Another science question need more nerds.

[Shadowferal]

Any mass at the speed of light would be infinite...which would take an infinite amount of energy to propel...

At least by Einstein.

[Wikiy]

It's a shady practice.

Either you're asking the students when they're too young; they don't know enough on the subject for the experience to be rewarding and they end up handing in a half-assed homework, or they are old enough and know enough, in which case it's a moot exercise, as the information would be better served during class. During a lecture, or spending 10 minutes on the subject in groups discussing in class with a teacher around would at least teach and reward me more than spending weeks on the task outside the classroom would.

It would be a great homework for some story telling (language) or some sort of arts class, though. But then again, I had grossly incompetent teachers so for me questioning a teacher is the first stop. "Higher thinking" and "exploration" wouldn't even be on their radar.

Maybe I just fail to see what such a homework would teach (the average student).

I honestly find more problems with the stopping and how would it affect the environment part than the part that someone is moving at speed of light. What was probably meant was that you're moving near the speed of light rather than at the speed of light.

The question of what would happen if air were to stop such a relatively large and massive object as a human, moving at near speed of light, as simple as it may sound, is actually probably more complex than the question of what happens exactly in the core of an exploding nuclear bomb. At least a nuclear bomb has spherical or cylindrical symmetry. But yeah, this is the reason why I think the question is inappropriate for schools - it would require proper, multi-month research (probably) to figure out.

[Elim Garak]

It’s like a self-contained paradox, what happens when an unstoppable force meets and immovable object, BUT AS THE SAME THING.

That's pretty easy - an unstoppable force bounces off an immovable object, force continues to be unstoppable, object stays immovable.

That's if you need them to collide - if not then the force can go into orbit around immovable object.

[AwkwardSquirtle]

Isn't that just limits?

- - - Updated - - -



I don't think mmo champion is the best place to have homework done

I dunno, you get a lot of nerds on gaming forums, some of them have to be science nerds.

[Shadowferal]

That's pretty easy - an unstoppable force bounces off an immovable object, force continues to be unstoppable, object stays immovable.

That's if you need them to collide - if not then the force can go into orbit around immovable object.

"Unstoppable" force is de facto "irresistible." It doesn't bounce off anything. It goes through everything.

Immovable object is de facto "impenetrable." Nothing goes through it.

This is why it's a paradox.

[May90]

I ve got a better question I was thinking of recently:

If you are on a spaceship that is moving at the speed of light (lets say point of origin is Earth, so in a line away from Earth) and you walk forward with a constant velocity. What happens then? Do you move at a speed (in a line away from Earth) that is higher than the speed of light?

Well, it is impossible. If you were on a spaceship moving at the speed of light, then essentially the spatial dimension along your movement would cease to exist, as would time, so you wouldn't be able to walk "forward" - there wouldn't be any "forward" to walk.

If the spaceship is moving at 0.99c relative to Earth, and you start walking forward in the ship at 0.99c, then your speed as seen from Earth will be approximately 0.99995c:

https://en.wikipedia.org/wiki/Veloci...ial_relativity

I would seriously question the teacher who thought the OP's question would be suitable homework. If I had gotten that homework I would have filed a strongly worded letter to the headmaster, clearly explaining why it's nothing short of bonkers.

Perhaps that was the goal of the assignment: to see how well the students understood the subject. If they understood it well, they can realize that the described scenario is impossible.

Isn't that just limits?

Well, yes... But those aren't some random limits. It's not like c is just some arbitrary speed chosen by the God which we can never overcome. It being c arises from the geometry of our space. Of course, why c is roughly 300,000 km/s - that's a more interesting question. Perhaps it is just the size of the fluctuation of something fundamental at the moment our Universe was created, and there might be other universes with other values or c, or even with other geometries entirely, like something crazy with 2 different time coordinates...

That is a mumbo-jumbo at this point though, and, beyond the scope of very marginal theoretical physics, it doesn't have many uses.

[Elim Garak]

"Unstoppable" force is de facto "irresistible." It doesn't bounce off anything. It goes through everything.

Immovable object is de facto "impenetrable." Nothing goes through it.

This is why it's a paradox.

Even that semantic twist can be overcome by quantum physics.

Force can let the object thru itself. Object is not penetrated - it penetrates the force. Force is not resisted either as it doesn't stop nor deviates in any way. No collision ever occurs between the two.

[May90]

The speed of light is 300,000 km/s due to our choice of units. This just means our choice of units are not natural. In cosmology c = 1.

