Why can’t anything go faster than the speed of light? | The History of the Speed of Light Part II

Figuring out that the speed of light was the ultimate speed limit of the universe is part of the late 19th-century and early 20th-century history of Physics. This is part 2 of my “history of the speed of light” series.
Part 1 - How did we measure the speed of light? • How did we measure the...
Michelson & Morley 1887 - www.ajsonline.org/content/s3-3...
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👩🏽‍💻 Dr Becky Smethurst is an astrophysicist researching galaxies and supermassive black holes at Christ Church at the University of Oxford.
drbecky.uk.com
rebeccasmethurst.co.uk

[@DrBecky]

Forgot to say photons (particles of light) are a special case - they have no mass. Hence why they can travel at the maximum speed with no trouble!

[@theCodyReeder]

Light has no mass?! But... then how do kugelblitz work? ;)

[@bimblinghill]

No mass, but momentum.. right? How does that work?

[@EnglishMike]

@@bimblinghill No momentum either, since momentum is mass * velocity and mass is zero.

[@theCodyReeder]

@@bimblinghill They do have relativistic mass just not rest mass. which is a distinction that has always bothered me since i've never observed a photon at rest.

[@xspotbox4400]

I will never forget first time when I've seen Superman shooting x-ray lasers from his eyes.

[@somedaypilot]

Please do more "history of ..." videos. It's fascinating to hear about how we figured these things out, and you're a wonderful teacher.

[@vikraal6974]

See Kathy loves Physics channel on KZbin, best channel so far on history of physics

[@DrBecky]

Thanks 🤗 these are my favourite to make too

[@turkosicsaba]

@@DrBecky Could you make a video about how we measured the distances to / sizes of the Sun and the Moon, using the transits of Mercury and Venus?

[@andrewmcfarland57]

I see the Royal Society and some BBC/PBS Nova documentaries in Becky's future :-)

[@jull1234]

History of science is an oft-forgotten topic. Too interesting for such a lack of general visibility.

[@handsfree1000]

I’ve been increasing in mass lately, guess I must be travelling too fast!

[@atoms_dancing]

Slow down buddy

[@SpaceCadet4Jesus]

No. You are NOT traveling too fast, in fact you are "relatively" stationary. The scientific reason why you are increasing in mass is because other objects are increasing their speed TOWARD you, namely, packets of food.

[@karma432]

When you start to approach infinite mass it's time to slow down.

[@stevenbaumann8692]

I’m so glad you showed how scientific failures can sometimes lead to the best discoveries. Us scientists should should really embrace our failures. Their experiment was only a partial failure., like you said, it paved the way for Einstein.

[@saltybits9954]

She never discovered anything

[@Salien1999]

In the grand scheme of things I wouldn't call it a failure at all. The experiment found a flaw in our model of the universe at the time. That's exactly how progress is made, and I can't believe anyone gives them flack for it to this day. How else do we find out the truth than to seek it?

[@keithmayes4358]

I love your style of presentation, your enthusiasm for your subject shines through and your explanations and analogies are so good my eyes hardly ever glaze over. Please keep ‘em coming!

[@Valdagast]

The only thing known to go faster than the speed of light is a cat who hears the treats bag.

[@fuckyshityfuckshit]

My girlfriend's cat could hear the treat bug rustle through 7 metres of led.

[@jamesdriscoll9405]

@@fuckyshityfuckshit 7 metres of led - is that a cover band?

[@DrBecky]

😂 love it!

[@lolsurprise7185]

Einsteins theory doesn't say nothing can move faster than light, it says that nothing with a velocity slower than light can be accelerated to a speed greater than that of c

[@theCodyReeder]

So thats why the cat gets to me before i've shaken the bag!

[@UkDave3856]

If only I'd had a physics teacher like you back in the 1980s, someone who explained the subject in an engaging way rather than just attempting to force the equations into our heads by making us repeat them over and over

[@saltybits9954]

She would be wrong then too.

[@bophame]

Same. 70’s. And the prof received an award for teaching. Oh, and he never erased. He just overwrote. Learning to teach it takes much longer than learning what it is.

[@saltybits9954]

@@bophame Bullshit. I bet youre a simp teacher beta male

[@ffggddss]

Nice treatment of this, but I have to take exception to your final argument. Mass of a body doesn't change when its kinetic energy increases; so in fact, *all* of the "extra energy" goes into increasing momentum. In SR, p ≠ mv; rather, p = γmv = mv/√(1-[v/c]²). In this view, the reason material bodies can never attain speed c, is that this would require infinite momentum. To adopt the stance that mass depends on velocity, requires the mass of *one and the same body* to depend on *the observer's state of motion,* and therefore, to be *different for different observers!* (Remembering that, in SR, all inertial frames are equally valid.) Which is clearly nonsensical. Physics is simpler if we stipulate that the mass of a body is a property of that body, not of the observer. This becomes clearer when it's all cast in terms of 4-vectors, in which the Minkowski metric applies: c²dτ² = c²dt² - dx² - dy² - dz² = dx₀² - dx₁² - dx₂² - dx₃² (in c=1 units) for time-like vectors ds² = -c²dt² + dx² + dy² + dz² = -dx₀² + dx₁² + dx₂² + dx₃² (in c=1 units) for space-like vectors With position 4-vector, ֿx = (ct, x, y, z) = (x₀, x₁, x₂, x₃), and proper time, τ (a 4-scalar = independent of choice of inertial frame!), the velocity 4-vector is ֿu = dֿx/dτ = (dt/dτ, dx/dτ, dy/dτ, dz/dτ) = (γc, dx/dτ, dy/dτ, dz/dτ) = (γ, dx₁/dτ, dx₂/dτ, dx₃/dτ), in c=1 units. Thus, ֿu = (γc, γv₁, γv₂, γv₃) = γ(1, v₁, v₂, v₃) . . . [c = 1] Its magnitude is c (or just 1, in c=1 units). This leads to a wonderful insight - every material body moves through *spacetime* at speed c! Then the energy-momentum 4-vector is just m (also a 4-scalar!) times the velocity 4-vector: ֿp = mֿu = (γmc, p₁, p₂, p₃) = (E/c, p₁, p₂, p₃) Its magnitude is mc (or just m, in c=1 units). Momentum (a 3-vector) is the 3 spatial components of ֿp. So applying the Minkowski metric to find the squared magnitude of this 4-vector, we obtain m²c² = E²/c² - (p₁² + p₂² + p₃²) = E²/c² - p² Multiplying by c² and rearranging, E² = m²c⁴ + p²c² ... just as you said; and which also covers the m=0 case of photons (E = pc), which you've added in your pinned comment. Fred

