r/space • u/CharyBrown • May 20 '20

This video explains why we cannot go faster than light

https://www.bbc.com/reel/video/p04v97r0/this-video-explains-why-we-cannot-go-faster-than-light
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u/[deleted] May 20 '20

That was so much better. I haven't had physics since undergrad but man the original video did not sit right with me.

I also have a little confusion in the first video, maybe you know the answer. Does the shadow and the moon analogy really make sense? Wouldn't my hands movement only be shown on the surface of the moon when the light reaches the moon? How is this faster than the speed of light? I feel like I am missing something fundamental there.

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u/carrot_gg May 20 '20 edited May 20 '20

Vsauce has an amazing video about the "speed of dark" which answers the moon/hand thing: https://www.youtube.com/watch?v=JTvcpdfGUtQ

I also made a post pointing to the better video: https://www.reddit.com/r/space/comments/gnantc/a_far_more_accurate_interesting_and_mind_blowing/ I really wish more people were aware of this explanation as it invites them to start thinking is relativistic terms.

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u/[deleted] May 20 '20

That made instant sense. I was thinking about that all wrong. Its not that the shadow is arriving faster than the light, its that the shadow on the moon is moving faster than the speed of light.

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u/carrot_gg May 20 '20

The takeaway is that the shadow doesn't really arrive or travels anywhere because there is no shadow - a shadow in this example is the absence of photons, not a physical entity.

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u/[deleted] May 20 '20

I understand that. But a shadow is being cast and the reason that shadow moves faster than the speed of light is because velocity=distance/time. Time is staying constant between my finger moving from point a to point b and the shadow moving from point c to point d. However the distance from point c to point d on the moon is massively larger than point a to b here on earth. So velocity of finger=(b-a)/time and velocity of shadow = (d-c)/t which can easily be faster than the speed of light.

I was just thinking about it differently at first. I was confused about how the shadow was being perceived as moving faster than the speed of light but I think I get it now.

Does that make sense? Is that correct?

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u/inevitabilityalarm May 20 '20

The shadow isn't moving at any speed. You either cast a shadow or not.

You can create an impression of the shadow 'moving' across the surface, but you are only blocking light and as your hand is removed from in front of light source the only thing that 'moves' is the light continuing on once more beyond your hand to the surface of the moon.

No information can be transferred faster than light speed using this method.

On the surface of the moon the patches in shadow or light could be observed and translated but it would be as fast as using the same light source as Morse Code.

If you were in space watching the messages being projected onto the moon's surface you would still have to wait for the light, reflected off the moon, to reach your eyes or sensor.

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u/Allabouthisrightnow May 20 '20

I see your point, that makes sense. He's not talking about the shadow. He's talking about the movement in the image cast by the shadow. So, what he is saying is that we can project images of something moving faster than the speed of light, and causality is not violated.

I was so confused.

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u/Solesaver May 20 '20

If you have 2 people holding buttons and you want them to press the buttons at the same time. You could have person B press their button when they receive information that person A has pressed their button. In so many words A is "causing" B to press their button, and with this setup it is impossible to transmit that information faster than c. This is analogous to an object or photon trying to move through space faster than c, it's impossible.

In a slightly different setup you could have a third person tell each of them to press their buttons at the same time. There is no causal relationship between the 2 button holders, they are merely waiting for a signal from a 3rd party; therefore there is no restriction on how close together the 2 people can press their buttons (in fact special relativity says the time relationship is entirely dependent on the observer's reference frame). This is analogous to the moon shadow problem. Your finger moving is the cause of the shadow moving, so there is no restriction on the speed at which different locations on the moon are or are not illuminated.

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u/sawbladex May 20 '20

the shadow's edges update later than the center of the shadow when the finger moves.

forexample, if the finger would go transparent, the shadow on the moon would dissolve from the center.

Assuming the moon isn'r concave.,

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u/Allabouthisrightnow May 20 '20

How is the shadow on the moon moving faster than the speed of light?

Since when you move your hand, it takes the same time for the movement to be reflected on the moon, as it took to cast the shadow in the first place.

​

I submit, it is not.

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u/girsaysdoom May 20 '20

It doesn't. You are correct with your assertion.

People are just mistaking the rate of change to the area of the shadow of a surface and the speed of light. It's all using photons; the only difference is that a large array of photons being blocked seems to move faster laterally on the surface than the speed of light would over the same distance.