Yes, but there could be, say, another universe in which it would be 5,000,000 km/s in our current units, due to the space-time scaling being different. Why is it exactly the value we are getting in our Universe? I doubt there is a definite answer to be found, and it likely has to do with probabilities.

[Vanyali]

I think the OP is asking what would happen if he stopped gradually over 1 meter. But I'm not sure if it would make a difference on the outcome.

1 meter at that speed is instantly.

[Shadowferal]

Even that semantic twist can be overcome by quantum physics.

That's an assumption.
Science demands proof.

[Dendrek]

That's an assumption.
Science demands proof.

You can't have proof of an impossible question. You cannot have an unstoppable force (infinite mass or infinite acceleration). You cannot have an immovable object (infinite mass or infinite inertia).

Additionally, even if either were possible, they would necessarily be black holes (one accelerating and the other not? Though it wouldn't be possible for one to be accelerating...). In that case, they would both swallow up anything they collided with. Additionally, they'd both be "unstoppable forces" because they'd have a gravitational pull that would be infinite across all space; the entire universe would be sucked into them. They'd also have a Schwarzschild Radius that is infinite, so the entire universe wouldn't even be "sucked" into them; it would already be inside.

[Shadowferal]

You can't have proof of an impossible question. You cannot have an unstoppable force (infinite mass or infinite acceleration). You cannot have an immovable object (infinite mass or infinite inertia).

Precisely my point.
That why:

Even that semantic twist can be overcome by quantum physics.

He'll never be able to prove such a presumption.

[Elim Garak]

Precisely my point.
That why:

He'll never be able to prove such a presumption.

You have to define both objects in real properties. Like "unmovable object is 1 cubic meter in size and is located there, it's made of this and it is colored pink". Same for the force. Until you do that I don't have to prove anything.

If it's a spherical object in vacuum and spherical force in vacuum - then I'm free to use spherical physics.

[Deleted]

Rel Mass = m / (1 - √(v^2 / c^2)) --- Rel Mass is relativistic mass, v is velocity, c is the speed of light. As v approaches c, Rel Mass approaches ∞

So if you are moving at the speed of light, you would have infinite relativistic mass, meaning it would take an infinite amount of force to stop you.

so your saying that a single sheet of wood, which is stopping light and casting a shadow, is able to withstand an infinite amount of force ?

[Azadina]

so your saying that a single sheet of wood, which is stopping light and casting a shadow, is able to withstand an infinite amount of force ?

That light has no mass.

[Orange Joe]

I've never understood this

How come a finite speed makes an object have infinite energy/mass?

Because of friction. The faster you go the more friction you have working against you.

[Twilight Cultist]

so your saying that a single sheet of wood, which is stopping light and casting a shadow, is able to withstand an infinite amount of force ?

Well, it isn't actually possible for anything with mass to travel at the speed of light, for multiple reasons. But theoretically, yes. If you somehow got a sheet of wood to travel at the speed of light and it remained a sheet of wood instead of separating into a bunch of elections and quarks, you would need an infinite amount of force to stop it.

[May90]

That equation does not apply to photons. Light has a finite amount of energy. The sheet of wood suffers very little force.

Yup. Light can travel at the speed of light (I know it is tautology ), because photons have zero rest mass. No matter what energy a photon has, it always will move at the speed of light, just its interaction with other particles will vary, depending on its energy.

That light has no mass.

Strictly speaking, photons don't have a rest mass, but they do have a "dynamic mass", proportional to their energy: E=mc^2. This mass doesn't experience gravitational force though, only gravitational lensing.

[Malfecto]

The reason you can't understand why reaching the speed of light affects mass and energy is because you are ignoring inertial frames and treating mass as rest mass and not relativistic mass. The speed of light is the same in all inertial frames in special relativity because it defines spacetime. Spacetime is not just energy and mass but distance and time as well. All of these must be considered in the inertial frame in order to understand the result.

Quoting from a better explanation than I can piece together in my distracted brain at the moment:

To accelerate you need to use energy. At low speeds the amount of energy you have to use to go faster seems to follow a linear relationship - put in a bit more energy and you'll go proportionally faster - but once you start approaching the speed of light the relationship between how much energy you use and how much faster you go trails off and instead your mass increases, your rate of time decreases and your length, in the direction you're traveling in, also decreases.