[@edwardlane1255]

can you spell out what the letters in u = dֿx/dτ stand for (what is u and what is being differentiated with respect too what) - otherwise fab comment - thanks

[@narfwhals7843]

@@edwardlane1255 u is the name for the four-velocity vector, dֿx is the difference in the four-position and dτ is the difference in "proper" time. The time measured by a comoving observer. Four-position is the four dimensional position in spacetime. And four-velocity is the velocity analog.

[@mikeferguson689]

Helpful video on the energy-momentum 4-vector and invariant mass: kzbin.info/www/bejne/jZ-ve6Kbd6qSeK8

[@joelvirolainen590]

The momentum has the relativistic correction term, so it's the momentum term increasing to infinity preventing the speed reaching light speed. The equation has two parts, the energy of the mass (staying the same) , and energy of the movement (the momentum part).

[@anuragthakur4341]

And time dilation and length contraction. Everything in SR basically.

[@dungeonseeker3087]

KZbin recommended this (actually part one) and I'm an instant sub. Fascinating topics presented clearly & factually. I'll be binge watching most of your back catalogue this weekend.

[@DSC800]

Adding that "P=MV" part really takes the fun out of future space travel.

[@daisyxfaith]

Dr. Becky, I'm absolutely obsessed with your channel! The way you present a topic is so engaging and is somehow simultaneously approachable without oversimplifying or dumbing anything down. You rock! :)

[@Robert-kx8mp]

Having been a physics major, I’ve heard the story many times, and whenever hearing it again I’m always amazed at Einstein.

[@saltybits9954]

Then you've been fooled more than once?

[@ecospider5]

I had two Epiphany’s in just an eight minute video. You are truly amazing. I have enjoyed understanding this at a high school level for 30 years. Randomly reading thing to see if I get something new but don’t find anything. 8 minutes of your video, my mind is blown and now I want to go do some math.

[@Michel-rh3nj]

Your Enthusiasm and clarity in explaining physics is amazing. For me, your videos are continuation of Richard Feynman lectures on physics, I’m hooked. Please keep those videos and books coming (audiobook is super)

[@ajkey2266]

When I don't care that it's blurry because the content is so good and so informative...

[@erictaylor5462]

I had a teacher explain it to me like this: According to Einstein, if we have two objects moving relative to each other, we can say that object 1 (O1) is moving and Object 2 (O2) is stationary or we can say O2 is moving and O1 is stationary. Both are correct, even though they seem mutually exclusive. So we could say that that matter is stationary and light is moving, or we could say light is stationary and matter is moving. So, imagine that light is stationary in space-time while matter moves through the combined space-time at the speed of light. If a bit of matter wants to move through space, it can only do so by using some of her velocity through time. This is why time slows down when you move through space. And it is why you can't go faster than light. Your total velocity through space-time must always equal 1C. There is a weird consequence to this. From the perspective of a photon, it never exists. A photon is created and absorbed at exactly the same time, even if that photon existed for billions of years from our perspective.

[@sennetor]

except when that "photon" passes through a double slit experiment and manages to demonstrate its existence in all places at once. I like your thinking though, it makes sense for SR.

[@erictaylor5462]

@@sennetor Remember, point of view is important. The photon does not move through time, so it is created and destroyed at exactly the same time from it's point of view. We see it exiting from our own point of view, which is why it can pass through a double slit and confuse the hell out of really smart people.

[@boggers]

I prefer to think of it as not going through time rather than not existing. It's also fun to consider that to a photon, the universe is 2D - collapsed to zero depth in the photon's direction of travel.

[@erictaylor5462]

@@boggers They are the same thing.

[@erictaylor5462]

@Zu_alt_fuer You understand that we are speaking metaphorically, right? Photons don't age, because they do experience time.

[@theCodyReeder]

Oh another one of thees videos?! Happy day!

[@sergei_gruntovsky]

Oh, I see why it's nice to have internet access on chicken hole base :)

[@kelleysimonds5945]

Like you have some right to bitch about other's videos. Hashtag - no respect..

[@JoeJumps]

The brilliance of your mind coupled with your lovely accent and a dash of your beauty. Makes you an absolute doll. I love how passionate you are about your work. You definitely got another subscriber out of me. Looking forward to more videos.

[@Doones51]

One of the best explanations i have heard. Thanks for being so clear, concise and speaking slow enough so i can absorb what you are saying.

[@robsbackyardastrophotograp8885]

I have known about the Michelson-Morley experiment, but I never knew they spun it to see a sinusoidal curve. Excellent explanations! I'd love to see more space science history here!

[@saltybits9954]

Michelson-Morley is B.S. another lie

[@veridicusmaximus6010]

@@saltybits9954 How's the Ice Wall?

[@saltybits9954]

@@veridicusmaximus6010 What does that have to do with anything we are discussing?

