Physics 62.1 Understanding Space, Time & Relativity (13 of 55) What Do You See at v=c?

In this video I will go on a limb and disagree with Einstein on what would you see traveling at the speed of light. When an observer is traveling at v=0.99c, the observer will see light traveling at the speed of the light from all directions. Einstein postulates that even when an observer is traveling AT THE SPEED OF LIGHT v=c, the observer will see light traveling at the speed of the light from all directions. I am going to go on a limb and respectfully disagree...

Next video in this series can be seen at:

[KuldeepYadav-rlfc]

If I have to explain your Mistake in terms of the Theory of Relativity - You have to understand that in the curved space-time fabric where you and the light wave coming behind you, both are following a motion in a straight line (that is curvy actually) - in this situation light will bend slowly with time. But as per your assumption of F reducing with D and later reaches to zero, then you gonna see the bent light in a straight line, now that's impossible because that particular light reached a particular bent - by moving at a particular velocity(c), and the motion of that light is dependent on the F values as well. So any of your motion with D that you think will drop the values of F - "must possess a different type of bent, which is not possible cause light has already achieved its motion's trajectory".

[The_man_himself_]

Surely for the person on the spaceship, time ticks on as normal. A stationary observer may 'observe' that their clock has stopped. For the spaceship, they will just see the universe shrinking to a dot and instantly travelling to the end of time. Scary!

[gilbertengler]

Excellent lectures!
But I have here a problem. When you move at the speed of light (viewed by an external observer), he will see that the time stopped for you, but I thought that the person going with the speed of light will not notice anything of this! So I would deduce that even when moving at the speed of light, time goes on for the person moving with the speed of light and so he can see the light around him as if nothing happened with a constant speed of c. I agree its more difficult to explain the frequency shifts! But sorry, afterall I am not a physicist!
Many thanks

[draven]

Michel van Biezen, yes I like your videos (physics maths etc.), however here I just want to comment on your concerns (@ 4:20). What you are really saying is that aboard the space ship (moving at c) if everything 'freezes' so all processes stop. If we rename those processes as simply 'change' then the arrow of time becomes synonymous with 'change'. Indeed, this is how I view the 'particulate arrow of time' (not a general case) but, however, we now have a quantum level problem. To solve that problem we need to...well lets just say it would go well beyond the scope of your tutorials...Ha, cutting edge 'what if' eh?

[KuldeepYadav-rlfc]

No Sir, you are Wrong.
I am not saying, I am any way closer to Einstein. But, I am posting this reply to help you.


In easiest words -
1- Initially if you are moving at a speed less than light, then there will be no portions of light coming from the source(B) behind you.


2- One major problem the gap of the intervals between a) when you have started your motion and the light source behind you(B) was turned on & b) the time when you had started to increase your speed to get closer to the speed of light - will decrease. This reduction is the compensation of 1/F.

[comicrelief]

When one says moving at a speed, one must say relative to what. Assuming person A is stationary, whatever that means, person B is moving relative to A. Even if B moves at c, the direction would have to be exactly away. A would have to be on the line containing the vector of B's travel and the angle of deviation would have to be kept exactly at zero. Is all of this possible?

[anbrugm]

Ok so, I have been watching your videos which I absolutely LOVE, so thanks for sharing, and my experience with space and time is only a few hours, but I have an input here based on knowledge from your other videos.

If you where able to reach the speed of light, any observer would see you as frozen in time, while you where infact operating normally. It has to be like this or you would find yourself moving 7 times slower than normal when travelling at 0, 99 C also. Yes? No?

[charlesbrightman]

Consider the following:
When we talk of the 'speed of light':
a. 'Speed' is distance divided by time.
b. 'Distance' is two points in space with space between those two points.
c. Both 'space' and 'time' have to have some sort of actual existent reality for 'speed' and 'speed of light' to actually exist in existent reality. (Other than just as a concept on paper).
d. What exactly is 'space'?
e. What exactly is 'time'?
f. If distance and/or time varies due to relativity, that would seem to affect the speed of light also, here again, speed being distance divided by time.
g. And if distance and/or time does not vary concerning the speed of light (here again speed being distance divided by time, distance being related to space), then how could that be unless the speed of light, and possibly light itself, makes up space and time itself? (Possibly space being energy itself, time being the flow of that energy). And other space and other time are relative to the maximum space and time of possibly infinite space and infinite time. Or in another context, an 'em' frequency that is either perfectly horizontal or perfectly vertical, basically the infinite zero or the infinite one. Everything else in existence is related to those two items that allows them to even exist.
h. Either distance (space) and time vary, even including when utilized in 'speed' and specifically the 'speed of light' or they do not.
i. And remember also, there is reality and there is our perception of reality which most probably doesn't match 100% absolute truth reality. While our perception of reality exists in absolute truth reality, it is not 100% absolute truth reality in and of itself. We truly do not know what we do not know.

[ericzeng]

I think it's length contraction rather than time dilation that the person B traveling at close to the speed of light across space will experience. B on board does not experience time dilation himself (only an observer A from the Earth will report B's time as slowing down compare to his). Traveling at any speed less than light, time always passes as "normal" to B, but the distance of the journey is contracted by a factor of the time dilation as observed by A. When B reaches the speed of light, space shrinks to a point, so he can reach any destination in "no time", literally.

