# Photons are a real buzzkill — they are always a no show to the party

**‘What is the universe like for a photon or a light beam?’**

*TLDR — photon doesn’t experience time or space — seems to exist outside of it*

If you were to ask this question on physics stack exchange, you would be brain shamed or worse snubbed entirely.

Einstein, at the age of 16, humoured himself with a similar thought experiment; now famously & fondly remembered as **‘Sitting on a light beam’**. The objective he had in mind was to reconcile** principle of equivalence** of physical laws of nature and widely accepted **belief that light traveled at c only in Ether**. His intuition rejected the idea that light had a preferred frame of reference (traveled at C only in ether) — though he had no real formal proof for it as **he hadn’t even read** Maxwells equation on electromagnetic propagation.

Before understanding the answer to this question, there are few basic fundamentals that need to be understood first.

Spacetime ≠ Space + Time. Spacetime is not how we understand it.

Our intuition is built & shaped by our experiences.

While traveling from your **home** to the nearest **Target** store you assume that the clock in your phone or the watch you carry keeps ticking the same way and is a reliable measure of time intervals. You simply don’t get confused as to what time it is or how much time has passed since you started from home — of course not because you spent ~$300 and you don’t want to look like an idiot who spend $300 on a watch — *get a phone may be next time*.

But is that how universe really works or is it how we think it works given our experiences ? Isn’t it obvious that moving from A to B has no bearing on the ticking of clock or that motion is/should be really independent of time ?

Our assumption of course is not true as has been disproved more than 100 +years ago. So when you move through space — *now hold your thought don’t be hasty, it simply means you moving from point A to point B — *you also move through time.

Damn! lets try again.

Moving from A to B also means you are gaining time. This means that the watch you so dearly hope is worth the spend, now doesn’t tick at the same rate as it did when you were home ( 1 second is now 0.8 seconds ). Of course the degree of difference depends on your speed from A to B.

So space is not really independent in itself. Space and time are knitted together in an eternal embrace.

Time clicks differently for you when you are moving through spacetime. And not just that, events happening in spacetime happen differently for observers traveling at different speeds. **Simultaneity is relative**. For you as you reached the store, some two other events also happened simultaneously but for your Mom those events were not simultaneous upon your arrival. And the interesting part is the distance you both measure having traveled is different.

This will make more sense in conjunction with the next section.

Speed of light is same in all frames of reference

Our human experience tells us that if you throw your watch with a velocity **V **while you are also moving with velocity **U** wrt to a stationary observer, then for the observer the velocity of the watch is **V+U **—* relative velocity*. But when you throw photons (switching on torch if you will) at velocity **C** while also moving with velocity **U, **then the stationary observer doesn’t record the speed of photon at **C+U.**

**Why, you ask?**

Well because of below

This is empirically proven to be something that is invariant in which ever frame of reference you chose to inhabit. The invariant here is the *C, **the speed of light. ***So light travels at C in all frames of reference.** Whether you don’t move or you move or you move 99.9999999999% the speed of light, light will still move at C with respect to you. **It might seem that the universe conspires to keep the speed of light same in all frames of references**, which it does. And the conspiracy is basically embedded in the geometry of spacetime.

A way to understand is to figure out the transformation of the reference frames between the two observers who are moving relative to one another (non-inertial movement or if the observers are not accelerating wrt to one another ). In simple terms how do you figure out stuff happening in their coordinate system — you need a way to translate events from their system to yours. This transformation is not a simple coordinate translation (Galilean transformation) — in fact that would sometimes give us speed of light > c and hence that is the incorrect transformation.

Transformation which preserves the speed of light in all reference frames is called **Lorentz transformation.**

A much more intuitive method is using **Minkowski spacetime graphs**

Above primary time & space axis arefrom the observers frame of reference and hence **ct & x** are perpendicular. We also obviously understand that in traveler's reference frame (blue lines) their own Time(**ct’**) and Space axis(**x’**) must also perpendicular. Its just that when you transform a moving reference frame to yours (observers), it looks like above. This happens primarily in such a way that **for both the observers the speed of light needs to be the same, **which means it makes 45 degrees with their respective coordinate system. But both black lines and blue lines are perpendicular to each other because they both represent independent coordinate systems.

Important thing to note is that since speed of **observer cannot ever reach to C** exactly, the top blue line (world line of traveler) can never cross the red line or its slope cannot be lesser that Tan 45 = 1

The faster you travel the more closer you come to the Line of light and further distorted are your time and space axis with respect to an observer at rest

Below is a minkowski spacetime map that depicts time distortion or time dilation.

So what does a photon experience as it travels ?

The above map helps us understand what might happen as traveler approaches closer to the speed of light. Both the blue axis bend towards the red line that is the speed of light. Substituting v=c gives us infinity and makes us uncomfortable. However all infinites are not to be understood as ‘wrong’, it simply means there is something unintuitive going on.

What it means is for the photon beam, if there was a clock on board it wouldn’t tick at all and it would not travel any distance — the entire distance is simply reduced to a point.

In other words from the perspective of the light, this is important, it doesn’t travel any distance and it travels in no time. But for an observer it seems that photon still takes some time to travel some distance ( Time = Distance/C)

**If the photon had any awareness**, then it would see the entire universe in a flash and it would spend no time doing so. It gets instantly absorbed as soon as it gets emitted **or in other words** it doesn’t spend any time traveling in the universe **or in other words** for the photon beam space and time don’t exist.

**It’s interesting to note that for observers on Earth, the light from sun takes ~8 minutes to reach. But for the photon it happens instantly**.