# Human & Speed of Light

1. Jul 29, 2004

### Sana

Dear Sir,
Can any one inform me and discuss about it that"If a body travels or comes with speed of light so can it'll be visible for human, can human see that body"? or simple "Can human see the body if it come with speed of light"?

Regards

Sana

2. Jul 29, 2004

### Reflector

Can't you speak english properly? I couldn't understand half of what you asked. And who are you addressing 'Dear Sir to?'.........

3. Jul 29, 2004

I think I understang your question (barelly). This is a paradox that Einstein had when he was thinking about light. If someone travels at the speed of light, and they hold out a mirror in front of them, can they see their image in the mirror? The answer is YES. The speed of light in a tricky subject. If you do the relativistic addition of welocity, you will get the ruslt that the light emmited out towards the mirror by your body is c, or the speed of light, so the person travellong at the speed of light in a spaceship wouldent even know it. This following is a calculation of the result.

u' = v + u / (1 + vu/c^2)
u' = 1c + 1c / (1 + (1c)(1c)/c^2)
u' = 2c / (1 + 1)
u' = 1c

4. Jul 29, 2004

### Tom Mattson

Staff Emeritus
If you don't understand the question, then don't bother with it.

Sana,

I've moved your post from the Quantum forum to the Relativity forum, because that's what this is really about. For those who don't understand, what Sana is asking about is this.

Consider a body that radiates in a part of the EM spectrum that is not visible (say, beyond either infrared or ultraviolet). The question is: Is it possible for the body to move at such a speed that the radiation is Doppler shifted to the visible spectrum? That is, is it possible that a body which is not visible to the naked eye can move so fast as to become visible?

Check out the following page from hyperphysics:

Relativistic Doppler Shift Calculator

It states the mathematical formulae, and has a neat little calculator that will spit out the Doppler-shifted results for you.

So, for instance, if you want to see how fast you would have to go for a red object (lsource=700*10-9nm) to disappear into the infrared (lobserved>700*10-9nm), then you just have to input those two parameters into the appropriate places and left-click outside the fields. The calculator will fill in all the other fields for you.

5. Jul 29, 2004

### Sana

Question is simply:
Can we see a body which travels with speed of light?

Sana

6. Jul 29, 2004

### Tom Mattson

Staff Emeritus
Oh, I guess Nenad was right.

The answer is: Sure you can. They're called photons!

7. Jul 29, 2004

Staff Emeritus
Up in Mkaku forum I answered Sana's question no. Our eyes interact with scattered photons to enable us to see other things, but we don't see the photons themselves.

8. Jul 29, 2004

### j8hart

When a photon arrives at your "eye" you "see" it.

However nothing can tell you it's coming, since nothing can out pace it.

If there was such a thing as a body that shines and also travels towards us at the speed of light we would not be able to see it coming.

Is that what you were asking?

9. Jul 29, 2004

### Tom Mattson

Staff Emeritus
That's one way to look at it. Another way to look at it would be to say that photons are the only thing that anyone ever really sees. But I don't think that's the issue so much as whether or not an object moving at c is detectable. And photons clearly are, else you wouldn't see the things that they scatter off of.

10. Jul 29, 2004

### reilly

It all depends. If the body is traveling toward you, the energy of the radiation/photons will burn you to a crisp - "blue shift",the relativistic doppler shift will give frequency -> infinity as v->c. A body going away from the observer at c will be invisible, due to the red shift.

Yes, we see via photons, but only ones in a limited part of the frequency spectrum, one that does not include 0 or abitrarily large frequencies.
Regards,
Reilly Atkinson

11. Jul 30, 2004

### Reflector

Oh I see. Sure you can. Photons travel at the speed of light and you can see them....

12. Aug 7, 2004

### sal

There's a problem, though, which is anything traveling at C is moving on a null geodesic, and time doesn't pass for that object. Therefore, if you were traveling at C, you could not see yourself in a mirror, nor do anything else, because your "clock" would stop.

But your calculation used the composition of velocities rule, which is derived from the Lorentz transforms, which are not valid for an observer moving at C. There's a divide by zero in there someplace if v=c, so you can't really conclude anything about how fast a photon would appear to travel from the PoV of another photon using CoV -- and that's really what you're trying to do here.

