# Why does light travel at light speed?

Nereid
Staff Emeritus
Gold Member
DrDaleCoxStudent said:
As you approach the speed of light time slows down. At the speed of light time stops.
I think you mean, 'as perceived by an observer in one frame, time in a moving frame is slower; however, within your own frame, to you time passes at the rate it always does, no matter what anyone else in other frames perceives' ... or something like that.

Gerinski
It seems to me that light (EM radiation) does not "travel" at any "speed".

The terms "travel" and "speed" imply the existence of space and time. There must be a distance in space to travel through, if 2 points are not separated by any distance you don't "travel" between them, you are simultaneously in both of them.
And the same about time, the term "speed" implies a measurement of a time interval, since speed will be the distance in space which has been covered divided by that time interval. If 2 events are not separated by any time interval, you can not have any "speed" between them, you experience both of them simultaneously.

For the EM radiation, space distance does not exist, space is shrinked to zero size so all the points which we perceive as being "swept" by a light beam are actually a single point for the radiation itself, all of those spacetime points (which we perceive as being separated by distance) are coexisting "at the same place".
Time too is frozen ( I think that means that rather than being shrinked to zero size it is extended to infinite size, which in fact amounts to much the same as being shrinked to zero size).
All of the events covered by the light beam (what we would picture as a diagonal line in a light-cone diagram) are actually "the same place at the same instant", they coexist. Light would not measure any distance nor time interval between them.

Therefore I would say that the "universe inhabited" by EM radiation is in fact a single point.
Rather than light "travelling at any speed", it's us (matter) that when "sweeping through" that point, from our perspective the point "unfolds" outwards and appears as extended events in spacetime (different places at different times).
All the "different" events along the course of that light-cone line, are just different perspectives of a single event, which shows us a different face as we look at it from different points of our extended spacetime.

The surface of the sun at the moment it emits a photon, the earth 8 minutes
later, alpha-centauri 4 years later ...., all are different faces of a single "thing" (although for what concerns us they are surely and very really different places at different times !) it's just that "what something is to us" may not necessarily be the same as "what it really is" when released free from our material constraints.
It's just our fate that we can not perceive all the faces of the point simultaneously, we are forced to see them one after another, forced to move to a different position in space and time to see a different face, if we are in the earth today looking at the sun, we have no way but to wait at least over 4 years if we want to see the "alpha-centauri face" of this event.

(would this suggest that in fact its the extended universe of matter the one that "travels" through a miriad of "radiation point-like "things" "?)

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Gerinski
Nereid said:
Originally Posted by DrDaleCoxStudent
As you approach the speed of light time slows down. At the speed of light time stops.

I think you mean, 'as perceived by an observer in one frame, time in a moving frame is slower; however, within your own frame, to you time passes at the rate it always does, no matter what anyone else in other frames perceives' ... or something like that.
I understand this relativity principle between material objects, but I'm not so sure it applies all the same for light itself .... If really so, then I guess all my reasoning above was incorrect and I'm sorry for confusing the subject even more.
But can you please reconfirm ? in the frame of reference of the light itself, space and time look exactly the same as for us? I mean, distances and time intervals have the same extension?
I read things like "it takes no time for light itself to travel between the event "sun at earth's time 2004" and the event "alpha-centauri at earth's time 2008", an hypotetical clock on the light beam would not measure any time interval"
If this is correct, then I guess my previous reasoning still holds

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look the light answer is that because it is light. but detailed is,as you might know that a photon has zero mass so a a very little energy willaccelerate it and will reach the ultimate speed thats of cource is 'c'.

Gerinski said:
I read things like "it takes no time for light itself to travel between the event "sun at earth's time 2004" and the event "alpha-centauri at earth's time 2008", an hypotetical clock on the light beam would not measure any time interval"
Light, once emitted, does not age. Once emitted, light does not pass through time. Light involves no motion through time, and all of its motion is constant motion through space. The statement "it takes no time" is a roundabout, and I consider somewhat confusing and misleading, way to make this statement.

I always thought that photons could not travel faster than light speed because the balance of the universe dictated that speed?

Gerinski
Prometheus said:
Light involves no motion through time, and all of its motion is constant motion through space
Thanks, this seems to support a bit my previous discussion, doesn' it?
Since the light wave is present in several points in spacetime (it's light-cone line), but it involves no motion through time, the term "travelling" does not seem appropiate for what light "does" through our specetime. WE perceive the "timeless" or "coexisting" presence of the light wave in those points as a movement or "travel", because those points appear to us separated in different places and different times.
But regardless of how we perceive it, as for light itself all those points in spacetime might be considered as being "together".

Again it seems to me that a "timeless -or atemporal- presence" is perceived by us as a motion, "at the speed of light".

Thanks so much for this clarifying discussion ! as you can guess I'm just an afficionado, and I always found the issue of the speed of light as one of the most intriguing!

jcsd
Gold Member
Be careful light does not have a rest frame, consequently statemnts like:"an hypotetical clock on the light beam would not measure any time interval" and "Light, once emitted, does not age", have no meaning in relativity.

best possible explanation is,
we know that there is light travels at ultimate speed, the maximum possible speed that could be attained in the universe. when you move past or toward the light you tries calculate the speed of light, which is constant. You are calculating something that happens instantaneously of course the value will be the same 'c'.

Chronos
Gold Member
Apologies for interrupting. jcsd gave the correct explanation.

Chronos
Gold Member
Clarification for bob: if c had an infinite velocity in our reference frame, the universe we observe would instantly collapse. That is what the Maxwell equations demand, and what Einstein realized when he made 'the biggest mistake of his career' by adding the cosmological constant. That Einstein dude turned out to be pretty smart.

I am a bit wary of a cosmological constant. First of all Einstein introduced it because he did not want an expanding universe, then he took it out, now its back again.
Could the electromagnetic force travel at any speed faster than light speed, say 200,000 mps and still keep within the parameters of the energy fluctuations allowed without destroying the balance of the universe?

Nereid
Staff Emeritus