[@veridicusmaximus6010]

@@saltybits9954 I was thinking you probably think the earth is flat so that one of those other lies was that the Earth is spherical.

[@saltybits9954]

@@veridicusmaximus6010 Well you're wrong again Einstein idiot.

[@connorwicks6647]

The increase of energy with velocity comes from the geometric properties of space-time itself and not mass. Mass is an invariant quantity, being the magnitude of the 4-vector, and shouldn't be applied to the time component of a 4-vector.

[@paulpaulin4104]

You r answer is perfectly right.Many physics teschers are still talking about relativistic mass.They are wrong. The same error appears in most physics books.

[@new-knowledge8040]

@@paulpaulin4104 That was what I stated in my YT videos. My videos basically show how even a high school dropout can discover the SR phenomena all by themselves, and derive the SR equations independently as well. However, the method of doing this, is unique. I soon found out that in the world of physics, anything new trying to replace the old, is violently opposed to such an enormous extent, that people in the world of physics, and many others, would rather focus upon this maintaining this violent opposition, rather than save the entire world from an upcoming deadly asteroid impact.

[@patrickrochel9827]

You and your videos are amazing! Keep it up, look forward to seeing these all week!

[@JordanALAllen]

Dr Becky I had no idea you had your own channel until now! Looking forward to catching up on your videos and seeing what’s to come... thanks for all you’ve taught me

[@nodoxplz]

Great video, other than when you say a body moving closer to the speed of light gets more massive. Relativistic Mass isn't actually mass, it's just a tool used to introduce relativity. Dr. Don on the Fermilab channel has a great video about it.

[@Robert-drisch]

But, if a Mass speeds towards the speed of light, where is the spaghetti Theory again...??? Visual

[@rogerschneider5971]

If what's increasing is inertia, and inertia is a measure of mass, is there really a distinction here?

[@DavidFMayerPhD]

"Empty Space" is filled with fields whose values are (close to) zero. Nonetheless, the fields are quite real and measurable to great precision. In his original paper, Einstein did NOT invoke the Michelson & Morley experiment. His rationale was this: In Classical Mechanics, coordinates transform according to "Galilean Transformation". In Maxwell's Electromagnetism, coordinates transform according to "Lorentz Transformation". It is absurd to imagine that natural coordinates transform differently for different phenomena. Hence, one of the two transformations must be incorrect. Since Maxwell predicts the speed of light correctly, he decided that the "Lorentz Transformation" must be correct. The "Lorentz Transformation" preserves the speed of light in all inertial frames. Also, in using the equation E^2=M^2*C^4 + P^2*C^2 (where ^ denotes exponentiation), Dr Becky mixed up REST MASS with TOTAL MASS. The equation is better written E^2 - P^2*C^2 =M^2*C^4 which is a Lorentz invariant, where M is the REST MASS which does not change with motion. In Einstein's equation E = M*C^2, M refers to the TOTAL mass even when the particle is moving relative to the observer. This also answers the frequently asked question, when one changes coordinate systems, energy changes. Where does the change in energy come from or go? This question is based on the false assumption that energy is a scalar. The answer is that E is NOT a scalar, but the time coordinate of a Lorentz 4-vector which transforms according to the "Lorentz Transformations". Sorry to be so pedantic.

[@NandR]

Dr., Why is E^2 - P^2*C^2 =M^2*C^4 a Lorentz invariant while E^2=M^2*C^4 + P^2*C^2 is not? My math is rusty.

[@DavidFMayerPhD]

@@NandR E is the time component of the Energy-Momentum four-vector, and momentum P constitutes the other three components. The dot product of a four-vector with itself is the modulus squared of the vector, and is invariant under Lorentz transformations.

[@NandR]

@@DavidFMayerPhD Thanks Dr.!

[@JJ8KK]

Thought I might share with Dr. Becky et al. a philosophical challenge to Einstein's theory of Time Dilation. She might be interested because it involves a definition of Time that I think she, as a physicist, would find difficult to criticize. The argument is that Time is nothing more and nothing less than the relative motion of all things in motion. We 'mark time' when we observe that certain motion events occur *_as_* some other (usually repetitive) motion event has occurred. To say that a plane covers a certain distance in a certain amount of 'time' is to say simply that the motion event we are observing occurred _as_ the sun moved across the sky a certain amount, as the earth rotated on its axis a certain fraction of a rotation, as the hands of a clock moved a certain distance, etc., etc. We would never have developed a sense of 'Time' if we had not noticed the regularity of certain motion events (e.g., the earth, moon, sun, stars) and had not made the 'guess' that nothing we can imagine will ever change that regularity of motion we've observed. The 'pace' of the motion we've experience is assumed to never change without some unexpected 'intervening' galactic [motion] event occurring that could alter it. This is why the notion of 'Time Dilation' proposed by Einstein is so problematic. He may have been right about a lot of his speculations, but this one is more than a little bit dubious. It is suggested by E that a spacecraft accelerated to a velocity near the speed of light would experience a 'slowing down' it's Time. What this necessarily means is that the _motion_ of all things in motion within the spacecraft would slow down _relative to the motion of the earth, sun, and moon_ back on earth. That is the only way we can accurately imagine the slowing down of time for a fast-travelling astronaut. What this means is that the motion of all the electrons orbiting nuclei in the spacecraft and in the astronaut's body would slow down relative to the electrons back on earth. Really? Where is the cause & effect? The linear acceleration of the spacecraft in a vacuum somehow causes the motion of all the subatomic particles within it to actually slow down? How so? If Time for an accelerated astronaut (IOW, hiser motion) was actually slowed down by some prior acceleration event, wouldn't hiser physical movements be slower than everyone else's, once the cabin doors were opened upon hiser return to earth? You say they'd be sped up again to the 'normal' motion pace experienced back on earth? What kind of event could be imagined that would "re-accelerate" the atoms that were slowed down by the spacecraft's acceleration? Acceleration _against_ its established inertial momentum? But I thought acceleration slows Time (motion) down? Logic tells us that the astronauts would not only be much younger than everyone back on earth, but they would also be observed to be physically moving _in slow-motion_ for nothing would have occurred that would have sped up their motion again. We can't simply assume that "something" would have 'automatically' occurred to restore their experience of 'Time' (their pace of experienced motion) back to what everyone else would be experiencing on earth. This is the kind of problem that is revealed when one attempts to remove the concept of "Time" from the more fundamental concept of Motion. Mathematically, you can create a Letter T that you say represents "Time" and put it into an equation, and then use mathematical operations to obtain 'equivalent' results. But if Time (galactic motion) is actually a universal *constant* , then pretending that it is actually a variable in an equation can surely give you some fantastic results (e.g., that Time Dilation can occur), but it is only because you have imaginatively assumed that the letter T actually represents a variable when in all observable reality, it is not and can never be anything other than a universal constant. Physicists use math to make predictions, but the equations they construct are ultimately dependent upon the conceptual 'values' they assign to the letters used in those equations, specifically, those which are assumed to be 'constants' and those which are assumed to be 'variables.' I claim that Einstein's Time Dilation Error arose from his decision to arbitrarily assume that T --- a constant --- should perhaps be thought of as a variable. The problem is that it is an assumption/guess that leads us down a path where the only way it is possible for us to accept predicted results is by professing a faith in *mathematical mysticism* . Defining the value of a letter in an equation as either a constant or a variable is an assumption---a guess---that may or may not be true, but it really does need to be justified by other assumptions/guesses that you have a high degree of confidence in. Einstein may have been every bit the genius that he is heralded as, but IMO, he got this one---the Time Dilation Hypothesis---wrong...

