:
I want to know what happens when something IS going the speed of light, not approaching. All massless particles trave
What happens if you assume 2+2=5? This may sound a little flip, but there is a serious point here.

What happens if you assume 2+2=5? This may sound a little flip, but there is a serious point here.
Hahaha thank you. I have missed out on something that has just become blatantly obvious to me. A reference frame cannot exist where a massless particle is at rest according to Einstein's postulate that light travels at the speed of light in ALL reference frames. Considering a reference frame that I am looking into is rather like saying what if 2+2=5. It doesn't, enough said.

Thank you but I understand the basic principle you are attempting to explain. I want to know what happens when something IS going the speed of light, not approaching. All massless particles travel at the speed of light. My question is in a frame of reference where a massless particle is at rest, does the definition of time break down? Does time in this frame of reference exist when v=c? I feel it does not.
There is no frame of reference for a massless particle (eg photon) simply because it cannot be held in a state of rest. It cannot accelerate, and thus cannot ever arrive into the same frame of any inertial entity.

My opinion is that the passage of time has no meaning for a photon, as it coexists everywhere in spacetime along its propagational path. It passes thru space and time only per the vantage of material entity, because a body of rest mass possesses a specific frame of reference and thus experiences the passage of time.

GrayGhost

Wannabenewton, i recommend you read the whole thread and carefully before posting.

If you travel at the speed of light you still see time moving by at the same rate for you, but an observer watching you at zero velocity will notice time passing by at an undefined (infinitely large) rate as observed by you.
and ecneics, time will pass by at the same rate regardless of what speed you travel at, as observed by you. your clock will allways tick a second as the time it takes to count 1001 or something of the same length.
the example i gave you is a countering paradox that requires us to change our definition of light travel. it tells us that time is a meaningless quantity for the photon, because say you travelled at the speed of light to mars and back, this trip will take 0 seconds for you, so how can you have ever taken the trip? how would you know or prove you have travelled that distance?
i never gave you an example as you approach c, i gave you the bottomline, if u travel at c, there will be problems all around the universe...

bcrowell
Staff Emeritus