To put it another way, if one photon is following another one, how fast does the one behind see the one in front pulling away? It'll be something like

dx/dtau

where 'x' is the distance between them, and 'tau' is the photon's proper time. But dx is zero (the distance between them doesn't change) and dtau is zero, because "proper time" doesn't pass for a photon -- so it's 0/0.

In other words, the answer, according to SR, is undefined.

13. Aug 8, 2004

Im preety shure it is defined. Most physics books will tell you that you will see your image when traveling at the speed of light. And in my calculation, there is no division by 0. Ill do it again for you in latex form.

$$u' = \frac {v + u} {1 + vu/c^2}$$

$$u' = \frac {1c + 1c} {1 + (1c)(1c)/c^2}$$

$$u' = \frac {2c} {1 + 1}$$

$$u' = 1c$$

14. Aug 8, 2004

### Tom McCurdy

Are photons pure energy? Well I supose everything is pure energy if your a string person, but I mean anything with mass can't go the speed of light so if you were continuingly pushing something it turns into energy so what determins what it would be if it were to travel at c?

Last edited: Aug 8, 2004
15. Aug 8, 2004

### sal

Show me a quote from any physics text indicating that velocity composition is well-defined for a frame of reference moving at C.

Perhaps I was not clear.

The formula you used, composition of velocities, is a consequence of the Lorentz transform.

The Lorentz transformation is not valid at C -- you cannot use it to shift your point of view to an FoR traveling at C. The transform incorporates gamma explicitly, and gamma->infinity as v->c.

In consequence, nothing derived from the Lorentz transform -- including the CoV rule -- is valid for v=c. The derivation of the composition of velocities law involves a divide-by-zero if one frame is moving at C, so the law itself is not valid for that case.

As to "most" physics books claiming you can see yourself in a mirror while traveling at C -- could you give an example of one? Since physics texts which treat relativity universally conclude that you can't travel at C, it would seem that any claim based on relativity theory about what you could do if you did travel at C must be vacuous, wouldn't you think?

In other words, you can't use a theory that says "X is impossible" to determine the behavior of "X" since, by definition, it's outside the domain of applicability of the theory.

The one thing SR does predict about travel at the speed of light is that proper time doesn't pass in that case. This prediction has consequences. In particular, massless particles cannot decay. Another consequence is that something traveling at C can't "do" anything -- photons are "frozen" except when they interact with other particles.

As an example of a consequence of the statement "time doesn't pass when you're moving at C", neutrino oscillations imply that nuetrinos have mass, because otherwise they would necessarily travel at C, and in that case they couldn't oscillate because something traveling at C can't "do" anything.

So, if you were a massless particle traveling at C, you could be hit by a mirror. But you couldn't look at the mirror, because that act of "looking" would imply some time had passed for you. Acts imply duration, and there isn't any when traveling at C.

16. Aug 8, 2004

### pmb_phy

There sure were a lot of answers to this very simple question. The problem is that you've assumed that a body, i.e. something which can emit or reflect photons, can travel at the speed of light. Since relativity says that is impossible the the question itself has no meaning. That was one of the more important conclusions Einstein's theory predicts.

Pete

17. Aug 8, 2004

### Neo

Hi Sal

Does this imply that photons, presuming they were created in the process of
Big Bang, are as old as the universe? "Immortalized photons?" Would this imply, do you think, that the usable energy of the universe would never run out and that photons cannot undergo entropy? Does this idea of "eternal photons" dismiss the possibility of "heat death?"

18. Aug 8, 2004

### sal

No, not at all -- photons can interact with other particles, and they can be absorbed; they just can't do anything spontaneously.

All it means is photons can't decay without the mediation of another particle, and nobody ever suggested they did that anyway, as far as I know.

The neutrino issue, on the other hand, is real and significant -- neutrinos were suspected of being massless and traveling at C, like photons. But determining that they oscillate -- or, in other words, they decay into other types of neutrinos -- shows that they can't be massless. If they were massless, then the neutrino half life would necessarily be infinite.

19. Aug 9, 2004

### Neo

Interesting...spontaneous processes require free energy. So does that imply that photons have no free energy? Or that they cannot use their free energy?

20. Aug 9, 2004

### sal

Dunno. I'm out of my depth once we step off the pier into the waters of thermodynamics.