[@charlesgantz5865]

One of the prime things that Einstein was trying to show was why a current in a wire produced a magnetic field. This is the electrodynamics part of "On the Electrodynamics of Moving Bodies". The first paragraph of Einstein's paper discusses this, and the reconciling of the Maxwell equations predictions with what is observed takes up the second half of his paper. The first half is the derivation of Special Relativity that is needed by the electrodynamics part of the paper..

[@Sad_King_Billy]

I've been trying to understand time dilation for years and this video finally made the concept click. Thank you!

[@theshowman8478]

Nice explanation Dr. Becky. Thanks.

[@ailblentyn]

You make two statements about Einstein's special relativity: 1) that it explains how light can travel without a medium 2) that it explains how light can travel with invariant speed. The second is true, but the first doesn't seem to make sense or be supported by you. Today, if we're using terms like "medium" in an ordinary sense, wouldn't we say that quantum field theories have actually deacribed what the medium of electromagnetic waves is.

[@ectoplasm12345]

Bump

[@new-knowledge8040]

In truth, Einstein's SR actually would explain why an aether/medium was not detectable, if there was one. It did not exclude such a possibility of there being an aether/medium. Watching the video, I kept waiting to hear the bit about how SR can explain how light can travel without a medium. Waiting and waiting and....

[@guyh3403]

I took off my glasses. There, everything back in focus!

[@Doones51]

Dr. Becky, you are very clear when explaining physics and much appreciated.

[@1_2_die2]

Dr. Becky, you are not out of context, but (again) out of focus. I doesn't really matter with the topic, but with your jewelry! Thanks for your great work and fantastic channel.

[@24sumo]

1 minute in, wow I’m following every word of this. I’m a genius. 9 minutes in, oh my god what is going on. Good video though

[@DrBecky]

😂

[@taanbrown4275]

:D Lol

[@dondickson5399]

same here

[@mxlexrd]

This video was great, apart from the part you said the m term in Einstein's equation increases. m in that equation is always a constant, increasing energy always increases momentum. To be clear, an object going 99.99% the speed of light has MUCH more momentum than an object going 99.9% the speed of light, not just 0.1% more.

[@Sableagle]

That annoying guy on Because Science did a video called "The Death Star's other fatal flaw" about the momentum of light. The other fatal flaw is recoil. It would be spectacular. I still don't get it. I went for molecular biology and weird things like bacteriophage mu, the virus that keeps forgetting to virus and acting like a plasmid instead and has extra space in its shell for carrying stolen bits of bacterial DNA to its next host because it doesn't need all that space for its own DNA because it has 4 genes overlapping on the same bit of its own chromosome.

[@victorfergn]

@@Sableagle O___o

[@Sableagle]

@@victorfergn That was about how I felt when I found out about that thing, too, only my eyebrows moved around a bit more. DNA works in threes, right? AAC-AGC-GCA-whatever. Well, if you have one gene with its first codon starting at base pair 100 and the next at 103 106 109 112 115 118 121 124 127 130 133 136 139 142 145 148 151 and so on, you can have another at 122 125 128 131 134 137 140 143 146 149 152 and so on and a third one at 138 141 144 147 150 153 and so on. The fourth gene on the same strand? Well, you know how DNA is a *double* helix, right? It's got one strand running one way and the complimentary strand running the other. Well, there's a gene on the *other* strand, too, running in the opposite direction. Doesn't help you feel any less O_o, does it? As for the part about forgetting to be a virus, it injects its DNA and ... er ... yeah, the bacterium now has two chromosomes. Sometimes they merge into one. Sometimes they split into two. Sometimes the split takes the entire virus chromosome and a bit of the host bacterium chromosome too. Sometimes it leave important bits behind. When it finally remembers it's a virus and activates the genes that make the bacterium turn itself into hundreds of copies of the virus, those copies take whatever bits got stolen with them and deliver them to other bacterial cells as free new genes. If they happen to be genes for a yellow pigment, the infected bacteria turn yellow. If they happen to be genes for antibiotic resistance, infected bacteria become antibiotic resistant.

