# When moving at the speed of light time stops

## Main Question or Discussion Point

If when you're moving at the speed of light time freezes, why then does it take light 8 minutes to reach the earth from the sun?

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ghwellsjr
Gold Member
If when you're moving at the speed of light time freezes, why then does it take light 8 minutes to reach the earth from the sun?
Do you think maybe it's because you're not moving at the speed of light?

What do you mean?

time flow is observer dependent in relativity. 8 minutes is measured from an observer on earth for example.

Pauli's exclusion principal???

Dale
Mentor
Pauli's exclusion principal???
How could that possibly be relevant?

You may want to read the FAQ on the rest frame of a photon:

Basically your opening statement "when you're moving at the speed of light time freezes" is fundamentally invalid.

phinds
Gold Member
2019 Award
If when you're moving at the speed of light time freezes, why then does it take light 8 minutes to reach the earth from the sun?
As has already been pointed out, you CAN'T move at the speed of light. However, light can, so the second part of your question is meaningful.

Do you understand the concept of distance = rate x time ? Rewritten as time = distance/rate, you could use this to figure out how long it would take you to go 1 mile if you are going 60 miles an hour. How about you apply this to light traveling from the sun to the earth.

EDIT: Do you understand that all I've done here is expand post #2 ?

basically Einstein's own question 'what happens if i move at speed of light' is not answered in his theory since nobody can move with that speed (no clock neither)

Okay, I think there is an interesting question here. I did read the FAQ before posting.

So far we know that because an axiom of the theory of special relativity is that light moves at c, we cannot use that theory to describe time dilation for a photon. One might still ask, DO we have a theory that suggests the meaning of time for a photon? I am still curious.

Or maybe I have misunderstood something?

Edit:

In the FAQ the last bit is "The concept doesn't make sense."

I guess I'm asking if in some other theoretical framework it does make sense?

Dale
Mentor
One might still ask, DO we have a theory that suggests the meaning of time for a photon?
Not that I am aware of.

in hofstadter's book goedel escher bach it is called the mu-method of answering a question by -deasking- it.

pervect
Staff Emeritus
One might still ask, DO we have a theory that suggests the meaning of time for a photon? I am still curious.
No. There is a sort of geometry that describes photon's worldlines, and one's ability to make "equally spaced" marks along them, even though one cannot assign a non-zero value to the spacing of the marks.

THis sort of geometry is called an "affine geometry".

It's an interesting topic, but it's a mistake to think of it as having all the properties of "time". That tends to lead only to self contradictions and confusion.

russ_watters
Mentor
What do you mean?
If when you're moving at the speed of light...
But you're not moving at the speed of light. You are stationary. The light is moving at the speed of light.

I'm not talking about me moving at the speed of light, I'm talking about light itself! Light is moving at the speed of light (obviously!) Shouldn't time freeze for light and arrive instantly - since time has stopped for it!?

pervect
Staff Emeritus
I'm not talking about me moving at the speed of light, I'm talking about light itself! Light is moving at the speed of light (obviously!) Shouldn't time freeze for light and arrive instantly - since time has stopped for it!?
You have some a notion of time, and you assume that light must have some sense of it too. And this idea is wrong.

Time is something that can be measured, and assigned values. This gives geometry a metric structure. We can say "the interval between point A and point B is C seconds".

The geometry of light has an affine nature - it doesn't have measurable "time intervals" at all. We can order A, B, and C, but we can't assign any meaningful numerical intervals to the "distance" between them.

The geometry of light has an affine nature - it doesn't have measurable "time intervals" at all. We can order A, B, and C, but we can't assign any meaningful numerical intervals to the "distance" between them.
You've been a great help, but from the last paragraph I only understood that time doesn't have measurable time intervals! But why?

pervect
Staff Emeritus
You've been a great help, but from the last paragraph I only understood that time doesn't have measurable time intervals! But why?
Suppose you have a nice standard plane polarized radio wave.

If you calculate the invariant time interval, also known as the "proper time" between any two points on the light wave according to relativity, the number you get will be zero.

However, any given observer can mark points along the wave at which the E-field is zero at any given time. And he'll find these points will be evenly spaced. This "even spacing" property happens in spite of the fact that all the proper time intervals are zero.

The distance from A to B, from A to C, and from B to C, measured using the invarinat interval, will all be zero. However, there is a unique point C such that AB and BC are "evenly spaced". This is the affine geometry of the light wave.

The exact spacing depends on the ight wave and the observer. One observer might see an AM radio wave with a 300 meter wavelength - a relativistically travelling observer might see it as much shorter, or longer, due to the doppler effect,

So light takes time to reach between two points because I (the observer) can feel/ measure time differently from the Light ??

Dale
Mentor
That seems like a good way to put it. In your reference frame you are not moving at the speed of light, you experience time, and in your frame light takes time to go from A to B. And light doesn't have a reference frame of its own.

Thank you all, you have been a great help.
Last post puts it straight and simple.

the lorentz transformation is singular for the speed of light i.e. Not defined

The time dilation formula is according to http://en.wikipedia.org/wiki/Time_dilation

T‘ = T * sqrt (1-v2/c2).

Set v=c, the proper time of light T’ will always be 0. The lorentz transformation is not defined for the speed of light, but the time dilation formula above is.

However, time is not frozen, the proper life time of a photon is zero.

ghwellsjr
Gold Member
Last edited:
bcrowell
Staff Emeritus
Gold Member
I don't think any of the previous posts have really addressed the basic misunderstanding invoved in the OP's question. The OP asked:

(1)
If when you're moving at the speed of light time freezes, why then does it take light 8 minutes to reach the earth from the sun?
The question shows a more basic misunderstanding of frames of reference. Suppose the OP had asked instead:

(2) "If when you're moving close to the speed of light time freezes, why then does it take neutrinos 8 minutes to reach the earth from the sun?"

The answer is that when we talk about the time taken for something to reach the earth from the sun, we're implicitly talking about the time as measured in the frame of the earth. But relativistic time dilation would relate the 8 minutes to the much longer time taken in the frame of the neutrino.

A question that didn't involve this misconception would be:

(3) If when you're moving close to the speed of light time freezes, does that mean that only a very short time passes in the frame of a neutrino that, in the earth's frane, takes 8 minutes to reach the earth?