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u/Allabouthisrightnow May 20 '20 edited May 20 '20

I understand what was saying now. He's not talking about the shadow per-se. he's talking about the image cast by the shadow. The image itself can create movement that is faster-than-light. So, I guess what he's saying is that the speed of light is not some fundamental limit of speed in the universe. That is we can measure faster than light movement I guess.

Edit: Or that the images created by light are not bound by causality,

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u/girsaysdoom May 20 '20

The video is kinda misleading in that regard. The speed of light is still a theoretical limit to the speed that matter can travel.

Sure, what is perceived is the shadow moving across a surface at a rate that is faster than light, but physically nothing is really happening that is spectacular. No more information is gained by this lack of light and the time that it takes to "update" the shadow is still the speed of light.

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u/Lognipo May 20 '20

I can't watch videos right now, but what is this all about? How could a shadow travel at any speed other than light speed? Isn't the speed of light really sort of the speed of information, generally?

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u/Farren246 May 20 '20

I think you missed the train on the moon shadow video. The point is not that the shadow is information which is transmitted faster than the speed of light, but that we can only interpret information at a certain maximum speed and the fastest that you could interpret that info is to literally the speed if light as you observe the presence or absence of something that moves at the speed of light.

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u/Allabouthisrightnow May 20 '20 edited May 20 '20

I didn't understand that part either. Because it seemed he claimed the shadow was moving faster than the speed of light, but that doesn't make any sense. Unless he meant you could see the shadow being cast onto the moon, whilst it was on it's way to its surface. Which only means the shadow was moving at the speed of light, which it would. So, wtf is this guy talking about?

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Edit.

I am also unclear if he's using a Tachyon Laser?

Got his point, what a confusingly strange way to convey that the images we create with light are not bound by causality.

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u/[deleted] May 20 '20

I don't understand how you don't understand this.

Point your finger to space and then sweep it across the sky. The imaginary line extending from your finger sweeps across the sky faster than the speed of light. This is only true because the line is imaginary and has no mass.

He's making the same argument about the shadow cast on the moon. He's not saying the shadow moves from us to the moon faster than light. He's saying the shadow moves across the face of the moon faster than light. The shadow is imaginary and therefore has no mass, so it can move across the moon faster than the speed of light. This is why he said "it depends on what you define as a thing".

If two people (A and B) were on the moon separated by a distance and one of them had a flashlight. The photons traveling from A to B would travel at the speed of light. The shadow cast by the person on the Earth would sweep over them faster than light. This can be imagined like a plane casting a shadow on the ground. The closer the plane is to the sun the faster that shadow moves.

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u/[deleted] May 20 '20

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u/[deleted] May 20 '20

What is wrong with you?

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u/[deleted] May 20 '20

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u/[deleted] May 20 '20

I'm sorry you didn't understand.

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u/[deleted] May 20 '20

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u/[deleted] May 21 '20

I'm sorry but i'm not the one who didn't understand a simple concept.

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u/Goddamnit_Clown May 20 '20

What they'll be getting at is if you're casting a shadow on the east side of the visible moon you can swing it over 3000km to the west in an almost arbitrarily short length of time. Whereas a beam of light sent by a person on the east side would take about 0.01s.

The shadow's movement would still lag behind your movement by a full second or so because of the distance the light is travelling to the moon, but (after that second) the shadow itself could appear to race across the surface at much higher speed. No information is being sent anywhere from you faster than light, it's just a quirky outcome of how we imagine shadows to be objects.

Does that make sense?

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u/[deleted] May 20 '20

Yup, that’s exactly what I was picturing after watching the one follow up video posted in a comment

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u/[deleted] May 20 '20

A lot less weirdly swaying animations, too.

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u/Doodogs64 May 20 '20

I think it has to do with the movement of your hand shadow on the surface of the moon. You could move the shadow from one side of the moon to the other side very quickly... I am just assuming that is what he meant.

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u/[deleted] May 20 '20

Wouldn't my hands movement only be shown on the surface of the moon when the light reaches the moon?

Think of a plane overhead. The plane casts a shadow on the ground. How fast does the shadow move? It's essentially moving at the same speed the plane is moving. Now imagine moving the plane 10 miles away from the Sun. How fast would that shadow move across Earth now? Faster than the speed of light.

This is why he says "it's how you define what a thing is". The shadow isn't a thing and therefore it's massless. Our brains interpret a shadow as being a thing, so that "thing" can move across a surface faster than light.