[@bvbinsane1vanity]

If you're going to try and correct a person with a phd, at least learn how to add. 99.9% + 0.1% is 100%. 99.9 + 0.09% = 99.99%

[@alwaysdisputin9930]

@@bvbinsane1vanity You're misunderstanding the OP's point. Nobody is talking about addition. The OP's saying that a small increase in speed gives rise to a huge increase in momentum. Thus DrBecky is wrong at 10:10

[@goukisan2869]

hi becky, great video. i am a big science enthusiast who randomly just discovered your channel. the bit at end when you go off-the-rail and start talking about your jewelry is so ridiculous, and yet the most enjoyable part of this video ( i subbed as soon as i saw it) Scientists have the bad habit of making it seem that we only have passion for advanced math, physics and cosmology. we need more people who can bridge the gap between science and our jewelries .

[@javierallende4639]

Excelent video Dr Becky, perfectly explained!

[@mikeferguson689]

“It is not good to introduce the concept of the mass M = m/(1−(v/c)^2)^1/2 of a moving body for which no clear definition can be given. It is better to introduce no other mass concept than the ’rest mass’ m. Instead of introducing M it is better to mention the expression for the momentum and energy of a body in motion.” -Albert Einstein (1948) As others have pointed out, your interpretation of the Energy-Momentum relation, E^2 = (mc^2)^2 + (pc)^2, is all wrong. The “m” term is the invariant “rest mass”, a constant. All energy of motion added shows up in the Lorentz transformed momentum which approaches infinity as v approaches c. p = mv/(1- (v/c)^2)^1/2

[@mikeferguson689]

Helpful explanation of 4-vector momentum and invariant mass: kzbin.info/www/bejne/jZ-ve6Kbd6qSeK8

[@johnneuman9470]

Could almost see my fingers red shift racing to hit play after notification of new Dr. B video! ❤️😀

[@cjmahar7595]

Or Blue shift depending on your point of view

[@AKMetCal]

I appreciate that this history begins with Roemer's restraint from giving a numerical value for the speed of light, and then the history ending with Michelson and Morley failing to measure the eather. It seems to me that they presented their observations without jumping to some weird conclusions.

[@mikefm4]

Beautifully explained. Your videos are superbly edited and top notch Thanks doctor!

[@MartinNatgeo]

I´m studing genetics but listening to you makes me feel like I mistake my career. You are awesome please continue this videos!

[@pedrosmith4529]

We just have to raise the speed limit, like they did on Futurama.

[@DunsmoreFamily]

If we made the speed of light 300,000 m/s then what would happen to both the distance of a meter and the weight of a kilogram? Seeing as how they are now codependent on each other.

[@menachemsalomon]

@@DunsmoreFamily The kilogram has been independently defined, although originally it was calculated as the mass of a liter (1000 cubic centimeters) of water. I think the issue is the purity and exact pH of the water. The answer to your question would still depend on the exact length of the second, and if it was indeed calibrated to 1/86400 the length of a mean solar day, and how that mean was determined.

[@fubaralakbar6800]

You don't want to do that. If you did, time would flow faster for everyone standing still. That would have some really fucked up consequences.

[@TheWeatherbuff]

Hi Dr. Becky! I'm just a low-life professional meteorologist, but also an amateur "space nut" :-) I just wanted to thank you for your great content. This is one of my favorite channels! Thanks for the information and entertainment.

[@mawnkey]

Michelson-Morley was an excellent experiment as it wound up disproving "established" science. So many people don't realize that science is about _disprovability_ in experimentation.

[@xilnes7166]

Great stuff.. love the history of how.... but I wonder why the late upload? and more importantly does that mean we aren’t getting a new video this week ? noooooooooo 😭😭😭😭if you have time pls make one more.... cant wait...🌑

[@wonderman8759]

I am wondering abt that too...

[@elenarlivingston352]

Yeah, I was thinking the same thing. Maybe she was busy. I mean after all she is a real professor not just a KZbinr. 😇

[@DrBecky]

It went up at 8pm UK time on a Wednesday like all my videos...

[@xilnes7166]

What I meant was we can never have enough of your videos...🤩LOL.. Thanks for all the great stuff..Always happy to see a new new upload , lights up my day...

[@erictaylor5462]

1:30 So they can't hear you scream, but they can see you screaming. Oh my, what a horrific image!

[@shawnmatthewpyper]

You would not be able to scream. Your blood would boil just before you burst.

[@alexdevisscher6784]

That must have been what Edvard Munch had in mind when he painted The Scream.

[@HAL-nt6vy]

@@shawnmatthewpyper You can survive for a couple minutes in complete vacuum and recover. Experiments on hero doggies taught us. Watch 2001: A Space Odyssey for an accurate depiction of someone surviving the vacuum of space sans helmet for a few seconds.

[@erictaylor5462]

@@HAL-nt6vy There was a Russian testing pressure suits when the suit failed. The guy passed out in just a few seconds, but the depressurized the chamber and he survived. No popping, but he did say the sensation from boiling saliva in his mouth felt strange.

[@HAL-nt6vy]

@@erictaylor5462 Bing tells me, ". . . research, conducted during the 1950s and 1960s, concluded that a human exposed to a vacuum will remain conscious for ten to fifteen seconds and can survive for up to 90 seconds with relatively minor and reversible side effects." Come November 3, 2020, we may want some vacuum to stop the SJW screaming when Donald Trump gets reelected.