[jeronimus]

The reasoning seems faulty to me, but i am certainly not as smart as Einstein or the professor in this video.

However, one could argue that if you are inside the rocket, no matter which speed you are going at, clocks local to you would be ticking just as always. That follows directly from the first postulate. There is no experiment one could do when moving at a constant speed within empty space, allowing him to figure out if he is moving or not.

edit: The first postulate has many formulations. One of which is that the laws of physics are the same in all inertial frames of reference (moving at a constant velocity relative to others)
Therefore, it is wrong to assume that when a spaceship would move at 0.99c as in the video, the clocks would slow down inside the spaceship when you are not specific.
The clocks would slow down only when observed by an observer watching the ship going at 0.99c. However, an observer INSIDE the spaceship would observe all clocks to be going at the same pace as usual.
IF an observer inside the spaceship would notice any difference, then the first postulate would be wrong, because then an observer INSIDE the spaceship would be able to tell that he is moving, which translates in the laws of physics NOT being the same within his inertial frame of reference.


That is true for v= 0.5c, v=0.8c, v=0.9c and even . So if we were to take the leap and allow v=c, it would seem reasonable to assume that even then, one could not do any experiment to figure out if he is moving or not. An observer inside the space ship simply would not know. Clocks would tick just as before for an observer inside the space ship.

Computers, clocks, other devices inside the spaceship, observed by an observer INSIDE the spaceship would work just as usual according to the first postulate of SR.
Hence, Einstein was correct in assuming that light would travel at C still when observed from within the spaceship.


An observer measuring the spaceship to be moving at v=c would observe everything inside the spaceship to be frozen in time.

The spaceship could reach any given spacetime location of an observer's lightcone who is measuring the spaceship to be travelling at v=c without any clock tick when observed from an observer measuring the spaceship going at v=c.

The mind mender here is that in order to reach any such location, the spaceship would have to instantaneously switch its direction while retaining v=c when observed from an observer measuring the ship to be going at v=c.
Similar to a photon which gets reflected by a mirror. The photon never ages when observed, and it can reach any given location within our lightcone without having aged.

But how does a photon "experience" all those direction changes from within its own frame, or how would an observer inside the rocket experience those, when they would basically all happen in an instant for an observer inside the rocket?


Also, just because a theory leads to accurate predictions locally, does not necessarily imply that spacetime or 4-space is properly described by this theory.

In particular, imagine two brothers within two spaceships traveling at a given velocity relative to each other and meeting locally at some point in spacetime.
Given their mother is sufficiently far away, they won't be able to agree on if she is alive or dead. We call it the "relativity of simultaneity" but there is no simultaneity at all.

Saying that different observers traveling at different velocities have a different simultaneity plane/axis is wrong in the sense that the name simultaneity implies something happening at the same time.

But this is not the case. Any observer can accelerate back and forth at his given spacetime location, shifting his so called simultaneity plane. Doing this acceleration dance, his cat being sufficiently far away on another planet would be switching from dead or alive depending on the direction he accelerates, which is absurd.
Hence, the so called simultaneity plane is certainly not a plane which describes what is happening at the same time. The naming is confusing at best.

In the end, there does not seem to be a way to resolve the issues above within the frame of SR. Hence, there seems to be no way to describe "the world out there" using SR, such that it would make sense to a thinking human being.

No doubt, Special Relativity and GR works great for LOCAL predictions as i said before (GPS sats etc), but they don't seem to work in describing the rest of world out there which isn't local to us.

[schifoso]

Your challenge to what to what Einstein believed is intriguing and I'll have to read up on what others say about this.

[hesokaheso]

Yes!! I totally agree! am watching your series this one and the other, ounce again your heling me through my courses thanks professor!

[filipvanlijsebetten]

I think you can erase the second part of the blackboard here. The question what happens when v=c, is a meaningless question. It's the same as asking what is north of the north pole or what is the color of Wednesday. You can only approach c asymptotically, since you have mass.

[albertomoldes]

I think there's a misunderstanding. The basis of special relativity is that nothing can move at speed of light. And because of that there exists a dilatation in time or contractions in distance. If something with mass could move at the speed of light time and space collapse because of the infinite energy required to move infinite mass. And even with infinite energy you can only get infinitely close to speed of light, but not reaching it.

[shahbazahmedansari]

Sir according to mass and velocity relationship, if a body travels with the velocity of light its mass would become infinite so it would require infinite energy to make it run with the velocity of light

[bebinashanty]

Is this situation practically faced by someone or if our body mechanisms stop then won't we die

[feelingzhakkaas]

As understood reach speed of light c you require infinite amount of energy.... how this is possible ?

[-tejasagi]

Sir, u took the liberty of the spaceship travelling at the speed of light.In practical world as per einstein nothing can travel at the speed of light due to mass energy relation.If u keep aside the suggested contradiction aside u will be still wrong, as u are dealing with einstein's problem using newtonian mechanics or not so advanced mathematics.if u take into account the the modern mathematics, the speed of light is constant.I think u made a mistake by underlooking the concept with doppler effect which could not be true for the relativistic universe.Doppler formulated his theory for a non relativistic universe, but still effective enough in the modern world.In a technologically advanced world the suggested formulas by doppler would fail.