Although this seems weird, it might help if you separate out the different factors of the situation. These are mass (from the matter comprising the object being accelerated) and distance and time (from velocity), and all of these are actually affected when you apply energy to something to make it move. As I said earlier, at low speeds, nearly all of the energy goes into moving the mass over the distance in the period of time, but as you get faster and faster the energy you put in begins to have more of an effect on the other factors instead i.e. mass, distance and time.

Now as to why there should be an upper speed limit is still one of the unsolved mysteries of the universe but it's been proved satisfactorily enough that as the velocity of something approaches the speed of light we do actually start to see the effects upon mass, distance and time. This has been demonstrated in lots of experiments, from comparing pairs of clocks at different altitudes, where the slight difference in gravitational strength simulates acceleration, to comparing clocks where one is moving and one is stationary. Perhaps the best demonstration though, is in particle accelerators such as the LHC (cough), where unstable particles start living much longer than they would normally do and where more energy is needed to keep them on course than would be the case if their mass didn't increase.

The relationship between an object's velocity and the relativistic effects such as time dilation, foreshortening and mass increase is actually quite simple and follows a circular sin law. It's perhaps easiest to show if we normalise the speed of light to '1' and use Pythagorus, where the hypotenuse represents the speed of light - 1, to get a factor we can multiply the normal rates of time and length by to get the relativistic values.

For example, if our velocity (v) is zero then the factor for our rate of time will be:
SQRT(c^2 - v^2)
= SQRT(1^2 - 0^2)
= SQRT(1)
= 1

So multiplying the normal rate of time by 1 gives us time passing at 100% of it's normal rate. However, if we've got up to half the speed of light, so v = 0.5, we get:
SQRT(1^2 - 0.5^2)
= SQRT(1 - 0.25)
= SQRT(0.75)
= 0.866025404

And now time will only be passing at 86.7025404% of it's normal rate i.e. slower. Relativistic length contraction follows the same rule but for mass increase you need to divide the normal mass by the factor instead of multiplying it, so the mass increase at 0.5 'c' would be 1.154700538 times it's normal mass.

Now if you try to get factors for speeds greater than the speed of light - say 1.5 times 'c' we'd get:
SQRT(1^2 - 1.5^2)
= SQRT(1 - 2.25)
= SQRT(-1.25)
= ERR

http://www.thenakedscientists.com/fo...7578#msg197578

You also have to understand momentum in special relativity is not the same as how we observe momentum under more general Newtonian mechanics.

https://en.wikipedia.org/wiki/Energy...entum_relation

So you have to define a lot of things to answer your question. Consider though that light travels through and around us all the time with ill effect, quite close to the speed of light. But in its inertial frame, light is massless radiation. If you were to suddenly accelerate your body to near the speed of light (let's say 99.9%), the energy needed would be:

E= 90kg * (.999*300,000 m/s)^2 or 80,838,081,000,000,000 Joules, or 8x10^16 Joules. *edit: Wrong order of magnitude row on the table) This would be the same as a megaton of TNT. I don't think your room would be safe.

Although...

given that you cross 1 meter at near the speed of light, you will generate 2.66x10^23 Watts or 266 Zettawatts, twice the luminosity of Wolf 359. Yea that room is toast man.

Another fun comparison is that you would exert 2 tenths of a Yottawatt, which is the energy created by Type II civilizations. These involve star lifting, collecting photons from feeding an accretion disk of a black hole, Dyson swarms and megascale antimatter processes.

- - - Updated - - -

Turns out you can, but it can't be in a vacuum.

I think he meant the normal state or human body you. Even so, Cherenkov radiation is moving faster than its excitation radiations phase velocity, which is slower than the speed of light but faster than the generating radiation's speed in the medium. Again, relativistic frames. Never does it reach or exceed c.

[May90]

E=mc^2 is not for a photon, you're looking for m=p/c and by extension m=h/λc.

I'm not sure what you mean by relativistic mass not experiencing a gravitational force: light cannot escape a black hole, for instance.

This expression still allows one to find the mass from energy. What you wrote is the same, just for momentum. E=pc is the same thing, essentially.

What I mean is, two photons flying by each other parallel to each other won't change their direction because of gravity, since there is no gravity between them: gravity is only formed by particles having a rest mass. Photons do change their directions in gravitational field due to the form of space-time changing, but it is not a gravitational attraction as such. A photon with relativistic mass of 1 eV will experience exactly the same gravitational lensing as a photon with relativistic mass of 1 TeV.