[@protoword10]

I watched many videos about this topic, but your explanation on this phenomena is quite clear and understandable. Thanks and you are great teacher as one of your subscriber mention it! It makes me so also to push bell button on your videos!

[@Nilguiri]

I think that's the first time I've heard the infinite mass at the speed of light thing so clearly explained in terms of E2 = m2c4 + p2c2 It all makes a lot more sense now, in fact I can see that it couldn't be any other way! Thanks, Dr. Becky!

[@kasession]

I try to test the theory of time slowing down as I speed up every day when I leave home at the last minute on my way to work. :-)

[@robertlamantin5088]

If nothing can overcome the speed of light, why my cat always ends to catch the red laser point ?

[@johnnyd3387]

My Dad used to say the only thing faster than the speed of light was the speed of thought. When your looking at the past looking at the stars. Imagine yourself at the farthest reaches of the universe and picture yourself there. I thought that was pretty cool. Love your fascinating show.

[@skhotzim_bacon]

Your dad sounds pretty awesome. I'm going to steal that saying for myself

[@themeatpopsicle]

Another outstanding explanation of something that doesn't often get communicated very well to non-technical folks!! Thanks!

[@orsonzedd]

It's not just a good idea, it's the law

[@MisterItchy]

They had a hypothesis and then, when it didn't hold up to testing, they abandoned the hypothesis? Didja hear that flat-earthers?

[@xspotbox4400]

My heroes.

[@anatomicallymodernhuman5175]

And dark energy chasers. 😁

[@MisterItchy]

@@anatomicallymodernhuman5175 I don't know what you mean? Are you saying that dark energy chasers should abandon the search?

[@anatomicallymodernhuman5175]

Mister Itchy , not at all. They and unicorn chasers should keep it up until they die. I just wish they’d quit wasting my tax money on it.

[@MisterItchy]

@@anatomicallymodernhuman5175 Like those damn scientists that cure cancer or make the breakthroughs in technology that make computers possible. Such a nuisance.

[@mini30coupe]

Love your show, your intellectual information and especially the new haircut!

[@BillGreenAZ]

I love your "phases of the moon" necklace!

[@Monothefox]

If M&M’s setup had been sensitive enough, they would have detected gravitational waves.

[@Shanajio8]

That would have freaked them out! ;)

[@mikeclarke952]

No. Their interferometer was just too small for that. The wavelength of gravitational waves is 100 times longer.

[@YodaWhat]

@@mikeclarke952 -- Eh, not quite. While MOST gravitational waves have very long wavelengths, in fact far too long for even our giant detectors, some grav waves do have short wavelengths, like those right at the end of the 'chirps' in the LIGO/VIRGO data. For those highest-frequency waves, _our giant grav wave detectors are actually too big!_

[@marsgal42]

As noted in an earlier work, "We need new physics!"

[@MauroHervas]

I can't thank you enough for your time and effort. I've been struggling to grasp the idea of relativity for years. I'm no engineer nor mathematician or anything related, just a guy from Chile that likes to learn stuff whenever is possible. Again, thanks. I'm a photographer though and my advice is get a canon camera. Those dual pixel sensors do an amazing job at keeping focus for you.

[@petebrandon8160]

Becky you are best! I love watching everything you talk about. Thank you so much!! X

[@ronik24]

Once more excellent quality content, presented in an interesting, pleasant way! But maybe try fixing the autofocus at the distance of your face before filming next video... :0)

[@robertclymer6948]

YES I want the pretty pretty face in focus!!!!

[@ronik24]

@@robertclymer6948 Don't be creepy ;-)

[@RedBatRacing]

She's going for the beautiful alien girl meets captain Kirk look.

[@billdecat855]

She explained the focus issue in the last videos doobly doos. She was too excited and sat forward in her chair, throwing her out of focus.

[@timbeaton5045]

ronik24 The whole point of, as you mentioned it, Autofocus is to FOLLOW focus, and as we know this is pretty variable in quality depending on camera, lens, etc. I think she set focus to MANUAL (you know like the Pros do(!)) and then sat forward of the static focus point. And A) she knows it, and B) she even annotated the video to apologise. What do you want? Blood???😀

[@francoislacombe9071]

"Can't get enough, cannot get enough, seriously." So, when are you getting that moon phases tattoo? 😂

[@petergreenwald9639]

Over many adult years, I read every Tom, Dick, and Harry's fabulous, paperback, science book. Some of it was intriguing. None of it helped my understanding. One day I, in trepidation, bought a copy of a paperback that was supposed to be Einstein's original paper. My goodness. Such clarity. Today I found your videos. My goodness. Such clarity.

[@tonyshort3650]

Love watching and learning from your videos, your a wonderful and beautiful teacher :)

[@iCH3At53D]

Hair looks good girl!

[@PetermusPrime]

Ah the ol' Michelson-Morley experiment, the Flat Earther's favorite thing to misrepresent.

[@anatomicallymodernhuman5175]

PetermusPrime , and geocentrists. Ran across a group of Thomist philosophy-based geocentrists recently. They were all about M-M and its antecedents.

[@PetermusPrime]

"Thomist philosophy" Had to look that one up.

[@Sableagle]

Yep. Expect the swarms of angry sock-accounts soon. They're like ants. Once one finds something they summon the rest.

[@josephbonham3045]

I was hoping you would address why the speed of light is the arbitrary number we ended up measuring and make it less random. You very clearly helped explain how we measured it, and why we can’t break it. Maybe it needs part three? I’m like the driver on the highway that sees 55 mph and thinks, “huh, I wonder why that number instead of 65 or 70 or 73?” And why so slow compared to the vastness of the universe?

[@BothHands1]

omg that little queen tidbit at the end just made my whole night 😂

[@olivergroning6421]

Sorry Becky, your explanation at the end for the relativistic energy-momentum relation is not correct. The correct relation is E^2=(m0c^2)^2+(pc)^2 it contains the rest mass m0, not a 'relativistic' mass m and doesn't change. You need to develop from the definition of group velocity vg=dE/dp and distinguish the non-relativistic and super-relativistic case: Non-relativistic m0c^2>>pc -> The dispersion relation approximates to E=m0c^2+p^2/(2m0) (first-order Taylor expansion) and accordingly vg=p/m0 (i.e. classically) Super-relativistic m0c^2

[@secularisrael]

I second that. Strongly. Becky, please consider rescinding or editing the video to correct for this.

[@justvisitingterra6459]

Hang on mate, You forgot something,

[@olivergroning6421]

As a little side note, a beautiful way to understand why nothing moves faster through space than the speed of light comes from general relativity (GR) and special relativity (SR) combined. The metric of empty space in GR actually means that all object moves at a constant speed (the speed of light) through the 4-dimensions of space-time. Photons just ‘choose’ to use all their speed-budget to move through space, which means they don’t move through time. An object not moving in space must ‘race’ with maximum speed through time. The rest energy E0 or rest mass m0=E0/c^2 is what propels us through time. As the photon has none, it is compelled to move only through space at full speed c. As massive objects always are propelled through time (even though very little a very high momentum) they can never reach full c in space. I hope you take this as constructive critique Becky and not as hair-splitting, because I like your channel a lot.

[@sawspitfire422]

I have no idea what you just said, but it sounds great :)

[@Initialgs]

Why are people even using the term ‘relativistic mass’ in the first place?

[@davidbaker8634]

Noooo not relativistic mass :(

[@hellosolo4962]

Thank you so much, you just helped me find my answers for the physics project i have been working on

[@mistyman591]

I discovered your vids yesterday.now i see a brand new one ! Lucky me

[@linkpatrick]

I feel like you did't really answer the question, "How do we know the speed of light is constant?" Could you perhaps do a video in Variable Light Theories and how it might provide an alternative explanation to Cosmic Inflation theory.

[@niaei]

You need to watch the previous video my friend.

[@chrisofnottingham]

No one knew it was constant but Einstein proposed it was was constant. This was still seen as controversial over a decade later but in the end the assumption it was a constant fixed things that were otherwise unfixable. Also, well before this, Maxwell did some amazing mathematical work where he showed mathematically that an electromagnetic wave had a fixed velocity based just on the electrical properties of a vacuum. This made no sense to him at the time because he had not used a reference frame. But in fact it was because the speed of an EM wave in a vacuum is the same for any reference frame.

[@boggers]

Yeah I agree, there is more to it. She mentioned time and distance changing with speed, but didn't follow through with that idea and switched to infinite mass at c instead. If you look at things from light's reference frame, time and distance both zero out at the speed of light. Another way of saying that - a photon's journey takes zero time and covers zero distance, according to the photon's reference frame. There's videos explaining the delayed choice double slit experiment that explain this concept pretty well.

[@Dragrath1]

Another interesting way to look at the rational behind the speed of light being constant that goes hand and hand with this that while position is naturally gauge invariant momentum is not unless you add an additional term which turns out to exactly reproduce Maxwell's equations. Noether's theorem tells us that for every symmetry there is a conserved quantity and for each conserved quantity there is a associated symmetry thus in this case the conserved quantity is the speed of light itself. Incidentally this theorem holds mathematically in any mathematical regime so through this we can be safely assured the speed of light can not change ever as a change in the speed of light would require a symmetry violation of gauge invariance meaning you could not establish values relative to other values i.e. you would need an absolute reference frame, the existence of which was ruled out by the Michelson and Morley experiment and subsequent replications. In other words we have to observable limits proved that the speed of light can only be constant as for it to vary there would need to be an absolute preferential reference frame i.e. the "aether" must exist in accordance of Noether's theorem. TDLR the speed of light can only vary if and only if there is a fundamental absolute reference frame which all observers are relative to. We have also proved that no such frame exists therefore it can be concluded the speed of light does not vary and more specifically in natural units the speed of light has no value whatsoever and is instead the reference from which all values are determined.

[@orlock20]

I think of it as a speed track that has three participants--proton, neutron and electron. Protons move at the speed of light. Electrons move at about 2,200 KmS. A fast neutron is considered to go 28,000 KmS. Track conditions affect the speed of these participants. Even protons slow down in different mediums including space.

[@kevingilchrist5920]

If we could experience being a photon, would we experience any existence at all?

[@lordpredator8855]

I think not. A photon doesn't experience existence because of the time dilation. As you go faster to the speed of light time slows down. But if you go at the speed of light time literally stops. There are simplified versions of a graph which connects the movement in space and standing still in space with time. If you don't move in space you are experiencing time OR moving slower than light you will also experience time. But if you're traveling at c you don't experience time at all. It's like being born and dieing at the same time in other words 0 seconds. Like being born and arriving at the destination with 0 seconds. But us humans we are expecting time because we travel slower than light. Hopefully i explained it good. If you like more explinations i suggest watching AnswerswithJoe FermiLab Edit(these chanes explain very good about light and diagrams)

[@kidwave1]

"Ultra High-Speed Camera" films PHOTONS IN MOTION! kzbin.info/www/bejne/qZ-2eoWAZ8ele80

[@richardeldridge8335]

Excellent video. "If the speed can't change... then it has to be the distance and the time." is brilliant. I now understand why there is time dilation. On another subject, if you want to memorize the speed of light, it's much easier to say 1 Planck length per Planck time.

[@stephenrice4208]

Question on one way light speed: 1. Can you take electron A and electron B and entangled them and separated them by a logical quantified distance that would allow for a very accurately measured speed of light. 2. located a light source at electron A. 3. Locate a timer and light sensor at electron B 4. used triggered change in electron A to simultaneously (or infinitely close), initiate the light source as well as the simultaneous change in electron B, and simultaneously (or infinitely close), use the change in electron B to start the timer, and when the light source hits the light sensor use the sensor to simultaneously(or infinitely close), stop the timer. 5. Then take the clock time and subtract the near simultaneous trigger time from electron A to the light source, and the near simultaneous trigger time from electron B to the timer, and the near simultaneous trigger time from the sensor stopping the timer, and thereby calculate one way speed of light across a quantified distance. Would this work to trigger the one way speed of light and could this time be used to better identify the speed of the second leg of the two way light speed measurement?

[@jneiberger]

I think I have a mental block regarding relativity and the speed of light. Over my lifetime, I've seen so many books, magazine articles, videos and TV shows about it, and I've listened to Great Courses audiobooks on relativity and time and such. It still just does not make sense to me. I feel like I start to grasp it, but then it slips away. It's so unintuitive.

[@alphacore4332]

your thoughts/emotions/etc are not made with light, so time slows down if you are moving light does not care about you, so when you are in slowmo it just keeps going you measure its speed while you are stuck in limbo and it looks to you like it's moving way faster than you are, because your watch is also in limbo people standing around watching you are wondering wtf you are doing, they see it as normal light you are trying to move at the speed of light but it still seems to be so much faster this is all because your watch is in the time bubble, and doesn't use light basically it is something like all of this nonsense^

[@new-knowledge8040]

Check my YT videos. I look at SR in a different way. I know it's a bad bad thing, yes a bad bad thing, but in my videos I reveal the foundation that creates the relativistic outcomes, meaning outcomes like frames of reference, length contraction, time dilation, etc. I say it's bad, because it seems as though the very idea of exposing the complete truth about anything in the world of physics, is 100% banned these days. They only allow you to reveal bits and pieces, just as is SR in general being exposed to the public in a fragmented manner, a manner that confuses the heck out of most people, all due to its entirety not being exposed.

[@adamrspears1981]

Teacher: "What is The Speed of Light?" Me: "1 Light-Year per year." Teacher: "OK Smarty Pants! Now seriously....what is The Speed of Light?" Me: "1 Light-Year per year!" Teacher: "I need a vacation!"

[@joesterling4299]

Yes! C defined in meters per second, and the meter defined as the distance light travels in 1/C seconds. Nobody tell me that circular reasoning is dumb again.

[@marinal2705]

This was by far the best explanation I have heard!

[@daztaylor]

Just stumbled upon your channel by chance. KZbin suggestions can be irrelevant at times but this has been a pleasant discovery. After your explanations within the video, the thing that surprised me most, was the archaic spelling of aether! ;-). Subscribed.

[@RedBatRacing]

Are you going for the beautiful alien girl meets Captain Kirk look with that soft focus?

[@pioneer_1148]

Dr Becky, fantastic physicist, excellent presenter, and god awful camerawoman

[@recifebra3]

You're an awesome explainer of history!! Please do more on electromagnetism or other subjects and Faraday, Maxwell, Plank and Heaviside.

[@stevecolclough]

I really enjoy your videos. Thank you for doing them.

[@Sakkura1]

Sounds like you're using the relativistic mass towards the end there. As far as I understand it, that concept is controversial in physics.

[@dwightalexander2648]

I know the speed of light is the universal speed limit, and as of now we don't have the capacity to increase it. But what if we put a huge radar speed gun and a huge speed limit sign just outside the stratosphere, so when the light from the sun and other celestial objects enters the earth it'll slow down along with the earth's time. I mean most people don't wanna grow old fast. NASA should fund this theory

[@DougKutner]

Thank you for your wonderful explanations!!!

[@jaimeriveras]

Very good explanation, thanks. Your new haircut looks great.

[@jibriel4918]

Even you, Dr. Smethurst, have the relativistic mass misconception.

[@silliconcarbon6637]

I tend to partially agree with you, because she does mention rest mass at 00:09:15, but doesn't refer to relativistic mass at 00:10:37. But could you explain your comment?

[@Lulzwhat]

First of all - we can definitely say that nothing can moves FTL only and ONLY when we reach it. And we aren't even close to it. So ¯\_(ツ)_/¯

[@juzoli]

Lulzwhat Yes we are VERY close to it. We reached 0.9999999C, which I think is pretty close.

[@Lulzwhat]

@@juzoli on which scale? Did we speed up at least a grain? No.

[@juzoli]

Lulzwhat Did you read what I wrote? 0.9999999c, what other scale do you need?

[@TheRourkesdrift]

Very nice video Dr Becky. X

[@synseer8484]

Love your content love your channel, thankyou for dumbing it down for me. #Happy Christmas Becky. - Syn

[@0517mshumer]

Thank you for the video, it's great. Love the hair cut!

[@junkmail4613]

0:30 Sitting so far forward makes your complexion as soft and perfect as your nice fuzzy sweater!!!

[@Muritaipet]

LMAO @ the girly astrophysicist, who does this amazing presentation on E=MC squared, so serious!! But loves her necklace too. Humans are funny, and Becky ........ you're wonderful

[@dungeonseeker3087]

Not sure if you take requests but one thing that really fascinates me is the idea that nothing is ever really empty, even a vacuum. Would love to see a video explaining the concept.

[@Anonymous-pm7jf]

Awesome explanation.. thank you.

[@RichardT2112]

Love the haircut ... video is brilliant as always!

[@JockMcBile]

Can't wait for Your New Book to come out in the New Year, over here. Yep, already Pre-Ordered

[@tigereye4c565]

thanks for the great work that you do.